Question

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Problem 2 1. What are the differences between a normal, mid-range water and a high-range water reducing admixture (superplasticizer)? (Reference: PCA 15th edition page 123) is to use water in exces of what is needed for cement bydration to maintain an adequate workability. What are the adverse effects of this excess water on harden concrete properties? (References: IMCP manual page 58 and PCA 15th edition page 125) 3. How can the addition of water reducing admixtures into concrete can control these adverse effects? (References: IMCP manual page 58 and PCA 15th edition page 125) 4. The water content calculated for a control mix is 250 lb. As per the ASTM C 494 requirements, what is the maximum water content allowed in a mix with a water-reducing admixture (Type A) under same conditions? 5. Suppose the waterreducing (Type A) admixture was added to concrete without reducing the original water content of 250 lb. What is the most significant impact of this action on fresh concrete properties? (Reference: PCA 15th edition page 125)

Answer 1

1.

Normal Water Reducers	Mid-Range Water Reducers	High-Range Water Reducers
When used as a water reducer it can reduce the water content by approximately 5% to 10%.	Mid-range water reducers provide significant water reduction between 6% and 12% for concretes with slumps of 125 mm to 200 mm (5 in. to 8 in.) without the retardation associated with high dosages of conventional (normal) water reducers.	A water reduction of 12% to 40% can be obtained using High-range water reducers admixtures.
It may be used as plasticizers to provide a moderate increase in workability.	Mid-range water reducers can be used to reduce stickiness and improve finishability, pumpability, and placeability of concretes containing silica fume and other supplementary cementing materials.	It can greatly reduce water demand and cement contents and make low water-cement ratio, high-strength concrete with normal or enhanced workability and to generate slumps greater than 150 mm (6 in.).
Normal water reducers are intended for concretes with slumps up to 100 mm or 125 mm but they are also used in combination with midrange and high-range water reducers in higher slump concrete mixtures.	Some can also entrain air and be used in low slump concretes (Nmai)

2.

Adverse effect of excess water in hardening of concrete:

Lower compressive strengths.

Segregation of the concrete mix under certain conditions resulting in variable quality throughout the concrete mass.

Cracking - with too much water, there will be lower tensile strength, and a tendency towards high shrinkage and subsequent cracking.

Dusting and scaling - bleeding of excess water brings too many fines to the surface of floors.

Sand streaks - excess water bleeding up the sides of forms washes out cement paste and leaves an unsightly streaked surface.

Contamination - too much water in concrete placed on grades causes contamination from the subgrade with the concrete leading to an array of quality problems.

Permeability - voids left as excess water evaporates invite water to seep through walls and floors.

3.

Adding a water-reducing admixture to concrete without also reducing the water content can produce a mixture with a higher slump. The rate of slump loss, however, is not reduced and in most cases is increased with the exception of polycarboxylate technology. Rapid slump loss results in reduced workability and less time to place concrete.

High-range water reducers are generally more effective than regular water-reducing admixtures in producing workable concrete. The effect of certain plasticizers in increasing workability or making flowing concrete is short lived, 30 to 60 minutes. This period is followed by a rapid loss in workability or slump loss. High temperatures can also aggravate slump loss. Due to their propensity for slump loss, these admixtures are sometimes added to the concrete mixer at the jobsite. They are available in liquid and powder form. Extended-slump-life plasticizers added at the batch plant help reduce slump-loss problems. An increase in strength is generally obtained with water reducing admixtures as the water-cement ratio is reduced. For concretes of equal cement content, air content, and slump, the 28-day strength of a water-reduced concrete containing a water reducer can be 10% to 25% greater than concrete without the admixture. Using a water reducer to reduce the cement and water content of a concrete mixture, while maintaining a constant water-cement ratio, can result in equal or reduced compressive strength, and can increase slump loss by a factor of two or more.

Water reducers decrease, increase, or have no effect on bleeding, depending on the chemical composition of the admixture. A significant reduction of bleeding can result with large reductions of water content; this can result in finishing difficulties on flat surfaces when rapid drying conditions are present. Tests have shown that some plasticized concretes bleed more than control concretes of equal water-cement ratio but plasticized concretes bleed significantly less than control concretes of equally high slump and higher water content. Despite reduction in water content, water-reducing admixtures may cause increases in drying shrinkage. Usually the effect of the water reducer on drying shrinkage is small when compared to other more significant factors that affect shrinkage cracking in concrete. High-slump, low-water- content, plasticized concrete tends to develop less drying shrinkage than a high-slump, high-water-content conventional concrete. However, high slump plasticized concrete has similar or higher drying shrinkage than conventional low-slump, low-water-content concrete.

4.

Normal water reducers when used as a water reducer, normal range, or conventional water reducers can reduce the water content by approximately 5% to 10%.

5.

Adding a water-reducing admixture to concrete without also reducing the water content can produce a mixture with a higher slump. The rate of slump loss, however, is not reduced and in most cases is increased, with the exception of polycarboxylate technology. Rapid slump loss results in reduced workability and less time to place concrete.