Explain Properties of Aggregates.

The Properties of Aggregates can be classified as:

1. Physical Properties
2. Mechanical Properties
3. Chemical Properties

Physical Properties:

 Some of the physical properties of Aggregates are:

1. Shape
2. Size
3. Texture
4. Specific Gravity
5. Bulk Density
6. Porosity
7. Bulking of Sand

Shape:

As per shape, Aggregates may be:

• Rounded Aggregates:Due to its minimum surface area, these type of aggregate gives good workability with lower W/C ratio also & require minimum cement paste for bonding. So it is considered best for economy point of view. But due to its poor interlocking, it is unsuitable for high strength concrete.
  
• Angular:
  The angular aggregates are superior to rounded aggregates from the following two points of view:
  
  • Angular aggregates exhibit a better interlocking effect in concrete, which property makes it superior in concret....

The Properties of Aggregates can be classified as:

1. Physical Properties
2. Mechanical Properties
3. Chemical Properties

Physical Properties:

 Some of the physical properties of Aggregates are:

1. Shape
2. Size
3. Texture
4. Specific Gravity
5. Bulk Density
6. Porosity
7. Bulking of Sand

Shape:

As per shape, Aggregates may be:

• Rounded Aggregates:Due to its minimum surface area, these type of aggregate gives good workability with lower W/C ratio also & require minimum cement paste for bonding. So it is considered best for economy point of view. But due to its poor interlocking, it is unsuitable for high strength concrete.
  
• Angular:
  The angular aggregates are superior to rounded aggregates from the following two points of view:
  
  • Angular aggregates exhibit a better interlocking effect in concrete, which property makes it superior in concrete used for roads and pavements.
  • The total surface area of rough textured angular aggregate is more than smooth rounded aggregate for the given volume. By having greater surface area, the angular aggregate may show higher bond strength than rounded aggregates.

The higher surface area of angular aggregate with rough texture requires more water for a given workability than rounded aggregates. This means that for a given set of conditions from the point of view of water/cement ratio and the consequent strength, rounded aggregate gives higher strength. Superimposing plus and minus points in favour and against these two kinds of aggregates it can be summed up as follows: 

For water/cement ratio below 0.4, the use of crushed aggregate has resulted in strength up to 38 per cent higher than the rounded aggregate. With an increase in water/cement ratio the influence of roughness of surface of the aggregate gets reduced, presumably because the strength of the paste itself becomes paramount, and at a water/cement ratio of 0.65, no difference in strength of concrete made with angular aggregate or rounded aggregate has
been observed.        

• Irregular: Aggregate comprising irregular aggregate type (between round & angular aggregate). Normally obtained from natural quarries. These type of aggregate gives workability & interlocking in between round & angular aggregate. These are also considered unsuitable for high strength concrete.
• Flaky: Aggregate having its least dimension less than 0.6 times its mean dimension is said to be flaky. It relatively comprises of thin particles. Due to its poor bonding & low strength, it is not considered suitable for medium or high strength concrete.
• Elongated: Aggregate having its larger dimension greater than 1.8 times its mean dimension is said to be elongated. It relatively comprises of long particles. Due to its poor bonding character, it is not considered suitable for medium or high strength concrete.

Size:

The largest maximum size of aggregate practicable to handle under a given set of conditions should be used. Perhaps, 80 mm size is the maximum size that could be conveniently used for concrete making. Using the largest possible maximum size will result in the

• reduction of the cement content
• reduction in water requirement
• reduction of drying shrinkage.

• Spacing of reinforcement
• Thickness of section;
• Clear cover;
• Mixing, handling and placing techniques.

Generally, the maximum size of aggregate should be as large as possible within the limits specified, but in any case not greater than one-fourth of the minimum thickness of the member.

Aggregates are divided into two categories from the consideration of size:

• Coarse aggregate and
• Fine aggregate.

Surface Texture:

Surface texture is the property, the measure of which depends upon the relative degree to which particle surfaces are polished or dull, smooth or rough.As surface smoothness increases, contact area decreases, hence a highly polished particle will have less bonding area with the matrix than a rough particle of the same volume. A smooth particle, however, will require a thinner layer of paste to lubricate its movements with respect to other aggregate particles. It will, therefore, permit denser packing for equal workability and hence, will require lower paste content than rough particles. It has been also shown by experiments that rough textured aggregate develops higher bond strength in tension than smooth textured aggregate. Surface texture characteristics of the aggregate as classified in IS: 383: 1970 is shown below.

