APPENDIX C-Continued 



pected to be encountered at the time of placement 

 These are often, but not always, reflected in speci- 

 fications for the job. 



2.3— The ability to tailor concrete properties to 

 job needs reflects technological developments 

 which have taken place, for the most part, since 

 the early 1900s. The use of the water-cement ratio 

 as a tool for estimating strength was recognized 

 about 1918. The remarkable improvement in du- 

 rability resulting from the entrainment of air 

 was recognized in the early 1940s. These two 

 significant developments in concrete technology 

 have been augmented by extensive research and 

 development in many related areas, including the 

 use of admixtures to counteract possible de- 

 ficiencies, develop special properties, or achiev^, 

 economy.* It is beyond the scope of this discus- 

 sion to review the theories of concrete propor- 

 tioning which have provided the background and 

 sound technical basis for the relatively simple 

 methods of this recommended practice. More de- 

 tailed information can be obtained from the list 

 of references. 



2.4 — Proportions calculated by any method 

 must always be considered subject to revision on 

 the basis of experience with trial batches. De- 

 pending on circumstances, the trial mixes may 

 be prepared in a laboratory or, perhaps pref- 

 erably, as full-size field batches. The latter pro- 

 cedure, when feasible, avoids possible pitfalls of 

 assuming that data from small batches mixed in 

 a laboratory environment will predict perfor- 

 mance under field conditions. Trial batch pro- 

 cedures and background testing are described in 

 Appendix 3. 



3. BASIC RELATIONSHIP 



3.1 — Concrete proportions must be selected to 

 provide necessary placeability, strength, dura- 

 bility, and density for the particular application. 

 Well established relationships governing these 

 properties are discussed briefly below, 



3.2— Placeabiiitj/ (including satisfactory finish- 

 ing properties) encompasses traits loosely ac- 

 cumulated in the terms "workability" and "con- 

 sistency." For the purpose of this discussion, 

 tuorkabilitj/ is considered to be that property of 

 concrete which determines its capacity to be 

 placed and consolidated properly and to be fin- 

 ished without harmful segregation. It embodies 

 such concepts as moldability, cohesiveness, and 

 compactability. It is affected by the" grading, 

 particle shape and proportions of aggregate, the 

 amount of cement, the presence of entrained 

 air, admixtures, and the consistency of the mix- 

 ture. Procedures in this recommended practice 

 permit these factors to be taken into account to 

 achieve satisfactory placeability economically. 



3.3 — Consistency, loosely defined, is the wet- 

 ness of the concrete mixture. It is measured in 

 terms of slump — the higher the slump the wetter 

 the mixture — and it affects the ease with 

 which the concrete will flow during placement. 

 It is related to but not synonymous with work- 

 ability. In properly proportioned concrete, the 

 unit water content required to produce a given 

 slump will depend on several factors. Water re- 

 quirement increases as aggregates become more 

 angular and rough textured (but this disad- 

 vantage may be offset by improvements in other 

 characteristics such as bond to cement paste). 

 Required mxing water decreases as the maxi- 

 mum size of well graded aggregate is increased. 

 It also decreases with the entrainment of air. 

 Mixing water requirement may often be sig- 

 nificantly reduced by certain admixtures. 



3.4 — Strength. Strength is an important char- 

 acteristic of concrete, but other characteristics 

 such as durability, permeability, and wear re- 

 sistance are often equally or more important. 

 These may be related to strength in a general 

 way but are also affected by factors not sig- 

 nificantly associated with strength. For a given 

 set of materials and conditions, concrete strength 

 is determined by the net quantity of water used 

 per unit quantity of cement. The net water con- 

 tent excludes water absorbed by the aggregates. 

 Differences in strength for a given water-cement 

 ratio may result from changes in: maximum size 

 of aggregate; grading, surface texture, shape, 

 strength, and stiffness of aggregate particles; dif- 

 ferences in cement types and sources; air con- 

 tent; and the use of admixtures which affect the 

 cement hydration process or develop cementitious 

 properties themselves. To the extent that these 

 effects are predictable in the general sense, they 

 are taken into account in this recommended prac- 

 tice. However, in view of their number and com- 

 plexity, it should be obvious that accurate pre- 

 dictions of strength must be based on trial 

 batches or experience with the materials to be 

 used. 



3.5 — Durability. Concrete must be able to en- 

 dure those exposures which may deprive it of its 

 serviceability^freezing and thawing, wetting and 

 drying, heating and cooling, chemicals, deicing 

 agents, and the like. Resistance to some of these 

 may be enhanced by use of special ingredients: 

 low-alkali cement, pozzolans, or selected aggre- 

 gate to prevent harmful expansion due to the 

 alkali-aggreeate reaction which occurs in some 

 are^is when concrete is exposed in a moist en- 

 vironment: sulfate resisting cement or pozzolans 

 for concrete exposed to seawater or sulfate- 



ACI STANDARD 



321 



