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.
Sunday, February 24, 2008
BULK DENSITY OF AGGREGATE
BULK DENSITY OF AGGREGATE
The bulk density of an aggregate is defined as the mass of the material in a given volume and is expressed in kg/litre.
• This volume includes the voids between, as well as the pores within, the particles.
• Bulk density is a function of particle shape, density, size, grading, and moisture content, as well as the method of packing the material (loose, vibrated, rodded) and varies not only for different materials, but for different sizes and gradings of a particular material.
• The bulk density of an aggregate can be used for judging the quality of aggregate by comparison with normal density for that type of aggregate.
• The bulk density is also required for converting proportion by weight into the proportions by volume.
Bulking of Fine Aggregate (Sand)
• The increasing in volume of a given weight of fine aggregate (sand) caused by the films of water pushing the sand particles apart.
• The extent of bulking depends on the percentage of moisture present in the sand and on its fineness.
• Figure 1 show shat the variation of percent bulking with moisture content.
Figure1. Effect of moisture content on the bulking of sand
• It seen that the bulking increases gradually with moisture content up to certain point and then begin to decrease with further addition of water due to the merging of films, until when the sand is inundated.
The bulk density of an aggregate is defined as the mass of the material in a given volume and is expressed in kg/litre.
• This volume includes the voids between, as well as the pores within, the particles.
• Bulk density is a function of particle shape, density, size, grading, and moisture content, as well as the method of packing the material (loose, vibrated, rodded) and varies not only for different materials, but for different sizes and gradings of a particular material.
• The bulk density of an aggregate can be used for judging the quality of aggregate by comparison with normal density for that type of aggregate.
• The bulk density is also required for converting proportion by weight into the proportions by volume.
Bulking of Fine Aggregate (Sand)
• The increasing in volume of a given weight of fine aggregate (sand) caused by the films of water pushing the sand particles apart.
• The extent of bulking depends on the percentage of moisture present in the sand and on its fineness.
• Figure 1 show shat the variation of percent bulking with moisture content.
Figure1. Effect of moisture content on the bulking of sand
• It seen that the bulking increases gradually with moisture content up to certain point and then begin to decrease with further addition of water due to the merging of films, until when the sand is inundated.
GRADING AGGREGATE
GRADING AGGREGATE
The grading of aggregate can be obtained by carrying out the sieve analysis test, and from this analysis, grading curves can be drawn. But, firstly, the quantity of material to be represented by the bulk sample has to carry out and this can be done by:-
a) Quartering by hand
b) Riffle box
c) Sample splitter
· The grading of an aggregate can have considerable effect on the workability and stability of concrete mix and is also an important factor in concrete mixed design. If all the particles of an aggregate are uniform size, the compacted mass will contain more voids and vice verse.
· The particle size distribution of a mass of aggregate should be such that the smaller particles fill the voids between the larger particles in order to produce dense concrete with needs less quantity of fine aggregate and cement paste. Therefore, it is essential that the coarse and fine aggregates be well graded to produce quality concrete.
· The grading of an aggregate is expressed in terms of percentage by weight retained or passing through a series of sieves taken in order. The sieves are arranged in such an order that the square opening is half for such smaller size.
From the grading curve, it indicates whether the grading of a given sample conforms to that specified, or is too coarse or too fine or deficient in particular size.
The reading of the grading curve will indicate the followings:-
1. If the actual grading curve is lower than the specified grading curve, the aggregate is coarser and segregation of mix might take place.
2. If the actual grading curve lies well above the specified curve, the aggregate is finer and more water will be required, thus increasing the quantity of cement also for a constant water cement ratio. Therefore, this is uneconomical.
3. If the actual grading curve is steeper than the specified, it indicates an excess of middle-size particles and leads to harsh mix.
4. If the actual grading curve is flatter than the specified grading curve, the aggregate will be deficient in middle size particles.
Surface Area of Aggregate
The surface area is affected by the maximum size of aggregate. The surface area will vary with the shape but is inversely proportional to the particle size. The mortar consisting of fine aggregate and cement should be slightly in excess of that just required to fill the voids in the coarse aggregate. Too coarse the aggregate results in harshness; too fine the aggregate required too large a water cement ratio for adequate workability. The surface area of aggregate also influence the amount f mixing water cement required. Generally, the water-cement ratio is fixed from strength consideration.
