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Concrete Technology | Early-age Cracking | Portland Cement Association (PCA)

Concrete Technology | Early-age Cracking | Portland Cement Association (PCA)

Early-Age Cracking

Early-Age Cracking
By Matthew D’Ambrosia¹ and Nathaniel Mohler²

Early-age cracking can be a significant problem in concrete. Volume changes in concrete will drive tensile stress development when they are restrained. Cracks can develop when the tensile stress exceeds the tensile strength, which is generally only 10% of the compressive strength. At early ages, this strength is still developing while stresses are generated by volume changes. Controlling the variables that affect volume change can minimize high stresses and cracking.

Mechanisms of Early-Age Volume Change
The volume of concrete begins to change shortly after it is cast. Early volume changes, within 24 hours, can influence tensile stress and crack formation in hardened concrete.

Chemical Shrinkage
Chemical shrinkage occurs due to the reduction in absolute volume of solids and liquids in the hydrating paste. Chemical shrinkage continues to occur as long as cement hydrates. After initial set, the paste resists deformation, causing the formation of voids in the microstructure.

Autogenous Shrinkage
Autogenous shrinkage is the dimensional change of cement paste, mortar, or concrete caused by chemical shrinkage (Figure 1). When internal relative humidity is reduced below a given threshold (i.e., extra water is not available), self-desiccation of the paste occurs, resulting in a uniform reduction of volume.

Figure 1 – Chemical shrinkage and autogenous shrinkage volume changes of fresh concrete. Not to scale.

Creep
Creep is the time-dependent deformation of concrete under sustained load. During early age, concrete creep is generally as much as 3-5 times higher than for mature concrete. Early load application due to construction forces or prestressing operations can therefore have a significant impact on total deformation. Furthermore, the magnitude of creep in tension is greater than in compression, and early tensile creep can be relied upon as a stress relaxation mechanism. Creep is influenced by drying or self-dessication at early age, and this synergy is often referred to as the Pickett Effect, after Gerald Pickett, a PCA researcher who discovered the phenomena in the 1940s (Pickett 1947).

Swelling
Concrete, mortar, and cement paste will sometimes swell when sealed or in the presence of external water. Swelling is generally caused by pore pressure, but can be accentuated by the formation of some expansive hydration products. The swelling is not significant, between 50-100 millionths at early ages; therefore, we will not be discussing swelling further.

Thermal Expansion
As cement hydrates, the reaction provides a significant amount of heat. In large elements, this heat is trapped and can induce significant expansion. When thermal changes are superimposed upon autogenous shrinkage at early age, cracking can occur. In particular, differential thermal stress can occur due to rapid cooling of massive concrete elements.

Testing of Early-Age Volume Changes

Chemical shrinkage test
Volume change due to chemical shrinkage can be estimated from the hydrated cement phases and their crystal densities or it can be determined by physical test. The physical test places a measured amount of lime-saturated water in an open pipet over a known amount of cement paste inside a closed container. The change in water level within the pipet indicates the change in volume due to chemical shrinkage (Figure 2).

Figure 2 – Test for chemical shrinkage of cement paste showing flask for cement paste and pipet for absorbed water measurement.

ASTM C157 - Modified for early age shrinkage
The standard drying shrinkage test for concrete can be modified to capture early age volume change by elimination of the curing period (usually 7-28 days) and beginning measurements as early as possible. Prisms may also be sealed after casting to provide an estimate of autogenous shrinkage. Surfaces should be sealed as quickly as possible to eliminate loss of moisture. It should be emphasized that autogenous shrinkage depends on temperature history (maturity) and will be different in a laboratory prism when compared to a larger concrete member in service under variable ambient temperatures. As with drying shrinkage measurements, the test result will not represent the actual shrinkage in the structure.

ASTM C1581 – Restrained Ring Shrinkage
The restrained ring shrinkage test consists of a concrete ring specimen 150 mm (6 in.) tall, 13 mm (0.5 in.) thick and 330 mm (13 in.) diameter that is cast surrounding an instrumented steel ring (Figure 3). The steel ring prevents the concrete from shrinking from the time that the concrete is first cast. Shrinkage stresses continue to grow as the concrete passes from initial set to final set and beyond. Tensile creep relaxation alleviates stress development and is considered beneficial at early age.The instrumented ring uses strain sensors to monitor the development of stress. If the shrinkage in the concrete is significant, the stresses will eventually cause cracking. The strain sensors provide an indicator of the cracking time, which is used to compare the cracking tendency between different concrete mixtures.

Figure 3 – Restrained ring shrinkage test setup. (Courtesy of CTLGroup)

ASTM C512 - Compressive Creep
The standard creep test consists of a frame and hydraulic loading system to apply constant stress to 150X300 mm (6x12 in.) cylindrical specimens (Figure 4). Deformation is monitored periodically over time and compard to compansion unloaded specimens to obtain the creep strain of the concrete, which can then be used to calculate the creep compliance, or “specific creep” of the material. Tests are typically started at 7 or 28 days of age, but this test can be modified for early age by starting the test as early as 24 hours. Sealed tests are used to evaluate “basic” creep and unsealed tests incorporate the Pickett Effect, or “drying” creep.

Figure 4 – Standard creep test frames. (Courtesy of CTLGroup)

Mitigating Early-Age Cracking

Optimization of aggregates to reduce total cementitious content
Since volume changes are more a function of the cement paste, rather than the more volume-stable aggregates, reducing the overall cementitious content is the best way to mitigate early-age volume changes. Typical concrete mixtures have gap-graded aggregates that leave significant void space for cement paste to fill. By optimizing the aggregate gradation across the entire spectrum, as opposed to the coarse and fine aggregates individually, the amount of paste required to surround each aggregate particle and fill the void space is minimized (Figure 5); thereby minimizing the effects of early-age volume change of the paste.

Figure 5 – A comparison of void space with different aggregate gradations.

Minimum w/cm ratio
Autogenous shrinkage increases with a decrease in water to cementitious materials ratio (w/cm). Concrete mixtures with a w/cm of 0.30 can experience autogenous shrinkage upwards of half of the normal drying shrinkage. Using the highest w/cm that still provides adequate strength and durability can reduce the impact of autogenous shrinkage.

Internal curing
Internal curing is a method by which water is encapsulated within a concrete mixture for continued release during the hydration process. Typical internal curing materials include high absorption lightweight aggregate particles and super-absorbent polymers. The self-dessication of the paste draws the water out of these particles to continue the hydration of the cement particles. This is particularly helpful in mitigating autogenous shrinkage of concrete mixtures with very low w/cm (0.30 or less).

Shrinkage-reducing admixtures
Shrinkage-reducing admixtures (SRAs) are typically used as mitigation of cracking and curling caused by drying shrinkage; however, SRAs can be utilized to mitigate autogenous shrinkage as well. The SRA, typically propylene glycol or polyoxyalkylene alkyl ether based, alters the surface tension of the pore water and reduces the stresses developed during desiccation, whether self-induced or by evaporation.

