3. concrete - past, present and future

8/10/2019 3. Concrete - Past, Present and Future

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The Institution of Engineers,

Malaysia

Universiti

Teknologi MARAUniversiti Malaya


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CONCRETE: PAST, PRESENT AND FUTURE

Dr Tam Chat Tim

Department of Civil & Environmental Engineering

National University of Singapore

e-mail: [email protected], fax: 65 6779 1635

Abstract

Historically the use of concrete in buildings may be grouped under three periods of practice, Past,

Present and Future. The Past dates back before design in concrete was codified in UK as D.S.I.R.

code in 1934. This is followed by the first issue of the British code of practice, CP 114: 1948 through

CP110: 1972 and BS 8110: 1985 until the current series of Eurocodes in the 2000s. The future coversfurther development of binders other than current series of clinker based cements, performance-based

criteria for codified durability design and achieving a sustainable concrete industry. Both the parts on

Past and Future are presented but the main emphasis is on the Present in view of the adoptionof EC2 with the challenges in tropical concreting. The achievement of high performance concrete in

all its three aspects: compressive strength, consistence and durability are more challenging in hot

tropical environment, either singly, in combination of two or all three high performances together.

Based on past fifty years of local experience and studies, selected major issues are discussed and

potential technical solutions proposed with consideration for both the cost effectiveness and locally

availability of technical resources.

1. INTRODUCTION

Dated around 7000 BC, lime concrete made from quicklime (by burning limestone), water and stone,

was found in 1985 in Israel. In later times, natural pozzolans, e.g. volcano ash, were activated by lime

for building purposes. Although this is the early form of cement, some structures of those ages, e.g.

Theatre in Pompeii (75 BC) are still standing to-day (BCA, 1999). This is in present day term a typeof pozzolanic cement. With the discovery of hydraulic cement by Joseph Aspin in 1824, theequivalent of modern day concrete was introduced into buildings. However, during the period from

the 1820s to early 1900, buildings were designed and constructed by various architects and buildersbased on their individual knowledge and expertise. Their experience was personal and various patents

were filed for the different approaches to the use of concrete in buildings. A good account of most of

these works up to 1935 in United Kingdom and United States of America can be found in a recent

publication by Trout (2013). This is considered as the Past of concrete in buildings. Around thistime, both in United Kingdom and United States of America, consensus had reached the stage of

codified design recommendations. The American Concrete Institute adopted the ACI 318 standard

for buildings in 1941 (ACI, 2004). In United Kingdom, the first code on design of concrete buildings

was published as DSIR code of practice in 1934 (BRE, 1934). This was followed by the first UK

code of practice for reinforced concrete, BS 114 (1948) and subsequently by CP 110 (1972) and itsupdated revisions BS 8110 (1985). It was only in the year 2000 onwards that the new Eurocodes are

published. These developments are presented as the Present use of concrete in structures. The

Future phase will include expected developments in both concrete as a construction material andperformance- based durability design.

2. PAST

The well-known ancient structures made with pozzolanic cement that are still standing today include

the Coliseum and the Pantheon in Rome and the Aqueduct at Pont du Gard in France. The invention

of Portland cement by Joseph Aspdin in 1824 brought concrete into a new phase of development

which continues to this day. However, in those early days, the use of concrete was based onproprietary systems of individual architects and builders. At that time some even considered concrete


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as not fit to be used above ground e.g. Dobson as quoted by Trout, (2013), until the introduction ofsteel reinforcement in concrete, its use had moved from purely plain concrete as foundations and

walls to whole buildings. One such system which included shear reinforcement was patented by

Hennebique in 1892. During this period, there is already the feeling that dependence on proprietary

systems hampered wide spread adoption of reinforced concrete. This is the beginning thateventually led to the development of codes of practice based on industry wide consensus of good

practice.

Professor Morice in his lecture at the inaugural meeting of The Concrete Society, UK on October 13,

1966 (as published in The Journal of The Concrete Society, V1, N1, January 1967), proposed the

following sequences in the technical development of concrete:

Period of practical experiment

Period of patents

Period of theoretical justification

Period of sound development

Some prominent researchers and their major contributions leading to publications of design codes and

concrete specifications in this period of time include:

Abrams, A.D.Design of concrete mixtures, Bulletin No. 1, PCA, 1918 (water-cement ratio concept)Faber, O. and Bowie, P.G.Reinforced concrete design - theory, 1

stEd. 1912, 2

ndEd. 1919

Faber, O.Reinforced concrete design - practice, 1920Cross, H.Analysis of continuous frames by distributing fixed end moments, Proc. ASCE, May 1930Cross, H. and Morgan, N.D.Continuous frames of reinforced concrete, 1932Reynolds, C.E.Reinforced concrete designers handbook, 1

stEd. 1932, latest Ed. 2009

William L. Scott, W.L. and Granville, W,H. Explanatory handbook on the code of practice forreinforced concrete 1934 (DSIR code, 1934, later CP 114, 1948 and revisions in 1957 and 1965)

Lea, F.M. and Desch, C.H. The chemistry of cement and concrete, 1935, 4thed. 1998, Ed. Peter C.

