fresh and early age concrete

Fresh and early age

concrete

SCT50 GROUP 14: ASSIGMENT 4

Word count: 5955

Date submitted: 28 May 2012

Group Members

Sivuyile Ngobozana(Group Leader) Makintane Mofokeng

Kevin Volmink Mfundo Taliwe Charlie Visser Pitso Reatile

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SCT 50 Group 14: Assignment 4 of 23

Summary

Concrete is in the fresh state while its shape can be changed and it can be compacted.

Concrete can remain in the fresh state normally for two to three hours. The length of this period

depends on various factors including temperature, cement type and content and admixture type

and content (CNCI, 2002)

Fresh cementitious materials behave as fluids with a yield stress which is the minimum stress

for irreversible deformation and flow. This yield stress can be measured using rheological tools

on the cement paste (Roussel, 2006)

Fresh concrete properties are very important in the performance of hardened concrete, so

proper attention needs to be taken and testing done on the fresh concrete to gather data that

will help understand the long term performance of the concrete.

Weather conditions especially temperature has a huge impact in fresh concrete during placing.

While low temperatures can result in delayed setting or retardation and decrease the rate of the

concrete early strength gain, high temperatures on the other hand will increase the rate of

hydration and result into premature stiffening of the concrete, plastic shrinkage cracking,

thermal stresses and cracking in large elements, permeability and also decrease the long-term

concrete strength and durability of the concrete. According to BS8500, the concrete temperature

shall not exceed 35 degrees Celsius at the time of delivery.

So proper testing and caution is imperative during delivery, placing, compaction and finishing of

fresh concrete as this will determine the long term durability of the concrete.


Table of Contents 1 The Assignment .................................................................................................................. 4

2 Introduction ......................................................................................................................... 5

3 Fresh concrete testing.......................................................... Error! Bookmark not defined.

3.1 Yield stress and plastic viscosity ................................... Error! Bookmark not defined.

3.1.1 Defination of yield stress and plastic viscosity ....... Error! Bookmark not defined.

3.1.2 Consistence of concrete by a single-point test ...... Error! Bookmark not defined.

3.2 Uses, advantages and disadvantages of the : ............... Error! Bookmark not defined.

Degree of compactability ...................................... Error! Bookmark not defined.

Flow table………………………………………………………………………………...8

V-funnel tests for fresh concrete……………………………………………………….8

3.2.1 Conclusions………………………………………………………………………………..8

3.3 Yield stress measurements ........................................... Error! Bookmark not defined.

3.3.1 Available methods for direct and indirect measurement of ... Error! Bookmark not

defined.

3.3.2 Relation to the single-point tests ........................... Error! Bookmark not defined.

3.4 Effect on plastic viscosity of : ........................................ Error! Bookmark not defined.

3.4.1 Superplasticizers ................................................... Error! Bookmark not defined.

3.4.2 Air-entraining admixtures ....................................... Error! Bookmark not defined.

4 Concrete for the construction of a windfarm ......................... Error! Bookmark not defined.

4.1 Considerations during placing to ensure the integrity of the base slab ........................14

4.2 Concise report for the contractor giving advice on:......................................................14

4.2.1 Suitable concrete compositions ...........................................................................14

4.2.1 Placing procedures ..............................................................................................14

4.2.1 Procedures for ensuring integrity of the slab after placing ....................................14

5 Methods for monitoring the concrete strength development in the base slabs .............. Error!

Bookmark not defined.


6 References .......................................................................... Error! Bookmark not defined.


1. The Assignment

Fresh and early age concrete

Question 1

(a) Define the terms yield stress and plastic viscosity in relation to the properties of fresh

concrete. Explain why the consistence of concrete cannot be fully characterized by a single-

point test.

(b) Discuss the uses, advantages and disadvantages of the degree of compactability, flow table

and V-funnel tests for fresh concrete.

(c) Describe the methods available for the direct and indirect measurement of yield stress and

explain how and why it is related to the single-point tests in (b) above.