Concreting procedures
Several concreting procedures can be used to minimize early-age volume changes. When autogenous shrinkage is a concern, the use of moist curing methods will help mitigate self-desiccation near the concrete surface. The use of a well-developed thermal control plan will mitigate the effects of thermal-based volume changes.

References
Kosmatka, Steven H.; Kerkhoff, Beatrix; and Panarese, William C.; Design and Control of Concrete Mixtures, EB001, 14th edition, Portland Cement Association, Skokie, Illinois, USA, 2002, 358 pages.

Pickett, Gerald, The Effect of Change in Moisture Content on the Creep of Concrete Under a Sustained Load, Research Department Bulletin RX020, Portland Cement Association, 1947.

Notes
¹Associate, CTLGroup
²Concrete Engineer, PCA

USEFUL WEBSITES ON CEMENT, CONCRETE AND THEIR SPECIFICATION

Source:  http://www.highwaysmaintenance.com/concbits.htm 
 The Idiots' Guide to Highways Maintenance
Copyright © 2000/10, C.J.Summers

THE ITEMS ARE LISTED (MORE OR LESS) IN ALPHABETIC ORDER, WITH VARIOUS SPECIFICATIONS AND DOCUMENTS THEN LISTED IN NUMERICAL ORDER

USEFUL WEBSITES ON CEMENT, CONCRETE AND THEIR SPECIFICATION I used to think I knew a bit about concrete, and perhaps I do, the basics, but with all the newly developed cement replacements and the many ways they are formulated to produce a number of different types of cement, I am unable to keep pace with regard to the intricacies of cement and concrete development and their specification. So it is good to find two websites that we all can use to find further information on concrete, its constituents, and probably most importantly all the new specifications and "Eurocodes" that apply to the use of concrete. These websites have sections specifically on the use of concrete in civil engineering. The Concrete Centre - www.concretecentre.com and Eurocode2 - www.eurocode2.info If you are involved in the structural engineering of highway networks it seems to me these websites will be very useful, but the codes and specifications that they list take some understanding, or that is how they appear to me.

AIR ENTRAINING AGENTS - CONCRETE AIR ENTRAINING AGENTS are particularly used in ROAD PAVEMENT CONCRETE. This is because entrained air produces very small discrete cavities in the CEMENT paste and these cavities do not normally fill with water even in a saturated concrete. They are thus able to relieve the hydraulic pressure developed in capillaries in the paste in the initial stages of freezing, i.e. it will prevent the spalling of concrete during frost.

ALKALI SILICA REACTION - CONCRETE FAILURE Some aggregates containing particular forms of silica may be susceptible to attack by alkalis originating from the cement or other sources, producing a gel type substance in an expansive reaction which can cause cracking and disruption of concrete. Damage to concrete will normally occur only when all the following are present together :- (a) a high moisture level within the concrete (b) a concrete with high reactive alkali content, or another source of reactive alkali (c) aggregate containing an alkali reactive constituent, Further information can be found in :- BS 5328:Part 1,Clause 4.2.4, and BRE Digest 330 Alkali aggregate reactions in concrete

BRE DIGEST 330 - ALKALI AGGREGATE REACTIONS IN CONCRETE

BRE Digest 363 - Sulphate and acid resistance of concrete in the ground This digest was issued in 1991 and supersedes BRE DIGEST 250, and gives guidance on concrete liable to sulphate attack.

BS EN 196-1 : 2005 : Methods of testing cement - Part 1 : Determination of strength This document describes the method for the determination of the compressive and, optionally, the flexural strength of cement mortar. The method applies to common cements and to other cements and materials, the standards for which call up this method . It may not apply to other cement types that have, for example, a very short initial setting time. The method is used for assessing whether the compressive strength of cement is in conformity with its specification and for validation testing of a CEN Standard sand, EN 196-1, or alternative compaction equipment. This document describes the reference equipment and procedure and allows alternative compaction equipment and procedures to be used provided that they have been validated in accordance with the appropriate provisions in this document. In the event of a dispute, only the reference equipment and procedure are to be used.

BS EN 196-2 : 2005 : Methods of testing cement - Part 2 : Chemical Analysis of cement This document specifies the methods for the chemical analysis of cement. It describes the reference methods and, in certain cases, an alternative method which can be considered to be equivalent. In the case of a dispute, only the reference methods are used. Any other methods may be used provided they are calibrated, either against the reference methods or against internationally accepted reference materials , in order to demonstrate their equivalence. This document describes methods which apply principally to cements, but which can also be applied to their constituent materials. They can also be applied to other materials, the standards for which call up these methods. Standard specifications state which methods are used.

BS EN 196-3 : 2005 : Methods of testing cement - Part 3 : Determination of setting times and soundness This document specifies the methods for determining standard consistence, setting times and soundness of cements. The method applies to common cements and to other cements and materials, the standards for which call up this method. It may not apply to other cement types that have, for example, a very short initial setting time. The method is used for assessing whether the setting time and soundness of a cement is in conformity with its specification. This document describes the reference methods and allows the use of alternative procedures and equipment, as indicated in notes, provided that they have been calibrated against the reference methods. In the event of a dispute, only the reference equipment and procedures are used.

BS EN 196-5 : 2005 : Methods of testing cement - Part 5 : Pozzolanicity test for pozzolanic cement This document specifies the method of measuring the pozzolanic cements conforming to EN 197-1. This document does not apply to Portland pozzolana cements or to pozzolanas.

BS EN 206-1 : 2000 : CONCRETE : PART 1 : SPECIFICATION, PERFORMANCE, PRODUCTION, AND CONFORMITY

BS 8500 - 1 : 2002 : CONCRETE - COMPLEMENTARY BRITISH STANDARD TO BS EN 206 - 1 : PART 1 : METHOD OF SPECIFYING AND GUIDANCE TO SPECIFIER

BS 8500 - 2 : 2002 : CONCRETE - COMPLEMENTARY BRITISH STANDARD TO BS EN 206 - 1 : PART 2 : SPECIFICATION FOR CONSTITUENT MATERIALS AND CONCRETE

BS EN 450-1 : 2005 - Fly ash for concrete - Part 1 : Definition, specifications and conformity criteria This document specifies requirements for the chemical and physical properties as well as quality control procedures for siliceous fly ash. It is, however, beyond the scope of this document to specify provisions governing the practical application of fly ash in the production of concrete.

BS EN 450-2 : 2005 - Fly ash for concrete - Part 2 : Conformity evaluation This document provides technical rules for the production control by the producer, including auto control testing of samples. It also provides rules for actions to be followed in the event of non-conformity, the procedure for the certification of conformity and requirements for dispatching centres.