1. Glassy: Eg: black flint
2. Smooth: Eg, chert, slate, marble etc
3. Crystalline: Eg, basalt, dolerite, granite etc
4. Granular: Eg, sand stone, oolite etc
5. Honeycombed or porous: Aggregate with porous surface texture. Eg, pumice
   

Measurement of Surface Texture:

 A large number of possible methods are available and this may be divided broadly into direct and indirect methods. Direct methods
includes:

1. making a cast of the surface and magnifying a section of this,
2. Tracing the irregularities by drawing a fine point over the surface and drawing a trace magnified by mechanical, optical, or electrical means,
3. getting a section through the aggregates and examining a magnified image.

1. measurement of the degree of dispersion of light falling on the surface,
   
2. determining the weight of a fine powder required to fill up the interstices of the surface to a truly smooth surface,
3. the rock surface is held against rubber surface at a standard pressure and the resistance to the flow of air between the two surfaces is measured.

Specific Gravity:

It is defined as the ratio of the weight of the certain volume of material to the weight of same volume of water. High specific gravity indicates good quality materials.

Bulking of Sand:

The increase in the volume of sand (fine aggregates) due to the presence of the moisture content is known as Bulking of sand. Any moisture content at the surface of aggregate forms a film around each particle which exerts surface tension  keeping the neighbouring particles away from it. Therefore, no point
contact is possible between the particles. This causes bulking of the volume.

It is interesting to note that the bulking increases with the increase in moisture content upto a certain limit and beyond that the further increase in the moisture content results in the decrease in the volume and at a moisture content representing saturation point, the fine aggregate shows no bulking. When the moisture content is increased beyond 8 to 10 %, the bulking of sand almost disappears.

Fine sands bulk greater than Coarse Sand since the fine grained sands have higher voids than medium and coarse grained sands due to which the percentage of moisture absorbed and hence the surface tension force increases and shows relatively more bulking.

alt="Explain the Properties of Aggregates."

The extent of bulking can be estimated by a simple field test. A sample of moist fine aggregate is filled into a measuring cylinder in the normal manner. Note down the level, say `h_1` . Pour water into the measuring cylinder and completely inundate the sand and shake it. Since the volume of the saturated sand is the same as that of the dry sand, the inundated sand completely offsets the bulking effect. Note down the level of the sand say, `h_2` . Then `h_1 ? h_2`
shows the bulking of the sample of sand under test.

Percentage of bulking `=(h_2-h_1)/h_2 *100`

Absorption and Moisture Content:

The ratio of the increase in weight to the weight of the dry sample expressed as percentage is known as absorption of aggregate.

The moisture condition defines the presence and amount of water in the pores and on the surface of the aggregate. There are four moisture conditions, as demonstrated in Figure.

alt="Explain the Properties of Aggregates."

1. Oven dry (OD): This condition is obtained by keeping the aggregate in an oven at a temperature of 110 degree Centigrade long enough to drive all water out from internal pores and hence reach a constant weight.
2. Air dry (AD): This condition is obtained by keeping the aggregate at ambient temperature and ambient humidity. Under such condition, pores inside of aggregate are partly filled with water. When aggregate is under either the OD or AD condition, it will absorb water during the concrete mixing process until the internal pores are fully filled with water.
3. Saturated surface dry (SSD): In this situation, the pores of the aggregate are fully filled with water and the surface is dry. This condition can be obtained by immersing coarse aggregate in water for 24 h followed by drying of the surface with a wet cloth. When the aggregate is under the SSD condition, it will neither absorb water nor give out water during the mixing process. Hence, it is a balanced condition and is used as the standard index for concrete mix design.
4. Wet (W): The pores of the aggregate are fully filled with water and the surface of the aggregate has a film of water. When aggregate is in a wet condition, it will give out water to the concrete mix during the mixing process. Since sand is usually obtained from a river, it is usually in a wet condition.
   

Porosity:

Less porous aggregates are preferred.

Soundness of Aggregate:

Soundness refers to the ability of aggregate to resist excessive changes in volume as a result of changes in physical conditions. These physical conditions that affect the soundness of aggregate are the freezing the thawing, variation in temperature, alternate wetting and drying under normal conditions and wetting and drying in salt water. Aggregates which are porous, weak and containing any undesirable extraneous matters undergo excessive volume
change when subjected to the above conditions. Aggregates which undergo more than the specified amount of volume change is said to be unsound aggregates. If concrete is liable to be exposed to the action of frost, the coarse and fine aggregate which are going to be used should be subjected to soundness test.