(d) Describe and explain the effect on plastic viscosity of:

(i) superplasticizers

(ii) air-entraining admixtures.

Question 2

A wind farm consisting of a large array of turbines is to be constructed over a two year period in

an exposed location in which the daytime summer temperatures can reach 35degC and the

nighttime winter temperatures can fall to -10degC. Each turbine has a concrete base slab 12m

x12m x 4m deep. The reinforcement is congested, particularly in the region around the base of

the column supporting the turbine. The required concrete strength class for the base slabs is

C50/60. The slabs will be constructed at a rate of one per week.

(a) Discuss and explain the issues that need to be considered when placing the concrete and

for ensuring the integrity of the base slab in the period after placing.

(b) Prepare a concise report for the contractor giving advice on:

(i) suitable concrete compositions

(ii) placing procedures

(iii) procedures for ensuring the integrity of the slab in the period after concrete casting.

(c) Discuss methods for monitoring the development of strength of the concrete in the base

slabs to determine when the strength specified by the designer for erection of the columns is

achieved. Recommend a preferred method, giving reasons for your choice.


2 Introduction

Fresh concrete properties are very important as they partly determine the concrete performance

in the hardened state and have an effect on the long term durability of the concrete. Fresh

concrete testing is necessary to ensure that these properties meet the requirements The degree

of compactibility, flow table and v-funnel are some of the tests that will be looked at, and uses,

advantages and disadvantages thereof. The other important factors that will impart the concrete

performance are placing, compaction and curing.

A wind farm consisting of large array of turbines is to be constructed over a two year period and

under some challenging conditions. There temperature fluctuations from -10 degrees Celsius

during night time to 35 degrees Celsius during day time and there is congestion of

reinforcement.

There are certain considerations that need to be taken into account when placing the concrete

in order to ensure the integrity of the base slab after placing. When placing concrete in any

environment there are certain precautions to be taken, firstly it is to ensure that the mix design is

fit for the purpose, hence a suitable concrete composition should be looked at. The other

important factor is how is the concrete going to be placed, especially in an exposed location as

described in this case; a proper placing procedure will address this.

At the end of the day the contractor has to ensure that he delivers a durable concrete as per the

specification hence a concise report advising the contractor on the above issues is necessary.

Like in any project, there will be challenges along the way and time is always of essence; so it is

imperative to avoid unnecessary delays. For instance the strength development on the base

slabs will need to be monitored to ensure that the concrete has reached the specified strength

before the erection of the columns commence.


3 Properties of Fresh Concrete

3.1 Yield Stress and plastic viscosity

The main properties of fresh concrete can be described in terms of Fluidity,

Compactability and Stability (cohesiveness). The first two of these properties, fluidity

and compactability, have commonly been combined into a general property called

workability or more recently as consistence. (Domone, 2010)

Two definitions for consistence (workability) are given below:

i. ‘that property of freshly mixed concrete or mortar which determines the ease and

homogeneity with which it can be mixed, placed, consolidated and finished’

(ACI, 1990)

ii. ‘that property determining the effort required to manipulate a freshly mixed

quantity of concrete with minimum loss of homogeneity’ (ASTM, 1993)

Neither of these definitions makes reference to a quantifiable or measurable property

and is therefore prone to subjectivity. Therefore it is important to be able to measure

consistence to more objectively understand and predict the properties of fresh

concrete.

3.1.1 Measurement of consistence

3.1.1.1 Fundamental Properties

Domone (2010) states that there is general agreement that the behaviour of fresh

paste, mortar and concrete all approximate reasonably closely to the Bingham model

as illustrated in Figure 1-1.

The yield stress (y), as shown below, is the shear stress at which movement occurs

(i.e. below this stress no movement occurs) and the plastic viscosity () is the rate at

which this movement occurs and the resulting relationship is called a flow curve

(Barnes, 2000).