BS EN 480-11 : 1999 : Admixtures for concrete, mortar and grout - Test methods Part 11 : Determination of air void characteristics in hardened concrete

BS 1014 - Specification for pigments for Portland cement and Portland cement products (pigmented portland cement, coloured concrete)

BS 1881 - Testing concrete : Part 128 : Methods for analysis of fresh concrete

BS 1881:Testing concrete was the British Standard covering the testing of concrete, and it came in many, many parts, such as:- BS 1881 : Part 101 : Method of sampling fresh concrete on site BS 1881 : Part 102 : Method of determination of slump BS 1881 : Part 106 : Methods for determination of air content of fresh concrete, BS 1881 : Part 111 : Method of normal curing of test specimens BS 1881 : Part 116 : Methods for determination of compressive strength of concrete cubes. BS 1881 : Part 108 : Method of making test cubes from fresh concrete. BS 1881 : Part 131 : Methods for testing cement in a reference concrete (Be aware that most of these standards have been superseded, however they do contain good information that could be the basis of good practice, that will keep you more or less safe for basic highways maintenance concrete use, and you will be able to understand them.)

BS 2499 : Specification for hot applied joint sealant for concrete pavements, and it comes in three parts, each individual part is included in the database. Part 1. Specification for joint sealants Part 2.Code of practice for the application and use of joint sealants. Part 3. Methods of test The complete standard gives recommendations for the preparation of joint sealant slots, the types of sealant and the application and site testing of hot-applied joint sealants for use in road joints. See also:- ***TRL RESEARCH REPORT 349***BS 5212:PART 1***BS 5212:PART 2***

BS 2499 : Hot-applied joint sealants for concrete pavements : Part 1. Specification for joint sealants This part of the standard specifies the requirements for hot-applied joint sealants for use in concrete roads. It also applies to hot-applied normal joint sealants in bituminous surfacing and between bituminous surfacing and concrete pavements. Shelf life, safe heating period, flow resistance, penetration and fuel resistance are all items covered in this part of the specification. See also:- ***TRL RESEARCH REPORT 349***BS 5212:PART 1***BS 5212:PART 2***

BS 2499 : Hot-applied joint sealant systems for concrete pavements : Part 2. Code of practice for the application and use of joint sealants. The above standard gives recommendations for the preparation of joint sealant slots and for the application of hot-applied joint sealants for use in road joints.

BS 2499 : Hot-applied joint sealant systems for concrete pavements : Part 3. Methods of test This part of the standard describes methods of test for hot-applied joint sealants for use in joints in roads, airfields and other concrete pavements. See also:- ***TRL RESEARCH REPORT 349***BS 5212:PART 1***BS 5212:PART 2***

BS 340 - Concrete kerbs  This British Standard is withdrawn, See :- BS 7263:PART1

BS 5075 : Concrete admixtures : Part 2 : Specification for air- entraining admixtures See also:- ***AIR ENTRAINING AGENTS***TRRL REPORT 363***

BS 5075 : Concrete admixtures : Part 1 : Specification for accelerating admixtures, retarding admixtures and water-reducing admixtures See also:- ***AIR ENTRAINING AGENTS***TRRL REPORT 363***

BS 5212 : Cold applied joint sealant systems for concrete pavements : Part 1 : Specification for joint sealants

BS 5212 : Cold applied joint sealant systems for concrete pavements : Part 2 : Code of practice for application and use of joint sealants

BS 5212 : Cold applied joint sealant systems for concrete pavements : Part 3 : Methods of Test

BS 5328 : Concrete This standard was an excellent set of volumes providing a great deal of information on concrete and its specification, and you needed access to these documents to be able to specify and place an order for the concrete you require. (These not too difficult to understand volumes have been superseded by BS EN 206-1, which is not at all easy to understand, hence BS 8500 : Parts 1 & 2. If you can still obtain copies of these standards, and you are "new" to concrete it may help to read these before you move on to BS EN 206-1. If you can obtain a set of these standards I suggest that you do so for the information that they contain.) It was in 4 parts :- Part 1 : Guide to specifying concrete. Part 2 : Method for specifying concrete. Part 3 : Specification for the procedures to be used in producing and transporting concrete. Part 4 : Specification for the procedures to be used in sampling, testing and assessing compliance of concrete.   Most CONCRETE is normally specified by stating 28 day strength, and aggregate size, but far more factors should be specified as stated in this specification. The many parts of BS 1881 dealt with the various ways of testing of concrete. See also:-**BS 1881**CONCRETE**STRUCTURAL CONCRETE**ROAD PAVEMENT CONCRETE**CONCRETE STRENGTH**

BS 5328 : Concrete : Part 2 : Methods for specifying concrete (mixes,  including amendments) It includes information on :- Designed mixes, Prescribed mixes, Standard mixes, and some good general information, worth having a copy for the information that it contains if you can still obtain one. This standard has now been superseded.

BS 5911-6 : 2004 : Concrete pipes and ancillary concrete products - Part 6 : Specification for road gullies and gully cover slabs This part of the standard specifies requirements and describes test methods for precast concrete road gullies manufactured from monolithic concrete or prefabricated sections of concrete. A gully outlet may incorporate a permanent former, with or without a jointing profile, for the connection of a pipeline. Requirements are also specified for gully cover slabs. The intended use for gullies is the interception of silt and debris from surface water being discharged into drainage systems. BS 5911-6 is a revision of BS 5911-230:1994, which is withdrawn on 31st. October 2004 when BS 5911-6 comes into effect.

BS 6089 : Guide to assessment of concrete strength in existing structures This standard gives information of tests that are able to determine strength of concrete already in a structure.

BS 6717 : Precast concrete paving blocks : Part 1 : Specification for paving blocks : Part 3 : Code of practice for laying See also:- ***BS 6677***BS 7533***

BS 7263 : Precast concrete flags, kerbs, channel edgings and quadrants : Part 1 : Specification This standard relates to the specification for precast concrete flags, kerbs, channels edgings and quadrants. It covers such criteria as dimensions, strength, water absorption, etc., and the methods by which these properties shall be tested. BS 7263:PART 1 has been prepared under the direction of the Road Engineering Standards Policy Committee and supersedes BS 340 and BS 368 which are withdrawn. See also:- ***BS 7263:PART 2***BS 435***

BS 7263 : Precast concrete flags, kerbs, channels, edgings and quadrants : Part 2 : Code of practice for laying See also:- ***BS 7263:PART 1***

BS 8000: Section 2 - Code of practice for concrete work

BS 8110 : Structural use of concrete : Part 1 : Code of practice for design and construction

BS 8110-2 : 1985 : Structural use of concrete - Part 2 : Code pf practice for special circumstances This part of BS 8110 gives recommendation for the design and construction of structural concrete that arise in special circumstances and are not covered in BS 8110-1. This part gives guidance on ultimate limit state calculations and the derivations of partial factors for safety, serviceability calculations with emphasis on deflections under loading and cracking. Further information for greater accuracy in predictions of the different strain components is presented. The need for movement joints is considered and recommendations are made for the provision and design of such joints. General guidance and broad principles relevant to the appraisal and testing of structures and components during construction are included.

