CHEMICAL ADMIXTURES FOR CONCRETE

types of chemical admixtures and there uses

1. CHEMICAL ADMIXTURES FOR CONCRETE K.Prashanth kumar

2. INTRODUCTION Concrete has become established superb building material over the years, the quality and consistency of which can be improved to meet the desired requirements by using appropriate chemical admixtures in the mix. Making quality concrete is a precise technology, and in many conditions when ordinary concrete would not be suitable, the technologist can ensure that the finished product meets this requirement by using the right admixtures in the right proportion. It is important that the operator has an intelligent appreciation of concrete admixtures and employs the correct admixture in the exact quantity specified in order to achieve the desired results. Broadly speaking, admixtures may be divided into various categories, namely, accelerators, retarders, water reducers, air-entraining agents, water proofers, pumping aids, plasticizers, superplasticizers etc. While admixtures, unlike cement, aggregate and water, are not an essential component of the concrete mix, they are an important and increasingly wide spread component in many countries, a mix that contains no admixture is now a days an exception.

3. What is Admixture? It is an optional ingredient of concrete which is added to modify the properties of fresh and hard concrete and grout materials as per some specific requirements. Addition of admixture may alter workability, pumping qualities, strength development, appearance etc. in fresh concrete and permeability, strength, durability etc. in hardened concrete. But use of chemical admixture is a must for producing high grade concrete. Chemical Admixtures

4. USES OF ADMIXTURE: Effect of Admixtures on the Properties of Fresh Concrete: To increase workability without increasing the water cement ratio. To retard or accelerate time of initial or final setting. To modify the rate of bleeding. To retard segregation or increase cohesion. To improve pumpability . To reduce the rate of loss of workability.

5. Effect of Admixtures on the Properties of Hardened Concrete: To accelerate the rate of strength development especially at early stage. To increase the strength To increase the durability To decrease the permeability To control expansion by alkali – aggregate reaction. To improve bond with reinforcement. To improve bond with old /new concrete. To improve impact or abrasion resistance. To inhibit corrosion of embedded reinforcement.

6. TYPES OF CHEMICAL ADMIXTURES: Water Reducing Admixtures Plasticizers. Super Plasticizers. Retarders and retarding admixtures. Accelerators and Accelerating Plasticizers Air-entraining admixtures. Water or damp proofing and permeability reducing admixtures. According to characteristics effects produced by them chemical admixtures used are as follows:

7. PLASTICIZERS(Water- reducing): Plasticizers are the organic or combination of organic and inorganic substances which reduces water content for certain degree of workability, when added in mix. The basic products consisting plasticizers are anionic surfactants (such as lignosulphonates , salts of sulphonates hydrocarbon) non-ionic surfactants (such as polyglycol esters, hydroxylated carboxylic acid products) and others such as carbohydrates, etc. among them calcium, sodium and ammonium lignosulphonates are commonly used. Plasticizers are mixed from 0.1% to 0.4% by weight of cement used and it reduces 5% to 15% of water with the increment of workability from 3 to 8cm slump. In the mix, the cement grains absorbs the plasticizers molecules and results change in the surface charge of the same sign which causes repulsive forces and makes the dispersion which increases plasticity and workability. Some Plasticizer also entrails the air but a good plasticizer is that which entrains air less than 2% only. The plasticizers are available in market in various brands with specifications for composition, dosages etc. Plasticizer

8. Uses of plasticizers: To achieve a higher strength by decreasing the water cement ratio at the same Workability as an admixture free mix. To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete. To increase the workability so as to ease placing in accessible locations. Water reduction more than 5% but less than 12% The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts. Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry. Retarding Effect : Plasticizers get adsorbed on the surface of the cement particles and form a thin sheath. This sheath inhibits the surface hydration reaction between water and cement as long as sufficient plasticizers molecules are available. become entrapped in hydration products.

9. SUPERPLASTICIZERS (High Range Water reducer): Normal water reducers are well established admixtures called plasticizers in concrete technology. A normal water reducer is capable of reducing water requirements by 10 to 15%. Higher water reductions, by incorporating larger amounts of these admixtures, result in undesirable effects on concrete like bleeding, segregation and hardening. So, a new class of water reducers, chemically different from the normal water reducer and capable of reducing water content by about 30% has been developed. The admixtures belonging to this class are known as super plasticizers. SUPERPLASTICIZERS No plasticizers With 0.3% superplasticizer

10. There are four types of super plasticizers which are generally used for concrete as given below. Sulphonated melamine – It is suitable in low temperature areas Sulphonated naphthalene – It is more suitable in high temperature areas Ligno sulphates – It is suitable for Indian conditions where temperature variation is high Carboxylated admixture – It is suitable where workability is required to be retained for large duration. Advantages of Superplasticizers : Significant water reduction Reduced cement contents Reduce water requirement by 12-30% Increased workability Reduced effort required for placement More effective use of cement More rapid rate of early strength development Increased long-term strength Reduced permeability

