High-workability mixes

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... 

The change in slump due to the addition of water is non-linear, i.e. the slump is more sensitive to water for highly workable mixes. 

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. 

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. 

In the cement concrete industry the recommended practice is to consolidate low slump mixes by vibration and high slump mixes with hand tools. For example, ASTM Method C 192 (6) specifies consolidating test specimens by vibration if the slump is less than 1 in. Relationships between workability and ease of placing sand-asphalt-sulfur mixes are being evaluated on various field projects to assess the workability requirements for mix placement with various types of equipment. 

For high workability or flowing concrete it is important to pay attention to the mix design so that the concrete will not bleed or segregate significantly and so that only the minimum quantity of superplasticizer is required. In particular, it may be necessan/to increase the proportion of fines by perhaps 5%. A trial mix is desirable. (See Bleeding above.)... 

Superplasticizers are often essential in order to obtain durable reinforced-con-crete structures. They allow a high workability, which is essential to achieve proper compaction of fresh concrete especially in slender elements or in the presence of dense reinforcement, without increasing the amount of mixing water. The increase in workabihty is then achieved without changing the w/c ratio and thus without affecting the strength and permeability of concrete. 

The use of WRAs (normal, mid-range and high-range) in ready-mixed and precast concrete to reduce water content and increase workability is described in the following pages. 

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