BS 812 : Testing aggregates : Part 123 : 1999 : Method for the determination of alkali - silica reactivity : Concrete prism method This standard specifies a method of measuring the expansion of concrete produced by alkali silica reaction. The method is designed as an accelerated laboratory test and is not meant to be a performance test for concrete.

BS 1881 : Testing concrete : Part 131 : Methods for testing cement in a reference concrete This standard gave methods for testing cement in a reference concrete, I.e. differing cements are mixed in the same proportions with the same constituents in the same way. The resulting concrete is judged by its constituency (slump) and compressive strength (100mm. Cubes).

BS 882 : Aggregates from natural sources for concrete. This standard covered the grading requirements of coarse, fine (sand), and all-in aggregate. It also covers the mechanical properties that are required of the aggregate and the information that shall be supplied about the aggregate by the supplier. Information on the aggregate will include such data as :- TEN PERCENT FINES VALUE, AGGREGATE IMPACT VALUE, FLAKINESS INDEX, GRADING, WATER ABSORPTION, SURFACE TEXTURE, PARTICLE DENSITY

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, as mixing water for concrete This standard supersedes BS 3148-1980, which will remain current until 1/12/2003, when it will be withdrawn.

BS EN 1170- 2 : Precast concrete products - Test method for glass fibre reinforced cement : Part 2 : Measuring the fibre content in fresh GRC, "Wash out test"

BS EN 1917 : 2002 : Concrete manholes and inspection chambers, unreinforced, steel fibre and reinforced This standard is a companion standard to BS EN 1919 "Concrete pipes and fittings, unreinforced, steel fibre and reinforced". This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by April 2003, and conflicting national standards shall be withdrawn at the latest by October 2004.

BS EN 12350 -1 : 2000 : TESTING FRESH CONCRETE : PART 1 : SAMPLING This supersedes BS 1881 : Part 101 : 1983, which is withdrawn

BS EN 12350-2 : 2000 : TESTING FRESH CONCRETE : PART 2 : SLUMP TEST This supersedes BS 1881 : Part 2 : 1983, which is withdrawn

BS EN 12350-3 : TESTING FRESH CONCRETE : PART 3 : VEBE TEST This supersedes BS 1881 : Part 104 : 1983, which is withdrawn.

BS EN 12350-4 : 2000 : TESTING FRESH CONCRETE : PART 4 : DEGREE OF COMPACTABILITY This supersedes BS 1881 : Part 103 : 1993, which is withdrawn.

BS EN 12350-5 : TESTING FRESH CONCRETE : PART 5 : FLOW TABLE TEST This supersedes BS 1881 : Part 105 : 1993, which is withdrawn.

BS EN 12350-6 : 2000 : TESTING FRESH CONCRETE : PART 6 : DENSITY This supersedes BS 1881 : Part 107: 1984, which is withdrawn.

BS EN 12390-1 : 2000 : Testing hardened concrete : Part 1 : Shape, dimensions and other requirements for specimens and moulds

BS EN 12390-2 : 2000 : TESTING HARDENED CONCRETE : PART 2 : MAKING AND CURING SPECIMENS FOR STRENGTH TESTS

BS EN 12390-3 : 2002 : TESTING HARDENED CONCRETE : PART 3 : COMPRESSIVE STRENGTH OF TEST SPECIMENS

BS EN 12390-5 : 2000 : TESTING HARDENED CONCRETE : PART 5 : FLEXURAL STRENGTH OF TEST SPECIMENS

BS EN 12390-6 : 2000 : TESTING HARDENED CONCRETE : PART 6 : TENSILE SPLITTING STRENGTH OF TEST SPECIMENS

BS EN 12390-7 : 2000 : TESTING HARDENED CONCRETE : PART 7 : DENSITY OF HARDENED CONCRETE

BS EN 12390-8 : 2000 : TESTING HARDENED CONCRETE : PART 8 : DEPTH OF PENETRATION OF WATER UNDER PRESSURE

BS EN 12504-1 : 2000 : TESTING CONCRETE IN STRUCTURES : PART 2 : CORED SPECIMENS - TAKING, EXAMINING AND TESTING IN COMPRESSION

BS EN 12504-3 : 2005 - Testing concrete in structures - Part 3 : Determination of pull-out force A range of different methods of measuring pull-out force are available. The measurement can be used to estimate in-situ strength, to determine when post tensioning can proceed, when forms and props can be removed, when winter protection and curing can be determined, or for comparative testing.

BS EN 12620 : 2002 + A1 : 2008 - Aggregates for Concrete BS EN 12620 specifies the properties of aggregates and filler aggregates obtained by processing natural, manufactured or recycled materials and mixtures of these aggregates for use in concrete. It covers aggregates having an oven dried particle density greater than 2,00 Mg/m3 (2 000 kg/m3) for all concrete, including concrete in conformity with BS EN 206-1 and concrete used in roads and other pavements and for use in precast concrete products. BS EN 12620 also covers recycled aggregate with densities between 1,50 Mg/m3 (1 500 kg/m3) and 2,00 Mg/m3 (2 000 kg/m3) with appropriate caveats and recycled fine aggregate (4 mm) with appropriate caveats. BS EN 12620 specifies that a quality control system is in place for use in factory production control and it provides for the evaluation of conformity of the products to this European Standard. This standard does not cover filler aggregates to be used as a constituent in cement or as other than inert filler aggregates for concrete. ( I acknowledge that this summary is copyright of BSI. ) PD 6682-1 : 2009 : Aggregates : Aggregates for concrete - Guidance on the use of BS EN 12620 (This part of PD 6682 gives guidance on the use of BS EN 12620 , and supersedes the 2003 edition.)

BS EN 13055 - 1 : 2002 : LIGHTWEIGHT AGGREGATES : PART 1 - LIGHTWEIGHT AGGREGATES FOR CONCRETE, MORTAR AND GROUT

BS EN 13375 - Flexible sheets for waterproofing - Waterproofing of concrete bridge decks and other concrete surfaces trafficable by vehicles - Specimen preparation This standard is one of a series of standards applicable to flexible sheets for waterproofing of concrete bridge decks and other concrete surfaces trafficable by vehicles. The document specifies the composition, the characteristics and the preparation procedure of the base specimen concrete slabs. The document also specifies the composition, the characteristics and the preparation procedure of different bituminous mixtures for the asphalt layer. It also covers the rules for the preparation of specimens.

