Aggregate INTRO

INTRODUCTION

• Aggregate is the component of a composite material used to resist compressive stress.
• Aggregate is used to describe the gravels, crushed stones and other materials which are mixed with cement and water to make concrete.
• Aggregate should be much smaller than the finished item for efficient filling, but have a wide variety of sizes.
• The particles of stone used to make concrete typically include both sand and gravel.
• The function of aggregate in concrete is as a mass of particles which are suitable for resisting action of applied load, abrasion and percolation of moisture and the action of weather. It is also to reduce the volume changes resulting from setting and hardening of concrete.
• Construction aggregate is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, and recycled concrete.
• Aggregates are a component of composite materials such as concrete and asphalt concrete; the aggregate serves as reinforcement to add strength to the overall composite material.
• Aggregates are also used as base material under foundations, roads, and railroads; as a stable foundation or road/rail base with predictable, uniform properties (e.g. to help prevent differential settling under the road or building), or as a low-cost extender that binds with more expensive cement or asphalt to form concrete.






Classification of aggregate
Aggregate can be classified into 3 categories, there are:
1. Normal aggregate - Crushed rock, sand and gravel, broken bricks.
2. Light weight aggregate - Pumice, expanded shale, expanded clay.
3. Heavy weight aggregate - Magnetite, hematite, limonite.

Normal aggregate:
• Having a specific gravity between about 2.5 and 3.0 and a bulk density in the range 1450 to 1750 kg/m³
• Can classified aggregates according size into
i. Coarse aggregate
ii. Fine aggregate(sand)
iii. All-in aggregate

Coarse Aggregate
• Materials retained on 5mm (3/16 inch) BS 410 test sieve.
• May described as uncrushed gravel or uncrushed stone, crushed stone or crushed gravel, and partially crushed gravel or stone.

Fine aggregates
• Materials passed through a 5mm (3/16 inch) BS 410 test sieve.
• Consider having a lower size limit of about 0.07mm, material between 0.06mm and 0.002mm is classified as slit, and smaller particles are called clay.
• May described as natural sand and crushed stone sand or crushing gravel sand.
• Normally be used when a trial mix has been made with the aggregate to determine its suitability for the particular purposes.



All-in aggregate
• Are materials composed of a mixture of coarse and fine aggregates.
• Normally not graded and used in unimportant work.

Light weight aggregates
• With a particle density of less than 2000kg/m³.
• Low density is main requirement.
• Has high water absorption.
• Require wetting before mixing in the mixer.
• The cover to reinforcement using light weight aggregates in concrete should be increased due to the increased permeability and rapid carbonation of concrete.

High density aggregate
• Ranging from 2.8 to 2.9 and unit weight from 2800 kg/m³ to 2900kg/m³ are used to make high density concrete.
• The compressive strength of these concretes is the order of 20 to 21 N/mm².
• The cement-aggregate ratio varies from 1:5 to 1:9 with a water-cement ratio 0.5 to 0.65.
• Produce dense and crack free concrete.
• Are not suitably graded
• Difficult to have adequate workability without segregation.