Figure 3-1 the flow curve of fresh concrete and the definitions of yield stress and plastic viscosity

(Domone 2010)

3.1.1.2 Factors affecting consistence

Lower values for y and indicate that the concrete mix is more fluid and in particular a

lower y translates in lower resistance to flow at low shear stresses for example under

self-weight when being poured (Domone, 2010). A lower value for results in a mix

that is less cohesive or ‘sticky’ and increased response when compacting with by

vibration, when localised shear rates can be large (Domone, 2010).

Domone (2010) further lists some important effects that the variation of mix proportions

and constituent materials has on y and as shown in Figure 1-2.

i. Increasing the water content while keeping the proportions of the other

constituents constant decreases y and in approximately similar proportions.

ii. Adding a plasticiser or super-plasticiser decreases y but leaves relatively

constant. In essence, the admixtures allow the particles to flow more easily but

in the same volume of water. The effect is more marked with super-plasticisers,

which can even increase , and can therefore be used to give greatly increased

flow properties under self-weight, while maintaining the cohesion of the mix.

iii. Increasing the paste content will normally increase and decrease y i.e. the mix

may start to flow more easily but will be more cohesive or ‘stickier’, and vice

versa.


iv. Replacing some of the cement with fly ash or ggbs will generally decrease y, but

may either increase or decrease , depending on the nature of the addition and

its interaction with the cement.

v. The small bubbles of air produced by air-entraining agents provide lubrication to

reduce the plastic viscosity, but at relatively constant yield stress.

Figure 3-2 Summary of the effect of varying the proportions of concrete constituents on the yield

stress and the plastic viscosity

In considering these effects it can be noted that the yield stress (y) and the plastic

viscosity () are independent properties, and different combinations can be obtained by

varying the mix constituents and their relative proportions. Both the yield stress and

plastic viscosity are required to define the behaviour of fresh concrete and hence

characterise its consistence.

3.1.2 Single point tests methods

Domone (2003) states that no single test or measurement can properly describe all of

the required properties of fresh concrete and hence recommends that the terminology

relating to workability (consistence) be divided into three classes as proposed by

Tattersall (1991) below:


Class 1: Qualitative, to be used in a general descriptive way without any attempt to

quantify, e.g. workability, flowability, compactability, stability, pumpability.

Class 2: Quantitative empirical, to be used as a simple quantitative statement of

behaviour in a particular set of circumstances, e.g. slump, flow table spread.

Class 3: Quantitative fundamental, to be used strictly in accordance with the

definitions in BS 5168: Glossary of rheological terms, e.g. viscosity, mobility,

fluidity, yield value.

Many tests have been devised and used over many years to produce quantitative

empirical values in Class 2 above. They give a single measurement i.e. one co-

ordinate on a flow curve, and are therefore often referred to as ‘single-point’ tests, to

distinguish them from the ‘two-point tests’ which give two measurements (Domone,

2003).

Domone (2003) states that single point tests are based on several different principles

and measure different properties and therefore a very wide degree of correlation is

obtained between them. He further compared different concrete mixes by ranking them

in order of increasing workability and different tests provided different ranking orders.

As a result single point tests can’t fully characterise concrete consistence as they only

give a single measurement of the different properties of consistence and are based on

different principles. These tests are however still widely used because of their simplicity

and ease of use both in the laboratory and on site (Domone, 2010).

3.2 Tests on fresh concrete

The following tests on fresh concrete are discussed below in Table 1-1 with regards

their uses, advantages and disadvantages: degree of compactability, flow table and v

funnel. A short description of the test procedure for each of these tests is given in

appendix A (can be moved to main text if word count allows).

Table 3-1 Discussion of tests on fresh concrete

Test Uses Advantages Disadvantages

Degree of

Compactability

Low, medium and high

consistence mixes.

Simple to carry out.

Suitable for site use.

Operator dependant.

Provides a single test

value.

Flow Table High to very high

consistence mixes.

Gives some indication

of tendency of mix to

segregate.

Operator sensitive.

No extra information

gained by jolting.

Cannot satisfactorily


measure bulk concrete

properties.

Provides a single test

value.