BS EN 13653 - Flexible sheets for waterproofing - Waterproofing of concrete bridge decks and other concrete surfaces trafficable by vehicles - Determination of shear strength This standard is one of a series of standards applicable to flexible sheets for waterproofing of concrete bridge decks and other concrete surfaces trafficable by vehicles. The document specifies a test method for the evaluation of the shear strength properties of the waterproofing sheet system applied to a concrete surface and with an asphalt layer.

BS EN 1338 : 2003 : Concrete paving blocks - Requirements and test methods This standard specifies materials, properties, requirements, dimensions, and test methods for unreinforced cement bound concrete paving blocks and complementary fittings. It is applicable to most but not all uses of these products, read the standard for particular information. This is a comprehensive standard covering in detail most important aspects relating to the specifying and testing of concrete paving blocks

BS EN 1339 : 2003 : Concrete paving flags - Requirements and test methods This standard specifies materials, properties, requirements and test methods for cement bound unreinforced concrete paving flags and complimentary fittings. It is applicable to precast concrete paving flags and complimentary fittings that are for use in trafficked paved areas and roof coverings.

BS EN 1340 : 2003 : Concrete kerb units - Requirements and test methods This standard specifies materials, properties, requirements and test methods for unreinforced, cement bound precast concrete kerb units, channels and complementary fittings, that are for use in trafficked paved areas and roof coverings.

BS EN 13579 : 2002 : Products and systems for the protection and repair of concrete structures - Test methods - Drying test for hydrophobic impregnation

BS EN 13580:2002:Products and systems for the protection and repair of concrete structures - Test methods - Water absorption and resistance to alkali for hydrophobic impregnations

BS EN 13581:2002:Products and systems for the protection and repair of concrete structures- Test methods - Determination of loss of mass of hydrophobic impregnated concrete after freeze-thaw salt stress

BS EN 13863-3 : 2004 : Concrete pavements - Part 3 : Test methods for the determination of the thickness of a concrete pavement from cores This document describes a method for the determination of the thickness of a concrete pavement by measurements of cores taken from the full depth of the pavement.

BS EN 13877-1 : 2004 : Concrete pavements - Part 1 : Materials This standard specifies requirements for, the constituents (concrete and other materials) of concrete pavements, and the properties of fresh and hardened concrete. This document is applicable to concrete pavements cast in-situ. Concrete that is compacted by rollers is NOT covered by this document. This document covers pavements for roads, motorways and airports, pedestrian footpaths, cycle tracks, storage areas, and in general all traffic bearing structures.

EN 13877-2 : 2004 : Concrete pavements - Part 2 : Functional requirements for concrete pavements This standard specifies requirements for concrete pavements cast in-situ and compacted by vibration. It also covers concrete sub-bases as well as wearing courses on bridges. This document covers concrete pavements in motorways, airfields, pedestrian streets, cycle tracks, storage areas and, in general, all traffic-bearing structures.

BS EN 13877-3 : 2004 : Concrete pavements - Part 3 : Specifications for dowels to be used in concrete pavements This document specifies the requirements for dowels to be used in cast "in situ" concrete pavements for roads, airfields and other trafficked areas.

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

BS EN 934-6 : PART 6 : ADMIXTURES FOR CONCRETE, MORTAR AND GROUT : PART 6 : SAMPLING, CONFORMITY CONTROL, EVALUATION OF CONFORMITY, MARKING AND LABELING

PD 6682-1 : 2003 : Aggregates : Aggregates for concrete - Guidance on the use of BS EN 12620 This part of PD 6682 gives guidance on the use of BS EN 12620 which specifies the properties of aggregates and filler aggregates obtained by processing, manufactured or recycled materials and mixtures of these aggregates for use in concrete. BS EN 12620 covers aggregates having an oven dried particle density greater than 2000kgs/cube metre for all concrete, including concrete conforming to BS EN 206-1, and concrete used in roads and other pavements, and for use in precast concrete products.

ISO 1920-1 (First edition 15/6/2004) : Testing of concrete - Part 1 : Sampling of fresh concrete This part of ISO 1920 specifies procedures for the sampling of fresh concrete. The samples are used for the testing of fresh concrete, or for making test specimens to determine the properties of hardened concrete.

ISO 1920-2 (First edition 15/04/2005) : Testing of concrete - Part 2 : Properties of fresh concrete This part of ISO 1920 specifies procedures for testing fresh concrete. It specifies the following test methods : determination of consistence (slump test, Vebe test, degree of compactibility, flow-table test and for high-fluidity concrete, the slump-flow test), determination of fresh density and determination of air content by the pressure-gauge method and by the water-column method.

ISO 1920-3 (First edition 01/10/2004) : Testing of concrete - Part 3 : Making and curing of test specimens This standard specifies the shape and dimensions of concrete test specimens for strength tests and the methods of making and curing these test specimens. This first edition of 1920-3 cancels and replaces ISO 1920:1976 and ISO 2736-2:1986 which have been technically revised.

ISO 1920-5 (First edition 15/10/2004) : Testing of concrete - Part 5 : Properties of hardened concrete other than strength This standard specifies procedures for testing properties of hardened concrete other than strength. This first edition of ISO 1920-5 cancels and replaces ISO 6275:1982 which has been technically revised.

ISO 1920-6 (First edition 01/10/2004) : Testing of concrete - Part 6 : Sampling, preparing and testing of concrete cores This standard specifies a method for taking cores from hardened concrete, their examination, preparation for testing and determination of compressive strength. The standard does not give guidance on the decision to drill cores or on the locations for drilling nor does it provide procedures for interpreting the core strength results. It is recommended that before coring, full agreement should be reached by all parties on the need for core testing and how the results should be interpreted.

BRE Report 279 : Sulphate and acid attack on concrete in the ground : recommended procedures for soil analysis

CEMENT BOUND MATERIAL - CBM - LEAN CONCRETE - SUB-BASE CBM 3 & CBM 4 are described under the LEAN CONCRETE heading. CEMENT BOUND MATERIAL also includes two grades of material weaker than the LEAN CONCRETE grades, these are CBM 1 and CBM 2, (see DfT Specification for Highway Works / "White Book", 1000 Series). CBM 1 and CBM 2 equate to the old "CBGB's", i.e. CEMENT BOUND GRANULAR BASE, this was a SUB-BASE or ROADBASE produced using naturally occurring material to which is added an appropriate amount of cement. CBGB/CBM 1 is usually produced on site by rotorvating the added cement into the naturally occurring material and compacting it with a suitable vibratory roller. Cubes are taken to make sure the correct strength is achieved by the material. ROADBASE/CBM 2 is usually produced by first stockpiling suitable material and then plant mixing and returning mixed material to a paver for laying. A change to the above classification is likely, and may have happened by the time you read this. See also:- ***LEAN CONCRETE***CEMENT STABILISATION***TRL CONTRACTOR REPORT 151***

COLD WEATHER WORKING - CONCRETE Fresh concrete MUST be protected from freezing conditions until it has set. Not until it has set, (i.e. the chemical process that is the hardening of concrete is substantially complete), is concrete safe from freezing and hence spalling when thawed.

