plasticity of fresh concrete, the amount of surface area of the solids per 

 unit of water volume, will determine the bleeding characteristics and 

 workability of concrete and grout. A low ratio of surface area of solids 

 to volume of water results in a thin and watery paste; consequently, the 

 aggregate particles are only slightly separated and the mixture lacks 

 plasticity and tends to segregate. The ratio of surface area of solids to 

 volume of water may be increased by increasing the amount of cement or by 

 adding a suitable mineral admixture to the mix. Admixtures that are 

 relatively chemically inert, such as ground quartz or limestone, cemetitious 

 materials such as natural cements, hydraulic limes or slag cements, and 

 pozzolans are commonly used. 



(12) Impermeability Admixtures . Concrete and grout are not 

 impermeable to the penetration of water; however, the terms "waterproofing" 

 or "damp proofing" have come to mean a reduction of rate of penetration of 

 water into dry concrete and grout. Admixtures comprised of fatty acids, 

 usually calcium or ammonium stearate or oleate, which also cause air 

 entrainment during mixing. Also used are mineral oils, asphalt emulsions, 

 and certain cut-back asphalts. 



(13) Corrosion Inhibiting Admixtures . In the manufacture of 

 certain concrete products containing steel, it might be desirable to 

 accelerate the rate of strength development by use of both a chemical 

 accelerator and heat. The latter is usually in the form of steam at 

 atmospheric pressure. When calcium chloride is used as the accelerator in 

 this type of curing, laboratory studies have found the rate of corrosion 

 of the steel to be accelerated. However, Arber and Vivian (1961) found 

 that certain compounds containing an oxidizable ion such as stannous 

 chloride, ferrous chloride, and sodium thiosulfate, act as accelerators as 

 does calcium chloride, but also appear to cause less corrosion than the 

 latter. Stannous chloride appeared to be the best of the products tried 

 and 2 percent of the salt by weight of cement was more effective than 1 

 percent, and as effective as greater amounts, both from the standpoint of 

 acceleration and resistance to corrosion. For effective use, the salt 

 must be added to the concrete in the stannous form and a dense concrete 

 must be used. 



(14) Color Admixtures . Pigments are often added to produce color 

 in the finished concrete. The requirements of suitable coloring admixtures 

 include: 



(a) color fastness when exposed to sunlight; 



(b) chemical stability in the presence of alkalinity produced 

 in the set cement; 



(c) no adverse effect on setting time or strength develop- 

 ment of the concrete; and 



(d) stability of color in autoclaved concrete products during 

 exposures to the conditions in the autoclave. 



Pigments frequently used to color concrete are: 



(a) Grays to black--black iron oxide, mineral black, carbon black, 



94 



