Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2011)

A paper by W. M. STERN, B.Sc.(Eng.), A.M.1. Struct.E. (Associate Member)

SPECIFYING READY MIXED CONCRETE SYNOPSIS

THIS

paper describes the suitability of ready ll1ixed concrete for different types of concrete structures, sOll1e of the precautions that ll1ust be taken, and discusses existing recoll1ll1endations for the use of transported concrete with particular reference to the recently published South African Bureau oj Standards sjJecijicalioll .'l.A.B.S. 878: 1967. The necessary conveying equipll1ent and ll1easuring devices which are now in use are described.

Introduction CONCRETE which is mixed at a central plant and then delivered ready for use to building sites by special agitator trucks is commonly known as ready mixed concrete. Such concrete should be regarded as a factory made product, but over the years it has been found that certain safeguards have to be adopted if the product is to conform to basic standards. Ready mixed concrete has been in use in parts of Europe and America for over 20 years. This method of concreting has shown particular advantages in urban areas and on congested building sites. Ready mixed concrete has also been used advantageously where only small quantities are required, or where it is required at intermittent intervals. About six years ago ready mixed concrete was introduced into South Africa and the industry has made rapid strides in the major cities of the country. The two principal categories of rcady mixed conerete are: I. The concrete is mixed at a central plant and the mix transported in an agitator truck which revolves slowly to prevent segregation and undue stiffening of mix

2. The materials are batched at a central plant, but mixed in a mixer truck either in transit to the site or immediately before any concrete is discharged. Such concrete is also called transit mixed COllcrete. Since agitating differs from mixing solely in the speed of rotation of the truck mixer (agit
opinions, on the permissi ble varia tion from these aecepted norms. This paper will endeavour to examine those aspects of a concrete specification which must be considered when ready mixed concrete is to be used. When considering any specification one must fil'st consider the type ofstrueture, the locality, environment and function. For the purposes of this paper, structures or structural elements will be divided into three classifications, for which certain specific requirements will be examined, namely: Structures type A

Here concrete is featured as an offshutter finished product, where maintenance work must be kept to a minimum and where a long durable life of structure is mandatory . This applies generally to structures for civil engineering works sueh as concrcte reservoirs, roads, bridges, silos, sewage structures, bunkers, canals, etc. and buildings. Structures type B

Structures as in type A, but located in a polluted atmospheric or marine environment with its corrosive influences, or surrounded by chemically deleterious groundwater conditions. This also includes normal buildings cladded with bricks, plaster or other materials. Structures type C

This refers to concrete structures where durability is not as essential as in types A and B, and where the environment is non-corrosive. Examples are c1added building structures, houses, concrete structures with offshutter finish built either for sub-economic or temporary purposes.

Specification items pertaining to ready mixed concrete The most important items to be examined are:

Concrete specification Just as a normal concrete mixed on site should comply with the general specifications covering the manufacture, handling and placement of concrete, standards should also apply to the ready mixed concrete, because the end product must be the same, namely a strong, durable concrete wi th a good finish. In certain aspects ready mixed concrete should exceed the requiremen ts of .the general concrete specifications, particularly in so far as batching facilities are concerned. In other aspects, however, caution must be exercised since some of the accepted norms in concrete handling do not always apply to ready mixed concrete, and there has been much discussion, with some differing DIE SIVIELE INGENIEUR in Suid-Afrlka -

Maar! 11168

Mixing and delivery time; Consistency and workability of concrete; Type of conveyance equipment. The specification for these three items must differ for each of the structures types A, Band C, if economics and good engineering judgment are to prevail, but opinions vary as to the degree of difference that should apply in each case. These three specification items will be discussed with reference to various existing specifications and shall then be applied to each of the structural types described above; it is further intended to show how all of these three specification items are inter-related for any structure. 31

