Effect of Maximum size of Aggregate on Strength of CONCRETE

Earlier it was thought that the use of larger size aggregate leads to higher strength.This was due to the fact that the larger the aggregate the lower is the total surface area and, therefore, the lower is the requirement of water for the given workability. So, a lower water/cement ratio can be used which will result in higher strength of Concrete.
However, later it was found that the use of larger size aggregate did not contribute to higher strength as expected from the theoretical considerations due to the following reasons..

1. The larger maximum size aggregate gives lower surface area for developments of gel bonds which is responsible for the lower strength of the concrete.
2. Secondly bigger aggregate size causes a more heterogeneity in the concrete which will prevent the uniform distribution of load when stressed.

When large size aggregate is used, due to internal bleeding, the transition zone will become much weaker due to the development of micro-cracks which result in lower compressive strength.
Generally, high strength concrete or rich concrete is adversely affected by the use of large size aggregate. But in lean mixes or weaker concrete the influence of size of the aggregate gets reduced. It is interesting to note that in lean mixes larger aggregate gives highest strength while in rich mixes it is the smaller aggregate which yields higher strength. The Fig.  below shows the influence of maximum size of aggregate on compressive strength of concrete..

 
Following Figure depicts the influence of size of aggregate on compressive strength of concrete for different w/c ratio.

AGGREGATE IMPACT VALUE TEST

    With Respect to concrete aggregates,toughness is usually considered the resistance of the material to failure by impact.Several attempts to develop a method of test for aggregates impact value have been made.The most successful and known test is described below..

APPARATUS
i) Impact testing machine conforming to IS: 2386 (Part IV) - 1963
ii) IS Sieves of sizes - 12.5mm, 10mm and 2.36mm
iii) A cylindrical metal measure of 75mm dia. and 50mm depth
iv) A tamping rod of 10mm circular cross section and 230mm length, rounded at one end
v) Oven

PREPARATION OF SAMPLE
i) The test sample should conform to the following grading:
- Passing through 12.5mm IS Sieve 100%
- Retention on 10mm IS Sieve 100%
ii) The sample should be oven-dried for 4hrs. at a temperature of 100 to 110^oC and cooled.
iii) The measure should be about one-third full with the prepared aggregates and tamped with 25 strokes of the tamping rod.A further similar quantity of aggregates should be added and a further tamping of 25 strokes given. The measure should finally be filled to overflow, tamped 25 times and the surplus aggregates struck off, using a tamping rod as a straight edge. The net weight of the aggregates in the measure should be determined to the nearest gram (Weight 'A').

PROCEDURE
i) The cup of the impact testing machine should be fixed firmly in position on the base of the machine and the whole of the test sample placed in it and compacted by 25 strokes of the tamping rod.
ii) The hammer should be raised to 380mm above the upper surface of the aggregates in the cup and allowed to fall freely onto the aggregates. The test sample should be subjected to a total of 15 such blows, each being delivered at an interval of not less than one second.

REPORTING OF RESULTS
i) The sample should be removed and sieved through a 2.36mm IS Sieve. The fraction passing through should be weighed (Weight 'B'). The fraction retained on the sieve should also be weighed (Weight 'C') and if the total weight (B+C) is less than the initial weight (A) by more than one gram, the result should be discarded and a fresh test done.
ii) The ratio of the weight of the fines formed to the total sample weight should be expressed as a percentage.

Aggregate impact value = B/A x 100%

iii) Two such tests should be carried out and the mean of the results should be reported.

A sample proforma for the record of the test results is given below..

    As per IS 283-1970 aggregate  impact value shall no exceed 45% by weight for aggregates used for concrete other than wearing surface and 30% for concrete of wearing surfaces (Run ways,Roads & Pavements)

WATER ABSORPTION TESTS OF AGGREAGATES

      Water Absorption is the percent of water necessary to add to the aggregate to obtain saturated surface dry (SSD) condition.

This test determines the water absorption of coarse aggregates as per IS: 2386 (Part III) - 1963.
APPARATUS
i) Wire basket - perforated, electroplated or plastic coated with wire hangers for suspending it from the balance
ii) Water-tight container for suspending the basket
iii) Dry soft absorbent cloth - 75cm x 45cm (2 nos.)
iv) Shallow tray of minimum 650 sq.cm area
v) Air-tight container of a capacity similar to the basket
vi) Oven
SAMPLE
A sample not less than 2000g should be used.

