Discover the versatility of 9 Types of Concrete Admixtures Most Commonly Used in this comprehensive guide. Learn how these additives enhance concrete properties for better construction outcomes.
Best Concrete Admixtures Commonly Used
Concrete admixtures, often unsung heroes in construction, play a pivotal role in shaping the durability and performance of concrete structures. In this guide, we look into the 9 Types of Concrete Admixtures Most Commonly Used, unlocking the secrets behind their applications and benefits.
In order to alter the qualities of the fresh or hardened concrete in any manner, an admixture, a substance, that is typically a liquid, is added to a batch of concrete during mixing in a quantity not more than 5% (ranges may vary from as low as 0.5% up to 5%) of the cement content of the concrete. A number of specific improvements are often added to the concrete by most admixtures, such as lowering the amount of free water required for a particular consistency level, etc. This decreases permeability and increases durability.
Admixtures are sometimes not only beneficial to utilize but absolutely necessary. Admixtures ought to be utilized only in situations where a high level of control is possible, as they are introduced to concrete mixtures in tiny amounts specified above. Concrete’s strength and other qualities may be negatively impacted by an admixture whose quantity in the concrete is more than the required dosage. Guidance for the requirements of admixtures can be found on BS EN 934-2. In this article, I will show the nine (9) most commonly used admixtures for concrete including examples of such.
This type of admixture can be classified into three: plasticizers, superplasticizers, and mid-range plasticizers.
Plasticizers or normal water-reducing admixtures or workability aids: This type of admixture permits the reduction in the water content of a given concrete mix without affecting the consistency or increasing the slump or flow of a given concrete mix without affecting the water content. Generally, plasticizers achieve their purpose by either increasing the slump of high-strength concrete by about 50 mm without affecting its strength to ensure the concrete is easy to place or by reducing the water content of concrete by about 10% without affecting the consistency especially in concrete prone to bleeding or segregation such as concrete made with angular aggregates or concrete made with low sand content or sand with low fines content. Examples of plasticizers are calcium, and ammonium lignosulphonates.
SuperPlasticizers or high-range water-reducing admixture: Similar to the plasticizers, superplasticizers are expected to achieve a high reduction in the water content of concrete without affecting the consistency or considerable increase in slump/flow of the concrete without affecting the water content. A flowing concrete is usually obtained by first producing concrete whose slump is in the range 50–90 mm, and then adding the superplasticizer, which increases the slump to over 200 mm. This is typically applied where reinforcement is particularly congested, making both placing and vibration difficult; and where large areas, such as slabs, would benefit from a flowing easily placed concrete.
When used to produce high-strength concrete, reductions in water content of as much as 30% can be obtained by using superplasticizers, compared to 10% with normal plasticizers: as a result, 1-day and 28-day strengths can be increased by as much as 50%. Such high-strength water-reduced concrete is used both for high-performance in situ concrete construction and for the manufacture of precast units, where the increased early strength allows earlier demoulding. Examples of superplasticizers are acrylic polymers, poly carboxylate, mulit-carbovylatethers.
Mid-range Plasticizer: This type of plasticizer can be used where either normal plasticizers or superplasticizers are not suitable. mid-range plasticizers reduce the water demand by up to 15%.
2. Accelerating Admixtures (accelerators)
Accelerating admixtures, also known as hardeners or anti-freezers act by increasing the initial rate of chemical reaction between the cement and the water so that the concrete stiffens, hardens, and develops strength more quickly. They have a negligible effect on consistency, and the 28-day strengths are seldom affected. Accelerating admixtures have been used mainly during cold weather, when the slowing down of the chemical reaction between cement and water at low temperatures could be offset by the increased speed of reaction resulting from the accelerator. Examples of accelerating admixtures are calcium chloride (prone to corrode steel because of chloride content), calcium formate, triethanolamine.
3. Retarding Admixtures
These slow down the initial reaction between cement and water by reducing the rate of water penetration to the cement. By slowing down the growth of the hydration products, the concrete stays workable longer than it otherwise would. The length of time during which concrete remains workable depends on its temperature, consistency class, and water/cement ratio, and on the amount of retarder used. They are used in warm weather, when the ambient temperature is higher than about 20oC, when a large concrete pour, will take several hours to complete, When the complexity of a slip-forming operation requires a slow rate of rise, and when there is a delay of more than 30 minutes between mixing and placing. Examples of retarding admixtures are calcium sulphate, starch, and salts of acids.
