Concrete Workability Tests

Workability Tests

• Slump test is widely used due to its simplicity and application.
• Compacting factor test is more precise and sensitive than the slump test, particularly useful for concrete mixes of very low workability.
• Compacting factor test measures the degree of compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height.
• Compacting factor is measured by the density ratio, i.e., the ratio of the density actually achieved in the test to the density of the same concrete fully compacted.

Compacting Factor Test

• The range of compaction factor is from 0.78 to 0.95.
• Concrete with high fluidity has a higher compaction factor.
• The compacting factor test measures the inherent characteristics of the concrete which relates very close to the workability requirements of concrete.

Kelly Ball Test

• The Kelly ball test measures the workability of concrete by measuring the penetration of a 152-mm-diameter hemisphere hammer into the fresh concrete.
• A concrete with higher consistency leads to a deeper ball penetration.

Segregation

• Segregation is the separation of the constituent materials of concrete.
• Segregation makes the concrete weaker, less durable, and leaves a poor surface finish.
• To avoid segregation, check the concrete is not 'too wet' or 'too dry', ensure proper mixing, and place the concrete as soon as possible.

Bleeding and Settlement

• Bleeding is the development of a layer of water at the top or surface of freshly placed concrete.
• Bleeding is caused by sedimentation (settlement) of solid particles and the simultaneous upward migration of water.
• Excessive bleeding increases the water-cement ratio near the top surface, creating a weak top layer with poor durability.
• The most effective means of reducing bleeding in concrete include reducing the water content, increasing the amount of cement, using finer cementitious materials, and using air-entrained concrete.

Process of Manufacture of Concrete

• Batching is the process of measuring quantities of concrete mixture ingredients by either mass or volume and introducing them into the mixer.
• Concrete can be placed in its final location by direct chute discharge from a truck mixer, conveyor belt, or truck-mounted pump and boom.
• Concrete can be pumped hundreds of meters vertically with these pumping methods.

Placing Concrete

• When placing concrete, be careful not to damage or move the formwork and reinforcement.
• Place concrete as near to its final position as possible.
• Start placing from the corners of the formwork or in the case of sloping site from the lowest level.

Compaction (Consolidation) of Concrete

• Compaction of concrete is the process adopted for expelling the entrapped air from the concrete.
• The lower the workability, the higher is the amount of air entrapped.
• Methods used for compacting the concrete include hand compaction, compaction by vibration, internal vibrator, and external vibrator.

Finishing of Concrete

• Concrete can be finished in many ways, depending on the intended service use.
• Finishing operations include screeding, bullfloating, floating, and troweling.
• Screeding or strikeoff is the process of cutting off excess concrete to bring the top surface of a slab to proper grade.
• Troweling should not be done on a surface that has not been floated.