Air Content and Compressive Strength

There was some speculation about whether those 179 kg per cubic metre rubber concrete pads (Mix-6) might disintegrate in any minute since its compressive strength was very low (see Table 11 ) as discovered later. But in the end, those pads still did what concrete was supposed to do, and have held up well since, though, with a very different and somewhat revealing characteristic in comparison with controlled concrete. 

The SRP project was to repair water cannel by spraying rubber shot-crete. The rubber content was 104.7 kg per cubic metre and the relevant information is given in Table 11.3, which again shows a high measured air content. 

Type Unit Weight, kg.m Slump, cm Air, % ASTM C173 [15] Air, % ASTM C231 [14] Strength 3rd day, MPa Strength 7th day, MPa Strength 28th day, MPa 

Total weight Volume excluding air, m Unit weight, g/cm Air,ASTMCl73 [15] 

Besides the two cases given here, it has been consistently observed that rubber crumbs do bring air into concrete, though the quantification can be very difficult. This increase in air content may act as a major contribution to the loss of compressive strength. 

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Suitable antifoamers are usually added to the latexes to prevent excess air entrainment. Figure 4.15 ° shows the effect of silicone emulsion-type antifoamer on the air content and compressive strength of the latex-modified mortars. Increased antifoamer content causes a pronounced decrease in the air content and an increase in the compressive strength. 

Figure 4.15 Effect of silicone emulsion-type antifoamer on air content and compressive strength of latex-modified mortars.

Surfactants enable the polymer particles to disperse effectively without coagulation in the mortar and concrete. Thus, mechanical and chemical stabilities of latexes are improved with an increase in the content of the surfactants selected as stabilizers. An excess of surfactant, however, may have an adverse effect on the strength because of the reduced latex film strength, the delayed cement hydration and excess air entrainment. Consequently, the latexes used as cement modifiers should have an optimum surfactant content (from 5 to 30% of the weight of total solids) to provide adequate strength. Suitable antifoamers are usually added to the latexes to prevent excess air entrainment increased dosages causes a drastic reduction in the air content and a concurrent increase in compressive strength [87, 92-94]. 

A CN-based corrosion inhibitor increases the early-age compressive strength. The effect on the ultimate strength was observed to be dependent on the amount of inhibitor added to the concrete. A CN-based corrosion inhibitor increases marginally the air content and the workability of the fresh concrete. 

At equal dosages, all water reducers are effective in producing concrete of equal or higher compressive strength than that of reference concrete. At equal cement content, air content, and slump, the water reducers increase the 28 day strength by about 10-20%. Table 9 shows the influence of lignosulfonate type admixture on the compressive strength of concrete.Water reduction varies between 5 and 8%. 

Unit weight, yield, and air content of concrete California Bearing Ratio (CBR) of laboratory-compactive soils Unconsolidated undrained compressive strength of cohesive soils in triaxial compression... 

Concrete which is produced using fine aggregates deficient at the fine end of grading, e.g. sea dredged aggregates, exhibit a tendency to bleed and segregate. The presence of a small amount of entrained air (2-4% by volume) leads to an improvement in cohesion, or mix stability. Alternatively, with mixes which are adequate in this respect, a reduction in sand content can be made when air is entrained without loss of cohesion. The amount that can be removed is approximately equal on a volume basis and leads to a reduction in water-cement ratio to minimize the effect of entrained air on compressive strength. 

Such effects increase with an increase in the polymer content or the polymer-cement ratio (the weight ratio of total solids in a polymer latex to the amount of cement in a latex-modified mortar or concrete mixture). However, at levels exceeding 20% by weight of the cement in the mixture, excessive air entrainment and discontinuities form in the monolithic network structure, resulting in a reduction of compressive strength and modulus [87, 94, 95]. 

Damp-proofing admixtures include soaps and fatty acids which react with the cement hydrates to modify workability, bleeding and settlement, air content, compressive strength and durability characteristics. Mix proportions, mix consistency, admixture dosage and poor mixing influence the effects produced by the admixture. In cement-rich mixes void content is often increased, resulting in increased permeability. Since the admixture... 

Masonry cement was developed to overcome the above problems. It is specified in ENV 413-1 [9.4]. Essentially it consists of Portland cement (at least 25 % in Type MC 5 and at least 40 % in the other Types) and an inorganic material (such as limestone), finely ground to more than 85 % passing 90 microns. Two of the four Types contain an air-entraining agent, while the other Types do not. The Standard specifies the characteristics of the cement, the fresh mortar and the compressive strengths. The limestone content of masonry cement can be over 70 % in Type MC 5 and over 55 % in the other Types. 

After the drying and modification stages, water is added, if required, to obtain the required moisture content for consolidation (i.e., just wet of the optimum moisture content [26.11]. The soil is then compacted to reduce the level of air voids to not more than 5 %. This ensures that the stabilisation reaction proceeds in the compacted state and results in a homogeneous, impermeable and stable layer. The stabilised layer has a low and acceptable shrink-swell potential, and improved compressive, tensile and flexural strengths. It also reduces the susceptibility of the stabilised layer to frost damage. 

WagnerP" expanded Powers and Brownyard s theory for ordinary cement paste,and developed a general expression to predict the compressive strength of latex-modified mortars, using the water-cement ratio and the content of entrained air ... 

For the purpose of developing the equations for the compressive strength prediction for latex-modified mortars and concretes, all-inclusive consideration of various factors such as polymer-cement ratio, water-cement ratio, and air content is required. Expanding Talbot s void theoryP on ordinary cement mortar and concrete, OhamaP P l defined binder-void ratio (a) or void-binder ratio (P), and empirically proposed the equations using a and p to predict the compressive strength of the latex-modified mortars and concretes as follows ... 

An amino- and ester-based organic inhibitor causes a decrease of the compressive strength by about 10%-20%. The air content was slightly increased. 

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