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Desirable properties in a concrete mix

• Ensure the concrete mix is workable which allows correct placement and consolidation
• Ensure the optimum desired quality of the hardened concrete is met which improves resistance to freezing and thawing, water tightness (low permeability) , wear resistance, and strength.
• The quality of the concrete is dependant mainly on the water to cement ratio, so water requirements should be minimised to reduce the cement requirement thereby reducing the overall cost.
• The mix should be a stiff as possible.
• Use the largest aggregate practicable.
• Use the optimum ratio of fines to coarse aggregate.
• A low water to cement ratio is the number one issue that affects concrete quality. Do not add too much water.

Components in concrete

• Portland Cement - the aggregate and sand is coated by a paste formed from the cement and water. This paste then hardens and binds the fines (sand) and aggregate together
• Water - this is needed to chemically react with the cement and to make the concrete workable.
• Aggregate - The fines are sand and gravel or crushed stone is the coarse aggregate (10, 20 & 40mm) in most mixes.

Admixtures in Concrete

• Accelerators - added to the concrete to reduce overall setting times and gain early strength. Calcium chloride is the lower cost accelerator, however a non-chloride accelerator should be specified if reinforcing has been used, to prevent corrosion of the steel.
• Retarder - used in hot weather to delay setting. Also used for difficult jobs and specialist finishes. May also act as a water reducer
• Fly Ash - improves workability, easier to finish and reduces the cement content.
• Air Entrainment - improves the workability of the concrete as well as the durability.