The soundness test consists of alternative immersion of carefully graded and weighed test sample in a solution of sodium or magnesium sulphate and oven drying it under specified conditions. The accumulation and growth of salt crystals in the pores of the particles is thought to produce disruptive internal forces similar to the action of freezing of water or crystallisation of salt. Loss in weight, is measured for a specified number of cycles. Soundness test is specified
in IS 2386 (Part V). As a general guide, it can be taken that the average loss of weight after 10 cycles should not exceed 12 per cent and 18 per cent when tested with sodium sulphate and magnesium sulphate respectively.
It may be pointed out that the sulphate soundness test might be used to accept aggregates but not to reject them, the assumption being that aggregates which will satisfactorily withstand the test are good while those which breakdown may or may not be bad. Unfortunately, the test is not reliable. Certain aggregates with extremely fine pore structure show almost no loss of weight. Conversely, certain aggregates that disintegrate readily in the sulphate test but produce concrete of high resistance to freezing and thawing. A low loss of weight usually. but not always, an evidence of good durability, whereas a high
loss of weight places the aggregate in questionable category.

Mechanical Properties:

 Some of the Mechanical properties of Aggregates are:

• Bond and Bond Strength
• Crushing Strength
• Abrasion Strength (Hardness)
• Impact Value (Toughness)
  

Bond and Bond Strength:

Bond is the interlocking Capacity of the aggregate and adhesion between aggregate and cement paste.

Bond strength is the resistance developed to split the aggregate particles from hardened cement paste.

Crushing Strength:

The ?aggregate crushing value? gives a relative measure of the resistance of an aggregate to crushing under a gradually applied compressive load. The standard aggregate crushing test is made on aggregate passing a 12.5 mm I.S. Sieve and retained on 10 mm I.S. Sieve. About 6.5 kg material consisting of aggregates passing 12.5 mm and retained on 10 mm sieve is taken. The aggregate in a surface dry condition is filled into the standard cylindrical measure in three layers approximately of equal depth. Each layer is tamped 25 times with the tamping rod and finally levelled off using the tamping rod as straight edge. The weight of the sample contained in the cylinder measure is taken (A). The same weight of the sample is taken for the subsequent repeat test.The cylinder of the test appartus with aggregate filled in a standard manner is put in position on the base-plate and the aggregate is carefully levelled and the
plunger inserted horizontally on this Aggregate Crushing Value Apparatus. surface. The plunger should not jam in the cylinder.
The appartus, with the test sample and plunger in position, is placed on the compression testing machine and is loaded uniformly upto a total load of 40 tons in 10 minutes time. The load is then released and the whole of the material removed from the cylinder and sieved on a 2.36 mm I.S. Sieve. The fraction passing the sieve is weighed (B).

The Aggregate Crushing Value `=B/A*100`

Where, A= weight of surface-dry sample taken in mould.

    and B= weight of fraction passing 2.36 mm sieve.

The aggregate crushing value should not be more than 45 per cent for aggregate used for concrete other than for wearing surfaces, and 30 per cent for concrete used for wearing surfaces such a runways, roads and air field pavements.

Abrasion Strength (Hardness):

It is the property by virtue of which the aggregate can resist the wearing and tearing effect. Hardness of aggregate is measured by following three methods:

1. Los Angel's Method
2. Darry Abrasion Test
3. Deval Attrition Test

However, the use of Los Angeles abrasion testing machine gives a better realistic picture of the abrasion resistance of the aggregate. This method is only described herein.

The test sample consist of clean aggregate which has ben dried in an oven at 105°C to 110°C and it should conform to one of the gradings shown in Table 3.22.

alt="Explain the Properties of Aggregates."

Test sample and abrasive charge are placed in the Los Angeles Abrasion testing machine and the machine is rotated at a speed of 20 to 33 rev/min. For gradings A , B , C and D , the machine is rotated for 500 revolutions. For gradings E , F and G , it is rotated 1000 revolutions. At the completion of the above number of revolution, the material is discharged from the machine and a preliminary separation of the sample made on a sieve coarser than 1.7 mm
IS Sieve. Finer portion is then sieved on a 1.7 mm IS Sieve. The material coarser than 1.7 mm IS Sieved is washed, dried in an oven at 105° to 110°C to a substantially constant weight and accurately weighed to the nearest gram.

