Presentation on theme: "AGGREGATES There are two types of aggregates Coarse Aggregates"— Presentation transcript:
1 AGGREGATESThere are two types of aggregatesCoarse AggregatesFine AggregatesFine and coarse aggregates make up the bulk of a concrete mixture.Sand, natural gravel and crushed stone are mainly used for this purpose.COURSE AGGREGATESIt 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 weightFINE AGGREGATESGrading limits of fine aggregates has been specified by grading zones, namely zone I, II, III and IVZone I - Fine aggregate is coarseZone II - Fine aggregate is finer than zone IZone III - Sand is finer than zone II sandZone Iv - is the finest.
3 GRADING OF AGGREGATESGrading 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.
4 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.
6 WATER TO CEMENT RATIO (W/C RATIO) 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 mixtureThe 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.
8 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 waysPrescribed 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 MixDesigned mix is used for structural works in terms of the grade.The grade denotes the 28days cube stength in N/mm2The strength of concrete is determined by casting it in 15cm cube moulds, curing it and testing it after 28 days.
9 GRADING OF CONCRETEIn 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 ConcreteStandard 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.
10 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/mixingCommon 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 concretePlasticizers/ 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.