V Funnel Self-compacting

concrete

3.2.1 Conclusions

Lower values for yield stress (y) and plastic viscosity () indicate that the concrete mix

is more fluid and in particular a lower y translates in lower resistance to flow at low

shear stresses and a lower value for results in a mix that is less cohesive. Yield

stress and the plastic viscosity are however independent properties and different

combinations can be obtained by varying the mix constituents and their relative

proportions. Therefore both yield stress and plastic viscosity are required to define the

behaviour of fresh concrete and hence characterise its consistence.

Single-point tests give a single measurement of the different properties of consistence

and can therefore not be used to fully characterise concrete consistence.


3.3 Yield Stress Measurement

Introduction

The hard, strong and durable concrete is achieved after the plastic period, however little

attention is paid to its fresh properties. The pumping, placing and compaction of concrete is

based on rheology, improved scientific approach has made it possible to predict the fresh

properties, select materials and design concrete of desirable performance. (Banfill, 2003)

Rheology is the scientific study of flow of matter and is concerned with the relationship among

stress, strain, rate of strain and time. The flow deals with the movement of adjacent elements

of liquids over each other. (Banfill, 2003)

The rotational rheometers are used to measure the shear stress of concrete at different shear

rates in order to measure the plastic stress and viscosity. (International Centre for Aggregate

Research, 2003)

3.3.1 Available methods for direct and indirect measurement

3.3.1.1 Tests on cement paste:

An instrument such as coaxial cylinder viscometer is the most commonly used for measuring

plastic stress and strain. (Newman and Choo, 2003)

The inner cylinder is rotated as indicated in figure 1 below and the torque required is measured.

The gap between the inner and outer cylinder is usually a few millimetres and sufficient for

cement paste. (International Centre for Aggregate Research, 2003)


Figure 1: A coaxial cylinder viscometer

3.3.1.2 Tests on concrete:

There are tests used to measure the rheology of concrete and they are used mostly in the

laboratory. Although small rheometers have been developed to be used on site, the availability

is limited due to the high cost. The rheometers for concrete must be capable of dealing with

large sample sizes. The distance between the inner and outer cylinder must be at least five

times the maximum aggregate size and the outer cylinder radius to the inner cylinder radius

should be 1.0 to 1.1 (International Centre for Aggregate Research, 2003).

3.3.1.3 Powers and Wiler Plastometer

The Powers and Wiler Plastometer has been designed to measure concrete, it can also be used

to measure paste and mortar. The design is based on the concept of coaxial cylinder

rheometer. The outside cylinder rotates while the inner cylinder measures the torque with

spring attached couple system. The device is shown in figure 2 below.

Figure 2: Plastometer

3.3.1.4 Tattersall Two-Point Workable Device

The two-point device was developed by G.H. Tattersall at Sheffield University in the 1970s after

determining that it would be inappropriate to measure the rheology using the coaxial cylinders

due to failure plane of concrete between the cylinders. The Tattersall two-point device uses the

impeller geometry to measure the rheology of concrete and the device has been refined over

the years by Tattersal and other researchers and is used intensively in concrete. The device

measures the torque required to turn the impeller in concrete. The two different impellers with

MH and LM system as indicated in figure 4 below are available to measure concrete based on


workability. The device is calibrated with Newtonian fluid of known viscosity and power law fluid

of known flow curve.

Figure 4: Helical Impeller (MH System) and Offset H-Impeller (LM System)

3.3.1.5 The BML viscometer

The BML viscometer is a coaxial cylinder rheometers which is dependent on Power and Wiler

plastometer and the Tattersall device. The outer cylinder rotates and the inner cylinder

measures the torque. The cylinder sizes can be changed to compensate for different concrete

stones sizes which are tested. The BML viscometer is suitable for concrete with slumps in

excess of 120mm and can be used for self compacting concrete and it has also been used for

concrete with slumps of 50-60mm. (ICAR, 2003)

3.3.2 Comparison to single-point test measurements

Only one value is measured in the single-point tests and the concrete is moving at a different

shear rate in each case. Each single-point test has an average shear which is similar to

determining one pint on the yield stress versus shear strain. (Newman and Choo, 2003)