Hewlett

The first guidance for concrete design in UK was the publication of the DSIR code of practice in 1934

(BRE, 1934). Although this document was revised in 1939 and 1948, the first British standard code

of practice for reinforced concrete, CP 114 was published in 1948 and revised in 1957 and 1965. The

edition of CP 114 (1957) introduced the concept of ultimate strength design via an approach which

simulated working stress design by means of adopting a load factor. This was not only misleadingin concept but also confusing to users to have two seemingly working stress equations for the same

reinforced concrete section. However, even the revised edition of CP 114 in 1965 still retained the

design method based on working stress (elastic design). This approach was also used in The UK

code of practice for prestressed concrete, CP 115 (1959, revised 1961). The introduction of limit state

design based on CP 110: 1972 put the situation back to a rational approach and further updated with

BS 8110: 1985. Since then this remains as the current state of design practice until the introduction ofthe Eurocodes in the 2000s.

Other notable publications include:

Johansen, K.W.Yield line theory, 1931 (English translation, C&CA, London, 1962)Whitney, C.Plastic theory of reinforced concrete, 1942 (Transaction of ASCE, V107, N1)Neville, A.M.Properties of concrete, 1stEd. 1963, 5hEd. 2011

Freyssinet, E.A general introduction to the idea of prestressing, 1949Magnel, G.Prestressed concrete, 1954

Lin, T.Y.Design of prestressed concrete structures, 1stEd. 1955, 3

rdEd. 1981 (co-author Burns, N.)

Further information on the historical development of concrete is available in these recent references:

ACI: A century of progress, ACI, 2004


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Trout, E.A.R.Some writers on concrete, Whittles, 201350

thEdition of Concrete, Journal of Concrete Society, UK, May 2013

3. PRESENT

The intervening period between 1985 and 2000 was a time when a large number of advances in both

analysis and design as well as materials technology took place. These advances are reflected in the

updated knowledge introduced into the current series of Eurocodes. For structures, the series of

Eurocodes for design and their related specifications and testing standards introduces a common basis

for the design of different construction materials with a more performance-based approach for

specifying materials. Some of these new approaches on concrete and its constituent materials only are

reviewed in the following sections. A full review of all the aspects is beyond the scope of this

presentation.

3.1 BS EN 1992

This series of design standards brings up-to-date knowledge for designing concrete structures. Theseries consists of the following standards:

BS EN 1992-1-1:2004, Eurocode 2: Design of concrete structures Part 1-1: General rules and rules

for buildings

BS EN 1992-1-2:2004, Eurocode 2: Design of concrete structures Part 1-2: General rules -Structural fire design

BS EN 1992-2: 2005, Eurocode 2: Design of concrete structuresPart 2: Concrete bridgesDesignand detail rules

BS EN 1992-3: 2006, Eurocode 2: Design of concrete structures Part 3: Liquid retaining andcontainment structures

3.1.1 Supporting standards

In support of the above design codes related EN standards for include:

BS EN 206-1: 2000, ConcretePart 1: Specification, performance and conformity(Revised BS EN 206: 2013 together with BS EN 12350 and BS EN 12390 series for method of

testing fresh and hardened concrete respectively)

BS 8500-1: 2006, Concrete Complementary British standard to BS EN 206-1 Part 1: Method ofspecifying and guidance for the specifier (Revision: BS 8500-1:2006+A1: 2013)

BS 8500-2: 2006, Concrete Complementary British standard to BS EN 206-1 Part 2 Specification for constituent materials and concrete (Revision: BS 8500-2:2006+A1: 2013)

BS EN 13670: 2009, Execution of concrete structures

BS EN 13791: 2007, Assessment of in-situ compressive strength in structures and precast concretecomponents

(BS EN 12504 series for method of testing concrete in structures)

BS 6089: 2010, Assessment of in-situ compressive strength in structures and precast concrete

componentsComplimentary guidance to that given in BS EN 13791

3.1.2 Supporting materials and products specifications and test standards

In support of the above standards, materials and product specification standardsinclude:

BS EN 197-1: 2000, Cement Part 1: Composition, specifications and conformity criteria forcommon cements (Revised BS EN 197- 2011)

BS EN 197-2: 2010, Cement Part 2: Conformity evaluation (BS EN 196 series on methods oftesting cement)


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BS EN 450-1: 2005, Fly ash for concretePart 1: Definition, specifications and conformity criteriaBS EN 450-2: 2005, Fly ash for concretePart 2: Conformity evaluation

BS EN 13263-1: 2005, Silica fume for concrete Part 1: Definition, requirements and conformitycriteria

BS EN 13263-2: 2005, Silica fume for concretePart 2: Conformity evaluationBS EN 15167-1: 2006, Ground granulated blast furnace slag for use in concrete, mortar and grout

Part 1: Definition, specifications and conformity criteria

BS EN 15167-2: 2006, Ground granulated blast furnace slag for use in concrete, mortar and grout Part 2: Conformity evaluation

BS EN 12620: 2002+A1: 2008, Aggregates for concrete

(BS EN 932, BS EN933, BS EN 1097 and BS EN 1744 series on methods of testing for

general, geometrical, mechanical and physical and chemical properties of aggregates

respectively)

BS EN 934-2: 2001, Admixtures for concrete, mortar and grout Part 2: Concrete admixtures

Definitions, requirements, conformity, marking and labelling

(BS EN 480 series for methods of testing admixtures)

BS EN 1008: 2002, Mixing water for concrete Specification for sampling, testing and assessing the

suitability of water, including water recovered from processes in the concrete industry, asmixing water for concrete

This presentation does not include the approaches for analysis and design of concrete structure, some

aspects of which are common for design of structures using other forms of construction materials. It

covers mainly issues related to concrete and its constituent materials as indicated under Supportingmaterials and products and test methods for concrete. Only selected topics that are new and thosereplacing current local practice are considered and highlighted as they enhance quality of concrete and

potential for sustainable concrete construction. Further details and other aspects of structural analysis

and design, control of constituent materials, production control systems, procedures and conformity

criteria are provided in the EN standards listed above. Testing methods related to concrete and its

constituent materials are also not included. They are provided under normative references in the

respective materials standards.

