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Journal of Asian Concrete Federation

Vol. 2, No. 2, Dec. 2016, pp. 95-101

ISSN 2465-7964 / eISSN 2465-7972

http://dx.doi.org/10.18702/acf.2016.12.2.2.95

Fire resistance calculation of existing concrete structures using

modified Tabulated Data Method

Duinkherjav Yagaanbuyant

*and Khishgee Radnaabazar

(Received: March 21, 2016; Accepted: October 24, 2016; Published online: December 31, 2016)

Abstract: During its service operation, a gradual reduction of the material strength and the load bearing ca-

pacity of a concrete structure is expected and the same is also true for its fire resistance capacity. Although it

is needed to estimate the remaining service life of the structure considering this reduction of fire resisting

capacity for the design for the building renovation or expansion, the proper calculation guidance or the

methodology still doesn’t exist in Mongolian national building code. This paper discusses the assessment of

the actual fire resistance capacity of the existing pre-cast concrete industrial/residential buildings that were

built during 1962-1999 in Ulaanbaatar, Mongolia. Modifications to the Tabulated data method are offered

based on the findings of the study. The estimated values of the actual fire resistance limit of PC walls and

slabs are proposed in tables and graphical forms that can be used in the calculation of the actual fire re-

sistance of the existing buildings in use to determine renovation requirements.

Keywords: pre-cast concrete, reinforcement, fire severity, inspection, Tabulated data method.

1. Background

During its service life of a concrete structure, a

gradual deterioration of structural performance in-

cluding a reduction of its fire resistance perfor-

mance is expected. When the deterioration is signif-

icant, the fire resistance capacity assessment of an

existing structure becomes one of the requirements

for further renovation and expansion. Roitman has

studied the fire resistance of existing buildings in

terms of effects of the reinforcement corrosion and

the damages to concrete, but the service life of the

structure has not been considered [1]. In this paper,

the operational condition and the service life of

structures are taken into account in the assessment

of the fire resistance capacity of concrete structures

for their further occupation.

In general the deterioration of the industrial

buildings can be often of major concern. But from

recent studies by the authors for the residential

buildings, even though their operational conditions

are not as severely reduced as those of the industrial

buildings, the accumulated deterioration in the

structures through years can have significant ad-

verse effects on its fire resistance capacity and

therefore should be included in the design of the

building expansion and/or the renovation. In recent

years such consideration of structural deterioration

reducing its fire resisting capacity has become even

more important due to the increased demand of

building expansion through addition of stories and

re-planning in urban areas in Ulaanbaatar. As the

currently used Mongolian national building code

doesn’t include a proper guideline to estimate this,

the authors offer a modification to currently used

Tabulated data method of the fire resistance based

on the actual service condition and the service life

of the existing concrete structures.

2. Fire resistance assessment methods

The fire resistance of reinforced concrete struc-

tures can be assessed experimentally through fire

tests of the actual structures or analytically by engi-

neering calculations [1~3]. A brief summary is giv-

en below.

2.1 Experimental Method

The results of the fire tests on the actual struc-

tures are most reliable and accurate for the assess-

ment of the fire resistance capacity. Usually the fire

tests are carried out using a fire chamber equipped

with the load testing machine and the furnace. Cal-

culation tables or tabulated data as developed from

Corresponding author Duinkherjav Yagaanbuyant is a Profes-

sor of the School of Civil Engineering and Architecture, The

Mongolian University of Science and Technology, Ulaanbaatar,

Mongolia.

Khishgee Radnaabazar is an Associate Professor of the School

of Civil Engineering and Architecture, The Mongolian Universi-

ty of Science and Technology, Ulaanbaatar, Mongolia.

95

the multiple test data to be used for the structural

assessment based on the structure’s geometry, ma-

terial type, and loading and boundary conditions.

The disadvantages of the fire test are time, cost,

energy consumption, and a need of special facility

and specialists which make the analytical methods

more favorable. The fire tests better be pursued for

selective structural elements or assemblages, a new

type of material or design of structural elements.

2.2 Analytical Methods

The analytical method of the fire resistance ca-

pacity determination of a structure can be carried

out in two steps: thermal analysis and static analysis.

The thermal analysis determines the developed

temperature field across the cross section of the

structure and the static analysis determines the load

bearing capacity of the structure under loading tak-

ing into account the effects of the changes of the

material properties due to elevated temperatures. In

general, the fire resistance capacity estimation is

carried out by incremental iteration method from

the initial ignition of the fire through thermal and

static analysis steps. In Fig. 1, the strength reduc-

tion over time is shown as an example of static

analysis result. The fire resistance limit , is con-

sidered as the time period of the ultimate load bear-

ing capacity reduction of the structure to its critical

state . In Fig. 2, a typical thermo-mechanical

scheme of a RC structure during fire exposure is

shown.

In BS EN 1992-2, three fire resistance

assessment methods are suggested: (1)Tabulated

data method, (2) Simplified calculation method, and

(3) Advanced methods. It is also specified that the

Tabulated data method can be used as an appendix

to the national standards.

In Russian and Mongolian practice, the

experimental method and above mentioned three

analytical methods are both used for assessment of

Fig. 1 – Typical scheme for the assessment of fire

resistance by strength reduction criterion at elevated

temperature [2]

Fig. 2 – Typical thermo-technical scheme:

1-fire, 2-concrete, 3-reinforcement, d-member

width, a-concrete cover depth, -critical temp.,

-fire temp., -temp. profile [2]

the structural fire resistance and, for this purpose,

the guidelines and procedures of ref. [7~9] are

followed. Currently, in Mongolia, the most

frequently used method for the fire design of a new

building is the Tabulated data method. But it cannot

be used for the design for the building expansion,

addition of stories in an existing building, re-

planning of the existing building. In the Tabulated

data method, the resistance limit of reinforced

concrete slabs and walls is given based on element

thickness and concrete cover depth of reinforcement,

but in the case of an existing building in use, actual

values of concrete cover and thickness could be

reduced due to defects in concrete cover and

reinforcement corrosion. Therefore certain

modifications are required in the use of the

tabulated method for the actual fire resistance

estimation of the existing buildings in use.

The Simplified calculation method is based on

applying the strength reduction factors at elevated

temperatures to simplified cross-sections for which

the thermal gradient through the structural member

is known or, alternatively, by utilizing a reduced

cross-section whereby a damaged zone is ignored in

the calculation and only the residual undamaged

part is included in the calculation of the resistance

[13]. The relevant parameters for the Simplified

calculation method are taken from the information

supplied on the material properties of the structural

component.

Generally the Advanced method includes the

numerical models for heat transfer from the fire into

the structural member and the mechanical response

models to determine the structural response of the

member or members to the effects of both

mechanical and thermal actions. General guidance

is available in the European Standard on the use of

the Advanced calculation models for the design of

concrete structures. Unlike the Tabulated data

96

Journal of Asian Concrete Federation, Vol. 2, No. 2, Dec. 2016

Table 1 – Fire resistance limit of reinforced concrete slabs [7]

Concrete type and slab characteristics Slab thickness t and concrete cover depth

of rebar a, mm

Fire resistance limit, min

15 30 60 90 120 150 180

Normal

Slab thickness t 30 50 80 100 120 140 155 One way or two way,

ly/lx≥1.5 a 10 15 25 35 45 60 70

Two way,ly/lx<1.5 a 10 10 10 15 20 30 40

NOTE: Normal weight concrete (2,000-2,600 kg/m3)

Table 2 – Fire resistance limit of load bearing walls [7]

Concrete type

Wall thickness tc and concrete cover depth of rebar a, mm

Fire resistance limit, min

30 60 90 120 150 180

Normal tc

a 100 10

120 15

140 20

160 30

200 30

240 30

method, the Advanced method is suitable for any

type of fire exposure. However, knowledge on the

degradation of the material properties for the

particular fire exposure condition is required.

2.3 Tabulated Data Method

From the guidelines of ref. [7,8], the fire re-

sistance of reinforced concrete slabs and walls

based on concrete type, element thickness, and con-

crete cover depth of reinforcement are shown in

Table 1 and Table 2. It must be noted that, in the

Tabulated data method, the resistance limit of rein-

forced concrete slabs and walls is given based on

element thickness and concrete cover depth of rein-

forcement, but in the field, other effects such as de-

fects in concrete cover, reinforcement corrosion,

and concrete strength actually play important roles

too [1,10].

3. Actual fire resistance of existing PC res-

idential buildings

From the tabulated data, the design limit of fire

resistance can be determined. But for the exist-

ing building in use, the actual fire resistance of the

structure is lower than this design value due to

the deteriorated state of the structures. The actual

fire resistance of the structure can be determined

by Eq. (1), where С1 is the reduction factor of the

service life and С2 is the reduction factor of the ac-

tual service conditions [1].

= С1 С2, (1)

The reduction factor С2 that accounts the service

conditions can be expanded further as С2,1;С2,2;...С2n

taking into account specific characteristics of the

condition such as concrete strength, corrosion,

Fig. 3 – Fire resistance limit of reinforced concrete

walls of pre-cast residential buildings based on ser-

vice condition and service life


slabs of pre-cast residential buildings based on ser-

vice condition and service life (ly/lx ≥ 1.5)


slabs of pre-cast residential buildings based on ser-

vice condition and service life (ly/lx< 1.5)

97


Table 3 – Fire resistance limit of reinforced concrete walls based on service condition and service life

Thickness tc and concrete

cover depth of rebar a, mm Fire resistance limit, hrs

0.5 1 1.5 2 2.5 3

tc, mm 100 120 140 160 200 240

a, mm 10 15 20 30 30 30

Ser

vic

e li

fe,

yea

rs

10 0.49 0.99 1.48 1.98 2.47 2.97

15 0.49 0.98 1.46 1.95 2.44 2.93

20 0.48 0.96 1.44 1.92 2.40 2.88

25 0.47 0.94 1.41 1.88 2.35 2.82

30 0.46 0.92 1.38 1.84 2.30 2.76

35 0.45 0.89 1.34 1.79 2.24 2.68

40 0.43 0.87 1.30 1.73 2.17 2.60

45 0.42 0.84 1.25 1.67 2.09 2.51

50 0.40 0.80 1.20 1.61 2.01 2.41

Table 4 – Fire resistance limit of reinforced concrete slabs based on service condition and service life

(boundary condition, ly/lx ≥ 1.5)

Thickness tc and concrete cov-

er depth of rebar a, mm

Fire resistance limit, hrs

0.25 0.5 1 1.5 2 2.5 3

tc, mm 30 50 80 100 120 140 155

a, mm 10 15 25 35 45 60 70

Ser

vic

e li

fe,

yea

rs

10 0.25 0.49 0.98 1.47 1.97 2.46 2.95

15 0.24 0.48 0.96 1.44 1.92 2.40 2.88

20 0.23 0.47 0.93 1.40 1.87 2.34 2.80

25 0.23 0.45 0.90 1.35 1.80 2.25 2.70

30 0.22 0.43 0.86 1.29 1.72 2.15 2.58

35 0.20 0.41 0.81 1.22 1.63 2.04 2.44

40 0.19 0.38 0.76 1.14 1.52 1.90 2.28

45 0.18 0.35 0.70 1.05 1.40 1.75 2.10

50 0.16 0.32 0.64 0.95 1.27 1.59 1.91

Table 5 – Fire resistance limit of reinforced concrete slabs based on service condition and service life

(boundary condition, ly/lx < 1.5)

Thickness tc and concrete cov-

er depth of rebar a, mm

Fire resistance limit, hrs

0.25 0.5 1 1.5 2 2.5 3

tc, mm 30 50 80 100 120 140 155

a, mm 10 10 10 15 20 30 40

Ser

vic

e li

fe,

yea

rs

10 0.25 0.49 0.98 1.47 1.97 2.46 2.95

15 0.24 0.48 0.96 1.44 1.92 2.40 2.88

20 0.23 0.47 0.93 1.40 1.87 2.34 2.80

25 0.23 0.45 0.90 1.35 1.80 2.25 2.70

30 0.22 0.43 0.86 1.29 1.72 2.15 2.58

35 0.20 0.41 0.81 1.22 1.63 2.04 2.44

40 0.19 0.38 0.76 1.14 1.52 1.90 2.28

45 0.18 0.35 0.70 1.05 1.40 1.75 2.10

50 0.16 0.32 0.64 0.95 1.27 1.59 1.91

98


freeze-thaw action etc. as shown in Eq. (2). In this

study, the concrete strength and the rebar corrosion

reduction factors are taken in the example calcula-

tion.

