AGGREGATES • There are two types of aggregates • Coarse Aggregates • Fine Aggregates • Fine and coarse aggregates make up the bulk of a concrete mixture. • Sand, natural gravel and crushed stone are mainly used for this purpose. • COURSE AGGREGATES • It is the aggregates whose particles completely pass through 7.5cm mesh sieve and which are entirely retained on 4.75mm sieve. • It is either measured by volume or weight • FINE AGGREGATES • Grading limits of fine aggregates has been specified by grading zones, namely zone I, II, III and IV • Zone I - Fine aggregate is coarse • Zone II - Fine aggregate is finer than zone I • Zone III - Sand is finer than zone II sand • Zone Iv - is the finest.
GRADING OF AGGREGATES • Grading of aggregates means particle size distribution of the aggregates. • Principle of grading is that the smaller size particle fill up the voids left in larger size particles. • Properly graded aggregate produce dense concrete. • The grading of aggregate is expressed In terms of percentages by weight retained on a series of sieves. • Grading detemines the workability of the mix, which controls segregation, bleeding, water-cement ratio, handling, placing and compacting.
SIGNIFICANCE OF BULKING OF SAND • Bulking of sand means increase in its volume. • Fine aggregates or sands, increase in volume when they possess some moisture. • Bulking is due to the formation of a thin film of water around the fine aggregate or sand particles. • Thickness of water film goes on increasing with more and more moisture and consequently, increase in volume continues. • But after certain percentage of water, volume of sand starts decreasing with increasing amount of water. • At certain percentage of water, increase in volume completely vanishes and volume of water occupied by sand becomes equal to the volume of dry sand. • See the next slide for the graphical representation of Bulking of Sand.
WATER • It is added to cement concrete to bring about the hydration of the cement and lubricate the aggregate particles. • Water which is suitable for drinking is usually considered safe for concreting purpose. • Water is measured by volume, litres per bag of cement. • WATER TO CEMENT RATIO (W/C RATIO) • It is the ratio of water to cement in a concrete mixture • The strength of concrete increases with decreasing water cement ratio. • Or in another words, lesser the volume of water in the concrete, the greater will be the strength of concrete. • See graphical representation on next slide.
MIX DESIGN/SPECIFICATIONS FOR CONCRETE/ CONCRETE MIX PROPORTIONING • Mix design is the process of proportioning the ingredients of concrete with the specified properties like strength, durability and cost. • Concrete when used on construction works is specified in one of the following ways • Prescribed Mix/ Nominal Mix. • The constituents of the mix should be in the ratio of 1:2:4 concrete, means one part of cement, two parts of fine aggregates and four parts of coarse aggregates. • For non-structural works e.g. boundary walls, building carrying low loads. • Designed Mix • Designed mix is used for structural works in terms of the grade. • The grade denotes the 28days cube stength in N/mm2 • The strength of concrete is determined by casting it in 15cm cube moulds, curing it and testing it after 28 days.
GRADING OF CONCRETE • In the designation of concrete “M”stands for mix design and numerical figure stands for characteristic compressive strength of 150mm cube at 28 days in N/mm2. • Minimum grade of concrete for reinforced concrete is M20. • Ordinary Concrete – Grades M10, M15, M20 are grouped as Ordinary Concrete • Standard Concrete – Grade M25, M30, M35, M40 and M50 are grouped as Standard Concrete. • High Strength Concrete – Grade M60, M65, M70 and M80 are grouped as High Strength Conrete.
ADDITIVES OR CHEMICAL ADMIXTURES • Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. • They are added at the time of batching/mixing • Common admixtures are listed below. • Accelerators – They speed up the hydration of the cement e.g CaCl2 NaCl. • Retarders – They slow the hydration of concrete. • Air Entraining Agent – Tiny air bubbles are added in concrete • Plasticizers/ Superplasticizers– Used to reduce the water content of a concrete thereby increases the workability and they have been called water reducers. • Pigments – Can be used to change the colour of concrete for aesthetics.