Concrete air-entrainment

Table 3.14 Bleeding of plain concrete, air-entrained concrete and air-entrained concrete containing a water-reducing agent...

Table 3.22 Freeze-thaw resistance of concrete air-entrained by different chemical types of admixture...

Total water—cement ratio Plain concrete Air-entrained concrete (4%) ... 

Admixtures are substances that are added during the mixing process in small quantities related to the mass of cement, in order to improve the properties of fresh or hardened concrete [7,8]. The most utilised admixtures are water reducers and superplasticizers that may be added to improve the workability of concrete or reduce the amount of mix water accelerators that are used to increase the rate of development of early strength of concrete set-retarders that reduce the setting time of concrete air-entraining agents that increase the freeze-thaw resistance of concrete (Section 3.1). Recently, corrosion inhibitors have been developed in order to increase the corrosion protection of embedded steel these will be treated in Chapter 13. 

Air-entrainment is the process whereby many small air bubbles are incorporated into concrete. Air-entrainment is essential for increasing the durability of concrete exposed to freeze thaw conditions. Air-entrainment improves workability of concrete and also decreases bleeding and segregation. Salts of wood resins, synthetic detergents, salts of sulfonated lignin, salts of petroleum acids, salts of proteinaceous materials, fatty and resinous... 

Naphthalenesulfonic acids are important chemical precursors for dye intermediates, wetting agents and dispersants, naphthols, and air-entrainment agents for concrete. The production of many intermediates used for making a2o, a2oic, and triphenylmethane dyes (qv) involves naphthalene sulfonation and one or more unit operations, eg, caustic fusion, nitration, reduction, or amination. 

Type V (High Sulfate Resistance). Type V Pordand cement is used in concrete exposed to severe sulfate attack of 1,500 to 10,000 ppm. Low concentrations of tricalcium aluminate [12042-78-3] give Type V its sulfate resistance. The sulfate resistance is improved with air entrainment and low water to cement ratios in the wet concrete. U.S. production of Type V Pordand cement in 1989 was 0.9% of the total Pordand cement production. 

Air-Entrainment Agents. Materials that are used to improve the abiUty of concrete to resist damage from freezing are generally known as air-entrainment agents. These surfactant admixtures (see Surfactants) produce a foam which persists in the mixed concrete, and serves to entrain many small spherical air voids that measure from 10 to 250 p.m in diameter. The air voids alleviate internal stresses in the concrete that may occur when the pore solution freezes. In practice, up to 10% air by volume may be entrained in concrete placed in severe environments. 

When concrete hardens, the cement paste shrinks. The gravei, of course, is rigid, so that smaii shrinkage cracks are created. It is found that air entrainment (mixing small bubbles of air into the concrete before pouring) helps prevent the cracks spreading. 

Fig. 1.3 The effect of air entrainment on compressive strength of concrete containing a water-reducing air-entraining agent and a normal air-entraining agent.

The presence of a water-reducing admixture can alter the air content of concrete, either as a deliberate measure (the air-entraining water-reducing admixtures) or as a side effect of the material in lowering the surface tension of the aqueous phase. 

It can be concluded from the assessment of the data in this section that inclusion into a concrete mix of a water-reducing admixture of the lignosulfonate, hydroxycarboxylic acid and air-entraining type should not lead to any deterioration in the durability of that concrete to freeze-thaw cycling. Indeed there are strong indications that, when used either as a means of reducing the water-cement ratio or, alternatively, of reducing the cement content, more durable concrete may result. 

The superplasticizers are a special category of water-reducing agents in that they are formulated from materials that allow much greater water reductions, or alternatively extreme workability of concrete in which they are incorporated. This is achieved without undesirable side effects such as excessive air entrainment or set retardation. 

When superplasticizers are used to reduce the water-cement ratio, normally any increase in air content will be minimal, especially in high-cement-content mixes. When it is a requirement to intentionally air-entrain such mixes, the dosage required to obtain a given air content is often considerably increased, presumably because of the reduced aqueous medium in the concrete. 

In the early days of the use of superplasticized concrete, some concerns were aired regarding the resistance of air-entrained concrete containing superplasticizers to freeze-thaw cycling. However, more recent research has indicated the following ... 

---