Superplasticizers water reduction

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. 

With the advent of high-performance concrete and the development of third-generation (mostly acrylic polymer-based) superplasticizers which provide significantly higher water reduction at flowable consistencies, this demarcation has blurred. Through the use of a mixture of admixtures it is now possible to obtain highly durable low water-cement ratio concretes that are nearly self-leveling and yet quite cohesive. 

Many concrete producers use a combination of superplasticizer and a conventional water-reducing, set-modifying or air-entraining admixture to achieve the desired performance. The superplasticizer provides the major portion of the required water reduction and the conventional admixture is added to achieve one or more of the following objectives (1) further water reduction (2) admixture economy (3) the desired air content (4) increased workability and (5) extension of set and workability. Coimnercial conventional admixture formulations used for this purpose are usually based on sodium lignosulfonates, hydroxycarboxylic acids or processed carbohydrates. Such combinations, besides reducing the dosage of the... 

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. 

Higher water reductions afforded by the use of superplasticizers enable the production of concrete having normal workability (75-90 mm slump) but... 

Previous work on superplasticized Portland cement concrete containing fly ash or blast furnace slag has shown that such mixes require 10% less admixture than reference Portland cement concrete to attain the same workability. Therefore, a given dosage may produce higher water reduction. The reason for the reduced admixture requirement has not been determined. It is probably due to the lowering (dilution) of the C3A content... 

Most often precast producers use finely ground cement such as ASTM Type III to obtain desired production cycles. The high water reduction and concomitant rapid strength gain afforded by the use of superplasticizers will enable the precast producer to change to the more economical Type I cement (Table 7.8). 

More substantial increases in the rate of strength development are obtained by the use of superplasticizers. As mentioned previously, because of the dramatic water reduction that can be achieved by the use of higher dosage of the superplasticizers, significant cement reduction or a change in the type of cement (from a high early Type 111 to a normal Type 1) is possible (Tables 7.28, 7.29 and 7.7). 

Higher water reductions produce cohesive mixes which are less susceptible to segregation, laitance and washout of cement paste. Superplasticizers permit the use of wider pipes for faster placing and allow cement reductions so that less heat is generated. 

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