Permeability water-reducing admixtures

This has already been dealt with in previous parts of this section where it can be seen that a reduction in the water-cement ratio by use of admixtures is beneficial in reducing the permeability of the concrete. Even when cement contents are reduced, whilst maintaining the workability and strength characteristics of corresponding mixes containing water-reducing admixtures, there is no deleterious effect on the permeability. 

The data presented in this section illustrate that, with the exception of those accelerating water-reducing admixtures containing calcium chloride, there is an abundance of evidence to support the conclusion that water-reducing admixtures of lignosulfonate chemical form certainly will not accelerate any kind of corrosion with reinforcement and, when used to reduce the water-cement ratio, will form a more permeable and durable protective cover for the reinforcement. In view of the chemical nature of the other types of materials such as the hydroxycarboxylic acids and hydroxylated polymers, it seems most likely that these materials too would have no deleterious effect in this respect. 

In water-retaining structures or basement concrete subject to high hydrostatic pressure, materials of this type are generally not beneficial. However, some dampproofing admixtures do contain water-reducing admixtures and will result in a reduction in permeability under an applied hydrostatic head. In addition, the reduced capillary size and quantity will increase the hydrostatic pressure required to enter the concrete surface (see later). 

The permeability of concrete is a guide to its durability (Section 1.5.2) but it can also be relevant to the design of structures which are intended to withstand a hydraulic head of water or other liquid. Extreme porosity is usually due to continuous passages in the concrete, due to poor compaction or cracks which can be minimized by the use of water-reducing admixtures to give increased workability whilst maintaining a low water-cement ratio. 

A normal water reducer lowers the water requirement to attain a given slump. Thus, for the same slump and a constant cement content use of lower w/c ratios, results in general improvement in strength, permeability, and durability. Alternatively, the desired slump is achieved without a change in w/c ratio by lowering the cement content. A water-reducing admixture may also be used to increase the slump to facilitate placements. According to ASTM, water reducers should be able to lower the water requirements by 5% of the control. The water reduction depends on the... 

Damp-proofing admixtures reduce the rate of adsorption of moisture into the pores of the concrete and retard the rate of transmission of water through unsaturated concrete. When combined with a water reducer such as a lignosulphonate they enable these damp-proofing properties to be combined with the benefits of denser and less permeable concrete achieved by water reduction. 

ASTM C 494 (Specification for Chemical Admixtures for Concrete) classifies several types of concrete admixtures. Of particulcir use for steel in concrete are the high range water reducers that are classified as type F or G admixtures. These chemicals significantly improve concrete workability at low w/c ratios, which is needed to produce low permeability concrete. 

They are applied on roofs, slabs on ground, basements, water-retaining structures, concrete blocks, and clay bricks. Waterproofing admixtures reduce the permeability of concrete. The dampproofing admixtures impart water repellency and reduce moisture migration by a capillary action. Examples of these admixtures are soaps and fatty acids which react with cement, conventional water reducers, methyl siliconates, etc. 

Admixtures are sometimes used to reduce permeability of concrete (80—82). These include pore-filling materials such as chalk. Fuller s earth, or talc water repellents such as mineral oil, asphalt, or wax emulsions organic polymers (acryHc latexes, epoxies) and salts of fatty acids, especially stearates. 

Air entrainment generally improves durability by reducing permeability. The resistance of hardened concrete to the action of frost and de-icing salts is considerably improved by the use of air-entraining admixtures. This is-achieved by the entrained-air bubbles acting as expansion chambers to accommodate the ice formed within the capillaries. Because the bubbles break up the continuity of the capillaries they also reduce permeability and water adsorption. 

Otherwise, for a w/c ratio < 0.5 only reduced water flow is possible. Then there is enough water for hydration and also enough space for hydration products, which swell during the process so that the capillary pores may have reduced permeability. These values for mortars and concretes depend on the mixture proportions and quality of components, and also on special admixtures added to modify the fluidity of the fresh mix. 

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