= ∙С1∙С2,1∙ С2,2∙ ..∙С2n (2)

To determine the reduction factor of the ser-

vice life С1, the damage state of structures should be

determined first through a detailed inspection of

each structure in terms of important characteristics

of structure for its technical serviceability such as

section dimension, strength, and cracking etc.

A total of 1,077 pre-cast concrete buildings

had been built in 1962-1999 in ten districts of

Ulaanbaatar. Buildings have 5 to 12 stories with the

structural pre-cast reinforced concrete wall and slab

panel system without framing. The inspections of

more than 60 buildings from these pre-cast concrete

buildings in 2001-2015 to assess the structural state

of the buildings have been carried out by the au-

thors. The structural deterioration in these buildings

was mainly caused by damaged concrete cover, re-

inforcement corrosion, and multiple freeze-thaw

action resulting in the reduced fire resistance. In

this paper, test results of 30 buildings located in 10th

Micro-district of Ulaanbaatar have been used. From

the study, it is observed that external walls, base-

ment floor slabs and walls, and roof slabs are more

damaged than the other structural elements due to

the reinforcement corrosions. These damage condi-

tions not only deteriorate the load bearing capacity

of the structures but also the fire resistance.

The damages of each structural element are reg-

istered and a damage degree is determined as a

ratio of the number of the damaged element to the

total number of elements in each structural type.

From the data, the correlation between the damage

degree and the service life of the structure t is

observed and the following relations are established

through regression equations:

External wall

(3)

Internal wall

ω = 0.0829 + 0.0019t+0.00001t2 (4)

Roof slab

(5)

Floor slab

(6)

Based on Eq. (4), Eq. (6), and Table 1 and Table 2,

the actual reduction of fire resistance of PC slabs and

walls can be determined. Results are shown in Figs. 3

through 5 and Tables 3 through 5.

4. Examples of calculation

4.1 Calculation of fire resistance of reinforced

concrete slab of 5-story residential building

Data: Basement floor reinforcement concrete slab,

5-story residential building located in the 15th

ho-

roolol, Ulaanbaatar. Slab dimension l = 5.4 m; b =

3.6 m; h = 0.12 m. Number of tensile rebars – 18

Class Вр-I, ds= 5 mm. Concrete class B15, concrete

cover depth from the rebar centroid δ = 0.02 m, ly/lx

= 5.4 / 3.6 =1.5

4-1-a Geometric characteristics required for the cal-

culation

By load bearing reduction “R”: а = δ + 0.5ds =

0.02 + 0.0025 = 0.0225 m

By insulation capacity “I”: h = 0.12 m

4-1-b Design fire resistance value for floor slab, 5

story pre-cast panel reinforced concrete building

By load bearing reduction “R”, Table 1: if a =

0.015 m then ( ) = 0.5 hr; if a = 0.025 m

then ( ) = 1.0 hr; а = 0.0225 then ( ) =

0.875 hr = 52.5 min

By insulation capacity “I”, Table 8 [7]: h =

0.12 m, (I)= 2 hr = 120 min

4-1-c Actual fire resistance based on service condi-

tion and life

Service life = 34 years

By load bearing reduction “R”, Table 3 or Fig

2: if a = 0.015 m then (R) = 0.41 hr; if a =

0.025 m then (R) = 0.91 hr; if а = 0.0225 m

then = 0.785 hr = 47.1 min

Actual fire resistance based on service condi-

tion and life: = 47.1∙0.92∙0.91 = 39.4 min

It is calculated that the actual fire resistance of

the floor slab is reduced by 25 percent based on the

service condition and service life.

4.2 Calculation of fire resistance of reinforced

concrete wall of 5-story residential building

Data: Reinforcement concrete partition wall, l = 5.4

m; b = 2.8 m; h = 0.14 m. Number of tensile rebars

– 18 Class A400, ds = 12 mm. Concrete class B15,

concrete cover depth δ = 0.015 m

4-2-a Wall shall be exposed to the fire from one

side. It is very rare for walls to experience fire from

both sides. If the wall is exposed to fire from both

sides then no deflection takes place and wall still be

working under compression [7].

4-2-b Design fire resistance limit of the wall is tak-

en from Table 2.

99


4-2-c Calculation of geometric characteristics re-

quired for the calculation

By load bearing capacity reduction “R”: а = δ

+ 0.5ds = 0.015 + 0.006 = 0.021 m

By insulation capacity “I”: h = 0.14 m

4-2-d Calculation of fire resistance of reinforced

concrete partition wall, 5-story pre-cast reinforced

concrete panel building

By load bearing capacity reduction “R”: Table

2 fire resistance limit: if a = 0.02 m then =

1.5 hr; if a = 0.03 m then = 2.0 hr; if а =

0.021 m then = 1.55 hr = 93 min

By insulation capacity“I”: Table.2. if h = 0.14

m, then (I)= 1.5 hr = 90 min

4-2-e Calculation of the actual fire resistance of

reinforced concrete partition wall based on service

condition and service life

Service life = 34 years

By load bearing capacity reduction “R”: Table

4 [7] if a = 0.02 m then = 1.1 hr; if a =

0.03 m then = 1.5 hr; if а = 0.021 m then

= 1.14 hr = 68.4 min. Considering reduc-

tion of service condition (reduction of concrete

strength and rebar corrosion), = 68.4

∙0.92∙0.91 = 57.2 min

It is calculated that the actual fire resistance of

the partition wall is reduced by 38 percent from the

design value based on the service condition and

service life.

5. Conclusions

During renovation and expansion of the exist-

ing buildings in use, not only the structural load

bearing capacity, and stability requirements but also

fire resistance requirements should be considered

and satisfied to provide safe living condition to oc-

cupants. The Tabulated data method is the most

widely used method for practicing engineers today

but its application for the calculation of the fire re-

sistance of structures of the existing buildings needs

certain modification that accounts the deteriorated

state of the structures due to the actual service con-

ditions. From this study, up to 25 to 38 percent re-

duction in the fire resistance of structures depend-

ing on the actual service condition and the service

life is observed. Based on the findings of the field

study of 30 pre-cast concrete buildings, the empiri-

cal regression equations that establish relations be-

tween damage degree of structural members and

their service life are proposed. Based on these equa-

tions, the estimated values of the actual fire re-

sistance limit of PC walls and slabs are suggested in

tables and graphical forms that can be used in the

calculation of the actual fire resistance of structures

of the existing buildings in use for their further ren-

ovation. Example calculations are also included.

References

1. Roitman, В.М., (2001) Engineering solution

for the assessment of fire resistance in the de-

sign of new building and building renovation,

Fire Safety and Science Publishing. (in Rus-

sian)

2. Federor, V.S., Levitskii, V.E., Molchadskii,

I.S., and Aleksandrov, A.V., (2004) Fire Re-

sistance and Fire safety of structures, ASV

Publishing. (in Russian)

3. Lennon, T., Moore, D.B., Wang, Y.C., and

Bailey, C.G., (2006) Designers’ guide to

EN1991-1-2, 1992-1-2, 1993-1-2 and 1994-1-2,

Handbook for the fire design of steel, compo-

site and concrete structures to the Eurocodes,

Thomas Telford.

4. Beeby, A.W. and Narayanan, R.S., (2005) De-

signers’ Guide to Eurocode 2: Design of con-

crete structures, Thomas Telford.

5. BS EN 1991-1-2 (2002) Eurocode 1: Actions

on structures - Part 1-2: General actions, Ac-

tions on structures exposed to fire, British

Standards Institution.

6. BS EN 1992-1-2 (2004) Eurocode 2: Design of

concrete structures - Part 1-2: General rules,

Structures fire design, British Standards Insti-

tution.

7. Guideline for the calculation of fire resistance

limit of structures, fire distribution limit by

structures and material fire ratings, SNiPII-2-

80 (1988), TsNIISK imeni V.V. Kucherinko,

Stroiizdat Publishing. (in Russian)

8. Guideline for the calculation of fire resistance

and fire safety of reinforced concrete structure,

STO 36554501-006-2006 (2008). (in Russian)

9. Rules for fire safety assurance of reinforced

concrete structures, STO36554501-006-2006

(2006) FGUP, NITs, Stroitelistvo Publishing.

(in Russian)

10. Reports submitted to Ulaanbaatar city gover-

nor’s office: Inspection of Residential Build-

ings (2014) Mongolian Univ. of Science and

Technologies, Ulaanbaatar. (in Mongolian)

11. Recommendations for the inspection and as-

sessment of technical state of large panel pre-

100


cast and masonry buildings (1988) TsNIISK

imeni V.V. Kucherinko. (in Russian)

12. Duinkherjav, Ya., (2016) The accumulation of

defects and damage of concrete structures of

existing pre-cast buildings, XIV Concrete

Structures Conference, Novosivirsk, Russia.

pp.18-22

13. Lennon, T., Rupasinghe, R., Canisius, G.,

Waleed, N., and Matthews S., (2007) Concrete

structures in fire, Performance, design and

analysis, Garston, Watford, Bre press.

101



Vol. 2, No. 2, Dec. 2016, pp. 102-109

ISSN 2465-7964 / eISSN 2465-7972


Effectiveness of recycled nylon fibers as reinforcing material

in mortar

ORASUTTHIKUL Shanya*; UNNO Daiki; YOKOTA Hiroshi; and HASHIMOTO Katsufumi

(Received: July 21, 2016; Accepted: November 9, 2016; Published online: December 31, 2016)

Abstract: Disposing of waste fishing nets has been a major issue in the sea environment. Although the stor-

age of such nets has not caused a serious safety hazard to date, it is important to find suitable recycling solu-

tions. In this study, the authors investigate the utilization of recycled waste fishing nets in fiber-reinforced

mortar and compare the mechanical properties of such mortar made with recycled waste fishing nets to those

of mortar made with PVA (polyvinyl alcohol) short fibers. Two types of recycled nylon fiber were investi-

gated: straight fiber and fiber with a knot at each end. The straight recycled nylon fibers were obtained by

manually cutting waste fishing nets to the lengths of 20 mm, 30 mm, and 40 mm, and adding them to mortar

at the volume ratios of 1.0%, 1.5%, and 2.0%. The 40-mm-long knotted fibers were added to mortar at the

volume ratios of 0.5%, 0.75%, and 1.0%. The mechanical test results showed improvements in first-crack

strength, toughness, and ductility for mortar reinforced with recycled nylon fibers. The addition of recycled

nylon fibers improves first-crack strength more than that PVA fibers do. However, the compressive strength

decreases with increase in fiber fraction, and decreases with increase in fiber aspect ratio.

Keywords: recycled nylon fiber, waste fishing net, recycled materials, fiber reinforced mortar.