3.2 Cementitious Materials

Besides cement that may be blended with other cementitious materials by producers, other

cementitious materials for concrete are covered under separate standards. The specification and

requirements for fly ash, blast furnace slag and silica fume are listed above. One or more of these

cementitious materials may be batched as a component of the total cementitious content at the mixer.

This approach is an alternative to pre-blended cement of the same composition. Guidance on the use

of additions is provided in EN 206-1 for taking into account in concrete composition with respect to

water/cement ratio and minimum cement content requirement, e.g. k-value concept.

3.2.1 Cements

The specifications and testing standards for concrete constituent materials are more performance-

based than current local standards. EN 197-1: 2010 covers 27 products of common cements in its

Table 1 and 7 products of sulfate resisting common cements in Table 2. Together they replace the

much smaller number of individual standards for each type of cement. Cements in Table 1 are

grouped into 5 main cement types as follows:

CEM l Portland cement

CEM II Portland-composite cement

CEM III Blast furnace cement

CEM IV Pozzolanic cement

CEM V Composite cement


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Within each main group a range of the main components and minor additional constituents at 0-5

percent by mass is indicated. For sulfate resisting common cements the 3 main cement types are as

follows:

Sulphate resisting Portland cement:

CEM I-SR 0 Sulphate resisting Portland cement (C3A content of the clinker = 0 %)

CEM I-SR 3 Sulphate resisting Portland cement (C3 CEM I-SR 5 Sulphate resisting Portland cement (C3A content of the clinke

Sulphate resisting blast furnace cements (no requirements on C3A content of the clinker):

CEM III/B-SR Sulfate resisting blast furnace cement

CEM III/C-SR Sulfate resisting blast furnace cement

Sulfate resisting pozzolanic cement (C3A cont

CEM IV/A-SR Sulfate resisting pozzolanic cementCEM IV/B-SR Sulfate resisting pozzolanic cement

Additionally, CEM III may have specific performance such as:

Low heat low early strength blast finance cement with a limited heat of hydration and designated as

LH.Sulfate resisting low early strength blast furnace cement in the strength class indicated by L

With the introduction of these changes specifiers need to have a deeper understanding of the

performance of different cements so as to select the most appropriate type for the requirements of

each specific project. Under specifying of requirements is obviously unsatisfactory but over

specifying requirements leads to unnecessary higher cost and impact on sustainable concreteutilization.

3.2.2 Supplementary Cementitious Materials

EN 206-1 defines addition as finely divided material used in concrete in order to improve certain

Type 1nearly inert additionsType IIpozzolanic or latent hydraulic additions

Fly ash and silica fume are pozzolanic additions and ground granulated blast furnace slag is also a

latent hydraulic addition. Other types of additions, e.g. metakaolin, rice hush ash etc may be usedafter establishment of suitability by a relevant national standard or provision in the place of use ofthe concrete which refers specifically to the use of the addition in concrete conforming to EN 206.

Restricted range of Portland cements and additions which have been combined in the concrete -1: 2006. Conformity procedure forcombinations under Annex A (normative) of BS 8500-2: 2006 is the procedure for establishing the

suitability of a combination enabling it to be counted fully towards the cement content and

water/cement ratio in designed concrete for durability requirements. However, there is no similar

guidance on heat of hydration requirement in low heat cement for temperature requirements. Result

from a single test sample is at best indicative of its heat of hydration level without any simple means

to relate it to the requirement in terms of characteristic value. User has to rely on assurance provided

by the blended cement producer, based on production conformity data.


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3.3 Concrete

The specification, performance and conformity for concrete are based on BS EN 206-1: 2000 and BS

8500-1 and BS 8500-2: 2006+A1: 2012, the two complementary British standards to BS EN 206-1,

Part 1: methods of specifying and guidance for the specifier and Part 2: specification of constituent

materials and concrete. Together they provide several new approaches and changes with great details.

Only items which are more commonly needed for specifying concrete and its constituent materials are

selected for presentation.

3.3.1 Types of Concrete

EN 206-1 defines tasks for the specifier, producer and user. It covers 3 types of concrete defined asfollows:

Designed concrete: concrete for which the required properties and additional characteristics arespecified to the producer who is responsible for providing a concrete conforming to the required

properties and additional characteristics

Prescribed concrete: concrete for which the composition of the concrete and the constituentmaterials to be used are specified to the producer who is responsible for providing a concrete with the

specified composition.

Standardized prescribed concrete: prescribed concrete for which the composition is given instandard valid in the place of use of the concrete

Two additional types of concrete have been introduced under BS 8500-1: 2006+A1: 2012:

Designated concrete: concrete with the requirement for the producer to hold product conformitycertification and therefore designated concretes are only applicable where third-party certification is

selected as the option in specifying the concrete.

Proprietary concrete: concrete for which the producer assures the performance subject to goodpractice in placing, compacting and curing and for which the producer is not required to declare the

composition.

