Question

case study topic: aggregate for concrete.

write a report about the following topic and follow the guidlines.

Abstract, introduction, history, experemental test and conclusion.

Answer 1

Case Study: Aggregate of Concrete

Abstract

Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. When aggregate is mixed with dry Portland cement and water, the mixture forms a fluid slurry that is easily poured and molded into shape. Most concrete is poured with reinforcing materials (such as rebar) embedded to provide tensile strength, yielding reinforced concrete. Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone are used mainly for this purpose. Recycled aggregates (from construction, demolition, and excavation waste) are increasingly used as partial replacements for natural aggregates, while a number of manufactured aggregates, including air-cooled blast furnace slag and bottom ash are also permitted.

The aggregates play an important role in the construction industry. There are many tests which are performed to check the quality of aggregates. Aggregates are a very important component of concrete, so the quality really matters when it comes to aggregates. The aggregate impact test value is a measure of resistance to sudden impact or shock, which may vary from its resistance to gradually applied compressive load.

Introduction to aggregates

Aggregates are a granular material used in construction. The most common natural aggregates of mineral origin are sand, gravel and crushed rock. A product by itself when used as railway ballast or armor stones, aggregates are also a raw material used in the manufacture of other vital construction products such as ready-mixed concrete (made of 80% aggregates), pre-cast products, asphalt (made of 95% aggregates), lime and cement (UEPG 2006). According to the source material aggregates can be classified as,

Natural aggregates, produced from mineral sources. Sand and gravel are natural aggregates resulting from rock erosion. Crushed rock is extracted from quarries.

Secondary aggregates, secondary materials arising from industrial processes.

Recycled aggregates, produced from processing material previously used in construction.

Natural aggregates come from rock of which there are three broad geological classifications: igneous, sedimentary and metamorphic.

Natural aggregates are extracted from natural deposits by quarrying and mining. Rock is blasted or dug and then reduced in size by series of crushers and screens to prepare for aggregate use. Sand and gravel are extracted from alluvial or marine deposits.

History

Ancient times

Mayan concrete at the ruins of Uxmal is referenced in Incidents of Travel in the Yucatán by John L. Stephens. "The roof is flat and had been covered with cement". "The floors were cement, in some places hard, but, by long exposure, broken, and now crumbling under the feet." "But throughout the wall was solid, and consisting of large stones imbedded in mortar, almost as hard as rock."

Small-scale production of concrete-like materials was pioneered by the Nabatean traders who occupied and controlled a series of oases and developed a small empire in the regions of southern Syria and northern Jordan from the 4th century BC. They discovered the advantages of hydraulic lime, with some self-cementing properties, by 700 BC. They built kilns to supply mortar for the construction of rubble masonry houses, concrete floors, and underground waterproof cisterns. They kept the cisterns secret as these enabled the Nabataeans to thrive in the desert. Some of these structures survive to this day.

Modern structural concrete differs from Roman concrete in two important details. First, its mix consistency is fluid and homogeneous, allowing it to be poured into forms rather than requiring hand-layering together with the placement of aggregate, which, in Roman practice, often consisted of rubble. Second, integral reinforcing steel gives modern concrete assemblies great strength in tension, whereas Roman concrete could depend only upon the strength of the concrete bonding to resist tension.

Reinforced concrete was invented in 1849 by Joseph Monier. The first house was built by François Coignet in 1853. The first concrete reinforced bridge was designed and built by Joseph Monier in 1875.

Experimental test
CRUSHING TEST: This test is mainly used for the pavements.This is test for the aggregates to know the amount of compressive stress which it can bear before failure or rupture.This test is standardized by IS:2386 part -iv and can be used to determine crushing strength of aggregates. The aggregate crushing value is resistance of aggregate under increasing crushing load.

The aggregates used for this test should pass through 12.5 mm sieve which retains on 10 mm are collected -washed and poured in a cylindrical specimen of dia 11.5 mm and 18 cm height also calculate the total sample weight as W1. The aggregate is filled in three equal layers and each layer is tamped 25 times with a tamping rod.

The specimen is kept in a UTM and a compressive load of 40 ton is applied until failure starting at rate of 4 tonnes/min.

Then crushed aggregates are sieved through 2.36 mm sieve. weigh the passing material and be it W2.  

Aggregate Crushing value = (W1/W2)*100

Conclusion

• The aggregate crushing value below 10 is considered as best aggregate for mainly roads.
• The aggregate crushing value above 35 is considered as weak aggregates.