1. Introduction

In recent decades, the world has been suffering

from the dumping of wastes, especially plastics left

in seas and oceans. Waste fishing nets account for

some of these wastes: 640,000 tons of fishing nets

are disposed of in the ocean annually [1]. As the

nets become totally entangled, separating them for

disposal is impractical. These nets can be harmful

to marine life, such as turtles, seals, and other ma-

rine mammals, which can become entangled and

suffer injury or drowning [2]. In addition, the ma-

rine food web could be disrupted. As abandoned

nets and plastic garbage tend to gather at or near the

surface of seas and oceans, they keep sunlight from

reaching small creatures such as planktons and al-

gae. Therefore, the animals that feed on these small

creatures are also directly affected

Most fishing nets are made of nylon and are

non-biodegradable. Even though their storage is not

hazardous, it is very important to find suitable ways

of recycling these nets. Spadea et al. [3] investigated

the use of waste fishing nets as recycled nylon fiber

for reinforced cement mortar. They found that add-

ing the fibers to mortar significantly improves me-

chanical properties, such as increased first-crack

strength (i.e., the modulus of rupture, MOR),

toughness, and ductility. Two decades later, many

researchers are studying the use of fiber reinforced

mortar (FRM) as a way of achieving higher MOR,

fracture toughness, impact resistance, and of con-

trolling shrinkage. The use of synthetic fibers such

as polyvinyl alcohol (PVA) and polypropylene was

found to be successful in significantly improving

the mechanical properties of the base material [4-7].

In recent years, many researchers have become in-

terested in using recycled materials [8-15]. Not only

are they interested in improving the mechanical

properties, but they are also concerned with achiev-

ing favorable environmental outcomes and realizing

economical products. Additionally, durability under

alkaline conditions is very important for the fibers,

if they are to enhance mortar composites. There is

some evidence in the literature that recycled nylon

fibers have excellent alkali resistance [3]. However,

a test by Ochi et al. [16] found that the tensile

strength of PVA fibers decreases by 44% from al-

kali exposure.

This study aims to investigate the effectiveness

of recycled nylon (R-nylon) fibers retrieved from

Corresponding author Shanya ORASUTTHIKUL is a Ph.D.

candidate at Hokkaido University, Hokkaido, Japan.

Daiki UNNO is a Master Student at Hokkaido University, Hok-

kaido, Japan.

Hiroshi YOKOTA is a Professor at Hokkaido University, Hok-

kaido, Japan.

Katsufumi HASHIMOTO is a Lecturer of Kyoto University,

Kyoto, Japan.

102

(a) Waste fishing net (b) Straight R-nylon fibers

(c) Knotted R-nylon fibers (d) PVA fibers

Fig. 1 – Types of fibers

Table 1 – Mechanical properties of fibers

Fiber

type

Diameter

(μm)

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Density

(g/cm3)

R-nylon

fiber 350 440 3.0 1.13

PVA 200 975 27 1.30

waste fishing nets to improve certain mechanical

properties of mortar. The authors address the prepa-

ration of R-nylon FRM and the identification of its

mechanical properties (e.g., compressive strength,

MOR, toughness, and residual strength) in compari-

son to those of PVA FRM. The effects of using dif-

ferent fiber volume ratios and lengths are analyzed.

2. Outline of Experimental Test

The waste fishing nets used in this study were

collected by fishermen in Hokkaido, Japan. The

fibers were manually cut in three different lengths

of 20 mm, 30 mm, and 40 mm, and were mixed in

mortar at several different volume fractions.

The volume fractions of the knotted R-nylon

fibers (1.0%, 1.5% and 2.0%) were lower than

those of the straight fibers (0.5%, 0.75%, and 1.0%),

because knotted R-nylon fibers tend to form tena-

cious clumps. The PVA fibers were included in the

mix at 1.0 vol.% and 1.5 vol.%. In the case of the

recycled nylon fiber, the authors carried out uniaxi-

al tensile tests according to ASTM C1557-03 stand-

ard [17] in order to determine the tensile strength

and Young’s modulus. The fiber types and the me-

chanical properties are given in Fig. 1 and Table 1,

respectively. Before the fibers were added, cement

and sand were mixed in a small mixer. Then, the

fibers were gently added to prevent the formation of

fiber balls, and all the dry components were mixed

by hand in order to obtain a uniform distribution of

fibers. Water was gradually added, and a mixing

machine was used at low speed for 2 minutes until a

homogeneous mixture was achieved. The mortar

was cast in prism molds of 40 mm 40 mm 160

mm for the bending test and in cylinders of 50 mm

diameter 100 mm in height for the compressive

strength test. The specimens were cured in water at

20˚C for 28 days and then tested. For the mortar

mixes, various combinations of fiber volume and

fiber aspect ratio were used as summarized in Table

2. The unreinforced mortar is notated as UR, and

the FRM specimens are notated as SRny (straight

recycled nylon fiber), KRny (knotted recycled ny-

lon fiber), and PVA (PVA fiber), followed by “fiber

length-volume fraction.”

3. Results and Discussions

3.1 Flowability

The flowability test was conducted in compli-

ance with ASTM C 1437 [18]. The flow diameters

were measured and are listed in Table 2. It can be

seen that flowability tends to decrease with increase

in fiber length and amount. A comparison of KRny

and SRny40 at 1.0 vol.%, shows that the FRMs

mixed with knotted fibers have greater flow diame-

ters, due to the balling of fibers, which leads to fi-

ber–mortar separation. Moreover, a comparison of

PVA and SRny30, which have similar aspect ratios,

shows fresh FRM containing PVA fibers to have a

smaller flow diameter than SRny30 FRMs has, at a

103


Table 2 – Test results of fresh mortar properties

Specimen type

Fiber fraction by volume (%)

Fiber length, L (mm)

Diameter, D (mm)

Aspect ratio (L/D)

Flow diameter (mm)

UR - - - - 262

KRny-0.5 0.5 40 0.35 114 247

KRny-0.75 0.75 40 0.35 114 233

KRny-1.0 1.0 40 0.35 114 227

SRny20-1.0 1.0 20 0.35 57 231

SRny20-1.5 1.5 20 0.35 57 226

SRny20-2.0 2.0 20 0.35 57 207

SRny30-1.0 1.0 30 0.35 86 229

SRny30-1.5 1.5 30 0.35 86 216

SRny30-2.0 2.0 30 0.35 86 195

SRny40-1.0 1.0 40 0.35 114 217

SRny40-1.5 1.5 40 0.35 114 207

SRny40-2.0 2.0 40 0.35 114 183

PVA-1.0 1.0 18 0.2 90 213

PVA-1.5 1.5 18 0.2 90 169

given volume fraction, due to greater number

of fibers.

3.2 Compressive strength

The compressive strengths of the FRMs are

presented in Table 3. The compressive test was

conducted in compliance with ASTM C 39 [19].

Table 3 indicates that the addition of the R-nylon

fibers causes a reduction in compressive strength of

the examined mortars up to 48%. Moreover, it can

be clearly seen that the compressive strength de-

creases with decrease in fiber length, and decreases

with increase in R-nylon fiber amount. An explana-

tion is that nylon fibers have a much lower modulus

of elasticity than mortar has, and therefore, the in-

clusion of fibers creates voids in the mortar [20].

Also, the addition of fibers, especially long fibers,

leads to increases in the volume of the interfacial

transition zone (ITZ), which results in the reduction

of strength and stiffness of FRM [21]. This might

suggest that the lower compressive strength of the

KRny FRM is the result of a greater reduction in

the modulus of elasticity of the FRM from the in-

clusion of knots as shown in Table 3. When the

specimens are subjected to compressive load, lateral

tensile strain in the mortar occurs due to the Pois-

son’s effect. As the load increases, the longer fibers

play a more important role in the mortar’s lateral

tensile strength than the shorter fibers play. These

longer fibers postpone crack enlargement by in-

creasing the mortar’s lateral tensile strength [13]. In

the case of the PVA fibers, the addition of fibers

results in a decrease in compressive strength of the

mortar, especially at greater amounts of addition.

An explanation is that the modulus of elasticity of

PVA fibers and mortar are almost the same, but the

fibers are much less dense. Therefore, the mortar

was filled with a high amount of fibers, which re-

sulted in reductions in the elastic modulus of the

mortar [22].

3.3 Flexural strength

After 28 days of curing, flexural strength tests

were conducted in accordance with ASTM C 293

[23]. The authors performed three-point bending

tests, and the peak load (Pcr), the first-crack strength

(i.e., MOR), and other results are listed in Table 3.

The results show that the addition of KR-nylon, SR-

nylon, and PVA fibers increases the MOR by up to

22%, 41%, and 22%, respectively. Spadea et al. [3]

and Orasutthikul et al. [24] proposed that using R-

nylon fibers from waste fishing nets is very effec-

tive at reinforcing mortar, with the MOR found to

be improved by up to 35% and 42% in the respec-

tive reports.

The first-crack strength can be calculated as

follows:

M = 3Pcrl/2bd2 (1)

where, M = modulus of rupture (MOR) or first-

crack strength; Pcr = maximum applied load;

104


Table 3 – Test results of compressive and flexural strength at 28 days

Specimen

type

Compressive strength test Flexural strength test

No. of specimens

f’c

(MPa)

SD

CV %△f’c

No. of specimens

Pcr

(kN)

MOR

(MPa)

SD CV %△MOR

UR 3 65.67 1.53 2.33 - 2 1.71 4.81 0.06 3.44 -

KRny-0.5 3 56.61 6.86 12.12 -13.80 2 2.08 5.86 0.12 5.66 21.95

KRny-0.75 3 55.96 11.43 20.42 -14.79 2 1.79 5.04 0.06 3.28 4.86

KRny-1.0 3 36.77 5.97 16.24 -44.00 2 1.58 4.45 0.00 0.00 -7.35

SRny20-1.0 3 52.56 1.18 2.24 -19.96 2 2.17 6.09 0.12 5.45 26.84

SRny20-1.5 3 48.66 2.05 4.21 -25.89 2 2.42 6.80 0.24 9.72 41.44

SRny20-2.0 3 34.14 1.70 4.99 -48.01 2 1.58 4.81 0.06 3.43 -0.09

SRny30-1.0 3 53.61 0.57 1.05 -18.36 2 2.04 5.74 0.18 8.66 19.42

SRny30-1.5 3 46.84 1.02 2.19 -28.67 2 2.00 5.63 0.12 5.85 17.06

SRny30-2.0 3 35.06 3.32 9.47 -46.61 2 1.54 4.34 0.18 11.46 -9.75

SRny40-1.0 3 55.54 2.84 5.12 -15.43 2 2.13 5.98 0.18 8.32 24.38

SRny40-1.5 3 48.24 0.57 1.19 -26.54 2 2.29 6.44 0.06 2.47 34.04

SRny40-2.0 3 35.27 1.07 3.02 -46.28 2 1.83 5.16 0.12 6.44 7.32

PVA-1.0 3 61.70 0.52 0.85 -5.87 2 2.04 5.74 0.06 2.87 19.49

PVA-1.5 3 59.57 5.61 9.41 -6.81 2 2.08 5.86 0.12 5.67 21.95

Note: f’c - compressive strength, SD - standard deviation, CV - coefficient of variation, %△fc - percent differ-

ence in compressive strength between control specimens (UR) and FRMs, △R - percent difference in modu-

lus of rupture between control specimens (UR) and FRMs.

b = specimen width; d = specimen depth; and l =

span length.

From Fig. 2(a) to (e), it can be clearly seen that

R-nylon FRM shows a significant reduction in load

after the first crack. This is because the R-nylon

fibers used in this study have a smooth surface, so

the friction interlocking between the fiber surface

and the mortar is poor, which results in low fric-

tional resistance to slippage. When PVA FRMs

were subjected to external load, the PVA fibers

tended to rupture before the fiber and the matrix

debonded from each other. This caused abrupt re-

ductions in the post-peak load of PVA FRMs to

nearly the same or slightly lower than that of

SRny40 FRM with a 2.0% fiber fraction when mid-

span deflection was increasing. After the bending

test, the fiber surface was examined to analyze the

frictional resistance and interfacial bond between

the fiber and the matrix. The R-nylon surface seen

in Fig. 3(a) shows the fiber had no serious change

in scratching as a result of low frictional resistance

between the fiber and the matrix. For the PVA fiber,

due to the stronger bond between the fiber and the

matrix, some cement paste is seen on the fiber sur-

face as shown in Fig. 3(b).

