waterproof tape. Curing compounds meeting ASTM Standard C309 seal the 

 moisture in the concrete and are economical and easy to apply; they may be 

 used where other methods are impracticable. The curing compound should be 

 covered with scuff -proof paper if a floor area must be used before curing 

 is completed. 



(4) Freezing and Thawing . Freezing and thawing damage is a serious 

 problem in northern climates, however, the mechanisms involved are now 

 fairly well understood. Exposing damp concrete to freezing and thawing 

 cycles is a severe test of the material, and poor concrete will certainly 

 fail. In pavements the damage is greatly accelerated by the use of deicing 

 salts, often resulting in severe scaling at the surface. Fortunately, air- 

 entrained concrete which is properly proportioned, manufactured, placed, 

 finished, and cured will almost always resist cyclic freezing for many 

 years. It should be recognized, however, that even good concrete may 

 suffer damage from cyclic freezing in unusual conditions, particularly 

 concrete which is kept in a state of nearly complete saturation. Also, in 

 cases where the concrete is saturated on the back side and exposed to air 

 on the front side, it may exhibit extremely variable behavior, ranging from 

 complete freedom from damage to total failure. 



There is general agreement that cement paste can be made completely 

 immune to damage from freezing temperatures by means of entrained air, 

 unless special exposure conditions result in filling of the air voids. 

 However, air entrainment alone does not preclude the possibility of damage 

 of concrete due to freezing. Freezing phenomena in aggregate particles 

 must also be taken into consideration (see test samples in Fig. 17), 



(a) Freezing in Aggregate Particles . Most rocks have port 

 sizes much larger than those in cement paste, and they expel water during 

 freezing. The size of the coarse aggregate has been shown to be an important 

 factor in its frost resistance. The critical size of rocks of good quality 

 range upwards from perhaps a quarter of an inch. However, some aggregates 

 (e.g., granite, basalt, diabase, quartzite, marble) have capacities for 

 freezable water so low that they do not produce stress when freezing 

 occurs, regardless of the particle size. The role of entrained air in 

 alleviating the effect of freezing in rock particles is minimal, 



(b) Overall Effects in Concrete . Without entrained air, the 

 paste matrix surrounding the aggregate particles may fail when it becomes 

 critically saturated and is frozen. However, if the matrix contains an 

 appropriate distribution of entrained air voids characterized by a spacing 

 factor less than about 200 micrometers (8 mils), freezing does not produee 

 destructive stress. If absorptive aggregates (such as structural light" 

 weight) are used and the concrete is in a continuously wet environment, 

 the concrete will probably fail when the coarse aggregate becomes satu- 

 rated. The pressure developed when the particles expel water during 

 freezing ruptures the particles and the -matrix. If the particle i# 



near the concrete surface, a popout can result. 



Whatever the absorption characteristics of a given aggregate, its rate 

 of absorption in concrete is limited by the rate at which water can paii 

 through its envelope of hardened cement paste. Because the coefficient ©i 

 permeability of hardened cement paste is lower as its cement content 



110 