The difference between the original weight and the final weight of the test sample is expressed as a percentage of the original weight of the test sample. This value is reported as the percentage of wear. The percentage of wear should not be more than 16 percent for concrete aggregates.

Impact Value (Toughness):

It is the ability of aggregate to resist the effect of sudden impact or shock and repeating loads.

The test sample consists of aggregate passing through 12.5 mm and retained on 10 mm I.S. Sieve. The aggregate shall be dried in an oven for a period of four hours at a temperature of 100°C to 110°C and cooled. The aggregate is filled about one-third full and tamped with 25 strokes by the tamping rod. A further similar quantity of aggregate is added and tamped in the standard manner. The measure is filled to over-flowing and then struck off level. The net
weight of the aggregate in the measure is determined (weight A ) and this weight of aggregate shall be used for the duplicate test on the same material.
The whole sample is filled into a cylindrical steel cup firmly fixed on the base of the machine. A hammer weighing about 14 kgs. is raised to a height of 380 mm above the upper surface of the aggregate in the cup and allowed to fall freely on the aggregate. The test sample shall be subjected to a total 15 such blows each being delivered at an interval of not less than one second. The crushed aggregate is removed from the cup and the whole of it is sieved on 2.36 mm I.S. Sieve. The fraction passing the sieve is weighed to an accuracy of 0.1 gm. (weight B). The fraction retained on the sieve is also weighed (weight C). If the total weight (B + C) is less than the initial weight A by more than one gm the result shall be discarded and a fresh test made. Two tests are made.

The ratio of the weight of fines formed to the total sample weight in each test is expressed as percentage.

Aggregate Impact Value `=B/A*100`

Where, A= weight of oven-dried sample.

and B= weight of fraction passing 2.36 mm I.S. Sieve.

The aggregate impact value should not be more than 45 per cent by weight for aggregates used for concrete other than wearing surfaces and 30 per cent by weight for concrete to be used as wearing surfaces, such as runways, roads and pavements.

Chemical Properties:

 Some of the Chemical properties of Aggregates are:

1. Alkali Aggregate Reaction

The reactivity of the aggregate is due to the presence of some forms of silica and carbonate in the aggregate that are chemically sensitive to the alkalis present in the cement. Accordingly, two forms of alkali- aggregate reactions are recognised:

1. Alkali-Silica Reaction
2. Alkali-Carbonate Reaction

Alkali-Silica Reaction:

The Alkali-Silica Reaction, commonly known as concrete cancer, is a swelling reaction that occurs over time in concrete between highly alkaline cement paste and the silica present in the aggregates.

The Alkali-Silica Reaction starts with the attack of alkali hydroxides derived from alkalies (K_2O,Na_2O) in the cement on the silicious materials in the aggregates. As a result, a hygroscopic alkali-silicate gel of unlimited swelling type is formed. This gel is surrounded and confined by the hydrated cement paste which acts as a semipermeable membrane. The tendency of the alkali silica gel to expand, by absorbing water, within this membrane creates an internal pressure leading to the expansion, cracking ans dispertion of the cement paste.

Alkali-Carbonate Reaction:

The Alkali-Carbonate Reaction is a process suspected for the degradation of concrete containing dolomite aggregate.

Alkali from cement might react with the dolomite crystals present in the aggregate inducing the production of brucite `Mg(OH)_2` and calcite `CaCO_3`.

Brucite could be responsible for the volumetric expansion after de-dolomotisation of water.

Some of the effect of Alkali-Aggregate Reaction are as follows:

1. The stress induced by the growth of silica gel results in the formation of cracks which in turns leads turns leads to the subsequent loss of strength and elasticity.
2. Alkali Aggregate reaction also accelerates other process of deterioration of concrete due to the formation of cracks.
3. Many destructive forces becomes operative on the concrete disrupted by alkali-aggregate reaction which will further hasten the total disintegration of concrete.

The Alkali-Aggregate Reaction can be controlled by :

1. Selection of non reactive aggregates.
2. By the use of low alkali cement.
3. By the use of corrective admixtures such as Pozzolona.
4. By controlling the Void space in concrete.
5. By controlling Moisture content and Temperature.
6. By using a leaner mix.
7. By the provision of air entrainment.
8. By the dilution of reactive aggregate with a non reactive one.
9. By the use of reactive aggregate in finely divided form.