Aggregates for ConcreteConcrete Technology

Introduction

• The proper selection of aggregates for use in concrete mixtures is critical to long term concrete performance.

• Aggregates generally occupy 60% to 75% of the concrete volume (70% to 85% by mass)

• strongly influence the concrete’s fresh and hardened properties, mixture proportions, and economy

• Aggregates must be clean, hard, strong, and durable particles that are largely free of absorbed chemicals, coatings of clay, and other fine materials in amounts that could affect hydrationand bond of the cement paste.

• Aggregate particles that are friable or capable of being split are undesirable.

• Aggregates containing appreciable amounts of soft and porous materials, including some varieties of siltstone, claystone, mudstone, shale and shaley rocks, should be avoided

Concrete aggregates are a mixture of rocks

and minerals

A mineral is a naturally occurring solid

substance with an orderly internal structure

and a chemical composition that ranges

within narrow limits

A rock is generally composed of several

minerals.

Rocks are classified as igneous,

sedimentary, or metamorphic, depending

on their geological origin.

Aggregate Classification

Normal weight

1200 kg/m3 to 1750 kg/m3 (75 lb/ft3 to 110 lb/ft3)

Light weight

560 kg/m3 to 1120 kg/m3

(35 lb/ft3 to 70 lb/ft3)

Heavy weight

typically over 2100 kg/m3 (130 lb/ft3)

Normal weight

• The most commonly used normal weight aggregates sand, gravel, and crushed stone, produce freshly mixed normal weight concrete with a density (unit weight) of 2200 kg/m3 to 2400 kg/m3 (140 lb/ft3 to 150 lb/ft3).

Light weight

• Lightwieght aggregates of expanded shale, clay, slate, and slag are used to produce structural lightweight concrete with a freshly mixed density ranging from about 1350 kg/m3 to 1850 kg/m3 (90 lb/ft3 to 120 lb/ft3).

• Other lightweight materials such as pumice, scoria, perlite, vermiculite, and diatomite are used to produce insulating lightweight concretes ranging in density from about 250 kg/m3 to 1450 kg/m3 (15 lb/ft3 to 90 lb/ft3).

Heavy weight

• Heavyweight materials such as barite, limonite, magnetite, ilmenite, hematite, iron, and steel punchings or shot are used to produce heavyweight concrete and radiationshielding concrete.

• Heavyweight aggregates produce concretes ranging in density from 2900 kg/m3 to 6100 kg/m3 (180 lb/ft3 to 380 lb/ft3).

Aggregate Sources

Natural aggregate

Manufactured aggregate

Recycled-Concrete aggregate

Marine-Dredged

aggregate

Natural Aggregate

• Gravel and sand are often a mixture of several minerals or rocks.

• Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed.

• Weathering and erosion of rocks produces particles of stone, gravel, sand, silt, and clay.

• Some natural aggregate deposits of gravel and sand can be readily used in concrete with minimal processing.

Natural Aggregate

• The quality (or soundness) of natural aggregate depends on the bedrock from which the particles were derived and the mechanism by which they were transported.

• Sand and gravel derived from igneous and metamorphic rocks tend to be sound, while sand and gravel derived from rocks rich in shale and siltstone are more likely to be unsound.

• Sand and gravel that have been smoothed by prolonged agitation in water usually are considered higher quality because they are harder and have a more rounded shape than less abraded sand and gravel.

Natural Aggregate

• However, the smooth surface of natural gravels can reduce the bond strength with the cement paste and reduce overall concrete strength.

Manufactured Aggregate

• Manufactured aggregate (including manufactured sand) is produced by crushing sound parent rock (igneous, sedimentary, or metamorphic) at stone crushing plants.

• Crushed air-cooled blast-furnace slag is also used as fine or coarse aggregate.

Manufactured Aggregate

• Manufactured aggregates differ from gravel and sand in their grading, shape, and texture.

• As a result of the crushing operation, manufactured aggregates often have a rough surface texture, are more angular in nature, tend to be cubical or elongated in shape (depending on the method of crushing), and more uniform in grade (size)

• Producers can provide high-quality material meeting specified particle shapes and gradations.

• In many cases, the particle elongation and layered flakes of manufactured sands can be reduced through appropriate crushing techniques.

Manufactured Aggregate

• Manufactured aggregates are less likely than gravel and sand to be contaminated by deleterious substances such as clay minerals or organic matter

• Some specifications permit higher fines content in manufactured sands because of the expectation of less clay contamination.

Recycled-Concrete Aggregate

• Recycled concrete is simply old concrete that has been crushed to produce aggregate.

• Recycled-concrete aggregate (RCA) is primarily used in pavement reconstruction.

• It has been satisfactorily used as aggregate in granular subbases, lean-concrete subbases, soil-cement, and in new concrete as the primary source of aggregate or as a partial replacement of new aggregate.

• Recycled-concrete aggregate generally has a higher absorption and a lower specific gravity than conventional aggregate

• Absorption values typically range from 3% to 10%, depending on the concrete being recycled

• this absorption rate lies between those for natural and lightweight aggregate.

• Absorption rates increase as coarse particle size decreases.

• The high absorption of the recycled aggregatemakes it necessary to addmore water to achieve the same workability and slump as concrete with conventional aggregates.

• Dry recycled aggregate absorbs water during and aftermixing.

• To avoid this increased absorption, RCA stockpiles should be prewetted or kept moist, as is the practice with lightweight aggregates.

Marine-Dredged Aggregate

• Marine-dredged aggregate fromtidal estuaries and sand and gravel fromthe seashore may be used with caution in concrete applications when other aggregate sources are not available.

• There are two primary concerns with aggregates obtained from seabeds: (1) seashells and (2) salt.

• Although seashells are a hardmaterial that can produce good quality concrete, a higher paste contentmay be required due to the angularity of the shells.

• Aggregate containing complete shells (uncrushed) should be avoided as their presence may result in voids in the concrete andmay lower the compressive strength.

Marine-Dredged Aggregate

• Marine-dredged aggregates often contain salt (chlorides) from the seawater.

• The highest salt content occurs in sands located just above the high-tide level, although the amount of salt on the aggregate is often not more than about 1% of the mass of themixing water.

• The presence of chlorides may affect the concrete by:• (1) altering the time of set,

• (2) increasing drying shrinkage,

• (3) significantly increasing the risk of corrosion of steel reinforcement, and

• (4) causing efflorescence.

• Generally, marine aggregates containing large amounts of chloride should not be used in reinforced concrete. To reduce the chloride content,marine-dredged aggregates can be washed with fresh water.

Aggregate Test Method

Aggregate Test Method

Home Work 04

• Find all available standards regarding aggregate quality tests!

• Among all characteristics of the aggregates, what are the effect of each one of them to concrete?

• Write your review based on available standards (ASTM, ACI, etc), and collect them all 48 hours after the class!