The requirements for designated concrete indicate it is limited to concrete strength class up to C40/50

and for intended working life of 50 years. These requirements are derived based on UK experience

and materials. Local users need to make appropriate judgement on the relevance/suitability of the

designated concrete for the intended use in the specific service environment conditions

Proprietary concrete permits innovative applications of new materials as well as marginal materials

for which no specific guidance has been established by common consensus. A performance-basedapproach may be adopted. Annex E (informative) of EN 206-1: 2010 provides guidance on theapplication of the equivalent performance concept of concrete properties. For each specificperformance, the level of performance (including its tolerance, or range) is to be stated in quantitative

terms in relation to the method of assessment. Characteristic value is to be adopted, where applicable.

3.3.2 Compressive Strength Classes

In the concrete design code, EN 1992-1-1 (Eurocode 2) the compressive strength adopted in the

design equations is the characteristic cylinder strength with specimen aspect ratio (length/diameter) of

2.0. However for conformity control the cube compressive strength (aspect ratio of 1.0) is the

alternate type of specimens. Although cylinder compressive strength is closer to the ideal uniaxial

strength (less end friction influence) but the testing of cube specimens offers some advantages for

production control testing as well as in relation to core testing. These include:


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(a) No end preparation, e.g. capping or grinding in the case of high strength concrete

(b) Core specimen from existing structures with length/diameter ratio of 1.0 is possible for

structural elements of thickness less than twice the diameter of the core

(c) Less volume of concrete for each standard test specimen as well as for core samples

(supports sustainability)

(d) Lower cost to produce test specimens and core samples as well as cost in disposal of

specimens after testing

The relationship between the characteristic cylinder compressive strength and characteristic cube

compressive strength is provided in EN 206-1 as Table 7 for normalweight and heavyweightconcrete and Table 8 for light-weight concrete. In Table 7 the relationship up to compressive strength

class of C55/67 is approximately based on characteristic cube strength = 1.25 characteristic cylinder

strength and above this, characteristic cube strength is 15 MPa higher than corresponding

characteristic cylinder strength. Table 7 includes compressive class of C100/115 although EN 1992-

1-1 adopts characteristic compressive strength class up to C90/105 only. The relationship for

characteristic compressive strength can be applied to test result (mean of two or more test specimens)

which may be above the characteristic strength.

For determination of compressive strength traditionally it is customary for specimen size to be 150

mm cubes and 150 mm x 300 mm cylinders. The rational for this may be for convenience rather than

technical. As early as BS 1881: Part 108: 1952, the requirement has been for specimen size to be at

least 4 times the maximum size aggregate. ASTM C 192-13 (2013) limits the ratio of cylinder

specimen diameter to maximum aggregate size to 3 only. This limit is also recommended in BS EN

12504-1: 2009 for the ratio of maximum aggregate size to diameter of core with addition information

of the effect of aggregate size and core diameter in Annex A (informative) of the standard. In general,

the maximum aggregate size for pavement concrete is 40 mm (1 in.) and for buildings 20 mm (

in.). Hence 150 mm cubes are able to cater for both maximum aggregate sizes. This choice for 150

mm cubes has gone on for over 6 decades and became a common requirement in specifications. Even

EN 206-1: 2010 only state the use of 150 mm cubes and 150 mm x 300 mm cylinders. However, BS8500-1: 2006 and the revised BS 8500-1: 2006+A1: 2012, Clause 12.2 Conformity control for

compressive strength states:

If conformity to the specified compressive strength class is determined using 100 mm cubes,the minimum characteristic 100 mm cube strength shall be that given for 150 mm cubes in BS

EN 206-1: 2000 Table 7 and BS EN 206-1: 2000 Table 8.

For the purpose of production control concretes of a limited range of strength classes using constituent

materials of the same type and source may form a concrete family for the purpose of production and

conformity control. Guidance is provided in Annex K (informative) of EN 206-1: 2013.

3.3.3 Consistence

The commonly used term workability is replaced by the term consistence. In addition to thetraditional tests of slump (S1-S4), Vebe time (V0-V4), degree of compactability (C0-C4) (replacing

compaction factor) and flow diameter (F1-F6), the revised EN 206: 2013 incorporates the additional

rules for self-compacting concrete (SCC) of EN 206-9 (withdrawn). The additional consistence

classes for SCC include slump flow diameter (SF1-SF3), viscosity indicated by t500 (VS1-VS3),

viscosity by V-funnel (VF1-VF2), passing ability by L-box (PL1-PL2), passing ability by J-ring (PJ1-

PJ2) and sieve segregation resistance (SR1-SR2). Details of test methods for consistence are given in

the EN 12350 series for Testing of Fresh Concrete (Parts 1 to 12). Development in testing of SCC is

continuing, e.g. Chan et al (2010) presented a modification to the J-ring test for assessing passing

ability and dynamic stability by T-box (Esmaeilkhanisan et al, 2014).


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For a given project only those tests that are required and the class for conformity need to be specified.

This is useful for the ready-mixed concrete producers to select the appropriate designed concrete that

the plant has developed (particularly if product certification is required by regulation, e.g. BCA

Singapore, or by project specification). Otherwise it is necessary to conduct initial testing of a new

designed concrete to meet the requirements in accordance with the process set out in EN 206-1: 2000,

Annex A (normative) - Initial Test. During initial production the rate of sampling (Table 13, EN 206-

1: 2000) and conformity criteria (Table 14, EN 206-1: 2000) are adopted until at least 35 test results

are obtained and analysed to meet requirements before production is placed under continuous

production control with its less demanding criteria (shown in the same tables) than initial production.