Maximum Size of Aggregate
In general, the larger the maximum size of aggregate, the smaller is the cement requirement for a particular water-cement ratio. This is due to the fact that the workability of concrete increase with the increase in the maximum size of aggregate.
· In the mass concrete, the use of a larger size aggregate is beneficial due to the lesser consumption of cement.
· The smaller the surface area of larger size aggregate, the water-cement ratio can be decrease which increase the strength
· The stress concentration in the mortar aggregate interface increasing with the maximum size of aggregate.
The grading of aggregate can be obtained by carrying out the sieve analysis test, and from this analysis, grading curves can be drawn. But, firstly, the quantity of material to be represented by the bulk sample has to carry out and this can be done by:-
a) Quartering by hand
b) Riffle box
c) Sample splitter
· The grading of an aggregate can have considerable effect on the workability and stability of concrete mix and is also an important factor in concrete mixed design. If all the particles of an aggregate are uniform size, the compacted mass will contain more voids and vice verse.
· The particle size distribution of a mass of aggregate should be such that the smaller particles fill the voids between the larger particles in order to produce dense concrete with needs less quantity of fine aggregate and cement paste. Therefore, it is essential that the coarse and fine aggregates be well graded to produce quality concrete.
· The grading of an aggregate is expressed in terms of percentage by weight retained or passing through a series of sieves taken in order. The sieves are arranged in such an order that the square opening is half for such smaller size.
From the grading curve, it indicates whether the grading of a given sample conforms to that specified, or is too coarse or too fine or deficient in particular size.
The reading of the grading curve will indicate the followings:-
1. If the actual grading curve is lower than the specified grading curve, the aggregate is coarser and segregation of mix might take place.
2. If the actual grading curve lies well above the specified curve, the aggregate is finer and more water will be required, thus increasing the quantity of cement also for a constant water cement ratio. Therefore, this is uneconomical.
3. If the actual grading curve is steeper than the specified, it indicates an excess of middle-size particles and leads to harsh mix.
4. If the actual grading curve is flatter than the specified grading curve, the aggregate will be deficient in middle size particles.
Surface Area of Aggregate
The surface area is affected by the maximum size of aggregate. The surface area will vary with the shape but is inversely proportional to the particle size. The mortar consisting of fine aggregate and cement should be slightly in excess of that just required to fill the voids in the coarse aggregate. Too coarse the aggregate results in harshness; too fine the aggregate required too large a water cement ratio for adequate workability. The surface area of aggregate also influence the amount f mixing water cement required. Generally, the water-cement ratio is fixed from strength consideration.
Maximum Size of Aggregate
In general, the larger the maximum size of aggregate, the smaller is the cement requirement for a particular water-cement ratio. This is due to the fact that the workability of concrete increase with the increase in the maximum size of aggregate.
· In the mass concrete, the use of a larger size aggregate is beneficial due to the lesser consumption of cement.
· The smaller the surface area of larger size aggregate, the water-cement ratio can be decrease which increase the strength
· The stress concentration in the mortar aggregate interface increasing with the maximum size of aggregate.
DURABILITY OF AGGREGATE
DURABILITY OF AGGREGATE
-Ability to with stand external or internal damaging attack or the soundness of the aggregate.
SOUNDNESS OF AGGREGATE
-Ability to resist excessive changes in volume due to changes in environmental condition (thermal changes, freezing and thawing).
-Aggregate to be unsound when volume changes result in deterioration of concrete.
-A method to determine the resistance to disintegration of aggregate by saturated solution of sodium sulphate (Na2SO4) or magnesium sulphate (MgSO4).
-Average loss of weight after test should not exceed 12%-18%.
ALKALI-AGGREGATE REACTION
-Reaction between the active silica constituents of the aggregate and the alkalis in cement.
- As a result, an alkali silicate gel is formed and internal pressure is developed cause cracking and disruption of cement paste.
-Rate of reaction depends on:
(a) Particle size and porosity of aggregate.
(b) Alkali content and fineness of cement.
(c) Non-evaporable water in the paste.
(d) Accelerated under alternate wetting and drying.
(e) Accelerated at temperature 10°c-40°c.
(f) Reduced by adding reactive silica in a finely powdered form.
THERMAL PROPERTIES OF AGGREGATE
-Affect durability of concrete.
-3 principal of thermal properties of aggregate:
(a) coefficient of thermal expansion
(b) specific heat
(c) thermal conductivity
- Coefficient of thermal expansion too much, large change in temperature may introduce differential movement and break the bond between aggregate and paste.