COMPACTING FACTOR TEST - CONCRETE This is a test for determining the WORKABILITY of concrete. See also:- ***WATER CEMENT RATIO***SLUMP TEST***CONCRETE WORKABILITY***

CONCRETE, A FEW BASIC NOTES Concrete is the product of mixing, aggregate, cement and water. The setting of concrete is a chemical reaction between the cement and the water. The reaction is known as hydration, it evolves heat as any chemical reaction, and it is irreversible. All concrete should be specified according to the information set down in, BS 5328:1990/91:Concrete, but now superseded, see item on BS 5328. B.S. 882 is the specification for aggregates from natural resources for concrete. Or the current specification/standard that has superseded the previously mentioned documents. Concrete for use in structures is covered in the DfT "Specification for Highway Works", (WHITE BOOK), in the 1700 series "Structural Concrete", and reading the current edition of this document will provide reference to the current specifications/standards for concrete. There are various specifications for cement according to what properties you require of the mixed concrete. The basic factors you will need to consider when specifying concrete are the required :- Strength Aggregate size Workability Cement Content The testing of concrete was covered in, BS 1881:Testing concrete, which consisted of many parts. See also :- ***BS 5328***BS 1881***BS 882***STRUCTURAL CONCRETE***

CONCRETE CUBES The taking of concrete cubes from fresh concrete as it is placed into structures is the normal way of controlling the quality of concrete that is supplied to a site. There is a delay of 28 days before results are known, as per the specification. If the strength of the concrete in the cubes is significantly below what is required the concrete has to be removed, (PERHAPS AFTER CORING). N.B. It is necessary to take extra cubes if you require early strengths. The concrete cubes are taken in machined steel moulds of 100mm.or 150mm. square. Correctly sampled fresh concrete is placed in the moulds and FULLY compacted, after the initial set (24 hours) the cubes are stripped, and placed in water at 20 deg.C Full details were to be found in :- BS 1881:Part 118,Method of making test cubes from fresh concrete. BS 1881:Part 111,Method of normal curing of test specimens (20 deg.C method) BS 1881:Part 116,Method for determination of compressive strength of concrete cubes but the above standards have now been superseded. See also:-***LEAN CONCRETE CUBES***

CONCRETE DESIGN In dealing with CONCRETE DESIGN there are many factors to consider such as :- strength, workability, water cement ratio, aggregate cement ratio, aggregate type, aggregate grading, cement type, purpose of concrete, etc., etc. I recommend anybody with a further interest in CONCRETE DESIGN to get hold of a copy of "Design of Normal Concrete Mixes" from H.M.S.O., as a start to learning about this subject, and it will show you how to design basic concrete mixes. Basically, but not always, in a CONCRETE DESIGN it is necessary to have a blend of fine and coarse aggregate which with the cement content will produce a high density with no voids. Cement and water will be in the correct amounts and with the correct WATER/CEMENT RATIO to produce the required strength and workability. See also :-***CONCRETE WORKABILITY***WATER CEMENT RATIO***

CONCRETE FAILURE - THAUMASITE ATTACK Thaumasite appears as a white pulpy mass which eats into concrete causing loss of strength and eventually exposing the steel reinforcement. It us associated with concrete produced using limestone aggregate, and placed in areas having sulphate bearing clays. The Highways Agency have established a "Thaumasite Expert Group" to investigate the problem and prepare recommendations. London, Oxford and Lower Lias clays cover an area of England from Gloucestershire and the South East to Humberside.

CONCRETE SOCIETY - ADDRESS Concrete Society, 3 Eatongate, 112 Windsor Road, Slough SL3 6PJ

CONCRETE SOCIETY TECHNICAL REPORT 30 - Alkali / silica reaction - minimizing the risk of damage to concrete

CONCRETE STRENGTH The strength of CONCRETE for specifying purposes is quoted as the 28 day strength that representative samples of fresh concrete when placed in machined moulds, (150mm. or 100mm.), and cured correctly, will attain when crushed, (i.e. COMPRESSIVE STRENGTH). The strength of CONCRETE is measured in NEWTONS/ SQ. MM., or MEGANEWTONS/ SQ. METRE, or even MEGAPASCALS. In fact it does not really matter what you use they all equate to the same thing and the numbers you quote will be the same. For example:- A 35 MEGANEWTON/SQ.METRE concrete = a 35 NEWTON/SQ.MM. concrete = a 35 MEGAPASCAL concrete. The actual strength of in situ concrete can be found by coring and crushing the cores obtained. Broadly a concrete with more cement will be a stronger concrete, BUT water plays a very important role in the strength of concrete, i.e. the water/cement ratio, and of course compaction/vibration. See also:- ***CONCRETE SPECIFYING***CONCRETE CUBES***WATER CEMENT RATIO***

CONCRETE WORKABILITY See SLUMP TEST, this is the most commonly used on site test for the workability of concrete, although there are others, one of them being the COMPACTING FACTOR TEST. CONCRETE needs to be WORKABLE to a greater or lesser degree according to the use the concrete is being put. Generally speaking concrete needs to be sufficiently WORKABLE to ensure complete compaction and hence maximum density is achievable. Concrete mixes with increased WORKABILITY have to be produced for situations where there is a lot of intricate steel reinforcing which MUST be completely covered and infilled. Also highly workable concrete mixes are produced to allow concrete to be pumped into place.. When producing WORKABLE concrete mixes particular attention MUST be paid to the WATER CEMENT RATIO. See also:-**CONCRETE**SLUMP TEST**COMPACTING FACTOR TEST**WATER CEMENT RATIO**

CRCR - CONTINUOUSLY REINFORCED CONCRETE ROADBASE This is an abbreviation for :- CONTINUOUSLY REINFORCED CONCRETE ROADBASE

DTP DESIGN MANUAL HD 32/94 - MAINTENANCE OF CONCRETE ROADS - CONCRETE SLABS This part of the DESIGN MANUAL FOR ROAD AND BRIDGES gives information on the maintenance, repair and strengthening of the various types of concrete slab used in concrete road construction, and it is recommended reading.

DTP DESIGN MANUAL HD 38/97 - CONCRETE SURFACING - WHISPER CONCRETE This part of the DESIGN MANUAL FOR ROAD AND BRIDGES includes information on CONCRETE SURFACING MATERIALS, in particular EXPOSED CONCRETE AGGREGATE SURFACE (EACS), this is often referred to as WHISPER CONCRETE.

