Water reduction, effect

Appropriate ranges of water reduction effectiveness (%) Nomenclature... 

As seen in the Fig. 4.4,t l the water-cement ratio of latex-modified concrete at a given slump is markedly reduced with an increase in the polymer-cement ratio. This water reduction effect is found to contribute to strength development and a drying shrinkage reduction. 

In the extensive literature on polymer drag reduction, it has occasionally been reported that a continuous thread of a high-concentration polymer solution injected into the axis of a pipe produces a drag-reduction effect on the water flow in the pipe [856]. The thread seems to persist through the length of the pipe and little, if any, diffusion of polymer to the walls of the pipe is apparent. 

Table 1.13 Effect of addition level of water-reducing admixtures on the water reduction ...

Table 1.14 Effect of varying the point of addition on workability and/or water reduction (after Dodson) ...

Table 1.15 Effect of water-reducing admixtures (0.1% by weight of cement) on the water reduction at a given workability (cement content = 300 kg m- ) (Maniscalco and Collepardi)...

Air-entraining water-reducing admixtures require special consideration the presence of entrained air leads to a reduction in compressive strength, whilst the water reduction results in a compensatory increase in strength. The effect can be quantified, however, by considering the amount of entrained air in terms of an equivalent volume of water to calculate the (air and water)-cement ratio. This new factor can be used to estimate the expected strength from Fig. 1.37. 

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. 

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]. 

One of the significant limitations of the use of first- and second-generation superplasticizers in ready-mixed flowing concrete is the rapid decrease in the initially achieved high workability and this constitutes one of the chief constraints to their wider acceptance. Therefore, a number of major producers of admixtures have sponsored active research to improve the workability retention characteristics of their superplasticizers. Some recent developments [48] have shown promise, among these are materials based on acrylate polymers (AP). The AP-based materials are reported to be more effective than SNF- or SMF-based surplasticizers in terms of water reduction, slump increase and slump retention. Figure 7.19 shows the remarkable improvement in the retention of workability produced by the AP type superplasticizer compared to an SNF type. 

High water reduction and plasticizing effects for concretes where low water-cement ratios and minimum cement contents are specified. 

The effectiveness of each admixture may vary, depending on its concentration in the concrete, the time of addition in the mixing cycle and various constituents of the concrete. Although each class of admixture is defined by its main effect (i.e. water reduction, set acceleration), it may have one or more secondary effects (retardation of set, increased bleeding, air entrainment) and its use may result in side effects. Side effects are those modifications of properties produced in the concrete that, even though unsought, are both inevitable and independent of an admixture s main function. Prior to selecting an admixture for an intended application, these... 

Reports of incompatibility between Portland cement and chemical admixtures have increased over the past 10 years. Rapid set, accelerated stiffening, increase in time of set, lack of water-reduction are some of the reported problems. Such effects produced by the interaction of the composition of the cement and that of the chemical admixtures has been often referred to as cement-admixture incompatibility . This is discussed below. 

Reddy GV, Singh RP (1985) Drag reduction effectiveness and shear stability of polymer-polymer and polymer-fiber mixtures in redrculatory turbulent flow of water Rheol Acta 24 296... 

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