Hardened concrete

Applicability Most hazardous waste slurried in water can be mixed directly with cement, and the suspended solids will be incorporated into the rigid matrices of the hardened concrete. This process is especially effective for waste with high levels of toxic metals since at the pH of the cement mixture, most multivalent cations are converted into insoluble hydroxides or carbonates. Metal ions also may be incorporated into the crystalline structure of the cement minerals that form. Materials in the waste (such as sulfides, asbestos, latex and solid plastic wastes) may actually increase the strength and stability of the waste concrete. It is also effective for high-volume, low-toxic, radioactive wastes. 

Splitting tensile strength of cylindrical concrete specimens Microscopic determination of parameters of the air-void system in hardened concrete... 

Air content of freshly mixed concrete by the pressure method Air content of freshly mixed concrete by the volumetric method Unit weight, yield, and air content of concrete Specific gravity, absorption, and voids in hardened concrete Resistance of concrete to rapid freezing and thawing Scaling resistance of concrete surfaces exposed to deicing chemicals... 

The presence of entrained air will, of course, be reflected in a reduced density in the plastic and hardened stage and its effect on subsequent properties of the hardened concrete will be discussed later. 

Although there are some anomalies in the literature, it is generally agreed that both types of volume deformation are a function of the same fundamental mechanism and that the influence of other factors such as admixtures will affect both shrinkage and creep in a similar manner. As outlined earlier, water-reducing admixtures can be used to obtain different effects on the plastic/hardened concrete and it is this factor, together with the admixture type, that is important in determining the effect on the volume deformations of concrete. 

Table 2.5 Mixture proportions and properties of fresh and hardened concrete (water reduced)...

Mix no. Properties of fresh concrete Air-Void determinations on hardened concrete ... 

This sequence is shown in Table 3.15, together with the resultant properties of the plastic and hardened concrete. The combined effects of sand and water reductions bring the cement content to approximately that of the comparative plain concrete so that 28-day strengths are similar. However, this is not consistent across all cement contents low-cement-content mixes tend to show an increase in strength, whilst higher cement content will show a slight decrease. This is illustrated by the data in Table 3.16 [31]. 

Dampproofing admixtures are formulated to affect the properties of the hardened concrete, and not those of concrete in its plastic state. In the case of materials based solely on calcium and aluminum stearates, stearic acid in solid or emulsion form, bitumens and hydrocarbon resins, there will be no effect on the properties of the plastic concrete with regard to air content, workability, mix design parameters, etc. When water-reducing admixtures or accelerators are included in the formulation, the effect on the concrete will be a function of the particular type of material used (see relevant section). The wax emulsions do appear to have an effect on the properties of the plastic concrete because of the lubrication effect of the very small... 

Compatibility with other admixtures due to the inherent surfactant properties exhibited by most of the materials in classes A, B and C, their use with other admixtures such as water reducers should be evaluated in terms of the side effects on the plastic and hardened concrete, viz. severe set retardation and excessive air entrainment. The molecules of materials with fewer surfactant properties compete with the superplasticizer for adsorption sites on the cement particle. This will result in a higher dosage of the superplasticizer. 

The application of these chemicals is not restricted to use as admixtures. Japanese investigators have carried out both laboratory and field trials in which the SRA was applied to the surface of the hardened concrete and... 

To improve the quality of hardened concrete such as increased early and long-term strengths and modulus of elasticity, decrease permeability (and hence inhibit corrosion of embedded steel reinforcement) and absorption, increase abrasion resistance and increase bond with reinforcement. 

Properties of hardened concrete Average compressive strength ... 

In bridge construction, the use of retarding admixtures produces uniformity in the rate of setting and lessens the risk of deflection in partially hardened concrete that may occur in continuously reinforced structures such as spandrel beams. The retarded concrete poured over the supports remains plastic until the final pours are placed at mid-span. 

Structures with a high degree of reinforcement are susceptible to loss of bond between steel and concrete. This is particularly true in situations where the vibration required to consolidate the concrete is transmitted along the reinforcement and could cause loss of bond in previously placed partially hardened concrete. The adjustment of admixture dosage as placement proceeds ensures that the concrete remains plastic throughout the entire pouring period. 

Segregation of the mix This mix can occur when larger doses of the admixture are added to offset the lack of initial fluidizing of the concrete or to reinstate workability. The segregation phenomenon is always related to a superplasticizer overdose, beyond the saturation point. In some instances the surface of the hardened concrete is covered with a white layer composed... 

Fig. 7.44 Effect of mixing time after addition of air-entraining admixture (mix 3.1) in combination with superplasticizer (mix 3.2) on the total air content in (a) fresh concrete and (b) hardened concrete (Mark and Gjoerv [131]).

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