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Mixing and delivery time . It is generally accepted that the facilities for mix design, materials storage, batching, mixing and quality control at a central batching plant under expert supervision can be and very often are superior to those found at a building site installation. Therefore this aspect will not be further discussed. A standard requirement for the placement of ordinary concrete which has been adopted by many C;H~1J.""CL in this country is S.A.B.S. 026:1958 1 clause (h), which states that 'concrete shall be placed in the' forms before loss of workability occun; and in no case more than 60 minutes after adding the water to the mix.' The Bureau of Reclamation (U.S.A.) in section 86 of their manua1 2 recommends a series of minimum mixing times, but warns that overmixing is o~jectionable because the grinding action increases fines, which requires more water to maintain consistency. Overmixing is also objectionable because it may drive out entrained air. The Bureau recommends, therefore, that mixing time shall not exceed three times the number of minutes given in its mixing time tabulation. For a six cubic yard mixer the maximum recommended mixing time is therefore nine minutes, to which must be added delivery time to the forms. In addition to these objections, overmixing allows increased evaporation of water from the mix which mean.s adding make-up water to maintain a workable mix, and may result in an undesirable high water to cement ratio. , ICb~com~s evident that such times as stated above are often impractical for ready mixed concrete operations. Specifications for ready mixed concrete have been devised to make allowance for the added transportation time from the batching plant to the building site. It is the opinion of the Author that, for normal ready mixed concrete operations the allowable mixing and delivery time of concrete should not exceed 90 minutes, unless procedures for long hauls are introduced; time being measured from the moment water is added to the mix. To support this view, the following authorities are quoted: (a) American Concrete Institute Standard A.C.I. 614-59. 3 Section IV clause 18 cautions on the variations of consistency when related to time, and the A.C.l. Manual of Concrete Inspection 4 bases its recommendations on'the A.S. T.Al. Standard C94:1i (b) A.S. T.M. Standard C941i requires that, under normal conditions, the concrete must be discharged from the mixer within 1thaun;. (c) The latest German recommendations 6 which have been adopted by most 'German public authorities also specify a maximum time of 90 minutes. Cd) The Swedish recommendations 7 issued in 1963 also specifY a maximum time of 90 minutes. (e) The Bureau of Reclamation's Concrete Manual,'). section 87 p. 261 states 'depending on weather conditions and the composition of the mix, the . concrete sometimes may be kept plastic .and workable for as long as I t hours by occasional turning in an agitator.' It continues on p. 262.to caution that 'unless adequate precautions are taken, troublesome 32'

conditions such as segregation and VariatIOns in consistencv mav occur to such an extent that control of the wat~r-ce~ent ratio may be lost.' The water-cement ratio more than any other factor in mix design will govern the strength, workability and durability of concrete, hence the time effect referred to above is most important. There are some authorities who appear to disagree with the maximum mL'i:ing and delivery time allowable. The recently published S.A.B.S. 878:1967 20 in clause 5.4 allows 3 hours, which is double the time specified by most of the aforementioned authorities. No limitations' on mixing time for type of concrete or structure are prescribed.

B.S.S. 1926-1962 8 allows two hours, but this will be further discussed with reference to data sheets 3 and 4 of the British Ready Alixed Concrete Association. 9 , 10 In data sheet 3 reference is made that 'concrete can in appropriate circumstances be agitated for considerably longer periods than the time permitted by the B.S.S. 1926-1962.'

Appropriate circumstances must surely include the type of structures to be cast, the functions of such structures, the atmospheric environment, type of mix, climatic conditions at time of casting, locality of ready mix plant and other influences. Different standards should be applied to say thinwalled water retaining structures requiring dense concrete and a good offshutter finish as compared with mass concrete floor slabs for housing projects. Hence for the different structural elements in the classifications such as types A, Band C, there should be corresponding different mixing time requirements. For instance mass . concrete foundations for any of the type A, B or C structures might have the time requirement based on recommendations for a type C structure even though these be foundations for a type A or B structure. Conversely a mass concrete foundation supporting type C structure might be classified and specified as a type B structure if sub-surface conditions are known to be chemically deleterious. The specifications pertaining to this question of mixing and delivery time might be relaxed in appropriate circumstances, for example, for a type C structure as compared with types A or B. Again there might be some slight relaxation for certain type B structures as compared with type A, but there should be no relaxation of time requirements for a type A structure. The summary conclusions from the investigation as extracted from data sheet 3 9 is as follows: 'The workability of concrete decreases with time of agitation and the compressive strength increases. For a time, the loss in workability is small and, in practice, it would only be necessary to increase the water content in order to maintain the required workability and strength; For very rich mixes, and for long periods of agitation, some reduction in strength would occur if the water content were increased to maintain the required workability; The drying shrinkage of concrete of low cement content is shown to be increased by prolonged agitation.' Referring to the portions denoted i~ i'talics. by the Author, one notes that the loss of workabilitv referred to can have serious consequences in th'e cOlfstruction of thinwalled concrete reservoirs, silos or. bunkers. Likewise the DIE SIVIELE INGENIEUR in Suid-AfriKa