3.3.4 Durability

Unlike structural design in concrete for which EN 1992-1-1 provides quantitative approach for each

specific action or combination of actions with partial safety factors for the actions and the resistance

required, durability design in concrete is still at the prescriptive level. Currently semi-empirical

approach with guidance for resisting environmental actions and recommended deem-to-satisfied

provisions for resisting qualitatively described (other than for sulfate in the ground) exposure classes

in BS 8500-1: 2006+A1: 2012, Annex A (informative) is intended for UK environment. Chemicalreactions are temperature dependent as indicated by Arrhenius equation and hence the guidance

provided in BS 8500-1: 2006+A1: 2012 needs to be modified for local tropical climate. An example

is the guidance in the Singapore standard SS 544-1 (2009), the equivalent standard to BS 8500-1:

2006 stated as follows:

The specifier should take into consideration the nature of the element, intended working life,

its importance and the cost of maintenance and repair to select the same or higher performance

concrete. Different elements in the same structure may be specified with different concrete to

optimise cost-effectiveness.

An alternative approach to selecting a higher performance concrete is to increase the cover thickness,

but not to the extent of significantly increasing flexural crack width to exceed the design limit. Acombination of both may lead to a more cost-effective solution. However, the current durability

design remains in a semi-quantitative approach with prescribed guidance on the type of cement, its

minimum content and maximum cement content for a given cover thickness. One major step is in

terms of the cover tolerance, the value of c to be specified is always positive, (unlike the typical 5mm intended for fixing tolerance). A positive tolerance for cover has significant influence on

durability and a very minor, if any, on the ultimate capacity of flexural elements and even less in

axially loaded elements, such as internal columns and load-bearing walls.

3.3.5 Effect of Tropical Climate

Most advances in concrete technology are initiated in the temperate regions and their implementation

in tropical climate often calls for appropriate adjustment for the effect of higher ambient temperaturesthroughout the year. Experience of past decades has indicated both challenges and opportunities for

innovative economic solutions, e.g. Tam et al (2008) including the sandwich concept proposed byTam et al (2002) in casting of thick raft foundations, and the effects of tropical climate on properties

of fresh and hardened concrete reviewed by Tam (2014). The major effects of higher ambient

temperature in tropical climate relate to reduction of consistence and the faster loss of consistence in

fresh concrete, potential plastic settlement and plastic shrinkage cracking. For thick sections and high

strength concrete, the issues involve producing initial placing concrete temperature well below

ambient temperature to avoid potential delayed ettringite formation at above 70OC in hardened

concrete and potential early thermal cracking due to high temperature differential between the warmer

interior of thick sections and its cooler outer zone. Although technical solutions such precooling of

constituent materials, injection of liquid nitrogen into fresh concrete and embedment of coolingsystems in hardened concrete may be adopted, but their availability currently is lacking due to their

high initial and operating cost as well as the infrequent demand for them making difficult the return of


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investing in such technologies. The availability of engineered chemical admixtures to provide desired

consistence and delay in setting and the use of ice and chilling water have enabled the use of

characteristic cube compressive strength up to 60 MPa at 28 days to meet most temperature

requirements. The introduction of low heat of hydration cements containing fly ash, ground

granulated blastfurnace slag and silica fume has also contributed to the solutions on temperature

limits. Challenges remain for strength classes above C50/60.

4. ASSSESSMENT OF IN-SITU COMPRESSIVE STRENGTH

Assessment of in-situ quality of concrete is one area for which less attention is given during

construction stage unless issues arise from unsatisfactory concrete supplied to the project or some

unforeseen events occurred on site which may have affected the concrete in the structure. On the

other hand, existing structures that have been in service for some time may need to be assessed for its

structural adequacy for change in usage or due to deterioration arising from lack of maintenance for

environmental actions. The use of core tests and some form of non-destructive testing (NDT)

methods have been in practice for a long time. However, a common consensus on the interpretation

of test results of core test and NDT methods is lacking. BS EN 13791 (2007) is the first standard thatsets out the linkage between these test results and the required compressive strength to EN 206-1

which is the basis for design to EN 1991-1-1: 2004. EN 13791: 2007, Table 1 provides the

minimum characteristic in-situ compressive strength for the EN 206-1 compressive strength. Theratio of in-situ characteristic strength to characteristic strength of standard specimens is set at 0.85.

The ratio is part of the partial safety factor for concrete, c, (EN 1991-1-1: 2004. Annex A). NationalAnnex NA (informative) to DIN EN 13791: 2008-05 (English version) provides Table NA.1 minimum characteristic in-situ compressive strength of light-weight concrete with a closed structure,

for compressive strength classes according to DIN EN 206-1 with the same ratio of in-situ

characteristic strength to characteristic strength of standard specimens (cylinder and cube) of 0.85 and

rounded to nearest 1 MPa.

In addition, the former BS 6089 (1981), which provided only guidance, it has been replaced by BS6089 (2010), Assessment of in-situ compressive strength in structures and precast concrete

components complementary guidance to that given in BS EN 13791: 2007. The new standardprovides not only additional guidance but also recommendations for interpreting test results. Some

new and useful topics from these documents are highlighted in this presentation and further details

and other areas are available in these two standards.