In the slump test the movement is at zero or small when the measurement is taken and hence

the relationship between yield stress and slump exists. (Newman and Choo, 2003)

3.4 Effects on plastic viscosity of:

i. The effect of superplasticizers on plastic viscosity of concrete

The fluidity of concrete increases with increased dosages of superplasticizers and the

reduction in plastic viscosity is less than that of yield stress; the superplasticiser also


maintains the viscosity. Large dosages of superplasticizers can result in different

effects.(Newman and Choo, 2003)

ii. The effect of air-entraining agents on plastic viscosity of concrete

The air-entraining agents increase the viscosity at the almost constant yield stress and

the behaviour is linked to the type and source of constituent material. (Newman and

Choo, 2003)

4. Concrete for the construction of a wind farm

Introduction

The hydration of cement is a chemical reaction that takes place when water is added to the

cement and produces heat. In cold weather, the cement hydration takes place slower than in

hot weather conditions. Placing concrete under hot or cold weather conditions can have

effect on freshly placed concrete. The minimum concrete temperature recommended by BS

8110 for the fresh concrete should not fall below 5 0C until the strength of 5 MPa is reached.

The time taken to reach the minimum strength will depend on the initial concrete

temperature, ambient conditions, sections size, mix constituents and insulation.

Weather condition needs special precaution when batching, transporting, placing, finishing,

curing and protecting concrete .Good concrete practice and proper planning are critical

when dealing with this environment. It requires an understanding of various factors that

affect concrete properties.

In plastic state concrete will freeze if its temperature fall below -50C.If plastic concrete

freezes, its potential strength can be reduced by more than 50% and its durability will

adversely be affected. Low concrete temperature has a major effect on the rate of cement

hydration, which results in slower setting and rate of strength gain.


4.1.1 Heat of hydration

As it has being stated that the base column is 12m X 12m X 4m, it would be expected that

the enormous heat of hydration be generated. The following precautionary measures during

design stage should be considered.

Even though GGBS is temperature sensitive, its reaction rate with cement increases with

increase in temperature (Newman and Choo 2003). Considering the environment and time

of concreting under weather conditions mentioned above, the heat of hydration will be of

significance to activating reactions of GGBS and cement. The AS 1379 requires that

concrete temperature at the point of delivery be within the range of 5 0C to 35 0C.

Aggregates makes up the bulk of the concrete and also has the highest heat capacity, they

have the greatest effect on the temperature of the freshly mixed concrete (ACI 1999).By

adding crushed ice, liquid nitrogen during hot weather conditions to freshly mixed concrete

will reduce the heat of hydration in concrete (ACI 1999).

In cold weather conditions where temperature falls below 50C, the mixing water should be

heated to accelerate the reaction of cementitious material (Richard and Neil 2002).

4.1.2 The placing of fresh concrete

Due to the blend of cement and extender, an extended setting time will be experienced due

to the temperature fall during night time. Wind breaks to prevent coldness and evaporation

during concrete placing will reduce thermal loss (Richard and Neil 2002). Before placing

concrete, formwork, reinforcement, prestressing steel and any surface with which the

concrete will be in contact must be kept free from ice and frost. Covering of formwork during

night time would be necessary.

The adequate access must be provided to ensure good placement. Adequate access along

the whole length of the formwork will be critical in achieving the good concrete placement.

Place the concrete in 450mm layers, and ensure the previous layer has been fully

compacted before placing the next one. Ensure the concrete does not strike the formwork

which may lead to concrete drying on the shutter and impairing the surface finish or the

reinforcement.

The formwork that is insulated to ensure the concrete temperature is maintained will be

advantageous. After concrete placement, the concrete must be protected to prevent early-


age freezing. The protection should remain in place until the concrete reach strength of

5N/mm2. .

The assignment has clearly stated that the placing of concrete is poured on conjested

steel reinforcement base column. Both conventional and self compacting concrete

can be used depending on how conjested is the reinforcement. To safe time and I

would advise the contractor to use self compacting concrete.

What is self compacting concrete?

