Normal Aggregate

DR. Khalid Alshafei

Normal Aggregate

• 75% of volume of concrete is occupied by aggregate → coarse & fine aggregates

Hence → aggregate affect the properties of concrete i.e. its physical, thermal & chemical properties influence the performance of concrete.

• Formation of aggregate: 1)aggregate is found in nature thru the process of weathering and abrasion or from crushing large mass of rock.

2) Artificial aggregate: ex. broken bricks & blast fume slag, however these are not good for concrete and they are free from lime.

Aggregate

• Properties of aggregate: These properties influence performance of concrete( strength, durability & structural performance)

- chemical & mineral composition - physical: shape, size, texture - specific gravity - hardness - strength - absorption

Aggregate

• How you determine good aggregate: by testing.• Strength of aggregate should not be less than

30-35 N.

• Classification of aggregate:1. Size classification; 2. petrographic or geological classification3. Shape & texture classification

Aggregate

• Size classification - coarse aggregate ranges between 10mm to 50mm,

typically 20mm. - fine aggregate: sand ≈0.07 mm- silt: between 0.06- 0.02 mm- Smaller particles termed as clay. • Petrographic / geological classification: It is the

classification of aggregate according to its common characteristics found in several groups of rock. ( table 3.1)

Aggregate

• Common minerals found in aggregate; - Silica, carbon, sulphate, iron sulphate, iron oxide.Shape & Texture classification(External

charecteristics) - The external shape of aggregate and surface texture

affect the properties of concrete. - Aggregate come in different shapes: rounded,

irregular, angular, flaky( thickness is smaller relative to two other dimension)( refer to table 3.2).

Aggregate

- Sea aggregate may contain shells whose content need to be controlled because they are brittle and they also reduce the workability of the mix.

- Surface texture classification: Surface texture of aggregate is based on the degree to which the particle surfaces are polished or dull, smooth or rough. Example: Glassy(ex. Slag),

Smooth(water worn), granular( fracture), rough, crystalline(contain crystal), honeycomb(contain pores & cavities).

- The shape & surface texture have a strong influence on the water requirement of the mix.

- Surface texture depends on hardness, grain size and pore characteristics of parent material.

Aggregate• Mechanical Properties1. Bond: rough texture result in greater adhesion or bond between the

particles and cement.- Quality of bond can be determined as good when crushed concrete specimen

contain some aggregate particles broken right through - Flexural strength is more affected by bond than compressive strength. - larger surface area of amore angular aggregate provide greater bond. 2. Strength: compressive strength of concrete cannot exceed the contained

aggregate. This is because of the failure of the bond of the cement and other particles in the concrete.

- Strength of aggregate determined from indirect tests: crushing strength of prepared rock samples, crushing value of bulk aggregate, and performance of aggregate in concrete.

. 3. Toughness: It is the resistance of aggregate to failure

by impact. The aggregate impact value is determined by the crushing value. ( refer to book for test procedure).

- maximum values of the average of duplicate samples: - 25% for heavy duty concrete. - 30% for concrete pavement - 45% for other concrete.4. Hardness: resistance to wear. It is assessed by using “ the aggregate abrasion value”

Aggregate

• Physical Properties1. Specific gravity ( or relative density) It is the ratio of mass of a unit volume of material to the

mass of the same volume of water at the stated temperature.

2. Porosity and absorption - Porosity: the ability to absorb water. - Permeability: the capability of porous material to permit

the flow of fluids through its pores spaces. - The Porosity, permeability, and absorption of aggregate

influence the bond between it and the cement paste.

Aggregate

Aggregate

- Water absorption is determined by measuring the decrease in mass of saturated & surface dry sample after oven drying for 24 hours. The ratio of the decrease in mass to the mass of the sample expressed as a percentage is termed absorption.

- The actual water absorption of the aggregate has to be deducted from the total water requirement of the mix to obtain the effective water/cement ratio, which controls both the workability and the strength of concrete.

Aggregate

3. Moisture Content It is the water in excess of the saturated and

surface-dry condition. Hence the total water content of a moist aggregate is equal to the sum of absorption and moisture content.