3.4 Toughness and residual strength

Toughness is an important mechanical charac-

teristic of FRM. In accordance with ASTM C 1018

[25], toughness indices I5, I10, and I20 were ob-

tained by dividing the area under the load-

deflection curve up to 3.0, 5.5, and 10.5 times the

first-crack deflection, respectively, by the area un-

der the curve up to the first-crack deflection (see

Fig. 4). Table 4 summarizes the toughness indices

and residual strength factors of FRMs at 28 days.

The addition of the fibers to the mortar appears to

afford outstanding improvements in toughness, es-

pecially for higher fiber fractions and greater fiber

lengths (see Fig. 5). The toughness and residual

strength of FRMs are dependent on fiber character-

istics, such as geometric shape, tensile strength, and

modulus of elasticity, as well as on the bond

strength between the fiber and the surrounding mor-

tar. SRny30 fiber and PVA fiber have similar aspect

ratios, but SRny30 FRMs have lower toughness

indices and residual strength factors than PVA

FRMs have. However, the fracture toughnesses of

105


0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0

0.50%

0.75%

1.00%

Lo

ad

(K

N)

Deflection (mm)

(a) Knotted R-nylon fiber

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0

20mm

30mm

40mm

Lo

ad

(K

N)

Deflection (mm)

(b) Straight R-nylon fiber with 1.0% fiber fraction

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0 0.5 1.0 1.5 2.0

20mm

30mm

40mm

Lo

ad

(K

N)

Deflection (mm)

(c) Straight R-nylon fiber with 1.5% fiber fraction

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0

20mm

30mm

40mm

Lo

ad

(K

N)

Deflection (mm)

(d) Straight R-nylon fiber with 2.0% fiber fraction

0.0

0.5

1.0

1.5

2.0

2.5

0.0 0.5 1.0 1.5 2.0

PV18-1.0%

PV18-1.5%

Lo

ad

(K

N)

Deflection (mm)

(e) PVA fiber

Fig. 2 – Load-midspan deflection curves of fiber-

reinforced mortar specimens

(a) before & after

(b) before & after

Fig. 3 – Fiber surface before and after the flexural

test: (a) R-nylon fiber, (b) PVA18 fiber

the SRny40 FRMs with a 2.0% fiber fraction

(SRny40-2.0) fall between those of the PVA FRMs

with a 1.0% fiber fraction (PVA-1.0) and the PVA

FRMs with a 1.5% fiber fraction (PVA-1.5). In this

study, the fiber fractions of the KRny fiber are low,

and therefore the toughness and residual strength of

the FRMs are not effectively improved. The PVA

fibers generally have a stronger bond to the sur-

rounding cementitious matrix because it is hydro-

philic. Redon et al. [26] found that small-diameter

PVA fibers ruptured before achieving their full

pullout length. Therefore, when the PVA FRMs

were loaded, the PVA fibers broke from tension

rather than from pullout.

4. Conclusions

In this study, the effectiveness and potential of

using waste fishing nets as recycled nylon fiber to

106


Table 4 – Toughness indices and residual strength factors at 28 days

Specimen

type

Toughness indices Residual strength factors

I5 I10 I20 R5,10 R10,20

KRny-0.5 1.61 1.82 2.42 4.20 6.02

KRny-0.75 1.56 2.20 4.00 12.80 18.01

KRny-1.0 1.91 3.27 6.21 27.20 29.43

SRny20-1.0 1.95 2.41 3.45 9.18 10.40

SRny20-1.5 2.40 3.38 5.23 19.60 18.50

SRny20-2.0 2.80 4.17 6.71 27.40 25.40

SRny30-1.0 1.97 2.91 4.79 18.80 18.80

SRny30-1.5 2.86 4.17 6.49 26.20 23.20

SRny30-2.0 3.05 4.23 7.29 23.60 30.61

SRny40-1.0 2.88 4.47 7.71 31.80 32.44

SRny40-1.5 2.49 4.25 7.80 35.18 35.50

SRny40-2.0 3.37 6.02 13.38 53.00 73.60

PVA-1.0 2.77 4.62 8.74 37.00 41.21

PVA-1.5 3.72 7.50 15.74 75.60 82.40

Fig. 4 – Toughness as defined by ASTM 1018

Fig. 5 – Toughness indices of FRMs

reinforce mortar were experimentally tested and

discussed. Compressive and three-point bending

tests were performed to investigate compressive

strength, peak load, modulus of rupture, toughness

indices, and residual strength factors. This study

found the effects of adding R-nylon fibers to mortar

to be as follows.

(1) The addition of R-nylon short fibers recycled

from waste fishing nets to mortar improves the

mechanical properties of the mortar, except for

compressive strength, which decreases as a re-

sult of that addition.

(2) The addition of R-nylon fibers, whether straight

or knotted, results in a reduction in mortar work-

ability. Mortar flowability tends to decrease with

increase in the fiber fraction. Mixes that incor-

porate longer fibers demonstrate smaller flow

diameters than those with shorter fibers.

(3) The addition of R-nylon fibers to mortar affords

a more ductile mode of failure than the addition

of plain mortar affords. Post-cracking ductility is

enhanced, which results in an increase in the

load carrying capacity of FRM in bending. The

post-peak loads are observed to be higher with

higher fiber fractions and longer fibers.

(4) Mechanical analyses show that the addition of

fibers leads to reductions in compressive

strength of 14-44% for KR-nylon fiber and 20-

48% for SR-nylon fiber. The MOR increased by

up to 22% for KRny FRM and by up to 41% for

SRny FRM.

(5) The addition of PVA fibers to mortar causes a

greater reduction in flow diameter than the addi-

tion of R-nylon fibers at a similar fiber aspect ra-

tio and the same fiber fraction.

(6) Post-peak load, toughness indices, and residual

strength factors differ significantly according to

fiber characteristics such as geometric shape,

tensile strength, and modulus of elasticity, as

well as according to the bond between the fiber

and the cementitious matrix. As evidenced by

107


the flexural test, R-nylon fiber shows a signifi-

cantly lower post-peak load than PVA fiber af-

fords, due to the smooth surface of R-nylon fi-

ber. Despite the stronger chemical bond between

the PVA fiber and the matrix, the PVA fibers

tended to rupture instead of pulling out. This

caused the post-peak load of the PVA FRM to

dramatically drop after the peak, due to breaking

of the fiber. However, R-nylon fiber shows

greater MOR improvement than PVA fiber af-

fords.

(7) For every type of fiber, its addition to the mortar

results in a decrease in compressive strength, es-

pecially the addition of fibers with a low modu-

lus of elasticity, such as R-nylon fiber.

It must be noted, however, that the R-nylon

FRMs analyzed in this study have been proven ben-

eficial in terms of mechanical properties as PVA

fiber, even if a higher amount of fibers may be re-

quired to match the performance. Moreover, the use

of waste fishing nets as recycled nylon fibers results

in an environmental benefit.

Acknowledgements

This study was supported by JSPS KAKENHI

Grant #26630208.

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109



Vol. 2, No. 2, Dec. 2016, pp. 110-116

ISSN 2465-7964 / eISSN 2465-7972


Relative evaluation of performance of limestone calcined clay

cement compared with Portland pozzolana cement

Ashok Kumar Tiwari* and Subrato Chowdhury

(Received: August 4, 2016; Accepted: November 6, 2016; Published online: December 31, 2016)

Abstract: Cement is most widely used binder to produce concrete and the most common construction mate-

rial today. Though concrete is a material with the lowest greenhouse emission, cement has the highest. With

the carbon footprint of cement accounting for over 7% of total world emissions, it becomes single most im-

portant material of environmental concern around the world. This concern has led to a search for lower car-

bon emitting binders and use of blended cements, incorporating large number of natural and industrial by-

products. This paper describes the performance of a composite cement binder consisting of calcined clay,

limestone, and Portland cement clinker as compared to a traditionally used fly ash based Portland pozzolana

cement. This study reports behavior of the two cement binders with respect to strength development, hydra-

tion, porosity of hydrated pastes, normal consistency, and admixture response with ageing. This study finds

that, though the clay based cement attains higher early age strength, the later age strength in mortar is lower

as compared to commercial fly ash based cement. Further the clay based cement has higher water demand,

but lower porosity compared to composite cement binder.

Keywords: binder, calcined clay, limestone, microstructure, hydration, porosity, water demand.

1. Introduction

Concrete is most widely used construction ma-

terial in the world today with over 25 billion tons

placed every year [1]. It is made from graded ag-

gregate system, cemented together by a binder,

normally called Portland cement. Often this con-

crete also includes various industrial by-products,

which may have either pozzolanic or self-cementing

properties. Modern concrete also has superplasti-

cizers to make the concrete workable and pumpable

for various applications. In spite of various devel-

opments, cement remains the only binder in con-

crete. The cement is highly energy intensive to pro-

duce and is responsible for about 7% of greenhouse

emissions in the world today [2]. Even though car-

bon footprint of concrete is lowest among building

materials used in construction (see Fig. 1), the same

is highest due to sheer volume of concrete being

used today. Researchers in academia and industry

have been working on reducing the carbon foot-

prints of cement and have been successful in reduc-

ing the same by about 20% in last one decade as

reported in World Cement Sustainability Initiative

Report published in 2012.

Cement production has been progressively in-

creasing and currently over 4 billion tonnes of ce-

ment is being produced annually across the world.

Realising the problems of greenhouse gases, the

cement industry has undergone a lot of changes.

The changes are in process engineering as well as in

variety of cements being produced. Today, utiliza-

tion of blended cements is usually preferred due to

their economic and technical benefits and indirect

advantages such as lower level of CO2 emissions by

reducing clinker production in plants. While the

emerging family of cementitious materials has been

expanding to a larger number, namely fly ash, silica

fume, calcined clay, metakaolin etc., materials like

fly ash (FA) and ground granulated blast furnace

slag (GGBS) are being widely used for cement pro-

duction.

The primary objective of usage of wide range

of cementitious materials, both natural and artificial

is to reduce the CO2 footprint and progressively

increase usage of non-bio degradable industrial

waste. These major industrial wastes are fly ash and

GGBS. Other minor industrial waste could be used

as cementitious material. Introduction of calcined

clay pozzolana as cementitious material has been

Corresponding author Ashok Kumar Tiwari is Vice President

of Concrete & Building Materials, UltraTech Cement Limited,

India.

Subrato Chowdhury is Former Joint President, UltraTech

Cement Limited, Inida.

110

Fig. 1 – CO2 discharge in production process of

construction materials

reported long back elsewhere globally. In India, it

began in late 1970s only, when BIS standard for

calcined clay became available for manufacturing

of Portland pozzolana cement [5]. Many publica-

tions report that performance of mortar and con-

crete composed with calcined clay closely compares

with mortar/concrete made with fly ash and GGBS.

The calcined clay is the potentially rich cementi-

tious material and its usage would greatly contrib-

ute in reducing carbon foot print.

The objective of this study is to evaluate the

performance of limestone calcine clay cement

(LC3) and commercially available Portland pozzo-

lana cement (PPC) towards understanding the prod-

ucts in greater details. The blended cements have

been in use widely for more than two decades in

reducing the CO2 footprint in relation to conven-

tionally used ordinary Portland cements. Most of

the countries today produce and use blended cement,

either binary or ternary blended for various con-

struction applications [3-7]. Accordingly the speci-

fication of cement are becoming performance ori-

ented rather than current one of prescription orient-

ed [8]. In India, due to large availability of fly ash

and to some extent GGBS, over 75% of cement

used is blended cement. The trend is similar across

the world and even in ordinary Portland cements,

performance improvers or minor additional com-

pounds such as GGBS, limestone, and fly ash are

replacing clinkers to the extent of up to 5% [9].

With these efforts, clinker conversion factor, which

is a measure of how much cement is produced per

unit clinker, has gone to 1.6 from traditional level

of 1.03 reducing carbon footprints by over 50%.

In countries, where fly ash is not abundantly

available, construction industry has been using lime-

stone powder as clinker substitute. Limestone re-

placement into Portland cement has been widely

studied for several years [9-11]. Limestone not only

works as micro filler, but also takes part in hydra-

tion process of clinker, improving workability,

strength, and durability [12]. The limestone can also

be used in ternary blends in combination with fly

ash, calcined clay and other pozzolana.

