Concrete Construction Practices The production of high-quality concrete does not rest solely on proper proportioning. The concrete placed in a structure must be of uniform quality, free of voids and discontinuities and adequately cured. Lack of sufficient attention to mixing, handling, and placing can result in poor concrete from a well- designed mix. Batching and mixing Batching Batching of aggregates and cements is best done by weight, since dispensing of solids on a volume basis can lead to gross errors. Only water and liquid admixtures can be measured accurately by volume. Look at Figure 11.1 of your text book for handling and string aggregates, figure 11.2 for methods of batching.

Mixing Thorough mixing is essential for the complete blending of the materials that are required for the production of homogeneous, uniform concrete. Not only does inadequate mixing result in lower strengths, but also in greater batch-to-batch and within-batch variations. However, overly long mixing times do not improve the quality of concrete and may severely limit the output of the batching plant. The optimum mixing time depends on: 1. The type of mixer. 2. The condition of the mixer. 3. The speed of rotation. 4. The size of the charge. 5. The nature of the constituents. Lean, dry or harsh mixes require more mixing than those made with rounded gravels.

Transportation Placement of Concrete

Properties of Fresh Concrete

Fresh concrete: from time of mixing to end of time concrete surface finished in its final location in the structure

Operations: batching, mixing, transporting, placing, compacting, surface finishing .

Treatment (curing) of in-placed concrete 6-10 hours after casting (placing) and during first few days of hardening is important.

Fresh state properties enormously affect hardened state properties. The potential strength and durability of

concrete of a given mix proportion is very dependent on the degree of its compaction.

The first 48 hours are very important for

the performance of the concrete structure.

It controls the long-term behavior,

influence (ultimate strength), (elastic modulus), creep, and durability.

Main properties of fresh concrete during mixing, transporting, placing and compacting.

Fluidity or consistency: capability of being

handled and of flowing into formwork and around any reinforcement, with assistance of compacting equipment.

Compact ability: air entrapped during mixing and

handling should be easily removed by compaction equipment, such as vibrators.

Stability or cohesiveness: fresh concrete should

remain homogeneous and uniform. No segregation of cement paste from aggregates (especially coarse ones) Fluidity & compact ability known as workability Higher workability concretes are easier to place and handle but obtaining higher workability by increasing water content decreases strength and durability .

Compaction of Concrete Finishing of Concrete

Workability Definition: Effort required to manipulate a concrete mixture with a minimum of segregation.

The amount of mechanical work or energy required to produce full compaction of the concrete without segregation or bleeding.

Workability measurement methods .

1. Slump test 2. Compacting factor test 3. Vebe test 4. Flow table test 1. Slump test - simplest