Contents 5 Concrete Additives

Commercial products vary from 25 to 45% w/w solids contents and addition levels required to produce concrete of almost self-compacting properties would be 1.0-3.0% by weight of cement. 

Concrete Additives. Melamine-formaldehyde resins can be sulphonated and then used in fresh concrete to create a plasticizing effect. The resin makes 1) concrete more flowable, thus improving concrete s workability and 2) allowing the water content to be reduced for faster drying times. The end result is an improved, stronger material that is structurally superior. 

The percolation argument is based on the idea that with an increasing Cr content an insoluble interlinked cliromium oxide network can fonn which is also protective by embedding the otherwise soluble iron oxide species. As the tlireshold composition for a high stability of the oxide film is strongly influenced by solution chemistry and is different for different dissolution reactions [73], a comprehensive model, however, cannot be based solely on geometrical considerations but has in addition to consider the dissolution chemistry in a concrete way. 

The specific electrical resistance of concrete can be measured by the method described in Section 3.5. Its value depends on the water/cement value, the type of cement (blast furnace, portland cement), the cement content, additives (flue ash), additional materials (polymers), the moisture content, salt content (chloride), the temperature and the age of the concrete. Comparisons are only meaningful for the... 

Unfortunately, the protection provided by concrete can be overcome by contamination of the concrete by chloride. Chloride, when entering the concrete as a contaminant of the mix constituents, is to a large extent (about 90%) complexed within the cement matrix and only a small percentage is free in the pore solutions. The present codes of practice ban the use of chloride-bearing additives and restrict the amount of chloride present in concrete. For normally reinforced concrete made with ordinary Portland cement it should be not more than 0.4% chloride ion with respect to the cement content weight/weight. 

The initial plastic state of the fresh concrete subsequent to the mixing process, where properties such as the air content, density and workability are normally measured by relevant standard tests, and utilized as a means of control of production. The magnitude of these properties is affected by the addition of water-reducing admixtures, either intentionally or as a side effect, which could result not only in a change in the characteristics in the plastic state, but could also be reflected in changed properties in the hardened state. 

When a normal, accelerating, or retarding water-reducing admixture is utilized to increase the workability of a concrete mix by direct addition, it would be reasonable to assume that the extent of the effect would be markedly affected by changes in mix design parameters such as cement content, aggregate size, shape and grading, and the water-cement ratio. A study of many hundreds of results, however, indicates that this is not the case and Fig. 

The temperature of the concrete has a significant influence on the amount of air entrained in concrete by a standard addition level of admixture the higher the temperature, the lower the air content. A typical set of results is shown in Fig. 3.19 [23]. The effect is more marked at higher slump values. 

Fig. 3.20 The effect of sand content on the air entrainment of concrete at two addition levels of air-entraining agent, the fineness modulus (FM) of the sand also being varied (Craven).

The presence of entrained air clearly makes the concrete more cohesive and allows a reduction in the quantity of fine aggregate, without increasing the tendency to segregate. The quantity of sand that can be removed from a concrete that is satisfactory in terms of cohesion prior to air entrainment is approximately 20 kg m 3 for each 1% of additional air content. 

The use of superplasticizers in air-entrained concrete has caused much debate. Two main problems are associated with superplasticized air-entrained concrete (1) a decrease in air content by 1-3% when slump is increased from 75 mm to 220 mm after the addition of the superpiasticizer to create flowing concrete, and (2) a change in the air void system to less desirable values. However, most investigators [10-11, 12] have shown that, although the air-void spacing factor required for adequate frost resistance is altered, the change did not necessarily affect the freeze-thaw durability of... 

Factors which affect the dosage rate are concrete temperature, initial slump (i.e. slump before the addition of the superplasticizer), cement type and content, the presence of other admixtures in the mix prior to the addition of the superplasticizer and the sequence of addition to the mix. The... 

Often it is difficult to adjust the mixture proportions to achieve desired design parameters for all properties of concrete. Consequently the properties of colloidal underwater concrete are controlled by the addition of three chemical admixtures. Minimum water-cement ratios range from 0.36 to 0.40. Cement and fine-aggregate contents are usually higher than corresponding mixes placed on land, and silica fume may be used in conjunction with a superplasticizer or conventional water reducers to reduce segregation. The key to a non-dispersible concrete with self-leveling characteristics is the successful optimization of the VEA with the superplasticizer used to increase the slump. 

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