This study compares the behavior of two

blended cements, one produced at small cement

production unit for this experimental study, the oth-

er one being commercially available fly ash based

Portland pozzolana cement. The tests were con-

ducted in accordance with Indian standards [13-

14].

2. Production of LC3 Cement

A limestone calcined clay cement (LC3) con-

sisting of limestone, calcined clay, and portland

cement clinker was used. Cement has 15% lime-

stone, 31% calcined clay, and about 50% portland

cement clinker with remainder being gypsum.

Properties of the raw materials are given in Table 1.

Limestone and calcined clay were ground separate-

ly and intermixed with ground clinker. A commer-

cially available Portland pozzolana cement (PPC),

made from fly ash conforming to Indian Standard

[5] were used for bench marking.

The clay was calcined in a rotary kiln at a tem-

perature of 900 degree Celsius for optimum calcina-

tion. A weight loss of 0.3% was observed and lime

reactivity of calcined clay was observed to be 7.8

MPa, which was significantly higher than the re-

quirements of pozzolanic materials and certainly

better than those of fly ash.

Table 1 – Oxide composition of raw materials

Element Clinker,

%

Lime-

stone,

%

Clay,

%

Gypsum,

%

SiO2 21.1 11.02 54.47 2.77

Fe2O3 4.32 1.55 4.93 0.36

Al2O3 4.65 2.53 27.29 0.62

CaO 65.16 44.24 0.06 32.62

MgO 2.13 1.96 0.13 1.20

SO3 0.77 0 0.01 38.75

Na2O 0.38 0.5 0.12 0.06

K2O 0.20 0.28 0.25 0.04

LOI 0.96 36.96 10.28 23.02

3. Materials and Test Methods

3.1 Cement

Cement, after proper sampling, was tested as

per Indian Standards [13-14] for the following

items:

fineness, by Blaine test apparatus;

full physical properties, including particle size

distribution and retentions on different sieve

111


sizes like 90 micron (R90), 75 micron (R75),

and 45 micron(R45);

full chemical analysis including insoluble resi-

due (IR), sulphate (SO3), loss on ignition (LOI),

magnesium oxide (MgO), total & soluble alka-

lis;

hydration study using calorimeter (ICP) at w/c

of 0.4 at 24, 72, 144, and 672 hours; and hydration study of LC3 by scanning electron

microscopy (SEM).

3.2 Cement paste

Workability retention study using marsh cone

and mini-slump

At w/c of 0.5 % by wt. without admixture and

at 0.4 with admixture

Fluorescent microscopy on cement paste po-

rosity

Mercury intrusion porosimeter Quanta chrome

MIP with high pressure 60,000 psi system for

porosity of paste

3.3 Cement mortar

Cement mortar with standard stand, ratio 1:3

for compressive strength

Cement pastes were tested for admixture com-

patibility using a marsh cone and retention was

measured using a mini slump. The mini-slump test

which was originally developed by Kantro [15] and

later modified by Zhor & Bremner [16], measures

the consistency of cement paste and is commonly

used for evaluating admixture-cement response for

flow and retention across the world. The mini-

slump cone is a small version of the slump cone.

The mini-slump cone is placed in the centre of a

piece of plane rigid and non-absorbent surface /

table. The paste was prepared at a water-binder ra-

tio (w/b) of 0.55 and retention of flow was meas-

ured up to 120 minutes.


A limestone calcined clay cement (LC3) con-

sisting of limestone, calcined clay, and Portland

cement clinker was used. Limestone and calcined

clay were ground separately and intermixed with

cement. A commercially available Portland pozzo-

lana cement (PPC), fly ash based, conforming to

Indian Standards [5] was used for bench marking.

Properties of cements, tested as per relevant Indian

standards [13-14] are given in Tables 2 and 4.

4.1 Cement properties

The chemical composition of cements is given

in Table 2 whereas the physical properties of

Table 2 – Composition of cement

Cement LOI IR SO3

LC3 7.18 22.68 2.15

PPC 2.78 24.23 2.83

Table 3 – Particle size distribution of cements

Sample d10 %,

µm

d50 %,

µm

d90 %,

µm

Mean

size, µm

LC3 1.26 13.11 57.48 22.25

PPC 1.70 18.09 51.80 23.04

Table 4 – Physical properties of cements

Cement

Blaine

fineness,

m2/kg

Normal

consisten-

cy, %

Compressive strength in

mortar, MPa,

at the age of

1

day

3

days

7

days

28

days

LC3 685 31 7.3 23.4 34.2 40.1

PPC 322 30 7.4 18.6 31.7 57.9

cement are listed in Tables 3 and 4. The LOI of

LC3 are much higher than the PPC. The higher LOI

is primarily contributed from the limestone addition.

The requisite variety of limestone for the purpose of

usage in LC3 are having intrinsic LOI in the range

of 32–37% typically whereas typical LOI of fly ash

is in the range of 0-6 %. SO3 range is 1.87–2.83%.

Crystals of tri-calcium silicate present in commer-

cially available cement are mixed with impurities

such as alkalis, sulphates, phosphorous, and host of

trace minerals. The hydration process of alite or tri-

calcium silicate initiates in presence of alkali sul-

phate or alkaline sulphate environment. Alite acti-

vates when sulphate concentration (SO3) of the sol-

id solution is close to 2% [17]. However PPC is

having SO3 concentration of 2.83% owing to possi-

ble usage of high sulphur bearing clinker. Insoluble

residue (IR) for all the cement is ranging 22.68–

24.23%. The contributory factors for IR are the per-

centage of fly ash addition and calcined clay.

The Blaine fineness of LC3 is significantly

higher in comparison with PPC, though mean parti-

cle size is similar (see Tables 3 and 4). The Blaine

fineness of the cement is influenced by the material

characteristics and the comminution system adopted.

Calcined clay and the limestone are vastly different

from clinker and fly ash as far as the material hard-

ness and grinding efficiency is concerned. In general,

the mean size of cements was similar, though d50 of

LC3 was significantly lower as compared to PPC.

The normal consistency of LC3 and PPC are in a

close range of 30–31% despite the fact that LC3

Blain fineness level is more than the double of PPC.

112


The compressive strength of the mortar cubes

of all cements are shown in Table 4. One day

strength of all the cement are in a very close range.

However 28 day strength of LC3 is remarkably

low in relation to PPC. This may be contributed to

lower clinker concentrations. The LC3 exhibited

early strength development up to 7 days and the

development between 7 days to 28 days was lower

in comparison with PPC.

The compressive strength development from 7

to 28 days of LC3 is 15% of 28 days strength

whereas, it is 46% for PPC. The hydration kinetics

is closely linked with the intrinsic material charac-

teristics. In the case of fly ash, the products of hy-

dration closely reassembles calcium–silica hydrates

produced by hydration of Portland cement. Howev-

er the reaction does not start until sometimes after

mixing. In the case of fly ash class F, this can be as

long as one week or even more. The glass material

in fly ash is broken down only when the pH value

of the pore solution is at least about 13 [18]. The

strength recovery of PPC from 7 days to 28 days is

much higher than in relation to LC3.

The lime reactivity of calcined clay was much

higher than fly ash resulting in higher demand of

Portlandite (CH) availability in the hydration system.

The LC3 cement exhibited higher one day strength

and higher recovery of strength from 1 to 7 days in

comparison with PPC. The higher rate of reactions

with available pore solution resulted in total con-

sumption of Portlandite generated in system. This

could cause availability of unreacted calcined clay.

Pozzolana cement resulted in lower growth or lower

strength recovery from 7 to 28 days. Reaction of

alumina and calcium carbonate with the progress of

hydration process of LC3 is possibly contributing to

improved early strength of LC3 up to 7 days in

comparison with PPC.

4.2 Hydration behavior

The isothermal (heat conduction) calorimetry

is an efficient tool to study the stages related to the

hydration of cement pastes or mortars at constant

temperature. The calorimeter continuously measures

and displays the heat flow related to the hydration

reactions taking place in the cement paste after

mixing. The respective cement was studied for

heat liberation using conduction calorimeter at a

w/c ratio of 0.40. The heat flow curve and the total

heat liberation curve are shown in Fig. 2 along with

that of PPC. The heat liberated at different age are

given in Table 5.

It has been observed that LC3 hydrated faster

with heat liberation almost double when compared

with PPC. However, the same tapers down after144

hours without significant gains till 28 days. PPC

catches up and surpasses LC3 at 28 days. This is in

0

50

100

150

200

250

0 50 100 150 200 250

Tota

l Hea

t R

elea

sed

, J/g

Time, Hrs

PPC LC3

Fig. 2 – Total heat released and heat flow of LC3

compared with PPC

Table 5 – Heat liberated for LC3 and PPC at differ-

ent age

Heat liberated,

J/gram of cement

Time, hours

24 72 144 672

LC3 100.1 178.2 205.1 215

PPC 68.3 142.3 165.8 230.4

agreement with compressive strength development

of the two cements.

4.3 SEM analysis of hydration products

Changes with time in the morphology and na-

ture of the hydration products of LC3 cement, at

water to cement ratio of 0.5, were studied by

scanning electron microscope. The microstructure

was observed at different intervals of hydration:

i.e. 1, 3, 7, and 28 days. The samples were stud-

ied in both fractured surface and polished section

using SE and BSD at variable pressure mode. The

hydration products such as calcium silicate hydrate

(C-S-H), portlandite (CH), ettringite (AFt), mono-

sulfate (AFm), C-S-A-H, limestone particles, and

deleterious materials like quartz and feldspar were

observed. The initial products at 1 day of hydration

are amorphous looking like fibrous shape. These

products mainly appear on the surface of the unhy-

drated grains, filling in void space as they grow.

Fibrous like C-S-H having size <200 nm, CH, Aft,

and AFm phases appear more at 1 day.

Interlocking structure of C-S-H and rod like

ettringite appear at 3 days. During the first 7 days of

113


Fig. 3 – Morphology of hydration products at 1 days

Fig. 5 – Morphology of hydration products at 7 days

hydration, the surface of the C3S grain are covered

by the radiating fibrous particles of C-S-

H,honeycomb structure, and interlocking space

between the grains.

As time of hydration increased, the fibrous

structure developed into a needle like C-S-H at 28

days. At 28 days hydration, the paste displayed a

massive tabular structure with platy with occasion-

ally fibrous hydration products. The morphology of

C-S-H is similar at 3 and 7 days.

Fig. 4 – Morphology of hydration products at

3 days

Fig. 6 – Morphology of hydration products

at 28 days

Both type I and II C-S-H were observed in all

the hydration period. The size of C-S-H increased

form 200 nm to 1.5 micron as the hydration period

in creased from 1 to 28 days. The morphology of

ettringite also changes from needle to rod like

structure as the hydration period increases. Lime-

stone particles are frequently present at all the ages

indicating incomplete reaction with cement parti-

cles. The amount of AFm and CH also decreases as

hydration period increases.

114


Fig. 7 – Flow behavior of LC3

Fig. 8 – Flow behavior of PPC

Fig. 9 – Flow retention of cement pastes

4.4 Porosity of cement pastes

The porosity of cement paste composed with

LC3 and PPC with ratio of water to cementitious

material 0.4 was determined using mercury intru-

sive porosimeter, both in terms of inter particle and

intraparticle porosity [19]. The results are listed in

Table 6. It is observed that hydrated LC3 paste has

lower porosity in comparison with that of PPC. The

total porosity of LC3 paste is 0.92% against 1.13%

of PPC.

4.5 Admixture demand of cement paste

Admixture compatibility of cements, tested

with common admixture showed that the optimum

dosage for LC3 was 1.4%, while it was 1.0% for

PPT. The flow behavior of cement paste showed

that LC3 has higher admixture demand, almost 40%

higher as compared to binary blend cements PPC

Table 6 – Porosity of cement paste by MIP

Cement

Porosity, %

Interparticle Intraparticle Total

LC3 0.844 0.074 0.918

PPC 1.248 0.087 1.133

(see Figs. 7 and 8), even though the water demand

for normal consistency was similar. This may be due

the higher fineness of LC3.

