Water-reducing agents effect

In the case of lignosulfonate water-reducing agents, the effectiveness in reducing the water-cement ratio diminishes with an increase in either the the C3A or alkali content. In a comparative experiment with three... 

The recorded data on lignosulfonate water-reducing agents indicate that, as far as freeze-thaw durability is concerned, because of the low water-cement ratios possible, an enhancement to the durability will invariably be obtained. When the admixtures are used to effect a reduction in the cement content, there are strong indications that a considerable enhancement of durability is obtained, presumably due to a reduction in the cement matrix which is the part of the concrete susceptible to frost damage. The higher aggregate content would therefore allow easier dissipation of stresses. 

Table 1.28 Effect of a lignosulfonate-based water-reducing agent on reinforcement bond under an appiied external voltage (after Kondo)...

Fig. 1.50 The effect of a hydroxycarboxyiic acid water-reducing agent on the creep of concrete when used to lower the water-cement ratio (Rodrigues).

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. 

The effect produced by the incorporation of a lignosulfonate-based water-reducing agent is shown in Table 7.6. The results were obtained from a series of mixes over an 8-month period by a ready-mix plant used in the production of concrete piles. Since the standard deviation of this particular plant was 5.0 MPa for mixes produced without the use of admixtures, it is evident that the use of the admixture resulted in reduced variability. These results indicate that in high workability mixes with cement contents in the median range, the admixture may cause an increase in the standard deviation. Thus in redesigning the mix to have a lower cement content in this class of concrete, adequate consideration should be given to this difference in standard deviation. Increased uniformity can be attained in this instance... 

However, it is felt that the intrinsic property of most water-reducing admixtures is to increase the creep and shrinkage of the concretes into which they are incorporated. This is minimized by utilizing the effect of the admixture to reduce the cement and, therefore, the paste content of the mix. For corresponding mixes with lignosulfonate water-reducing agents, the FIP/CEB recommendation for creep computation can be used with confidence because of the incorporation of a cement content/water-cement ratio function in the calculation. 

Earlier work [37] by one of the authors indicated that superplasticizers of the SNF and SMF type were less strongly adsorbed onto the hydrating cement than normal water-reducing agents and this was used to explain why there was less retardation by the superplasticizers. This theory was flawed, in that later work showed that the part of the SNF that was effective (the polymer)... 

An increase in the flow of the mix will have a beneficial effect on compaction and, as a result, compressive strength will improve. The presence of the water-reducing agent may also improve the early strength by affecting the setting and hardening rate of such mixes. 

Figure 4- exhibits the effects of nonionic surfactant content, HLB and antifoamer on the flexural and compressive strengths of epoxy-modified mortars with a modified polyamide-amine hardener content of 55%> a polymer-cement ratio of 4-0% and a water-reducing agent content of 3 0 %, which are the optimum values recommended from the above test results. The flexural strength of the epoxy-modified mortars with a nonionic surfactant of HLB 9 5... 

Effects of water-reducing agent content and polymer-cement ratio on flexural and compressive strengths of epoxy-modified mortars. 

Epoxy-modified mortars which are prepared by mixing most popular bisphenol A-type epoxy resin with commercial modified polyamide-amine hardener, polyalkyl aryl sulfonate-type water-reducing agent, polyoxyethylene nonylphenol ether-type nonionic surfactant and silicone emulsion-type antifoamer into cement mortar have excellent properties comparable to ordinary polymer-modified mortars using latex-type cement modifiers[5]. The optimum mix proportions of the epoxy-modified mortars are shown in Table 2. Their disadvantage is a need of much higher polymer-cement ratio than the ordinary polymer-modified mortars. Therefore, the development of low-cost, effective dispersants is expected in the near future. 

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