- Specific heat increases with moisture content of concrete.
- Thermal conductivity of heat depends upon composition of concrete.
- Thermal conductivity of heat high when lower water content of the mix and normal aggregate than light weight aggregate.
DELETERIOUS SUBSTANCES IN AGGREGATE
-Affect strength, workability and long term performance of concrete.
-Total amount of deleterious material should not >5%.
IMPURITIES
-Form of organic matter which interfaces with the processes of hydration of cement, more likely present in fine aggregate than coarse aggregate.
-Test called Colorimeter Test of A.S.T.M standard.
COATING
-Clay may be present in aggregate in the form of surface coating.
-Coating prevents development of good bond between aggregate and cement.
UNSOUND PARTICLES
Salt contamination
Unsound particles – those that fail to maintain their integrity, those that lead to disruptive expansion on freezing or even on exposure to water. (clay lumps, wood and coal)
Finally, these materials will increase specific surface of aggregate and more water required to wet all particle in mix then reduce strength and durability of concrete.
-Ability to with stand external or internal damaging attack or the soundness of the aggregate.
SOUNDNESS OF AGGREGATE
-Ability to resist excessive changes in volume due to changes in environmental condition (thermal changes, freezing and thawing).
-Aggregate to be unsound when volume changes result in deterioration of concrete.
-A method to determine the resistance to disintegration of aggregate by saturated solution of sodium sulphate (Na2SO4) or magnesium sulphate (MgSO4).
-Average loss of weight after test should not exceed 12%-18%.
ALKALI-AGGREGATE REACTION
-Reaction between the active silica constituents of the aggregate and the alkalis in cement.
- As a result, an alkali silicate gel is formed and internal pressure is developed cause cracking and disruption of cement paste.
-Rate of reaction depends on:
(a) Particle size and porosity of aggregate.
(b) Alkali content and fineness of cement.
(c) Non-evaporable water in the paste.
(d) Accelerated under alternate wetting and drying.
(e) Accelerated at temperature 10°c-40°c.
(f) Reduced by adding reactive silica in a finely powdered form.
THERMAL PROPERTIES OF AGGREGATE
-Affect durability of concrete.
-3 principal of thermal properties of aggregate:
(a) coefficient of thermal expansion
(b) specific heat
(c) thermal conductivity
- Coefficient of thermal expansion too much, large change in temperature may introduce differential movement and break the bond between aggregate and paste.
- Specific heat increases with moisture content of concrete.
- Thermal conductivity of heat depends upon composition of concrete.
- Thermal conductivity of heat high when lower water content of the mix and normal aggregate than light weight aggregate.
DELETERIOUS SUBSTANCES IN AGGREGATE
-Affect strength, workability and long term performance of concrete.
-Total amount of deleterious material should not >5%.
IMPURITIES
-Form of organic matter which interfaces with the processes of hydration of cement, more likely present in fine aggregate than coarse aggregate.
-Test called Colorimeter Test of A.S.T.M standard.
COATING
-Clay may be present in aggregate in the form of surface coating.
-Coating prevents development of good bond between aggregate and cement.
UNSOUND PARTICLES
Salt contamination
Unsound particles – those that fail to maintain their integrity, those that lead to disruptive expansion on freezing or even on exposure to water. (clay lumps, wood and coal)
Finally, these materials will increase specific surface of aggregate and more water required to wet all particle in mix then reduce strength and durability of concrete.
Physical Properties of Aggregate
PHYSICAL PROPERTIES OF CONCRETE
Strength
·Strength of concrete cannot exceed strength of bulk aggregate contained in it.
·Strength of aggregate is sufficient if it is stronger than that of the concrete made with them.
·In practice, majority of normal weight aggregates are considerably stronger than concrete.
·The strength of concrete does not normally exceed 80N/mm square and is generally between 20 and 50 N/mm square, the strength of the aggregates commonly used is in the range of 70 to 350 N/mm square.
·A good value of average crushing strength of aggregate is 200 N/mm square. In general, igeous rocks are very much stronger than sedimentary and metamorphic rocks.
·Test to determine strength: -
1. Aggregate crushing value
2. Aggregate impact value
3. Ten percent fines value (125mm to 10mm particles)
Hardness
·The ability of aggregate to withstand wear or load or pressure applied.
·Depend on the type of parent rock
·Test : abrasion test (BS 812 : Part 113 : 1990)
·A satisfactory aggregate should have an abrasion value of not more than 30% for aggregates used for wearing surfaces and 50% for aggregates used for non wearing surface.
