Admixtures

Admixtures are material added to concrete mix in order to modify its properties. Usually it is not more than 5% by mass of cement and should always follow the manufacturer recommendations. Additives refers to a substance which is added at the cement manufacturing stage.

Chemical admixtures: Air-entraining agents: protects concrete from the deleterious effects of freezing and thawing. Plasticizers: water reducers. superplastecizers Retarders Accelerators

Accelerators Accelerators Admixtures used to accelerate the rate of setting and hardening of concrete. As they increase the rate of early strength development. They are useful for: quick repairs, where early removal of form work required for use and for concreting in cold weather. Types of accelerators : 1- calcium chloride 2- sodium carbonate 3- aluminum chloride 4- potassium carbonate 5- ferric salts

Advantages of using calcium chloride(CaCl₂): Early strength development. Used when concrete placed at low temperatures. When urgent repair work is required. It acts as a catalyst in the hydration of C₃S and C₂S Increase the resistance of concrete to erosion and abrasion.

Calcium chloride may be added to rapid-hardening(type III) and to ordinary portland (type I) cement Calcium chloride must not be used with high alumina cement. Long-term strength of concrete is unaffected.

Effect of quantity of CaCl₂ on the mix: - It accelerates setting, however overdose can cause flash set. - To calculate the quantity required it can be assumed that the addition of 1% of calcium chloride ( as a fraction of the mass of cement) affects the rate of hardening as much as a rise in temperature of 6° C. A calcium chloride content of 1 to 2 percent is generally sufficient.

Calcium chloride should be uniformly distributed throughout the mix by dissolving the admixture in mixing water then adding it to the concrete mix.

Disadvantages of calcium chloride: It reduces the resistance of cement to sulphate attack. Increases the risk of alkali aggregate reaction. Increases shrinkage and creep of concrete. Lowers the resistance of air-entrained concrete to freezing and thawing at later ages. Increase the plastic shrinkage because the plastic stage is extended. It has been found to cause corrosion to reinforcement in certain cases.

What is relevant with respect to corrosion is the chloride ion, Cl¯ What is relevant with respect to corrosion is the chloride ion, Cl¯. All sources of the ion , including what presence on the surface of marine aggregate should be taken into account. It is observed that 1.56g of CaCl₂ corresponds to 1 g of chloride ion. Risk of corrosion can be prevented by the use of very rapid hardening cements or of chloride free admixtures.

retarders Retarders: Admixtures used to delay the setting and hardening of concrete. Generally used in hot weather. Where it is used: In hot weather In mass concrete where high temperature can reduce the normal setting and hardening time. In ready mixed concrete to gain more time for delivering concrete to far sites In preventing the formation of cold joints between successive lifts. Where an architectural surface finish of exposed aggregate is required.

Material used as retarding admixtures: Sugar, carbohydrate derivatives, soluble zink salts and soluble borates. 0.05 percent of sugar by mass of cement will delay the setting time by about 4 hours. A large quantity of sugar, 0.2 to 1 percent, of the mass of cement, will prevent the setting of cement. This is useful in case of malfunctioning of a concrete mixer.

Use of retarding admixtures reduces the early strength but later the rate of strength development is higher, so the longer-term strength is not much different.

Plasticizers Plasticizers(water reducers) Purposes of using plasticizers: Achieve a higher strength by reducing Water/cement ratio. Achieve the same workability by decreasing the cement content. Increase the workability. Dosage of plasticizers used in concrete is only a fraction of one percent of the mass of cement.

Plasticizers Reduction of water varies between 5 and 15 percent and depends on the following: Cement content. Aggregate type. Pozzolans and air-entraining agent if present. To achieve optimum properties it is essential to carry out trial mixes.

Side effects of plasticizers: Increase in strength at early ages due to a greater surface area of cement exposed to hydration. Improvement of long term strength because of a more uniform distribution of the cement throughout the concrete. Improvement in durability of concrete.

Superplasticizers Superplastisizer is more effective type of water-reducing admixtures and known as high range water reducers. Its dosage level is higher than plasticizers and its side effects are lower because they do not entrain a significant amount of air. Use of Superplastisizer Where flowing concrete is required in situations where placing is inaccessible. Where very rapid placing is required. Where very high strength concrete is required, using normal workability but a very low water/cement ratio.

How Superplastisizers work? Superplastisizers cause the cement to dispers through the action of the sulfonic acid being adsorbed on the surface of cement particles, causing them to become negatively charged and thus mutually repulsive. With use of Superplastisizer, 25 to 35 percent of water reduction is possible to achieve high strength(compared with about one-half that value for plasticizers)

The improved workability produced by superplastecizers is of short duration and thus there is a high rate of slump loss: after some 30 to 90 minutes the workability returns to normal. For this reason, the superplasticizers should be added to the mix immediately prior to placing. The only real disadvantage of superplasticizres is their high cost.

flowing concrete: it is high slump, easy flow concrete achieved by adding water reducers.