Aggregate• Deleterious substances found in aggregate 1. Impurities: interfere with the process of hydration of cement.2. Coatings: ( ex. Clay) prevent the development of good bond

between aggregate & cement paste.3. Sulphate and chloride salts.4. Other weak or unsound particles.

• Organic Impurities: It consists of products of decay of vegetables matter & usually present in sand rather than coarse aggregate and it is easily removed by washing. It interferes with hydration process.

Aggregate• Clay: Clay is present in aggregate in the form of surface coating

which interfere with the bond between the aggregate and cement paste.

- clay and very fine materials should not be present in large quantities because, due to their fineness and therefore large surface area, they increase the amount of water necessary to wet all the particles in the mix.

• Sieve analysis: It is the process of dividing a sample of aggregate into fractions of same particle size. Its purpose is to determine the grading or size distribution of the aggregate.

- 5mm is the dividing line between the fine and coarse aggregate.

Aggregate

• Fineness modulus(FM): It is the sum of the cumulative percentages retained on the sieve of the standard series , divided by 100.

FM is usually calculated for the fine aggregate rather than for course aggregate.

- Typical values range from 2.3 and 3.0, a higher value indicate a coarser grading. FM detects the variation s in the aggregate from the same source which affect the workability of the fresh concrete.

Aggregate

• Grading: Particle size distribution. - Grading is important because it affects workability.

However strength is independent grading.Main influences factors on workability:1. Surface area of the aggregate: determines the

amount of water necessary to wet all the solids.2. The relative volume occupied by the aggregate.3. The tendency to segregation.4. The amount of fines in the mix.

Aggregate

• Surface area is measured in terms of specific surface i.e. the ratio of the surface of all the particles to their volume.

• A larger particle size has a lower specific surface, hence water requirement decreases.

• The relative volume of the aggregate affects workability. An economic requirement is that the aggregate occupies as a large a relative volume as possible since it is cheaper than cement paste.

aggregate

• The workability is improved when there is an excess of paste to fill voids in the sand, and also an excess of mortar to fill the voids in the coarse aggregate because the fine material lubricate the larger particles.

• In case grading of aggregate extended to a larger maximum size, this will lower the water requirement of the mix, hence water/ cement ratio can be reduced which lead to increase in concrete strength.

Aggregate

• However there is a limit of maximum aggregate size due to the following:

1 - Decrease in water will lower bond area and discontinuities introduced by very large particles. Hence concrete becomes grossly heterogeneous with resulting lowering of strength.

2- In structural concrete , the maximum size is restricted to 25 mm because of the size of concrete section and of spacing of reinforcement.

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• The maximum aggregate size should be smaller by 5 mm than the horizontal bar spacing and smaller than 2/3 of the vertical spacing.

• In conclusion it is important to use aggregate with a grading such that a reasonable workability and minimum segregation are obtained in order to produce a strong and economical concrete.

Aggregate

• Gap-graded aggregateAggregate is gap-graded when intermediate sizes are

absent from the gradation curve. On the grading curve, gap-grading is represented by a horizontal line over the range of sizes omitted( see fig. 3.5).

Gap-graded aggregate is distincted from continuously graded conventional aggregate.

Gap grading mixes are used to obtain uniform textures of exposed-aggregate concrete.

Aggregate

• Local (white) aggregate in Bahrain is not used in structural concrete because it contains high chloride and react with alkalis like K₂O, Na₂O which cause disintegration of concrete and affect its strength. In Bahrain we use Ras Alkhaima(Rak) or Saudi aggregate.

• Sizes of fine and coarse aggregate ; - fine aggregate: 0-5mm - coarse aggregate: 5-50mm

Aggregate

• Sphericity is one of aspect of the shape of coarse aggregate and defined as a function of the ratio of the surface area of the particle to its volume(specific surface)

• Aggregate particle can be both flaky and elongated.

• Soundness of aggregate is aggregate’s resistance to disintegration by weathering and, in particular, freeze-thaw cycles.

Aggregate

• Shape of aggregate , along with grading, is the most important consideration for a high quality concrete aggregate, as it effects the workability, finishability and water/cement ratio of concrete.

• Well size distribution of the aggregate particles (smaller particles added in the voids between the larger ones) increase concrete density.

Fineness modulus

Aggregate