Concrete Construction and Strength

Curing Temperature and Strength Gain

• Higher curing temperature leads to earlier strength gain in concrete, but may decrease 28-day strength
• Lower curing temperature results in slower strength gain

Types of Joints in Concrete Construction

• Isolation Joints: permit horizontal and vertical differential movements, used around perimeter of floors, columns, and slabs-on-ground
• Contraction Joints: provide for movement in the plane of a slab or wall, induce controlled cracking due to drying and thermal shrinkage
• Construction Joints: stopping points in the construction process, bond new concrete to existing concrete, and permit no movement

Measuring Workability of Concrete

• Common tests to measure workability: Slump Test, Compacting Factor Test, Flow Test, Kelly Ball Test, and Vee Bee Consistometer Test
• Slump Test: most commonly used method, measures consistency of concrete, not suitable for very wet or very dry concrete

Slump Test

• Apparatus: metallic mould in the form of a frustum of a cone with specific dimensions
• Types of Slump:
• Collapse Slump: concrete collapses completely, indicating too wet or high workability mix
• Shear Slump: top portion of concrete shears off and slips sideways
• True Slump: measures the deformability of concrete

Bleeding and Settlement

• Bleeding: development of a layer of water at the top or surface of freshly placed concrete
• Caused by sedimentation of solid particles and upward migration of water
• Excessive bleeding increases water-cement ratio near the top surface, creating a weak top layer with poor durability

Reducing Bleeding in Concrete

• Reduce water content, water-cementitious material ratio, and slump
• Increase the amount of cement, finer cementitious materials, and supplementary cementing materials
• Use blended hydraulic cements, air-entrained concrete, or increase the amount of fines in the sand

Process of Manufacture of Concrete

• Batching: measuring quantities of concrete mixture ingredients by mass or volume and introducing them into the mixer
• Mixing: thorough mixing of materials is essential for uniform concrete, can be done by hand or machine

Mixing Concrete

• Hand Mixing: practised for small scale, unimportant concrete works, with 10% more cement added to cater for inferior concrete
• Machine Mixing: ready mixed concrete is batched and mixed at a concrete plant and delivered to the project

Retempering (Remixing) Concrete

• Remixing concrete with addition of required quantity of water, and sometimes a small quantity of extra cement

Transporting Concrete

• Methods of transportation: mortar pan, wheelbarrow, crane, truck mixer, belt conveyors, chute, pump and pipeline, skip and hoist, and helicopter
• Precaution: maintain homogeneity obtained at the time of mixing while transporting to the final place of deposition