4.1 Core Test

Assessment of in-situ compressive strength directly from testing of cores constitutes the reference

method, although indirect test methods may also be used. Unlike the guidance in BS 6089 (1981)

when the partial safety for concrete was to be at least 1.2 compared to its value of 1.5 in design (ratio

= 0.80) but with no clear guidance on if the in-situ value needs to the characteristic strength (the riskis much higher in using mean in-situ strength). EN 13791: 2007 adoption of the ratio of 0.85 is based

on in-situ characteristic strength and hence significantly more conservative than basing on guidance of

the former BS 6089 (1981) withdrawn in 2010, replaced with EN 13791. The advantage that thecore specimen needs to be of aspect ratio 1.0 for assessment based on cube compressive strength has

been mentioned in 3.3.2 above. For in-situ strength to be determined from cores, testing a core withequal length and a nominal diameter of 100 mm gives a strength value equivalent to the strength value

of a 150 mm cube manufactured and cured under the same conditions. For the equivalent strength ofa 150 mm x 300 mm cylinder the required aspect ratio for a core needs to be equal to 2.0. For this

reason, it is preferable to adoption the cube as the standard specimen for determination of

compressive strength in production control and conformity assessment.


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4.1.1 Factors influencing core strength

Annex A (informative) of BS EN 13791 (2007) provides guidance on some of the factors influencing

the measured core strength. In addition to the correction factors for length/diameter ratio given in the

National Annex NA (informative) Guidance on the use of BS EN 12504-1: 2009, Testing of hardened

concrete Part 1: Cored specimens Taking, examining and testing in compression, the followingfactors are selected for highlighting:

Direction of drillingnormally the core result should not be modified for direction of drillingrelative to direction of casting (difference of 8% in BS 6089: 1981 and BS 1881: Part 120:1983, typically between 0 to 8% indicated in Annex A (informative) of EN 13791: 2007).

The correction is intended to cater for the lack of homogeneity for in-situ concrete with respect to

depth of pour, particularly in deep sections. However, in recent years designed concretes have

compositions that are less prone to segregation during compaction by vibration and less settling

before hardening hence the factor may not apply. In the case of self-compaction concrete, such a

correction may lead to non-conservation estimation of in-situ concrete strength for cores drilled

perpendicular to direction of casting (referred to as horizontal cores).

The other factor is the moisture state of the specimen at the time of strength determination:

Moisture content except for where it is not feasible, core shall be exposed to a laboratory

atmosphere for at least 3 days prior to testing. Strength of a saturated core is 10-15% lowerthan a comparable air-dried core (moisture content normally between 8-12%).

National Annex NA (informative) to DIN EN 13791: 2008-05 (English version) requires testing to be

carried out on air-dried cores (conditioned for at least 12 hours in standard laboratory atmosphere

prior to testing). It also states that the compressive strength of an air-dried core having a diameter of100 mm or 150 mm may be deemed equivalent to that of a saturated 150 mm cube.

Reinforcementcores used to measure the strength of concrete should not contain reinforcingbar. National Annex NA (informative) of BS EN 12504-1: 2009 provides formulae for

transverse reinforcing bars which can be expected to result in a reduction in measured strength.

It is to be noted that Concrete Society Report No. 11 (1987) recommended that where the required

correction is 5% or less the formulae can be accepted as being of adequate reliability and correctiongreater than 10% cannot be regarded as a reliable measure of the concrete strength. The tests of Loo

et al (1989) confirmed that embedded transverse reinforcement reduces the strength of cores with a

length/diameter ratio of 2, but at a ratio of 1, embedded steel has no effect on the measured strength of

the core. This is another situation in support of the preference to adopt the cube as the standard

specimen for determination of compressive strength in production control and conformity assessment.

4.1.2 Assessment by Testing of Cores

Two approaches are provided in EN 13791: 2009 for direct assessment with cores. Approach A

applies where at least 15 cores are available from a test region consisting of several test locations.

Approach B applies where 3 to 14 cores are available. It is important to note the definition for test

location and for test region:

Test locationlimited area selected for measurements used to estimate one test result, which isto be used in the estimation of in-situ compressive strength

Teat region one or several structural elements, or precast concrete components assumed orknown to be from the same population


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The choice of structural elements that may be deemed to be from the same population is important to

avoid potential outliers in the test results when information on the concrete used is lacking, e.g. in an

old structure.

Approach AThe estimated in-situ characteristic strength of the test region is the lower value of:

fck,is = fm(n),isk2x s, or fck,is = fis,lowest + 4

where s is the standard deviation of the test results or 2.0 MPa, whichever is the higher value,

ksis given in national provisions or, if no value is given, taken as 1.48

Approach BThe estimated in-situ characteristic strength of the test region is the lower value of:

fck,is = fm(n),isk, or fck,is = fis,lowest + 4

The margin k depends on the number n of test results and the appropriate value indicated below based

on Table 2 of BS EN 13791 (2007) and relating it to the k factor in Approach A:

Table 1k factor

n k (Value of s relative to k2= 1.48 x s in Approach A)

10 to 14 5 3.58

7 to 9 6 4.05

3 to 6 7 4.73

As expected, for smaller number of cores tested, a more conservative interpretation has to be adopted.

However, the value of the standard deviation sin Approach A may be higher than the relative value

indicated for Approach B when close to 15 cores are tested.

4.1.3 Assessment by Indirect Methods

Two alternative methods for assessment of in-situ compressive strength are provided:

Alternative 1Direct correlation with cores (sub-clause 8.2)Alternative 2Calibration with cores for a limited strength range using established relationship

Alternative 1 requires at least 18 pairs of results, 18 core test results and 18 indirect test results from

the same test locations covering the range of strength of interest to establish the relationship. The

relationship is established by regression analysis to obtain a best fit curve and determined as the lower

ten percentile of strength. The use of this relationship gives a safety level where 90% of the strength

values are expected to be higher than the estimated value.

