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PROPERTIES OF CONCRETE ARTICLE Effects of aggregates on properties of concrete • SUBMITTED TO: • Dr. Ayub Elahi • SUBMITTED BY: • • • • • 2K9-SCET-02/CIVIL 2K9-SCET-26/CIVIL 2K9-SCET-07/CIVIL 2K9-SCET-52/CIVIL 2K9-SCET-54/CIVIL • Aggregates Granular mineral material(such as sand, gravel, crushed stone) used with a bonding medium (such as cement or clay) to make concrete, plaster, or terrazzo mixture. Sum minerals are given below: • Minerals Silica (ex. Quartz) • Silicates (ex. Clay) • Carbonate (ex. Calcite, dolomite). Types of aggregates 1.Fine aggregates 2.Coarse aggregats Properties of aggregates and their effects on concrete • Grading: Grading is the distribution of particles among various sizes. Grading is usually expressed in terms of cumulative percentage passing each sieve. Coarse Aggregate Grading: • Usually more water and cement is required for small-size aggregate than for large sizes,due to an increase in total aggregate surface area. • Grading sieves • The optimum maximum size of coarse aggregate for higher strength depends on 1.Relative strength of the cement paste 2. Cement-aggregate bond 3.Strength of the aggregate particles • Effect of Size on concrete: • Variation in the size of Aggregates changes the water demand, cement content, micro-cracking (strength) in concrete. It also effects pumpability, and durability of concrete. Effect of Size on concrete • The larger the maximum size of the coarse aggregate, the lower the water demand of the mix. The maximum size of aggregates is important parameters in any concrete mix. its affect relative proportions in mix, workability, economy, porosity and shrinkage of concrete etc Effect of Grading on concrete • Grading of aggregates depends on the proportions of coarse and fine aggregate. If grading of aggregate is varied, it also changes cement paste content (cost economy), workability of the mix, density and porosity Maximum aggregate size, they affect • Relative aggregate proportions • Cement and water requirement • Workability • Pump ability • Economy Fineness modulus • is a measurement of the coarseness or fineness of a given aggregate. For a fine aggregate, FM is calculated from the cumulative percentages of aggregate retained on sieves No.4,8, 16, 30, 50, 100, 3/8in., 3/4in., 1.5in., 3in., and 6in., divided by 100. The Fineness Modulus should be between 2.3 and 3.1. Effect of Fineness on Density of Concrete • After measuring the slump, several 150 mm cubes were filled. These were cured in water tank for 28 days. After curing, each cube was weighed using electronic balance and density of concrete was calculated. Texture • The surface texture of aggregate can be either smooth or rough Effects of Surface Texture • The surface texture of aggregate can be either smooth or rough. A smooth surface can improve workability, yet a rougher surface generates a stronger bond between the paste and the aggregate creating a higher strength • Rough-textured and elongated particles require more cement paste to produce workable concrete mixtures, thus increasing the cost • The shape and texture of aggregate affects the properties of fresh concrete more than hardened concrete. Concrete is more workable when smooth and rounded aggregate is used instead of rough angular or elongated aggregate. Most natural sands and gravel from riverbeds or seashores are smooth and rounded and are excellent aggregates. Crushed stone produces much more angular and elongated aggregates, which have a higher surface-to-volume ratio, better bond characteristics but require more cement paste to produce a workable mixture. Absorption and Surface Moisture • The absorption and surface moisture of aggregates should be determined using ASTM C 70, C127, C128, and C 566 so that the total water content of the concrete can be controlled and the batch weights determined The moisture conditions of aggregates are: • Oven dry • Air dry • Saturated surface dry (SSD) • Damp or wet Effects of absorption & Surface Moisture • that using saturated coarse aggregate in the mix, always yields higher shrinkage strain than is dry aggregate. There about 10% increase in drying shrinkage for all mixes, except in uncrushed gravel.The increase for uncrushed gravel was about 3% and that could be resulted from the low water absorption (0.98%) and smooth surfaces which make no difference between dry and saturated condition. Effects of absorption & Surface Moisture • Using dry aggregate causeds the water/cement ratio in the vicinity of the aggregate particles to be less than else where because aggregate absorbed water to reach saturated surface-dry condition Moisture conditions of aggregate • The reduction in water content would improve the bond between cement mortar and aggregate and reduce the interface zone porosity and microcracking (2), and consequently,increases the restraining action of aggregate against drying shrinkage.The same influence was observed with respect to the elasticity modulus,compressive, and splitting tensile strength of concrete. For porcilinite aggregate, the effect of moisture content was more obvious on elasticity modulus and splitting tensile Refrences • www.ads google.com • NEVILLE, A.M, 'Properties of concrete', IV edition, Pearson Education Pvt. Ltd. 2005. • www.engg pedia .com • http://books.google.com.pk/books?id=TNzk4WA6GoC&lpg=PA412&ots=EV2yFWPT9A&dq= effects%20of%20fineness%20modulous%20on %20conc&pg=PA412#v=onepage&q&f=false • Slides of lecture