The aggregates are important concrete ingredients. Aggregates should possess excellent mechanical properties to ensure that the concrete mix will develop the required strength. Aggregates effects are not limited to concrete strength. also, the durability of concrete can be affected greatly by aggregates
Bonding of aggregates to cement paste is a critical property. The bonding depends on the shape and surface texture of aggregates. Aggregate with rough surface will develop a good bond with cement paste.
Similarly, a large surface area with angular shape will produce a high bond with cement paste. A good bond between aggregates and cement paste will not affect significantly the concrete compressive strength, but a good bond will improve the flexural strength of concrete.
There are no direct tests to evaluate the bond property. In general, when the bond is good, not all aggregates will separate from the concrete mix. Some of the aggregates will be broken.
However, if a large number of aggregates broken. This indicates that the aggregates are weak.
The compressive strength of concrete is affected significantly by the aggregates. If the aggregates that contain weak aggregates particles can substantially reduce the concrete mixture strength.
There is no direct test to determine the strength of aggregates, but there are indirect tests that can be used to evaluate the strength of aggregates. Crushing strength of prepared rock sample, crushing strength of bulk aggregates and the performance of aggregates in concrete can be used to assess the strength of the aggregate.
Crushing of prepared rock sample is not used widely. But we should note that a good value of the crushing of a prepared rock sample is 200 Mpa. However, many good aggregates the value can be up to 80 Mpa.
The used aggregates should have a higher strength than the normal concrete strength because the actual stress on the individual aggregates is higher than the nominal compressive strength. Another test used to evaluate the strength of aggregates is the bulk aggregate crushing test.
The material used for this test should pass 14 mm (0.5 in) sieve and retain on 10.0 mm (3/8 in). The aggregate sample should be dried at 100-110 C for 4 hours. Then the material will be placed on a mold and tampered.
A plunger will be placed on top of the sample, and a load of 400 kn (40 tons) will be applied over the gross-area of the plunger (pressure of 22.1 Mpa). The load will be increased gradually for 10 minutes. Then the plunger is removed, and the sample is sieved at 2.36 mm size sieve. The ratio of the mass of aggregate passing 2.36 mm sieve to the total mass of the sample is the aggregate crushing value.
Toughness is the resistance of aggregate to failure by the impact. The aggregate impact value can be determined for bulk aggregate. BS:812-112: 1990 provides full details for the toughness test. These tests will be related to the crushing value of aggregate. The tested aggregate can be surface dray or saturated.
The size of the aggregate should be passing 14 mm and retaining on 10 mm. Then the aggregate sample is placed on the mold and subjected to impact by a hammer falling 15 times under its weight. The aggregate sample then sieved by 2.36 mm sieve, and the ratio of the mass of aggregate passing 2.36 mm sieve to the total mass of soil sampled is determined.
The test result will determine the toughness of aggregate. Aggregate with a 25% crushing value can be used for heavy-duty concrete. For aggregate with a 30% crushing value can be used for concrete pavement.
Hardness is an important property of aggregate, and it is used to determine the resistance of an aggregate to wear. Hardness can be determined by the Los angles abrasion test.