Assessment for each test region is based on at least 15 test locations with the mean and standard

deviation calculated from the test results or 3.0 MPa whichever is higher.

The in-situ characteristic compressive strength of the test region is the lower of

fck,is = fm(n),is1.48 x s, or fck,is = fis,lowest + 4, where s is the standard deviation of test results

Alternative 2 makes use of a relationship determined from a limited number of cores and a basic curve

provided in EN 13791 (for rebound hammer test, ultrasonic pulse velocity test and pull-out force test).

For each selected test region should have at least 9 test locations for both indirect test and cores. The

basic curve forms the basis for calculating the shift, f to obtain the established relationship byusing the difference between the core strength and the value from the basic curve for the indirect


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method used f = fis fR, v or F and the mean fm(n) for the n results and the sample standarddeviation s. The shift, is schematically shown in Figure 1 of EN 13791 (Figure A.1 of Annex A),

with f = fm(n)k1x s where k1is taken from the table shown below.

Table 2(Table 3 of BS EN 13791)Coefficient k1dependent on the number of paired tests

Number of paired test results (n) Coefficient, k19 1,67

10 1,62

11 1,58

12 1,55

13 1,52

14 1,50

1,48

The basic curves are shown graphically (Figures 2 to 4 in EN 13791, 2007) as well as presented in the

following mathematical functions of the curves for numerical calculation:

(Rebound hammer, Figure 2, EN 13791)):

fR = 1,25 x R R = 1,73 x R

(Ultrasonic pulse velocity, Figure 3, EN 13791):

fv = 62,5 x v2

(Pull-out force, Figure 4, EN 13791):

fF= 1.33 x (F N)

The approach adopted for Alternative 1 for assessment of in-situ compressive strength also applies to

the use of Alternative 2, i.e. based on at least 15 test locations for each test region and method for

establishing the in-situ characteristic compressive strength. The minimum characteristic in-situ

compressive strength for EN 206-1 compressive strength classes are provided in Table 3 (Table 1 of

BS EN 13791 (2007).

One of the objectives in assessment of in-situ compressive strength is to establishing relationship by

indirect methods to select test locations to cover the range of strength of interest. One suggested

approach is included as Annex B (informative) Sample test plan in SS 592: 2013, the Singapore

complementary standard to SS EN 13791: 2009. A multi-stage plan is proposed to assist in selecting

potentially upper, middle and lower range of strength values. More guidance on the approach is

provided by Tam (2006).

EN 13791 (2007) also provides assessment where conformity of concrete based on standard tests is indoubt, e.g. from limited number of identity test values for concrete supplied under certification. The

purpose is to ascertain if the in-situ concrete is with adequate strength and the concrete in the test

region conformed to EN 206-1 but not to verify if the concrete is in compliance with the specified

characteristic strength based on testing of standard test specimens.

For a test region comprising many batches of concrete with 15 or more core data the requirements are:

fm(n),is ck + 1.48 x s) and fis,lowest ck4) MPa where s = standard deviation

Alternatively, by agreement between the parties, where there are 15 or more indirect test data

(preferably from ultrasonic pulse velocity tests) and at least two cores taken from the locations that

indicate the lower strengths, the region may be deemed to contain concrete with adequate strength, if:

fis,lowest ck4) MPa


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Table 3Minimum characteristic in-situ compressive strength for EN 206 compressive strengthclasses

Compressive strength

class according to EN

206-1

Ratio of in-situ characteristic strength to

characteristic strength of standard

specimens

Minimum

characteristic in-situ

strength (N/mm2)

fck, is, cyl fck, is, cube

C8/10 0,85 7 9

C12/15 0,85 10 13

C16/20 0,85 14 17

C20/25 0,85 17 21

C25/30 0,85 21 26

C30/37 0,85 26 31

C35/45 0,85 30 38

C40/50 0,85 34 43

C45/55 0,85 38 47

C50/60 0,85 43 51

C55/67 0,85 47 57C60/75 0,85 51 64

C70/85 0,85 60 72

C80/95 0,85 68 81

C90/105 0,85 77 89

C100/115 0,85 85 98NOTE 1 The in-situ compressive strength may be less than that measured on standard test specimenstaken from the same batch of concrete

NOTE 2 The ratio 0.85 is part of cin EN 1992-1-1: 2004

When the test region is deemed to contain concrete with adequate strength, the concrete shall be

deemed to have come from a conforming population. Where the strength is less than 0,85(fck4)MPa, the design assumptions are not valid and the structure should be assessed for structural

adequacy based on the estimated in-situ compressive strength.

5. FUTURE

Although the recent introduction of approaches based on Eurocode approach or its equivalent has

contributed to a significant update on the use of concrete in structures as indicate in the selected topics

presented above, there is amble room for further development. The current activities in drafting of

performance-based durability criteria for corrosion due to carbonation and ingress of chloride will

eventually lead to quantitative models for the design against each of these exposure conditions and

their combination. However, it will be necessary for each climatic region to gather sufficient data onthe environmental action and the long term performance of specific concretes to calibrate the design

formulae under consideration. Temperature is a major factor as all chemical reactions are

temperature dependent. Carbonation may be simplified to one based on the square root of time

approach but the carbonation constant and the required concrete are factors yet to reach common

consensus. Chloride ingress is likely to be in terms of Ficks second law but the factors such as watersoluble or acid soluble chloride in the concrete is to be adopted and the required concrete for adequate

performance are factors to reach common consensus. Local data on both environmental action and

history of concrete performance are lacking and it is time to initiate such long term monitoring to

enable the developed models to be applied.