Self Compacting Concrete is defined as a concrete that fills a form by its own weight, with

minimum or no vibration. The SCC is characterised by the high level of flow without

segregation, high deformability, ability to pass the space between reinforcement bars without

aggregate interlocking and exhibiting a good level of robustness ( Felekoglu et al, 2006).

The name “Self Compacting” clearly describes the properties of the concrete. Due to the

SCC deformability and self compacting properties the mix has enabled the construction

industry to be able to construct complex shaped structures, steel dense structures and

structures with difficult access with greater comfort than when the conventional concrete is

used. However to produce the SCC mix is not easy as the mix is sensitive to its design and it

requires strict controls (Ouchi et al, 2003).

The mix design and production of the SCC is made possible by the use of super plasticisers

that increase the workability of the concrete and enable the flow without segregation. In

some case a small dose of viscosity modifying agents is used to enhance the properties of

the mix.

4.1.3 Curing conditions

Curing should commence immediately after the slab has been casted by means of the

following different methods depending on weather conditions and state of concrete, ponding

with water, continuous spray mist, covering with plastic sheeting or sprayed on curing

compounds.


4.1.4 Concrete temperature during Production

During production, the concrete mixing temperature will be controlled by heating the mixing

water. Since the temperature of concrete affect the rate of slump loss and may affect the

performance of admixture, temperature fluctuation can result in variable behavior of

individual batch. Premature contact of very hot water and concentrated quantity of cement

may cause flash setting and cement balls in the truck mixers. The following mixing

procedure will be followed when mixing concrete.

When delivering the concrete with the mixer truck; the drum must rotate slowly to minimize

the temperature losses during delivery. (Petersons, 1966)

5. Methods of monitoring the strength development of concrete

In large-volume pours, the avoidance of excessive temperature rises and differentials is essential to minimize problems of thermal cracking. (Newman, 2003). After temperature limits are specified , temperature monitoring is required by specification. Other reasons for monitoring strength development to eliminate failures during construction were due to overestimation of the early-age strength of concrete, which resulted in early removal of formworks when the partially matured concrete was not strong enough to support upper levels of the structure. In order to observe the property changes of a concrete at early age, a non-destructive test (NDT) method that can continuously monitor the concrete behavior throughout its setting and hardening procedure is highly desired. Besides the quality control issues, it is also necessary to have an simple and easily accessible equipment setup, so that the measurement can be easily applied to various structures, such as pavements, walls, columns and floor slabs.

Methods Lok-test The Lok-test system is used to obtain a reliable estimate of the in-place strength of concrete in newly cast structures in accordance with the pullout test method described in ASTM C900, BS 1881:207, or EN 12504-3. Two principal uses of LOK-TEST are for:


Determining whether in-place concrete strength is sufficient for early application of loads, such as due to formwork removal, application of prestressing.

Determining whether the in-place strength is sufficient for terminating curing and thermal protection. Evaluating the quality of the critical cover layer protecting the reinforcement in the finished structure.

Principle A steel disc, 25 mm in diameter at a depth of 25 mm, is pulled centrally against a 55 mm diameter counter pressure ring bearing on the surface. The force required to pullout the insert is measured. The concrete in the strut between the disc and the counter pressure ring is subjected to a compressive load. Therefore the pullout force is related directly to the compressive strength. Loading is performed either to a required force, in which case the test is nondestructive, or to the peak-load, which results in a slightly raised, 55-mm diameter crack on the surface. The disc is cast into concrete either by attaching it to formwork before placing concrete or by inserting it manually into the fresh concrete. Correlation and Accuracy of Estimated Strength Lok-test provides an accurate estimate of in-place strength because the peak pullout force has a well-defined correlation to compressive strength measured using standard cylinders or cubes. More than 30 years of correlation experience from all over the world indicates close agreement, suggesting that one general correlation is applicable for all normal density concrete mixtures, as shown below. A different correlation, however, has been found for concrete made with lightweight (low density) aggregate. (Petersen,2003). Advantages of Lol-test method

Quick, inexpensive, provide good correlation.

