Polymer/cement ratio

The properties of a latex depend on the nature of polymers in the latex, particularly the monomer ratio in copolymers and the type and amount of plasticizers. The monomer ratio affects the strengths of the latex modified mortars to the same extent as the polymer-cement ratio [87, 92]. Mechanical and chemical stability, bubbling and coalescence on drying all depend on the type and amount of surfactants and antifoamers and the size of dispersed polymer particles. It is important that the use of selected antifoamers and surfactants as stabilizers or emulsifiers produces no adverse effect on cement hydration. 

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]. 

Although the mix design of latex-modified mortar and concrete is done in much the same way as that of ordinary mortar and concrete, properties such as workability, strength, extendibihty, adhesion, watertightness and chemical resistance are controlled by the polymer-cement ratio rather... 

The presence of the cement hydrate/polymer comatrix in LMM and LMC confers superior properties, such as high tensile and flexural strengths, excellent adhesion, high waterproofhess, high abrasion resistance and good chemical resistance, when compared to ordinary cement mortar and concrete. The degree of these improvements however depends on polymer type, polymer-cement ratio, water-cement ratio, air content and curing conditions. Some of the properties affected by these factors are discussed below [87, 88, 93-95]. 

The water content, required to produce a given slump, can be significantly reduced when a latex admixture is used and the extent of the reduction increases with an increase in the polymer-cement ratio. This effect is due to... 

The nature of the polymer latex is determined by the monomer ratio in the copolymer and this property of the latex affects strength values in manner similar to that obtained with the polymer-cement ratio. The effects of polymer-cement ratio on strength are presented in Table 6.14 [87]. 

Most latex-modified mortars and concretes have good adhesion to most substrates (tile, stone, brick, steel and aged concrete) compared to conventional mortar and concrete. In general, bond strength in tension and flexure increases with an increase in the polymer-cement ratio,... 

Table 6.14 Effect of polymer-cement ratio on mortar properties (Ohama et al.)...

The significant differences in the moduli of latexes and the cement hydrates (elastic moduli 0.001-10 GPa and 10-30 GPa respectively) causes most LMMs and LMCs to have a higher deformability and elasticity than ordinary cement mortar and concrete. Depending on the polymer type and polymer-cement ratio, the deformability and elastic modulus tends to initially increase with an increase in the polymer-cement ratio and subsequently decrease at higher ratios. Poisson s ratio however is only marginally affected [87, 94, 98]. 

The large pores (ranging from 0.01 pm to 0.1 pm) are sealed by the continuous polymer film formed in the comatrix of LMM and LMC. Consequently, they show reduced water absorption, water permeability and water vapor transmission over ordinary cement mortar and concrete and this effect increases with an increase in polymer content and polymer-cement ratio (Fig. 6.15). The improved water permeability also improves the resistance to chloride ion entry and hence corrosion mitigation [87]. 

The reduction in porosity, decreased water content, and air entrainment that results when latexes are used in mortar and concrete mixes make them much more resistant to freezing and thawing conditions than conventional mortar and concrete. Figure 6.17 presents the freeze-thaw durability in water (-18 to 4°C) of combined water-and dry-cured SBR-, PAE- and EVA-modified mortars [98], The frost resistance of mortars made with these latexes is markedly improved even at polymer-cement ratios of 5%. However, an increase in the polymer-cement ratio does not necessarily produce further improvement in freeze-thaw resistance. EMM and EMC, when exposed to outdoor conditions involving freeze-thaw, UV radiation and carbonation show better weatherability when compared with conventional mortar and concrete. 

LMC is used in underwater concrete for both new construction and repair. The important requirements to obtain antiwashout capability, such as segregation resistance, flowability, self-leveling characteristics and lower bleeding are provided by the addition of viscosity-enhancing polymeric admixtures at polymer-cement ratios of 0.2-2.0%. These admixtures are water-soluble polymers, and classified under two groups, viz., cellulose types such as methyl cellulose and hydroxy ethyl cellulose and polyacrylamide types such as polyacrylamide and polyacrylamide-sodium acrylate [101]. 

The values can be affected by the porosity of the substrate, and its service conditions. An increase in abrasion resistance is dependent on the type of latex and polymer-cement ratio used. In general, abrasion resistance is considerably improved with an increase in polymer-cement ratio. 

The pore structure of latex-modified systems is influenced by the type of polymer in the latexes used and polymer-cement ratio as discussed in detail later. The total porosity or pore volume generally tends to decrease with an increase in the polymer-cement ratio. This contributes to improvements in the impermeability, resistance to carbonation, and freeze-thaw durability. 

The mix proportions of mostlatex-modified mortars are in the range of the cement-fine aggregate ratio = 1 2 to 1 3 (by weight), the polymer-cement ratio of 5 to 20% and the water-cement ratio of 30 to 60%, depending on the required workability. The standard mix proportions for the latex-modified mortars for various applications are shown in Table 3.6.[2]... 

The mix proportions of most latex-modified concretes cannot be easily determined in the same manner as those of latex-modified mortars because of many factors considered in the mix design. Normally, the polymer-cement ratio of the latex-modified concrete ranges from 5 to 15%, and the water-cement ratio from 30 to 50%. A rational mix design system developed for the latex-modified concrete by Ohama is described below. 

C, P, W, S, G Weight of cement, polymer, water, sand, and gravel per unit volume of latex-modified concrete, respectively (kg/m ), i.e., unit cement content, unit polymer content, imit water content, unit sand content, and unit gravel content Polymer-cement ratio (by weight)... 

Regardless of polymer type, the compressive strength (ac) of latex-modified concrete can be predicted at polymer-cement ratios of 5, 10,15and 20% by using the binder-void ratio (a) as follows ... 

Polymer-cement ratio (P/C) from 0.05 to 0.20 (5 to 20 wt% of polymer with respect to cement)... 

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