11. Disadvantages of Superplasticizers : Additional admixture cost (the concrete in-place cost may be reduced) Slump loss greater than conventional concrete Modification of air-entraining admixture dosage Less responsive with some cement Mild discoloration of light-coloured concrete

12. RETARDERS: These admixtures are used where setting time of concrete need to be delayed. Retarder delays the hydration process but doesn’t affect the eventual process. Initial setting time can be delayed by more than 3 hours. The main application of retarding admixtures is in eliminating the cold joints and controlling the setting time of concrete. RETARDERS

13. Advantages of Retarders: Retarding admixtures delay hydration of cement. To delay or extend the setting time of cement paste in concrete. These are helpful for concrete that has to be transported to long distance, and helpful in placing the concrete at high temperatures. Used in casting and consolidating large number of pours without the formation of cold joints Used in grouting oil wells.

14. These are used to accelerate the setting time and hardening process of cement in concrete. These are used in concreting under flowing water and in road repair works so that work can be put to use as earliest as possible. ACCELERATORS: Accelerating admixtures can be divided into groups based on their performance and application : Set Accelerating Admixtures : Reduce the time for the mix to change from the plastic to the hardened state. Set accelerators have relatively limited use, mainly to produce an early set . 2. Hardening Accelerators : Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least 130% at 48 hours. Hardening accelerators find use where early stripping of shuttering or very early access to pavements is required. They are often used in combination with a high range water reducer, especially in cold conditions.

15. Advantages : Reduced bleeding, Earlier finishing, Improved protection against early exposure to freezing and thawing, Earlier use of structure, Reduction of protection time to achieve a given quality, Early removal of form, and In some case, early load application. Shortens the setting time of cement and therefore increases the rate of gain of strength. Enables earlier release from precast moulds thus speeding production. Reduces segregation and increase density and compressive strength. Cures concrete faster and therefore uniform curing in winter and summer can be achieved. Early use of concrete floors by accelerating the setting of concrete. Reduces water requirements, bleeding, shrinkage and time required for initial set. ACCELERATORS

16. AIR-ENTRAINING ADMIXTURES: Air-entraining admixtures cause small stable bubbles of air to form uniformly through a concrete mix. The bubbles are mostly below 1 mm diameter with a high proportion below 0.3 mm. The benefits of entraining air in the concrete include increased resistance to freeze-thaw degradation, increased cohesion (resulting in less bleed and segregation) and improved compaction in low-workability mixes AIR-ENTRAINING ADMIXTURES

17. Air entraining agents : Natural wood resins Animal and vegetable fats and oils such as tallow, olive oil and their fatty acids such as stearic and oleic acids Various wetting agents such as alkali salts or sulphonated organic compounds Water soluble soaps of resins acid Miscellaneous materials such as sodium salts of petroleum sulphonic acids, hydrogen peroxide and aluminium powder, etc . Factors affecting air entrainment : Type and quantity of air entraining agents used Water cement ratio of mix Type and grading of aggregates Mixing time Temperature Type of cement Influence of compaction Admixtures other than air entraining agents used

18. Effect of air entrainment on the properties of concrete : Air entrainment will effect directly the following three properties of concrete Increased resistance to freezing and thawing Improvement in workability Reduction in strength Incidentally Air entrainment will affect the properties in following ways : Reduces the tendencies of segregation Reduces the bleeding and laitance Decreases the permeability Increases the resistance to chemical attack Permits reduction in sand content, water content, cost, & heat of hydration Reduces unit weight, alkali aggregate reaction, the modulus of elasticity

19. WATERPROOFING ADMIXTURES : It is essential to water proof a structure to prevent the seepage of water for its durability. provide water proofing solutions for basements, sunken portions, roofs, terrace gardens and expansion joints etc. several ways of water proofing depending on a structure, site and conditions viz.: • Polymer Modified Cementitious Membranes • Polyurethane Membranes • Box Type Method using Kota/Agra Stone • Injection Grouting • APP modified bituminous membranes / Trafelt • EPD(ethylene-propylene-diene) Membrane • PVC Membrane Damp proofing treatment in building: Treatment to foundation. Treatment to floors. Treatment to walls. Treatment to flat roof . Treatment to parapet wall. Treatment to pitched roof . WATERPROOFING ADMIXTURE

20. Function: The main function is to reduce either the surface adsorption into the concrete and the passage of water through the hardened concrete. To achieve this, most products function in one or more of the following ways : Reducing the size, number and continuity of the capillary pore structure Blocking the capillary pore structure Lining the capillaries with a hydrophobic material to prevent water being drawn in by absorption / capillary suction

21. CONCLUSION: Plasticizers, superplasticizers, & Air-entraining admixtures is mostly used. Superplasticizers is widely used all over the world. India is catching up with the use of superplasticizer in the construction of high rise buildings, long span brides Because it reduced cement content, increased workability, & increased strength. Superplasticizers is reduce the water content and increase the strength.