In terms of new concrete construction the current knowledge on assessment of in-situ should be

applied to determine the quality of concrete achieved in different site conditions, both in terms ofstructural safety and durability performance. Concrete in the cover zone of structural elements needs


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to be assessed differently from the bulk concrete, as the former affect durability and the latter

structural adequacy. An appropriately established correlation between rebound hammer hardness is

more suitable for assessing cover concrete, whereas that for ultra-sonic pulse velocity is more related

to the bulk concrete. Such correlation may begin from the stage of initial testing in the laboratory for

new concrete and further monitored for in-situ concrete for various site conditions on such factors as

degree of compaction and curing history.

6. CONCLUDING REMARKS

The short review on history of the development on the use of concrete in structures from the past to

the present has indicated the progress made so far. Some indications of the needs for the future have

been highlighted. It is up to the combined effort of all the stakeholders in the concrete industry to

take up the challenge to ensure that sustainable concrete industry will be achieved for the economic

development of the whole world in the years to come for the future generations of human kind.

7. REFERENCES

ACI (2004), ACI: A century of progress, American Concrete Institute, Farmington Hill, MI, 123pp

ASTM C192/C192M 13, (2013), Standard Practice for Making and Curing Concrete TestSpecimens in the Laboratory, Vol. 04.02, ASTM International, W. Conshohocken, PA., 2013

BCA (1999), Concrete through the ages, British Cement Association, Berkshire, UK, 37pp

BRE (1934), DSIR Code of practice for reinforced concrete, Department of Scientific & Industrial

Research, UK, 1934

BS 114 (1948), The structural use of concrete in buildings, British Standards Institution, London,

1948

BS 6089 (1981), Guide to the assessment of concrete strength in existing buildings, British Standards

Institutions, London, 1981 (withdrawn in 2010)

BS 6089 (2010), Assessment of in-situ compressive strength in structures and precast concretecomponentscomplementary guidance to that given in BS EN 13791: 2007, British Standards

Institutions, London, 2010.

BS 8110 (1985), Structural use of concrete, British Standards Institution, London, 1985

BS EN 206 (2013), Concrete Specification, performance, production and conformity, British

Standards Institution, London, 2013

BS EN 12504-1 (2009), Testing concrete in structures, Core specimens, Taking, examining and

testing in compression, British Standards Institution, London, 2009.

BS EN 13791 (2007), Assessment of in-situ compressive strength in structures and precast concrete

components, British Standards Institution, London, 2007

Chan, K.D., Ong, K.C.G. and Tam, C.T., (2010), Passing ability of SCCimproved method based onthe P-ring, Proc. 35

thOWICS, 25-27 August, 2010, Singapore, CI-Premier, Singapore, 2010,

pp9-16.Concrete Society, (1987), Technical Report No. 11, Concrete core testing for strength, London, 1987,

44pp

CP 110 (1972), The structural use of concrete, British Standards Institution, London, 1972

CP 115 (1959), The structural use of prestressed concrete in buildings, British Standards Institution,

London, 1959.

Esmaeikhanisan, B., Feys, D., Khayat, K.H. and Yahia, A., New test method to evaluate dynamic

stability of self-consolidating concrete, ACT J. of Mat., May-June, 2014, pp299-307

Loo, Y.H., Tan, C.W. and Tam, C.T. (1989), Effects of embedded reinforcement on measured

strength of concrete cylinders, Magazine of Concrete Research, Vol. 41, No. 146, 1989, pp11-

18.

SS 544-1 (2009), ConcreteComplementary Singapore standard to SS EN 206-1Part 1: Method ofspecifying and guidance for the specifier, SPRING, Singapore, 2009


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SS 592 (2013), Assessment of in-situ compressive strength in structures and precast concrete

components complementary guidance to that given in SS EN 13791: 2009, SPRING,

Singapore, 2013

SS EN 13791 (2009), Assessment of in-situ compressive strength in structures and precast concrete

components, SPRING, Singapore, 2009

Tam, C.T, (2006), Monitoring of in-situ quality of concretefrom construction to demolition, SpecialOWICS Lecture, Proc. 10th Int. Conf. on Inspection, Appraisal, Repair & Maintenance of

Structures, 25-26 Oct. 2006, Hong Kong, CI-Premier, Singapore, pp1-8

Tam, C. T. (2014), Challenges and opportunities in tropical concreting, Invited Paper, 2nd

International Conference on Sustainable Civil Engineering Structures and Construction

Materials, 23-25, September, 2014, Yogyakarta, Indonesia , 2014 (accepted for publication).

Tam, C.T., Harsono, E. and Swaddiwuddipong, S., (2008), Concreting in the tropics: precautions and

opportunities, Keynote Paper, Proc. 7th International Congress, Concrete: Constructions

Sustainable Option, Conference 3: Concrete Durability: Achievement and Enhancement, Theme

1: Retaining and Extending Performance, Dundee, Scotland, 8-12 July, 2008, pp

Tam, C.T., Swaddiwuddipong, S., Ho, D.W.S. and Seow, S.S., (2002), Strategy for casting of raft

foundations in tropical climate, Journal, Institution of Engineers, Singapore, Singapore, V42,

N6, pp6-11Trout, Edwin, A.R., (2013), Some writers on concrete, Whittles Publishing, UK, 279pp

ANNEX A

Figure A.1

(Figure A.1 corresponds to Figure 1 of BS EN 13791: 2007)

3. concrete - past, present and future

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