Small changes in strength can be detected.

Correlation is not mix specific. Disadvantages of Lol-test method

Some preplanning needed.

Some local damage Capo –test The CAPO-TEST permits performing pullout tests on existing structures without the need of preinstalled inserts. Capo-test provides a pullout system similar to the LOK-TEST system for accurate on-site estimates of compressive strength. Procedures for performing post-installed pullout tests, such as Capo-test, are included in ASTM C900.


Typical applications of the Capo-test include the following:

Quality control of the finished structure

Verification of in-place strength when strength of standard-cured specimens fails to meet acceptance criteria

Estimating residual strength of concrete in existing structures

Evaluation of fire-damaged structures

Integrity of structures Principle When selecting the location for a Capo-test, ensure that reinforcing bars are not within the failure region. The surface at the test location is ground flat and a 18.4 mm hole is cored perpendicular to the surface. A recess (slot) is routed in the hole to a diameter of 25 mm and at a depth of 25 mm. A split ring is expanded in the recess and pulled out using a pull machine reacting against a 55 mm diameter counter pressure ring. As in the Lok-test, the concrete in the strut between the expanded ring and the counter pressure ring is in compression. Hence, the ultimate pullout force is related directly to compressive strength. The test is performed until the conic frustum between the expanded ring and the inner diameter of the counter pressure is dislodged. Thus there is minor surface damage, which should be repaired for aesthetic reasons or to avoid potential durability problems. Correlation and Accuracy of Estimated Strength Several investigations have shown that the pullout strength measured by the Capo-test is essentially the same as the pullout strength measured by Lok-test. Based on testing experiences and laboratory studies, it has been found that the accuracy of the compressive strength estimated by the Capo-test using the general correlations is similar to results obtained with the Lok-test. (Moczko, 2002) Advantages of Capo-test method

Less preplanning required, inexpensive and provide good correlation.

Correlation is not mix specific.

Useful for supplementary tests Disadvantages of Capo-test method

Slower than Lok-test.

Some local damage.

Surface preparation may be needed.

Maturity measurement The concrete maturity method is a proven strength estimation technique (ASTM C 1074) that accounts for the effects of time and temperature on the strength development of in-place concrete. This method gives a continuous estimate of concrete strength during the curing period. (Gresser, 2012)


The maturity method is a technique to account for the combined effects of time and temperature on the strength development of concrete. The method provides a relatively simple approach for making reliable estimates of in-place strength during construction. The temperature history is used to calculate a quantity called the maturity index. For each concrete mixture, the relationship between strength and the maturity index is established beforehand. The strength relationship and the measured in-place maturity index are used to estimate the in-place strength Principle

Determination of the appropriate maturity function for the specific concrete that will be used in

construction. Determination of the relationship between compressive strength and the maturity

index. Measurement of the in-place maturity index and estimation of the in-place strength.

Cylindrical concrete specimens are prepared using the same as the concrete to be used in

construction. Temperature sensors are embedded at the centers of at least two cylinders. The

sensors are connected to recording devices. The specimens are cured in a moist curing room.

Compression tests are performed on at least two specimens at ages of 1, 3, 7, 14, and 28 days.

At the time of testing, the average maturity value for the instrumented specimens is recorded. If

maturity instruments are used, the average of the displayed values is recorded (Barnes, 2007).

Advantages of Capo-test method

Relatively inexpensive

Disadvantages of Capo-test method

Correlation with strength is mix specific.

Limpet pull-off test

Limpet pull-off test measures the pull-off strength of the surface zone of concrete and the bond

strength of patch repairs with minimal damage. It is an exceptionally good tool for assessing the

progressive deterioration in these cases when exposed to normal or aggressive environments.