4. Pozzolanic Admixtures
Pozzolans are a broad class of siliceous or siliceous and aluminous materials which possess little or no cementitious properties but when they are in finely divided form and subjected to hydration reaction with water, they react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties such as calcium silicate hydrate. Pozzolans or pozzolanic materials used as admixtures could be naturally occurring or manufactured artificially. Naturally occurring ones are shale, pumicite, volcanic stuffs, etc. Artificial pozzolans include rice husk ash (RHA), bone ash (BA), groundnut shell ash (GSA), oil palm empty fruit bunch ash (OPEFBA), coconut coir ash (CCA), silica fume, blast furnace slag, etc. Most of these products have found usefulness in concrete production
5. Damp Proofing Admixtures
Damp proofing admixtures, also known as water-resisting admixtures or permeability-resisting admixtures are the types of admixtures that stop water from passing through hardened concrete or passage through unsaturated concrete. These types of admixtures are necessary because the prevention of water penetration is one of the most important properties of hardened concrete. Damp-proofing admixtures should be impervious, durable, capable of bearing load, dimensionally stable, and should be free from sulphates, chlorides & nitrates. These admixtures do not have a significant effect on the setting time of concrete; however, they help to ensure a reduction in the bleeding of concrete, a reduction in drying shrinkage, and an improvement in the durability of the concrete. Damp proofing admixtures can be available in liquid, powder, or paste form. Examples of liquid form are hot bitumen while mastic asphalt comes into the class of paste.
6. Gas Forming Admixtures
Gas-forming admixtures is a kind of concrete admixtures which reacts in slurry, releasing gas, and forming fine and uniform pore, this makes aerated concrete have porous structure. These admixtures generate or liberate bubbles of gas in the fresh mixture during the hydration process before the initial set of the cement paste matrix takes place. Through the generation and liberation of bubbles in the mix, they help to maintain concrete’s initial volume, counteracting settlement and bleeding. Examples of common gas-forming admixtures are aluminum powder, activated carbon, and hydrogen peroxide which at higher volumes can be used to make lightweight concrete.
7. Air Entraining Admixtures
Air-entraining admixtures could be synthetic or organic resins that create tiny air bubbles in concrete by introducing a regulated volume of air. The diameter and dispersion of the bubbles should be approximately 50 microns. An air-entraining admixture is primarily used because the hardened concrete’s resistance to freezing and thawing is increased by the presence of small bubbles in the material. This resistance is further enhanced when de-icing salts and fluids are used. When the water in capillary gaps freezes, it expands and threatens to rupture, causing saturated concrete, which is what most outdoor paving will be. If the concrete is air-entrained, the air bubbles, which intersect the capillaries, stay unfilled with water even when the concrete is saturated. Thus, the bubbles act as pressure relief valves and cushion the expansive effect by providing voids into which the water can expand as it freezes, without disrupting the concrete. When the ice melts, surface tension effects draw the water back.
When concrete is saturated, air-entrained concrete retains its water-free state in the intersections of its capillaries. By creating spaces into which the water can expand as it freezes without causing damage to the concrete, the bubbles serve as pressure relief valves and buffer the expansive effect. The water in the bubbles is drawn back out by surface tension effects when the ice melts. For all external paving applications, including major highways, airfield runways, garage drives, and pathways that are likely to be exposed to extreme freezing temperatures and de-icing agents, air-entrained concrete must to be specified and utilized. Examples of air-entraining admixtures are tributyl phosphate, silicones, and water-insoluble alcohols.
8. Grouting Admixtures
Grout is a composite material generally consisting of water, cement, and sand. Grouting admixtures are admixtures that are added to mortar and grout to improve workability, durability, and setting time performance. They also impact shrinkage compensation, high flowability, and pumpability to the concrete mix. They equally help to control expansion and minimise water demand. These characteristics are very useful for the concreting of pieces of complicated geometry or with great concentration of reinforcements, as well as for pumping concrete and in prefabrication. Grouting admixtures come in two types: accelerators and retarders. Examples of accelerators are calcium chloride and triethanolamine; examples of retarders are mucic acid and gypsum.
9. Colouring Admixtures
Colouring admixtures are admixtures used to dye mortar, cement, render, and floor screeds to achieve the right colour. The products are usually available in either powder or liquid form and it can be mixed as a dry or wet mix. To generate a consistent hue for every batch you need to make, they are simple to use and the pigment will blend into the mix consistently. Numerous distinct colours might be effortlessly incorporated into the mix. Dry sand or cement mixes can have dyes applied to them by hand or by machine before water is added to mix the mixture. It is important to weigh the cement and cement dye instead of estimating by volume to get the best colour match possible. Lower the amount of colour you add to the mixture if you’d like it to turn out a lighter hue. Colouring admixtures generally increase the value and aesthetic appeal to concrete, with excellent color uniformity. In addition, depending on the contents of the materials, they also offer ultraviolet (UV) resistance, long-lasting vibrancy, and enhanced finishing characteristics. Examples of colouring admixtures are red oxide, hydroxide of iron, and barium manganite.
The world of concrete admixtures is diverse and transformative. From enhancing workability to ensuring durability and sustainability, the 9 Best Concrete Admixtures Commonly Used offer a myriad of possibilities for construction professionals. As you embark on your next project, consider the unique advantages each admixture brings, shaping concrete into a versatile and resilient building material.