4.6 Retention of flow

Retention of flow of cement paste shows that

LC3 has a low initial flow and poor retention as

compared to PPC (see Fig. 9). This may be due to

fine limestone powder and higher fineness of LC3.

5. Conclusions

This study observed that the limestone calcined

clay cement behaves differently as compared to

commercially available fly ash based blended ce-

ment. The compressive strength development of

limestone calcined clay cement is also different to

fly ash based blended cement in terms of low im-

provement at later ages. The hydration behavior

shows a distinct difference in terms of high heat

evolution at early age, unconventional of blended

cements. Not only the cement paste has a high water

demand, resulting in lower workability, but also it

needs higher dosage of superplasticizers, if used.

Porosity of LC3 cement paste is lower than that of

PPC paste showing an improved durability.

Acknowledgements

The paper is based on the study at Central R&D

of UltraTech Cement Limited at Taloja Centre, un-

der the leadership of the authors. Acknowledge-

ments are due to all scientist who have contributed

to the study in one or another way. Dr. Suresh K.,

Dr. Mohan Medhe, Dr. Narendra Kumar, Manish

Kuchya, Er. Sandeep Keskar, Dr. Arunachala Sa-

dangi need special mention for analytical support.

References

1. Schokker, A. J., (2012) The Sustainable Con-

crete Guide-Applications, US Green Concrete

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3. BS EN 197-1 (2000) Cement - Part 1: Compo-

sition, specifications and conformity criteria for

common cements.

115


4. IS:455 (2015) Portland Slag Cement Specifica-

tion, Bureau of Indian Standards, New Delhi.

5. IS:1498 - Part I & II (2015) Portland Pozzolana

Cement (fly ash based & Calcine Clay based),

Bureau of Indian Standards, New Delhi.

6. IS:16415 (2015) Composite Cement, Bureau

of Indian Standards, New Delhi.

7. AS:3972 Type GP (2010) General purpose and

blended cements, Standards Australia Limited.

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tives and status of fly ash in India, fly ash

sources, applications & potential environmental

impact, Nova publications, pp. 315.

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limestone blended cement, Cement and Con-

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zolanic cement, Cement and Concrete Research,

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carbonate in cement hydration, Cement and

Concrete Research, 37, pp. 551-558.

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hydraulic cement, Bureau of Indian Standards,

New Delhi (Reaff. 2005).

14. IS: 4031 (1988 & 1989) Methods of physical

test for hydraulic cement (Pt 2-6), Bureau of

Indian Standards, New Delhi (Reaff. 2003-

2009).

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Reducing Admixtures on Properties of Cement

Paste, A Miniature Slump Test," Cement, Con-

crete and Aggregates, 1(2), pp.95-102.

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in Evaluation of Lignosulphonates and Con-

crete Admixtures, Fourth CANMET/ACI/JCI

Conference: Advances in Concrete Technology,

SP 179-58, American Concrete Institute, pp.

1011-1042.

17. Neville, A. M., (2011) Properties of Concrete,

5th edition, Prentice Hall.

18. Taylor, H. F. W., (1997) Cement Chemistry,

Thomas Telford.

19. ASTM D4404 - 10 Standard Test Method for

Determination of Pore Volume and Pore Vol-

ume Distribution of Soil and Rock by Mercury

Intrusion Porosimetry, P.A.

116



Vol. 2, No. 2, Dec. 2016, pp. 117-127

ISSN 2465-7964 / eISSN 2465-7972


Experimental investigation of the use of CFRP grid for shear

strengthening of RC beams

Ngoc Linh Vu *; Kimitaka Uji; and Vu Dung Tran

(Received: August 4, 2016; Accepted: November 11, 2016; Published online: December 31, 2016)

Abstract: Carbon fiber reinforced polymer (CFRP) sheet and grid have been widely applied to improve both

shear and flexural strength of reinforced concrete (RC) beams. Compared to CFRP sheet, CFRP grid and

sprayed mortar have advantages in dealing with the damaged concrete surface. This work examined the ef-

fectiveness of CFRP grid and sprayed mortar in enhancing the shear capacity of RC beams. Three RC beams

were fabricated and two of them were strengthened by CFRP grid and sprayed mortar. Then four-points

bending test was carried out to collect the data on the behavior of CFRP grid and stirrups in the three beams.

The results are presented and discussed in this paper. This study evaluated the strengthening effectiveness of

CFRP grid and sprayed mortar and the behavior of stirrups and CFRP grid. The difference in behaviors be-

tween stirrups and CFRP grid was analyzed.

Keywords: CFRP grid, sprayed mortar, stirrup, shear strengthening, RC beam.

1. Introduction

After a period of rapid economic growth, re-

pairing and retrofitting of existing infrastructures

that have been aging rapidly, such as buildings,

bridges, and tunnels, have been among the most

important civil engineering challenges all over the

world. For example, in Japan, the percentage of

highway bridges that are more than 50 years old

was approximately 18% in 2013 and will be 43% in

2023. For tunnels, this percentage is 20% and 34%,

respectively [1]. The other reason for the demand

for strengthening and rehabilitation of structures is

the upgrading of their load carrying capacity and

resistance to withstand underestimated loads, to

ameliorate the increased perceived risk from earth-

quakes. Since 1990s, carbon fiber reinforced poly-

mer (CFRP) has been used in civil infrastructures to

increase the load carrying capacity due to the lim-

ited durability of traditional materials. Using CFRP

has been considered as a very effective strengthen-

ing and rehabilitation method for such work. CFRP

has many advantageous engineering characteristics

such as high strength-to-weight ratio and stiffness-

to-weight ratio, corrosion resistance, and ease of

application and construction. CFRP currently plays

a key role in strengthening and retrofitting concrete

structures in Japan. The area of CFRP sheet used

was 980,000 m2 in 2006 and 1,310,000 m

2 in 2013

[2]. CFRP is usually used in the form of sheet. The

consumption of CFRP in the form of grid was only

about 5% of that of CFRP sheets in 2006. CFRP

grid and sprayed mortar have advantages in dealing

with damaged concrete surface. Scheme of CFRP

grid application is shown in Fig. 1.

Fig. 1 – Schematics of CFRP grid reinforcement

application

Many experimental studies have been conduct-

ed over the past decade to study the performance of

concrete beams strengthened in shear with external-

ly bonded FRP composites. Bukhari [3] reviewed

the existing design guidelines for strengthening

continuous beams in shear with CFRP sheets and

proposed a modification to Concrete Society Tech-

nical Report. Chen [4] studied the shear behaviour

of RC beams with FRP grid and concluded that RC

beams strengthened with CFRP grid have good

shear behaviour in terms of both increasing th

Corresponding author Ngoc Linh Vu is a Ph.D. candidate of

Dept. of Civil and Environmental Engineering, Tokyo Metro-

politan Univ., Japan, is a lecturer of Dept. of Civil Engineer-

ing, Univ. of Transport and Communications, Vietnam

Kimitaka Uji is a Professor of Dept. of Civil and Environmen-

tal Engineering, Tokyo Metropolitan Univ., Japan

Vu Dung Tran is a Doctor of Dept. of Civil and Environmen-

tal Engineering, Tokyo Metropolitan Univ., Japan

117

Table 1 – Mix proportion of concrete

Gravel

(mm)

Slump

(mm)

W/C

(%)

Air

(%)

Weight (kg/m3)

Water Cement Sand Gravel Admixture

20 120 55.6 4.5 158 284 792 1126 1.3

Table 2 – Properties of polymer and epoxy primer

Properties Polymer Epoxy primer

Main component SBR synthetic rubber vinyl acetate-ethylene copolymer

Solid content 45-46 (% by weight) 45-48 (% by weight)

Appearance white milky liquid white milky liquid

Viscosity below 50 (mPa.s) 500-2000 (mPa.s)

pH 8.0-9.0 4.5-6.5

Density 1.0 (g/cm3) 1.06 (g/cm

3)

Table 3 – Mechanical properties of concrete and mortar

Type Compressive strength (N/mm2) Elastic modulus (kN/mm

2) Tensile strength (N/mm

2)

Concrete 34.1 31.9 2.92

Mortar 36.7 31.0 2.87

Table 4 – Mechanical properties of reinforcing bars and CFRP

shear capacity and controlling of the crack width.

Guo [5] examined the effect of shear capacity of

RC beams reinforced with a haunch using the PCM

shotcrete method with CFRP grid and investigated

the adhesive properties of the reinforced interface

between PCM and the existing concrete. Steffen [6]

concluded that the use of CFRP grid could provide

a quick, efficient method for providing corrosion

resistant concrete deck reinforcement. CFRP grid

reinforcement represents a suitable replacement for

steel rebars in some concrete structural members

subjected to aggressive environmental conditions

that accelerate corrosion of the steel reinforcement

and cause deterioration of the structures [7]. These

studies have not focused on the difference in behav-

iours between stirrups and CFRP grid. The main

purpose of this research was to study the capacity of

CFRP grid and the behavior of stirrups and CFRP

grid in shear strengthening. This paper illustrates

the use of CFRP grid combined with sprayed mor tar to improve the shear strength of RC beams.

The existing guidelines are related to the applica-

tion of CFRP sheet, and there are no specific guide-

lines for the application of CFRP grid. The material

factor in calculating the effectiveness of CFRP grid

in providing shear resistance capacity was evaluated.

Based on the experimental result, the effectiveness

of CFRP grid and sprayed mortar in enhancing the

shear strength of RC beams was also evaluated. The

difference in behavior between stirrups and CFRP

grid was analyzed in detail.

2. Experimental Program

Three RC beams were fabricated and tested.

The control RC beam was not strengthened while

the two other beams were shear strengthened by

CFRP grid and sprayed mortar. The three RC

beams were designed such that their flexural

strength was much higher than their shear strength

to ensure that failure was controlled by the shear

force.

Type Size Area

(mm2)

Yield

strength

(N/mm2)

Tensile

strength

(N/mm2)

Elastic

modulus

(N/mm2)

10cm

10cm

Strains gauges

Rebar

D32 794.2 389 587 2 105

D10 71.33 413 561 2 105

D6 31.67 417 570 2 105

CFRP grid CR8 26.5 * 1400** 1 105*

NOTE: * CFRP grid has no yield strength; ** manufacturer’s data. Fig. 2 – CFRP CR8

118


2.1 Materials

The three RC beams were fabricated using

ready-mixed concrete with a compressive strength

of 34.1 N/mm2 and maximum aggregate size of 20

mm. In the concrete mix, high-early-strength

Portland cement was used and the water-cement

ratio was 0.556 with the addition of water-reducing

and air-entraining admixtures. The other parameters

of the concrete mix are shown in Table 1. The

mortar to be sprayed was made by mixing 25 kg

premixed mortar, 1.21 kg polymer, and 4.1 kg

water. In order to increase interface adhesion,

epoxy primer was applied to the surface of concrete

before spraying mortar. The properties of the

polymer and epoxy primer are shown in

Table 2.

D32 was used as the main reinforcing bar in

the tension zone. D6 and D10 were used as the

stirrup and reinforcing bar, respectively. The CFRP

grid used in this test was CFRP-CR8 (The CR8

label is given by the manufacturer) with a grid

spacing of 100 mm 100 mm. The mechanical

properties of concrete and mortar are given in Table

3. Mechanical properties of reinforcing bars and

CFRP grid are listed in Table 4.