EARLY STRIPPING OF CONCRETE SHUTTERING - CEMENT REPLACEMENT It is not uncommon for contractor and engineer to agree for the early stripping of shuttering providing concrete strengths obtained from EXTRA cubes taken at the time of the pour indicate the concrete has sufficient early strength. There can be a problem if a contractor who is familiar with using concrete made entirely with OPC cement, changes to a cement that has a CEMENT REPLACEMENT additive. (The change is made because the concrete is slightly cheaper.) The CEMENT REPLACEMENT is usually GROUND GRANULATED BLAST FURNACE CEMENT, 28 day strengths of the two concretes will be much the same, BUT 7 day strength of GGBS cement will be substantially lower. Be aware of this and bring it to the attention of the contractor before it becomes an issue. See also:- ***GROUND GRANULATED BLASTFURNACE CEMENT***

FAST TRACK CONCRETE PAVING - FTCP FTCP is a process developed in IOWA, USA which allows a concrete road to be opened to traffic within 24 hours of construction instead of the 2 to 3 weeks normally required. It may not lend itself to weather conditions in the U.K.. The high early strength of FTCP is achieved partly by the use of a rapid hardening cement and partly by high temperature curing. The high temperature curing is achieved by covering the concrete with thermally insulated blankets which maintain the initial high temperature of the concrete derived from the heat of hydration of the cement within the mass of the concrete. See also:- ***TRL RESEARCH REPORT 355***

FOAMED CONCRETE - TRENCH REINSTATEMENT FOAMED CONCRETE is a lightweight concrete made by blending cement, sand or fly-ash, and water with a PREFORMED foam, i.e. an already created foam before mixing. It is becoming increasingly used for TRENCH REINSTATEMENT, especially narrow trenches. For trenches, it is specified in the SPECIFICATION FOR THE REINSTATEMENT OF OPENINGS IN HIGHWAYS. FOAMED CONCRETE is self levelling and needs no compacting. It MUST have a density higher than 1050kg/cubic metre so that it will displace water in trenches. FOAMED CONCRETE has a maximum fairly low strength, as well as a minimum strength, so that if re-excavation is needed it will not be too difficult to remove. Until FOAMED CONCRETE has achieved an initial set the material must be treated as a liquid and appropriate SAFETY PRECAUTIONS taken. ESPECIALLY to exclude children and pets to prevent drowning.

GOOD BOOKS ON HIGHWAY MAINTENANCE - CONCRETE These include :- 1) DTP Design Manual HD 31/94, Maintenance of Concrete Roads See also:- ***GOOD BOOKS***DTP DESIGN MANUAL HD 31/94***

INTERPAVE - CONCRETE BLOCK PAVING - KERBS "INTERPAVE" is the the name of :- THE PRECAST CONCRETE AND KERB ASSOCIATION The address :- 60, Charles Street, Leicester , LE1 1FB Tel. 0116 253 6161 Fax. 0116 251 4568 "INTERPAVE" produce some excellent information sheets on block paving, and kerbs channels and edgings, also information on pavement design, construction and reinstatement after utility work in the road. See also:- ***BS7533***

LEAN CONCRETE - CBM - CEMENT BOUND MATERIAL (still useful information but now superseded when specifying) LEAN CONCRETE is a relatively low strength concrete (6.5 - 15.0 MN., depending on grade), which is used as a roadbase material. LEAN CONCRETE is no longer called LEAN CONCRETE from a specifying point of view, it is called CEMENT BOUND MATERIAL (CBM). CEMENT BOUND MATERIAL 3 (CBM3), the weaker grade with a 7 day strength of 6.5 to 10.0 MN., CEMENT BOUND MATERIAL 4 (CBM4) with a 7 day strength of 10.0 to 15.0 MN.. See also :-***ROADBASE***LEAN CONCRETE CUBES***CEMENT BOUND MATERIAL***

LEAN CONCRETE CUBES - CEMENT BOUND MATERIAL (still useful information but now superseded when specifying) Remember LEAN CONCRETE is now known as CEMENT BOUND MATERIAL (CBM). The basic difference between "ordinary" cubes and LEAN CONCRETE cubes is that the compaction of the LEAN CONCRETE is performed using a vibrating hammer with a square compacting foot, precise details of the method are in Clause 1040 of the DOT Specification for Highway Works See also:- ***LEAN CONCRETE***CEMENT BOUND MATERIAL***

PD 6534:1993 - CONCRETE - PERFORMANCE - PRODUCTION This document is published as a guide note to ENV 206, and it gives guidance on CONCRETE, with regards to performance, production, placing and compliance criteria.

POROUS CONCRETE SURFACING FOR ROADS This material is still in the development stage, and I presume the concrete industry is endeavouring to produce a concrete material to compete with POROUS ASPHALT. This report covers the development, design, production, trials and testing on this material. See also:- ***TRL CONTRACTOR REPORT 320***

POROUS NO FINES CONCRETE The specification for this concrete can be found in :- The Specification for Highway Works, Volume 1, Series 2600, Miscellaneous. Clause 2603. "No fines" concrete is used in situations where a material with a relatively high structural strength is required but is also free draining. There was/is a more helpful description/specification for Porous No Fines Concrete in Clause 1617 (Concrete for Structures) in the 1976 (Blue Book) copy of Specification for Road and Bridge Works.This stated that the cement should be OPC, and the aggregate 37.5mm. single size. The ratio of aggregate to cement should be 10:1 by mass. The concrete could be mixed by machine , or hand, to produce a concrete of uniform colour and consistency. The quantity of of water used should be no more than was needed to coat all the aggregate particles without forming excess grout. The concrete should only be compacted by hand.

ROAD PAVEMENT CONCRETE The subject of ROAD PAVEMENT CONCRETE at the level at which this guide is aimed is best covered by the :- 1000 Series : Road Pavements-Concrete and Cement-Bound Materials, of the DfT Specification for Highway Works, (White Book). This Series includes such items as:- Grades of concrete, constituents of concrete, air content, density, strength, workability, trial mixes, mixing concrete, transport and delivery, laying, compacting, curing, testing. It also covers construction practices associated with concrete road pavements. See also:- ***DfT DESIGN MANUAL HD 32/94***

SLUMP TEST - CONCRETE WORKABILITY The SLUMP test is an on site test for determining the workability of concrete. It is performed by filling a cone shaped cylinder with concrete, through its narrower open end, its broader open end being on a flat surface. When the cone is lifted up the concrete inside will collapse, the degree to which the concrete column collapses compared to the original height of the cone is the figure given as the slump. The SLUMP will vary according to the desired WORKABILITY of the concrete, but a slump of 75mm. to 100mm. is usually about right, BUT check the contract specification for the specified figure. This test is fully covered in :- B.S.1881 : Testing Concrete : Part 102 : Method of determination of slump. Basically if the cone of concrete when freestanding is erect the concrete is "stiff" and not very workable, if you have a "collapse" the concrete is too wet and the WATER/CEMENT RATIO could be impaired and hence the concrete will be weak. !   !   !   NEVER ADD WATER TO A "READYMIX" CONCRETE WITHOUT THE APPROVAL OF THE ENGINEER   !   !   ! See ***CONCRETE WORKABILITY***WATER CEMENT RATIO***