PROCEDURE
i) The sample should be thoroughly washed to remove finer particles and dust, drained and then placed in the wire basket and immersed in distilled water at a temperature between 22 and 32^oC.
ii) After immersion, the entrapped air should be removed by lifting the basket and allowing it to drop 25 times in 25 seconds. The basket and sample should remain immersed for a period of 24 + ½ hrs. afterwards.
iii) The basket and aggregates should then be removed from the water, allowed to drain for a few minutes, after which the aggregates should be gently emptied from the basket on to one of the dry clothes and gently surface-dried with the cloth, transferring it to a second dry cloth when the first would remove no further moisture. The aggregates should be spread on the second cloth and exposed to the atmosphere away from direct sunlight till it appears to be completely surface-dry. The aggregates should be weighed (Weight 'A').
iv) The aggregates should then be placed in an oven at a temperature of 100 to 110^oC for 24hrs. It should then be removed from the oven, cooled and weighed (Weight 'B').

REPORTING OF RESULTS
Water absorption = (A – B)/B x 100%
Two such tests should be done and the individual and mean results should be reported.

Aggregates

Aggregates are inert granular materials such as sand, gravel or crushed stone that are an end product in their own right. They are also the raw materials that, along with water and hydraulic binders, are an essential ingredient in concrete. For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete.

Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into several distinct categories, and are either coarse or fine:

Coarse aggregates

Coarse aggregates are particles greater than 4.75mm, but generally range between 9.5mm to 37.5mm in diameter. They can either be from Primary, Secondary or Recycled sources. Primary, or 'virgin', aggregates are either Land- or Marine-Won. Gravel is a coarse marine-won aggregate; land-won coarse aggregates include gravel and crushed rock. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder.

    
Coarse Aggregates

Fine Aggregates 

Fine aggregate are basically sands won from the land or the marine environment. They are less than 4.75mm. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 9.5mm sieve. As with coarse aggregates these can be from Primary, Secondary or Recycled sources.

  Sand

AGGREGATE CRUSHING VALUE

       Aggregate Crushing Value Test is for determining the aggregate crushing value of coarse aggregates.
APPARATUS 

i) Cylindrical measure and plunger
ii) Compression testing machine
iii) IS Sieves of sizes - 12.5mm, 10mm and 2.36mm

PROCEDURE
i) The aggregates passing through 12.5mm and retained on 10mm IS Sieve are oven-dried at a temperature of 100 to 110oC for 3 to 4hrs.
ii) The cylinder of the apparatus is filled in 3 layers, each layer tamped with 25 strokes of a tamping rod.
iii) The weight of aggregates is measured (Weight 'A').
iv) The surface of the aggregates is then levelled and the plunger inserted. The apparatus is then placed in the compression testing machine and loaded at a uniform rate so as to achieve 40t load in 10 minutes. After this, the load is released.
v) The sample is then sieved through a 2.36mm IS Sieve and the fraction passing through the sieve is weighed (Weight 'B').
vi) Two tests should be conducted.

REPORTING OF RESULTS
             Aggregate crushing value = B/A x 100%
      The result should be recorded to the first decimal place and the mean of the two results reported.

AGGREGATE ABRASION VALUE

This test helps to determine the abrasion value of coarse aggregates as per IS: 2386 (Part IV) – 1963.

The apparatus used in this test are Los Angles abrasion testing machine, IS Sieve of size – 1.7mm, Abrasive charge – 12 nos. cast iron or steel spheres approximately 48mm dia. and each weighing between 390 and 445g ensuring that the total weight of charge is 5000 +25g and Oven.

Sample Preparation

The test sample should consist of clean aggregates which has been dried in an oven at 105 to 110oC to a substantially constant weight and should conform to one of the gradings shown in the table below:

Procedure to determine Aggregate Abrasion Value

The test sample and the abrasive charge should be placed in the Los Angles abrasion testing machine and the machine rotated at a speed of 20 to 33 revolutions/minute for 1000 revolutions. At the completion of the test, the material should be discharged and sieved through 1.70mm IS Sieve.

Reporting of Results

i) The material coarser than 1.70mm IS Sieve should be washed, dried in an oven at a temperature of 100 to 110^oC to a constant weight and weighed (Weight ‘B’).

ii) The proportion of loss between weight ‘A’ and weight ‘B’ of the test sample should be expressed as a percentage of the original weight of the test sample. This value should be reported as,
Aggregate abrasion value = (A-B)/B x 100%.