2.2 Test Specimens

The RC beams had the dimensions 200 mm

500 mm 2,750 mm. They were labelled RC beam

1 (control beam), RC beam 2, and RC beam 3. RC

beam 1 and RC beam 2 were reinforced with 6D32

longitudinal rebars at the bottom and 2D10 longitu-

dinal rebars on the top. The stirrup spacing was 200

mm (see

Fig. 4). In RC beam 1, D10 bar was used for

stirrups while, in RC beam 2, D6 bar was used as

stirrups, which is smaller than the type used in the

RC beam 1. With the assumption that the shear

strength in this beam was lost due to corrosion, the

cross-sectional area of the stirrups was reduced. RC

beam 3 was reinforced with 6D32 longitudinal re-

bars at the bottom. RC beam 3 had no stirrups in

order to evaluate the capacity of CFRP grid for en-

hancing shear resistance without stirrups. RC beam

2 and RC beam 3 were strengthened by CFRP grid

and sprayed mortar. The CFRP grid was placed

along the beam web and mortar was sprayed to cre-

ate an additional 20 mm layer on both sides of the

two beams.

Table 5 shows the total shear reinforcement ar-

ea of each beam. Reinforcement area in RC beam 1

is the total of cross sections of D10, while, in RC

beam 3, it is the total cross sections of CFRP grid

(vertical grid only) and, in RC beam 2, it is the total

of cross sections of D6 and CFRP grid in 200 mm

beam length of each beam.

Fig. 3 – Reinforcing bars and stirrups

2.3 Casting RC Beams

First, the RC beams were cast in wooden

formwork and cured by moisture-retaining cover

(see Fig. 5). Eight days after the RC beam 2 and RC

beam 3 were cast, their web’s surface was sand

blasted. The roughness of the beam surface after

sand blasting was 0.15 mm. Four days later, CFRP

grid was fixed on both sides of the beam by steel

bolt anchors. On the next day, epoxy primer was

applied. The roughness of the beam surface after

applying epoxy was 0.13 mm. After the epoxy layer

had dried, the repair mortar was sprayed (see Fig. 6).

Finally, a curing compound was sprayed on the

mortar surface. Tests of the RC beams were carried

out at the age of 27th, 28

th, and 29

th day.

2.4 Instrumentation and Test method

Four-point-bending test was performed (see

Fig. 7) on the three beams, with a span length (L) of

2,350 mm and a shear span (a) of 1,100 mm. The

effective depth (d) was 423 mm. The shear span to

effective depth (a/d) ratio was 2.6. During the test,

when the first flexural crack and the first diagonal

crack appeared, the specimens were unloaded to

mark the cracks and take photographs. After that

the load was continually increased until the beams

failed. The failure processes were monitored by

strain gauges installed on concrete, reinforcing bars

and the CFRP grid, and by displacement transduc-

ers placed at mid-span and two end supports of each

beam. Fig. 8 through Fig. 10 show locations of the

displacement transducers and strain gauges installed

on rebar, concrete, and CFRP grid in the three RC

beams.


3.1 Load – displacement behavior, crack devel-

opment and failure mode Fig. 8 through 10 describe cracks generated

during the test. The cracks shown in bold lines were

the largest cracks at the ultimate state of each beam.

119


Fig. 4 – Longitudinal and transverse cross-sections of three RC beams and strengthening scheme using

CFRP grid and sprayed mortar

Table 5 - Total cross-sectional area of shear reinforcement: CFRP grid and stirrups

No. Shear reinforcement Total reinforcement area

(mm2/200 mm length)

Ratio of shear reinforce-

ment

(%)

RC beam 1 D10 @200 mm 142.66 0.37%

RC beam 2 D6 @200 mm and CFRP grid CR8 @100 mm 169.34 0.35%

RC beam 3 CFRP grid CR8 @100 mm 105.60 0.22%

NOTE: As - area of shear reinforcement (mm2); b - beam width (bRC1 = 200 mm; bRC2 = bRC3 = 240 mm); s - shear rein-

forcement spacing (mm).

Fig. 5 – RC beams fabrication

Fig. 6 – Fixing CFRP grid and spraying mortar

120


Fig. 7 – Beam loading

Based on the observation of cracks for the three

beams, it is supposed that all beam failures were

due to diagonal tension. The test results of all

beams are summarized in Table 6. RC beam 2 ex-

hibited the highest maximum load (757 kN) while

RC beam 3 exhibited the lowest (617 kN) and RC

beam 1 showed a value between the two extremes

(690 kN). Compared to the control beam (RC beam

1), the maximum test load of RC beam 2 was higher

by 9.7% while that of RC beam 3 was lower by

10.6%.

Table 6 – Summary of test results

Beam index Crack-

ing

load

(kN)

Maximum load

(kN) Max.

mid-

span

displ.

(mm)

Design Test

RC beam 1 225 354 690 8.4 RC beam 2 300 697 757 7.4 RC beam 3 300 656 617 7.1

The total cross-sectional area of shear rein-

forcement placed in 200-mm beam length was

142.6 mm2 (two bars of D10) in RC beam 1, while,

in RC beam 2 and RC beam 3, it was 169.3 mm2

(two rebars of D6 and four bars of CFRP CR8) and

105.6 mm2

(four bars of CFRP CR8), equivalent to

118.7% and 74.3% of that of RC beam 1, respec-

tively. The ratio of shear reinforcement of RC beam

1 was 0.37%, while, in RC beam 2 and RC beam 3,

it was 0.35% and 0.22%, equivalent to 94.6% and

59.5%, respectively. The total cross-sectional areas

of reinforcing bars and CFRP grid and the ratios of

shear reinforcement are shown in Table 5. From test

results, in two cases of strengthening, CFRP grid

and sprayed mortar proved the effectiveness for

enhancing the shear capacity. It could be concluded

that concrete, rebar, mortar, and CFRP grid worked

well together.

Fig. 11 shows the load versus mid-span dis-

placement curves for the three beams. The maxi-

mum mid-span displacements of the three beams

are listed in Table 6. RC beam 1 had the highest

mid-span displacement of 8.4 mm while RC beam 2

and RC beam 3 had lower values of 7.4 mm and 7.1

mm (lower than 11.9% and 15.5% compared to RC

beam 1), respectively. Each load versus mid-span

displacement curve (see Fig. 11) could be divided

into two stages. The first stage is before the crack-

ing (225 kN for RC beam 1, 300 kN for RC beam 2

and RC beam 3). In this stage, RC beam 2 and RC

beam 3 showed higher stiffness than RC beam 1

because their width was larger (240 mm) compared

to that of RC beam 1 (200 mm). In the second stage

after the cracking, there is a distinct difference be-

tween RC beam 1 and RC beam 3. The stiffness of

RC beam 3 became lower than that of RC beam 1

due to cracks developed and the cross-section re-

duced. The Young’s modulus of the stirrups is

higher than that of CFRP grid (200,000 N/mm2 ver-

sus 100,000 N/mm2, respectively, in Table 4). RC

beam 3 had CFRP grid but no stirrups. Therefore

stirrups provide higher stiffness to the beam and

result in smaller displacement value compared with

the CFRP grid. In general, after strengthened by

CFRP grid and sprayed mortar, both RC beam 2

and RC beam 3 had higher stiffness, ductility char-

acteristic, and the cracking loads were also im-

proved (except in the final period for RC beam 3,

when the load was over 500 kN).

3.2 Behavior of stirrups in RC beam 1 and

CFRP grid in RC beam 3

When the load increased, the RC beams de-

formed, cracks appeared and developed, and the

beams failed. The behavior of the stirrups in RC

beam 1 and that of the CFRP grid in RC beam 3

were different. Fig. 12 illustrates the load versus

strain curves of stirrups in RC beam 1 and CFRP

grid in RC beam 3. Strain gauges S3 and S4 with

locations shown in Fig. 8 were installed on the stir-

rups in RC beam 1. Strain gauges G13 and G35

with locations shown in Fig. 10 were installed on

the CFRP grid in RC beam 3. These strain gauges

were near the cracks in each beam.

From Fig. 12, in general, strain values on

CFRP grid in RC beam 3 are smaller than those on

stirrups in RC beam 1. The strain recorded by G13

was the highest in RC beam 3 and the strain record-

ed by S3 was the highest in RC beam 1. These

gauges were close to the ultimate cracks in each

beam (see Fig. 8 and Fig. 10). The load versus

strain curves consist of 3 stages: In the first stage,

there are no cracks in each beam. Cracking load of

RC beam 3 was around 300 kN while, for RC beam

1, it was around 225 kN (see Fig. 13). In this stage,

at the same load levels, the strain of G35 was

smaller than that of S4 and the slope of the experi-

mental load-strain curves of RC beam 3 was higher

121


Fig. 8 – Locations of strain gauges on stirrups and experimental cracks in RC beam 1

Fig. 9 – Locations of strain gauges on stirrups and experimental cracks in RC beam 2

Fig. 10 – Locations of strain gauges on CFRP grid and experimental cracks in RC beam 3

than that of RC beam 1. The reason was the beam

width of RC beam 3 was 240 mm (two layers of 20

mm including CFRP grid and sprayed mortar were

applied on two sides of the RC beam) compared to

the 200 mm width of RC beam 1. Thus, the bending

stiffness of RC beam 3 increased by about 10%.

In the second stage, when the diagonal cracks

appeared and developed, the length and the width of

the cracks increased, the cross-sectional area re-

duced gradually, and the shear capacity of both

beams provided by concrete and mortar reduced.

The shear strength of RC beam 1 was mainly pro-

vided by stirrups whereas that of RC beam 3 was

mainly provided by CFRP grid. Moreover, the

Young’s modulus of CFRP grid (1 105 N/mm

2) is

a half of that of the stirrup (2 105 N/mm

2) as

shown in Table 4. Therefore, strains in RC beam 3

are higher than those in RC beam 1 as shown in Fig.

13 and Fig. 14.

In the third stage, when the load continued in-

creasing, the stress on the stirrup reached the yield

122


Fig. 11 – Load vs. mid-span displacement curves

Fig. 12 – Load vs. strain curves of stirrups in RC beam 1 and CFRP grid in RC beam 3

Fig. 13 – Load vs. strain curves of S4 (RC beam 1)

and G35 (RC beam 3)

Fig. 14 – Load vs. strain curves of S3 (RC beam 1)

and G13 (RC beam 3)

123


strength. From the values given in Table 4, the stir-

rup is expected to yield at a strain of

= 2,085 10-6

. In test, stir-

rup S3yielded at a strain of 2,180 10-6

equivalent

to a load of 475 kN as shown in Fig. 14. When the

stirrup yielded, the modulus of shear reinforcing

bars reduced rapidly, whereas CFRP grid was elas-

tic until fracture. This is the reason why strains in

RC beam 3 were smaller than those in RC beam 1.

3.3 Difference in behavior between CFRP grid

and stirrups in RC beam 2 Bonding between concrete and sprayed mortar

is one of the most important factors that influence

the effectiveness of reinforcement. The difference

in behaviours between the stirrups and CFRP grid

at the same positions in RC beam 2 may provide the

information on the bonding between concrete and

sprayed mortar. The strain gauges on CFRP grid G4,

G14, G30, and G35 and those on the stirrups S1, S2,

S3, and S4 were respectively at the same positions

in RC beam 2. Locations of these strain gauges are

shown in Fig. 9. The data of the bending test are

listed in Table 7. Fig. 15 illustrates the load versus

strain curves of the stirrups and CFRP grid in RC

beam 2.

The strain curves for the load between 300 kN

and 550 kN (see Table 7 and Fig. 15) show the fol-

lowing behaviour: When the load is greater than

350 kN, the strain values on the stirrup are higher

than that on the CFRP grid (except stirrup S1),

S3/G30 and S4/G35 are higher than S2/G14. At a

load greater than 700 kN, the difference in strains

between stirrup S4 and CFRP grid G35 becomes

larger. At the ultimate stage, the strain recorded by

S4 is about 5 times higher than that by G35 (4.9

times higher at a load of 750 kN). Thus, strain

gauge readings at the same position differed

significantly. This observation proved that, these

materials (the CFRP grid and the stirrup) no longer

worked together and the bonding between concrete

and mortar was damaged. The possible reasons are

as follows: First, when the strain on stirrups S2, S3,

and S4 was greater than 2,085 10-6

(as calculated

in section 3.2), the stirrups yielded and deformed

rapidly after this point. Second, when the load was

increased from 320 to 425 kN, cracks appeared and

propagated towards the loading point, the

compression area of the cross-section reduced.