. Maart 1968

Reproduced by Sabinet Gateway under licence granted by the Publisher (dated 2011)

reduction of strength and increase of drying shrinkage, could have serious detrimental effects and all this, according to data sheet 3, would be due to prolonged agitation. In the case of a cladded building framework, these detrimental effects are not necessarily of a serious nature. It is interesting to note in this data sheet the relation· ship between compressive strength and workability in terms of agitation time. Note that in a concrete mix with an aggregate to cement ratio of 4.5 (e.g. 1:1 t:3 mix) the 28 day strength of concrete with an initial slump of one inch is decreased after one hour of agitation, and in the case where the aggregate to cement ratio is 6 (e.g. I :2 :4 mix) with an initial slump of one inch, the 28 day strength is decreased after two hours of agitation. At first this would appear to contradict a previous statement in the data sheet that compressive strength increases with agitation time. As will be shown later, however, this depends on the agitation time as well as the mix proportions. With a 1:2:4 concrete mix it is possible that agitation may increase its strength during the first two hours and thereafter cause a strength loss as shown on curve I-I of Fig. 11. Even in initially wet mixes with an initial slump of five inches, the 28 day strengths for the two mixes described above are decreased after two hours and three hours agitation respectively. Such mixes would, however, normally only be used in type C structures, as most reinforced concrete in type A and B structures should be vibrated, which implies a slump range of one inch to three inches - concrete with a five inch slump is unsuitable for vibration. The re.tempering of ready mixed agitated concrete according to data sheet 3 produces slightly greater detrimental effects, which are all the more serious when applied to types A and B structures described previously. Data sheet 4 10 of the British Ready Mixed Concrete Association discusses concreting in hot weather, a con· dition often encountered. This data sheet pin-points the problems of workability decrease and the greater build up of heat of hydration leading possibly to thermal cracking on cooling. This problem is also well known. The recommendation made in the data sheet is that 'the concreting operation should be completed as quickly as possible ... ' and cautions that 'fresh concrete may not fully bond to concrete which has partly hardened. This would lead to a weakness in the unit.' The implications of such defects are serious in any concrete, but even more so in civil engineering structures. Surely, therefore, long hauls in such instances must be avoided unless special provisions are made? The S.A.B.S.20 specification is very vague about water content for concrete. It does not specify any form of control on the water-cement ratio, and clause 5-9 permits considerable mix tolerances far too great, in the opinion of the Author, to be accepted as a standard for type A and B structures. According to clause 6.2 the actual water content and percentage of air entrainment need not be stated., No limitations are imposed for retempering concrete. It should be noted that none of the quoted standards confine their recommendations to any particular type or classes of concrete structure. They are intended primarily for 'the building industry, where the use of ready mixed concrete has been far greater than in the civil engineering industry. The reasons for this, amongst others, are the confined spaces for batching plants on building sites in metropolitan areas, time and DIE SIVIELE INGENIEUR in·Suid-Afrika -' Maart 1968

cost of erecting batching plants for a limited concrete requirement, and that many building contractors do not have proper facilities for designing their own concrete mixes. Since long-haul methods are based on delivery time requirements, a more detailed investigation than that already described was made concerning the effects of prolonged mixing and delivery. Referring to pages 123 and 124 of Betontechnische Berichte 1965,18 Dr. Wischers, who is a leading concrete technologist at the Concrete Research Institute in Dusseldorf, describes these effects in greater length and reference is made to Figs. 9, 10 and II, from his paper. The Author has converted certain metric units and terms into their English equivalents, for ease of reference.

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