bld101aggregatesall.05.06 - University of Ulster, Faculty...

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Str 400 N/mm 2 University of Ulster, Faculty of Engineering, School of the Built Environment Desirable Properties In Aggregates When a cement paste sets and hardens is subject to considerable shrinkage. Fine and coarse aggregates are used in concrete to limit the possibility of shrinkage and to provide a durable, inert and inexpensive filler. The stones chosen for aggregate in normal concrete usually fulfil the following: 1. They provide adequate bond with the cement paste. This is important in reinforced concrete members in bending. A rough surface, such as crushed rock gives a good bond. Interlocking of cement particles and aggregate also contribute to the bond. Very smooth stone, with less possibility for grip to develop, will give poorer bond. If bond is good then a concrete cube crushed in a test apparatus will show some aggregate broken through. No accepted test is yet available. Bond will improve with the age of the concrete. 2. The Aggregate should have adequate strength . The strength of the concrete is dependent on the aggregate strength, so the latter requires to be greater by a fair measure than the concrete strength. It is difficult to adequately test the aggregate strength. Average crushing values of good aggregate are 200 N/mm 2 . Concrete strengths generally vary between 25-60 N/mm 2 . The strength of the aggregate needs to be higher than the concrete strengths because local stresses between particles will be higher than the average applied stress. Relation between compressive strength of parent rock and crushing value of Aggregate from the same rock. - Neville. 3. Abrasion Resistance Resistance to abrasion and attrition is required during mixing, handling and transportation. Resistance to polishing is also required for stones to be used as road surfacing. Toughness ie resistance to crushing and impact loads during the compaction process and under traffic. It is necessary that the stone will maintain these properties, and its strength, during its service life and it should not be damaged by degradation. BS812 includes tests for these properties. 4. Absence of Impurities (i) Chemically reactive minerals ie Alkali Silica Reaction. (ii) Salts. Chlorides in marine aggregates which can give corrosion. BLD101J ©DJL/JG Oct 05 1 40 N/mm 2 Crushing value is higher in weaker rocks
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University of Ulster, Faculty of Engineering, School of the Built Environment (iii) Clay, silt, dust, organic matter which can load to poor bonding, shrinkage and may inhibit hydration. (iv) Flaky minerals such as mica, which will split on cleavage planes at low stresses. They are prone to disintegrate chemically. (v)
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bld101aggregatesall.05.06 - University of Ulster, Faculty...

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