Toughness
·Resistance of aggregate to failure by impact.
·Can be determined by Aggregate Impact Test (MS 30 : Part 10 : 1995)
·Aggregate impact value must not exceed 45% by weight for aggregate used for concrete other than those used for wearing surfaces and 30% for concrete in wearing surfaces.
Durability
·Ability to withstand external or internal damaging attack a.k.a. soundness of aggregate.
·Can be obtained by Soundness Test (BS 812 : Part 121 : 1989)
·Aggregate with high modulus of elasticity generally produces concrete with high modulus of elasticity.
·Modulus of elasticity of aggregate affects the magnitude of creep and shrinkage of concrete.
·Compressibility of aggregate would reduce distress in concrete during its volume changes while a strong and rigid aggregate might lead to the cracking of the surrounding cement paste.
·Thus aggregate of moderate/low strength and modulus of elasticity can be vulnerable in preserving the durability of concrete.
Porosity
·Aggregate normally have pores of various size
·Aggregate will absorb water when it is dry but normally release water in the concrete mix when it is wet.
·Amount of water and its rate of permeation depend on size and volume of aggregate.
·Since 75% of concrete is aggregate, aggregate porosity affects directly porosity of concrete.
Water Absorption
·Is the weight of water absorbed by an oven dry aggregate in reaching the saturated and surface dry condition.
·The water absorption is express as percentage of the weight of the dry aggregate.
·It is determined by measuring the increase in weight of an oven dry sample immersed in water for 24 hours and weighed at a saturated and surface dry condition.
Strength
·Strength of concrete cannot exceed strength of bulk aggregate contained in it.
·Strength of aggregate is sufficient if it is stronger than that of the concrete made with them.
·In practice, majority of normal weight aggregates are considerably stronger than concrete.
·The strength of concrete does not normally exceed 80N/mm square and is generally between 20 and 50 N/mm square, the strength of the aggregates commonly used is in the range of 70 to 350 N/mm square.
·A good value of average crushing strength of aggregate is 200 N/mm square. In general, igeous rocks are very much stronger than sedimentary and metamorphic rocks.
·Test to determine strength: -
1. Aggregate crushing value
2. Aggregate impact value
3. Ten percent fines value (125mm to 10mm particles)
Hardness
·The ability of aggregate to withstand wear or load or pressure applied.
·Depend on the type of parent rock
·Test : abrasion test (BS 812 : Part 113 : 1990)
·A satisfactory aggregate should have an abrasion value of not more than 30% for aggregates used for wearing surfaces and 50% for aggregates used for non wearing surface.
Toughness
·Resistance of aggregate to failure by impact.
·Can be determined by Aggregate Impact Test (MS 30 : Part 10 : 1995)
·Aggregate impact value must not exceed 45% by weight for aggregate used for concrete other than those used for wearing surfaces and 30% for concrete in wearing surfaces.
Durability
·Ability to withstand external or internal damaging attack a.k.a. soundness of aggregate.
·Can be obtained by Soundness Test (BS 812 : Part 121 : 1989)
·Aggregate with high modulus of elasticity generally produces concrete with high modulus of elasticity.
·Modulus of elasticity of aggregate affects the magnitude of creep and shrinkage of concrete.
·Compressibility of aggregate would reduce distress in concrete during its volume changes while a strong and rigid aggregate might lead to the cracking of the surrounding cement paste.
·Thus aggregate of moderate/low strength and modulus of elasticity can be vulnerable in preserving the durability of concrete.
Porosity
·Aggregate normally have pores of various size
·Aggregate will absorb water when it is dry but normally release water in the concrete mix when it is wet.
·Amount of water and its rate of permeation depend on size and volume of aggregate.
·Since 75% of concrete is aggregate, aggregate porosity affects directly porosity of concrete.
Water Absorption
·Is the weight of water absorbed by an oven dry aggregate in reaching the saturated and surface dry condition.
·The water absorption is express as percentage of the weight of the dry aggregate.
·It is determined by measuring the increase in weight of an oven dry sample immersed in water for 24 hours and weighed at a saturated and surface dry condition.
Wednesday, February 20, 2008
Aggregate .... (-.-) ZzZzZ...
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Hope you all ENJOY it
Will be updating it soon. Peace!!
Hope you all ENJOY it
Will be updating it soon. Peace!!
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