STRUCTURAL CONCRETE The subject of STRUCTURAL CONCRETE is huge. Therefore for the purpose of initially finding the information associated with HIGHWAY MAINTENANCE it is best to use the "1700 Series" of the DfT Specification for Highway Works (WHITE BOOK). The "1700 Series" will provide a great deal of information on the necessary requirements concrete must fulfil for satisfactory use in STRUCTURES. But best to buy yourself a good BASIC book on concrete and concrete design for information actually about concrete. "Design of Normal Concrete Mixes", is one such book and is, or used to be, available from H.M.S.O.. See also:-***CONCRETE***CONCRETE DESIGN***CONCRETE SPECIFYING***EARLY STRIPPING OF CONCRETE SHUTTERING***

SULPHATE CONTENT - CONCRETE - LIME STABILISATION It is important to know the sulphate content of the soil, ground water and any imported material that will be in contact with concrete structures or purchased product (e.g. drainage pipes) because sulphates will attack concrete produced from O.P.C.. It is possible to avoid this problem by using cements that will produce concrete that resists sulphate attack. E.g. would you believe SULPHATE RESISTING CEMENT. Building Research Station Digest 90, "Concrete in sulphate-bearing soils and ground waters", from H.M.S.O. will provide much information on this subject. SULPHATE CONTENT of a soil ALSO relates to the suitability of a soil for LIME STABILISATION. Sulphate in a soil can combine with calcium to form gypsum which absorbs a large amount of water of crystalisation causing the road to "heave".

TRL CONTRACTOR REPORT 320 - POROUS CONCRETE ROADS

TRL Report 303 - The evaluation of tests for repair materials used on concrete pavements This report covers a range of laboratory tests to establish the likely performance of the various materials on the market, a number of representative types of material have have been tested and conclusions drawn. It was also emphasised the extreme importance of good workmanship and correct weather conditions at the time of the work.

TRL REPORT 329 : Use of non-destructive testing for the assessment of newly constructed concrete pavements This was a study of tests undertaken on the surface of pavement concrete that were considered able to determine the strength of pavement concrete, and most tests gave a fair degree of accuracy in indicating early strength of the concrete.

TRL RESEARCH REPORT 349 - CONCRETE JOINT SEALANTS This is a report to study the performance of 18 different concrete joint sealants, and results obtained both in the laboratory and on site are tabulated and discussed. This is a useful piece of reading for anybody considering work on concrete joints. See also:- ***BS 2499***

TRL RESEARCH REPORT 355 - FAST TRACK CONCRETE PAVING This is a very useful report on this subject describing the process, with mix designs and details of a full scale trial. See also:- ***FAST TRACK CONCRETE PAVING***

TRRL - BRE - DESIGN OF NORMAL CONCRETE MIXES This is an old, 1975, publication, it is however an excellent basic document on concrete design, and to be recommended if copies are still available. (This is quite often the case if you take the time to contact the TRL Publications department.)

TRRL REPORT 612 : Continuously reinforced concrete pavements : a report of the study group. This is an excellent study, giving good information on this type of carriageway.

TRRL Report LR 363 : Air entrained concretes : a survey of factors affecting air content and a study of concrete workability AIR ENTRAINING AGENTS are included in CONCRETE to prevent damage by frost and de-icing salts, in CONCRETE PAVEMENTS. Also to increase WORKABILITY in STRUCTURAL CONCRETE, it is also claimed to reduce segregation, bleeding and shrinkage of concrete. This report is a study of factors affecting air content and its affect on CONCRETE, it is an excellent report and recommended reading on this subject. See also:- ***CONCRETE***CONCRETE WORKABILITY***STRUCTURAL CONCRETE***BS 1881:PART 106***

WATER CEMENT RATIO - CONCRETE STRENGTH - ADDING WATER ON SITE Not wishing to make this guide too technical this is an important point about concrete that you should know because actions taken on site to improve CONCRETE WORKABILITY will alter WATER CEMENT RATIO and SO decrease concrete strength. Everything else remaining the same the higher the WATER CEMENT RATIO the weaker the concrete will be, i.e. the same concrete with a 0.75 W/C Ratio will be weaker than if it had a 0.5 W/C Ratio, assuming there was sufficient water to enable the complete hydration of the cement and for full compaction of the concrete to take place. It is VERY IMPORTANT not to add water on site to concrete that has been designed to achieve a certain specified strength, at least not without the Engineers' approval. ADDING MORE WATER THAN NEEDED TO PRODUCE FULL COMPACTION DECREASES STRENGTH. It is IMPORTANT to be aware that W/C Ratio is usually expressed as a percentage of 1, e.g. a W/C Ratio of 0.45 (by weight) means 45% of the weight of CEMENT will be the TOTAL quantity (weight) of FREE water in the CONCRETE. See also:-**CONCRETE WORKABILITY**

WHY DID IT FAIL ? - CONCRETE This is a HUGE subject, but here are a few of the more simple reasons, and ALL these DO happen:- (1) Did you specify the correct concrete for the situation where it is to be used. (2) Was the type of concrete you specified actually delivered. (3) Was the concrete of the required workability. (4) Was the concrete correctly compacted, i.e. sufficient agitation/vibration when placing the concrete to ensure maximum density but not too much to cause segregation. (5) Was the concrete placed within its specified time limit. (6) Was water added to the concrete, (thus weakening the concrete), without the Engineers approval. (7) Was the concrete protected from drying out. (8) Was the concrete protected from frost/freezing. MOST IMPORTANTLY do not automatically assume because your CONCRETE CUBE strengths have failed that the concrete mass they have come from is below specification, confirm by taking cores. See also:- ***CONCRETE STRENGTH***CONCRETE CUBES***

WHY DID IT FAIL ? - CONCRETE CUBES - THE TESTING PROCESS The following are a few reasons why the concrete cube itself has failed rather than the concrete it is supposed to represent. When this type of failure occurs it is necessary to take cores of the actual concrete on site for further testing. (1) Was the concrete from which the cube was made correctly sampled. (2) Was the concrete in the cube mould correctly compacted. (3) Was the cube allowed the correct time for its initial set before stripping to avoid damage. (4) Was the cube protected from drying out and from frost/freezing during initial set. (5) Was the cube correctly cured at the correct temperature. (6) Was the cube mould and hence cube truly square. (7) Was the cube crushed at the correct rate. See also:- ***SAMPLING***CONCRETE CUBES***BS 1881***CONCRETE STRENGTH***