Most of these cracks were located on the same posi-

tions as S2, S3, and S4 (see Fig. 9). Once a crack

crossed the locations of stirrups and CFRP grid, the

adhesion of mortar to concrete surface was also

damaged.

0

100

200

300

400

500

600

700

800

0 2000 4000 6000 8000

Load(kN)

Strain(μ)

G35

S4

a) Stirrup S4 and CFRP grid G35

0

100

200

300

400

500

600

700

800

0 5000 10000 15000

Load(kN)

Strain(μ)

G30

S3

b) Stirrup S3 and CFRP grid G30

Fig. 15 – Load vs. strain curves of stirrup and

CFRP grid in RC beam 2

3.4 Contribution of CFRP grid in shear

strengthening of RC beams

Design shear capacity values were calculated

for different values of shear strength as shown in

Table 6. Flexural cracks appeared when the flexural

cracking strength of concrete was exceeded by the

applied stress. After that, if the stress reached the

shear strength of concrete, diagonal cracks would

occur. The design shear capacity of RC beam 1 is

the sum of shear resistances contributed by concrete

and reinforcement while those of RC beam 2 and

RC beam 3 are the sums of shear resistances con-

tributed by concrete, reinforcement, and CFRP grid.

According to Eq. (9.2.3) of Ref [8], the design shear

capacity was calculated using following equations:

(1)

124


Table 7 – Comparison of strains on stirrups and CFRP grid in RC beam 2

Load

(kN)

Stirrup – Comparison

G6 G14 G30 G35 S1 S2 S3 S4 S1/G6 S2/G14 S3/G30 S4/G35

50 1.0 1.0 −2.0 3.0 2.0 2.0 1.5 1.5 2.0 2.0 −0.8 0.5

100 8.0 8.0 0.1 9.0 8.0 7.0 5.5 6.0 1.0 0.9 55.0 0.7

150 12.0 11.0 1.0 14.0 15.0 8.5 7.5 7.5 1.3 0.8 7.5 0.5

200 13.0 13.0 −3.0 15.0 17.0 8.5 6.0 7.0 1.3 0.7 −2.0 0.5

250 14.0 16.0 2.0 15.0 21.5 9.0 7.0 6.5 1.5 0.6 3.5 0.4

300 18.0 13.0 20.0 12.0 29.0 6.0 45.5 4.5 1.6 0.5 2.3 0.4

350 4.0 −14.0 418.0 154.0 32.5 5.0 592.0 247.5 8.1 −0.4 1.4 1.6

400 126.0 1111.0 1064.0 392.0 57.0 1248.0 1822.0 854.0 0.5 1.1 1.7 2.2

450 338.0 1785.0 1358.0 640.0 158.5 2264.0 3246.0 1443.5 0.5 1.3 2.4 2.3

500 636.0 2305.0 1789.0 1027.0 438.0 4046.5 5592.0 2091.0 0.7 1.8 3.1 2.0

550 864.0 2748.0 2126.0 1232.0 632.0 6475.0 7606.0 2417.5 0.7 2.4 3.6 2.0

600 1234.0 3273.0 2622.0 1467.0 782.0 9382.0 10198.5 2905.5 0.6 2.9 3.9 2.0

650 1451.0 3759.0 3168.0 1596.0 826.5 11489.5 12621.0 3714.0 0.6 3.1 4.0 2.3

700 1573.0 4243.0 3882.0 1645.0 864.5 13727.5 15415.5 5050.0 0.5 3.2 4.0 3.1

Design shear capacity in RC beam 1:

(2)


(3)


(4)

- Vcd: design shear capacity without shear rein-

forcement

(5)

(N/mm2) where fvcd 0.72

(N/mm2)

(d in mm) when d >1.5, d is

taken as 1.5

when p >1.5, p is taken as

1.5

n=1

bw: web width

d: effective depth

pv=As/(bw d)

where, As - area of tension reinforcement

(mm2); f'cd - design compressive strength of

concrete (N/mm2); f'cd is taken as 34.1 N/mm

2

with concrete and 36.7 N/mm2 with mortar (see

Table 3); b - member factor, may generally be

taken as 1.3; Vcd1 - shear resistance contributed

by concrete; Vcd2 - shear resistance contributed

by concrete and sprayed mortar; Vcd3 - shear re-

sistance contributed by concrete and sprayed

mortar.

- Vsd: design shear capacity of shear reinforcement,

taken from Eq.(9.2.6) of Ref [8]

(6)

where, Aw - total area of shear reinforcement

placed in Ss; fwvd - design yield strength of shear

reinforcement (yield strength of stirrup, D10 for

RC beam 1 and D6 for RC beam 2, was taken,

fwvd of RC beam 1 is as 413 N/mm2 and fwvd of

RC beam 2 is 417 N/mm2 in Table 4); z - dis-

tance from location of compressive resultant to

centroid of tension steel, which may be taken as

d/1.15; Ss - spacing of shear reinforcement; b -

member factor, may generally be taken as 1.1.

- VCFRP: design shear capacity of CFRP grid. In

Japan, there is no standard specification for

shear strengthening concrete structure using

CFRP grid. In this case, the CFRP grid was tak-

en as shear reinforcement.

According to Eq.(9.2.6) of Ref [8], the following

equation was suggested for calculating VCFRP.

(7)

where, Aw - total area of shear reinforcement

placed in Ss; fCFRP - design yield strength of

125


shear reinforcement (In this experiment, there is

no yield strength of CFRP and CFRP can be

considered as a linear-elastic material. fCFRP was

taken as tensile strength, equal to 1,400

N/mm2); b - member factor of CFRP material

(as there is no standard specification for CFRP

grid, the safety ratio of 1.3 for CFRP sheet tak-

en from “Guideline for repair and strengthening

using CFRP sheet for concrete structure” [9]

was used)

According to test results in Table 6, RC beam

1 and RC beam 2 showed the maximum test loads

of 690 kN and 757 kN, respectively, much higher

than design loads of 354 kN and 697 kN, respec-

tively, due to the following reasons. First, the value

of the design load is calculated for linear-elastic

material, while at the load of 690 kN, the stirrup in

RC beam 1 already yielded; second, there is safety

ratio included in the formula in standard specifica-

tions. For RC beam 3, the design loads are higher

than the experimental maximum load, due to the

following reasons. First, applying the formula for

stirrups to calculate shear strength of CFRP grid

was not appropriate as some coefficients in the

formula were not reasonable for CFRP grid, the

stirrup has yield strength but CFRP grid does not.

Second, when computing the design shear strength,

it was assumed that the CFRP grid would work at

full capacity. Actually, the maximum stress in

CFRP grid was less than 1,400 N/mm2 as shown in

Table 4. At the ultimate state of strengthened beams,

the maximum load of RC beam 3 was 617 kN and

the maximum strain values of CFRP grid in RC

beam 3 were recorded by strain gauges G13 and

G35 (location of strain gauges are shown in Fig. 10).

The maximum strains were 11,943 10-6

and

6,377 10-6

as shown in Fig. 16, respectively. The

stress on the CFRP grid was calculated as follows.

The maximum stress values on the CFRP grid

recorded by G13 and G35 were 85.3% and 45.5%

of the tensile strength of CFRP grid (1,400 N/mm2)

as shown in Table 4. Guideline for repair and

strengthening using CFRP sheet for concrete struc-

ture [9] suggests that the material factor for calcu-

lating CFRP sheet capacity is 1.3. There is no men-

tion of the corresponding value for CFRP grid. Here,

using material factor of 1.3, the shear design load of

RC beam 3 would be 656 kN, which is not appro-

priate. In the present research work, a coefficient of

1.5 is proposed, indicating that the CFRP grid

works at 66.7% of its tensile strength. Using the

coefficient of 1.5, the design load will be 637 kN

for RC beam 2 and 600 kN for RC beam 3.

4. Conclusions

Experiments were conducted to study the per-

formance of CFRP grid and sprayed mortar for

shear strengthening of RC beams. CFRP grid con-

sists of vertical and horizontal components but this

study only focused on researching the vertical com-

ponent of the CFRP grid. Based on the collected

data, the following conclusions were drawn.

(1) CFRP grid and sprayed mortar could be signif-

icantly effective in shear strengthening. In this

experiment, RC beams strengthened with

CFRP CR8 and sprayed mortar (RC beam 2

and RC beam 3) attained 110.7% and 89.4% of

shear capacity compared with the ultimate load

of the control beam (RC beam 1). The cross-

sectional area of reinforcing materials in RC

beam 2 and RC beam 3 were equivalent to

118.7% and 74.3%, respectively, of the cross-

sectional area of stirrups in RC beam 1. In

general, the stiffness, ductility characteristic,

and the cracking load of the strengthened RC

beams (RC beam 2 and RC beam 3) using

CFRP grid and spayed mortar were improved.

(2) The behavior between the CFRP grid and the

stirrups reflects the bonding between concrete

and sprayed mortar. It is one of the most im-

portant factors that influences the reinforce-

ment effectiveness. According to experimental

results, when the load increased, variations of

the stirrup strains and the CFRP grid strains at

the same position in a strengthened RC beam

had similar tendencies. After the stirrups yield-

ed and the cracks developed in a strengthened

RC beam, the bonding between concrete and

sprayed mortar was considerably affected.

(3) In an application of CFRP grid and sprayed

mortar, CFRP grid could not work at 100% of

the tensile strength. Ref [9] reports the material

factor of 1.3 for calculating CFRP sheet capac-

ity, but the maximum stress of CFRP grid was

85.3% of the tensile strength in our experi-

ments. Therefore, a material factor of 1.5 is

proposed for the CFRP grid when applying the

formula in Ref [8] to calculate the shear

strength of a RC beam strengthened with

CFRP grid and sprayed mortar. Using a coeffi-

cient of 1.5 means the CFRP grid works at

66.7% of its capacity.

126


It is concluded that the shear-strengthening of

RC beams by CFRP grid and sprayed mortar is ef-

fective. CFRP grid and sprayed mortar are useful in

strengthening and retrofitting of concrete structures.

CFRP grid can support or replace stirrups in RC

beams in providing shear strength. The availability

of CFRP grid and sprayed mortar should be more

intensively investigated and widely applied in con-

crete structures in the future.

References 1. White paper on land, infrastructure, transport

and tourism in Japan (2014), 123 pp.

2. "CFRP Repair and Reinforcement Method

Technology Research Association"

http://tampoken.com/04.html (accessed on Dec.

5, 2016)

3. I.A. Bukhari, (2010) “Shear strengthening of

reinforced concrete beams with CFRP,” Maga-

zine of Concrete Research, (1), pp. 62.

4. W.Y. Chen, (2010) “The shear behaviour of

beams strengthened with FRP grid,” Proceed-

ings of the 5th International Conference on FRP

Composites in Civil Engineering.

5. R. Guo, (2014) “Shearing Capacity and adhe-

sive Properties of Reinforce Interface for RC

beam with haunch retrofitted by CFRP grid

with spraying PCM shotcrete,” Memoirs of the

Faculty of Engineering, Kyushu University.

6. R. Steffen, (2003) Design and Constructability

of a CFRP Grid Reinforced Bridge Deck, High

Performance Materials in Bridges, pp. 106-116.

7. B. Zhang, (2004) “Behaviour of one-way con-

crete slabs reinforced with CFRP grid rein-

forcements,” Construction and Building Mate-

rials, 18(8), October 2004, pp. 625–635.

8. JSCE Guidelines for Concrete, N0.15. (2007)

Standard Specifications for Concrete Struc-

tures, “Design,” Japan Society of Civil Engi-

neers, pp. 156.

9. JSCE – Concrete Committee (2000) Guideline

for repair and strengthening using CFRP sheet

for concrete structure.

127


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