Principle Two basic approaches can be used. One is where the metal disc is attached directly to the concrete surface. This approach is used to make an assessment of the strength of a body of concrete where there is no reason to believe that its surface is materially different to the rest of the mass. Typically, it might be used to judge when it is safe to strike formwork. The second approach is where partial coring of the surface is adopted. If the concrete surface is carbonated or altered and, therefore, having different physical properties compared with the


interior, a much more valid strength value is obtained by coring to a depth below the affected layer thereby causing the failure surface to occur in the unaffected mass. This second approach has particular value in assessing and assuring the strength of the interface between base concrete and surface repair material. If the partial core is continued below the interface and the failure is on the interface, a direct measure of the bond strength of the interface is obtained. Advantages of Capo-test method

Inexpensive to perform.

No pre-planning required.

Superficial damage only.

Disadvantages of Capo-test method

Correlation is mix specific.

Time needed for bonding to surface.

Surface preparation needed.

Poor correlation between pull-off force and concrete strength Lok-test will be the preferred method as it is quick to use and is inexpensive. What is more important is the reliability of the results and this method provides good correlation. The results can be updated quickly as small changes in the results will be detected which is important for decision making. The disadvantages of using this method are really minimal as small surface damage can be repaired.


References

ACI (1990). Cement and concrete terminology, ACI 116R-90, American Concrete

Institute, Detroit, USA.

American Concrete Institute (1999). Construction Practices and Inspection. Hot weather

concreting, Farmington Hills, USA.

ASTM (1993). Standard definitions and terms relating to concrete and concrete

aggregates, Specification C125-93, American Society for Testing and Materials. West

Conshohocken, USA.

Barnes H.A. (2000). A Handbook of Elementary Rheology, University of Wales, Wales,

pp 59

Banfill PFG, 2003, The rheology of fresh cement and concrete – A review,p1

BS 8110-1:1997: Design of structural use of concrete in building.

Cement and Concrete Institute (2002), School of Concrete Technology, Properties of fresh concrete , Midrand, 2002, pge 1

Domone P. (2010). Part 3: Concrete. In Construction Materials: their Nature and

Behaviour. 4th edition, (eds Domone P and Illston J), Spon, London, pp. 120-126

Domone P. (2003). Part 1: Fresh Concrete. In Advanced Concrete Technology, Vol 2:

Concrete Properties (eds Newman J and Choo BS), Elsevier, Oxford, pp. 1/4-1/29.

Early age concrete strength assessment, htt://www.construct .org .uk [Accessed 14 May 2012]

Fulton’s concrete technology, Ninth edition, Cement & Concrete Institute, Chapter 15,

Thermal properties of concrete and temperature development at early ages in large

concrete elements, Yunus Ballin and Peter Graham, Midrand, 2009, pp.273-282.

Felekoglu, B. Turkel, S. Baradan, B. (March 2006) .Effects of water/cement ratio on the

fresh and hardened properties of self-compacting concrete, Turkey.

International Centre for Aggregate Research, 2003, p43-55

John Newman and Ban Choo (2003), Advanced concrete technology, Contituents

Materials:Elsevier, Butterworth-Heinemann pg 3/18 -3/20.

John Newman and Ban Choo (2003), Advanced concrete technology, Concrete

properties:Elsevier, Butterworth-Heinemann pg 5/11 -5/17.


References (cont’d)

J Newman and B Seng Choo (2003), Advanced Concrete Technology, Processes Concrete mix design Butterworth-Heinemann, Oxford pp. 13/1-12

Moczko, A., 2002. Comparison Between Compressive Strength Tests From Cores, CAPO-TEST and Schmidt Hammer,” Wroclaw Technical University, Poland

Newman J and Choo BS, 2003, Advanced concrete technology, p11/13-27

Ouchi, M.2003. Self Compacting Concrete Development, Application and Investigations.

Japan

Petersons, N., 1966, Concrete Quality Control and Authorization of Ready Mixed

Concrete

Rouseel,N, 2006. Correlation between yield stress and slump;Comparison between

numerical simulations and concrete rheometers results, Paris

Richard and Neil .(September 2002). The Concrete Society. Cold weather concreting

Technical report No. 129 pg 55-56.

www.concreteconstruction .net/testing/testing-in-place-concrete.aspx [Accessed 14 May 2012]

www.pcte.com.au

fresh and early age concrete

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