Procedure for Crushing Concrete Cubes: Determining Compressive Strength
The compressive strength test of concrete cubes is a fundamental procedure in construction quality control, providing critical data on the structural integrity and load-bearing capacity of concrete mixes. This standardized test, governed by specifications like ASTM C39/C39M or EN 12390-3, involves subjecting carefully prepared and cured concrete cubes to increasing compressive load until failure. The resulting maximum load divided by the cross-sectional area gives the compressive strength value. Here’s a detailed breakdown of the essential procedure:
1. Preparation of Test Specimens:
Moulds: Use rigid, non-absorbent, and leak-proof cube moulds (typically steel or cast iron), conforming to standard dimensions (e.g., 150mm or 100mm sides). Ensure internal faces are clean, smooth, lightly oiled (with a non-reactive mineral oil), and correctly assembled without gaps.
Sampling: Obtain a representative sample of freshly mixed concrete from the batch according to relevant standards (e.g., ASTM C172).
Filling: Fill the moulds in layers (typically two or three layers for standard cubes). Compact each layer thoroughly using either:
Rodding: Use a standardized tamping rod (16mm diameter, ~600mm long) with rounded ends. Rod each layer uniformly across its surface for the specified number of strokes per layer.
Vibration: Place the mould on a vibrating table operating at specified frequency and amplitude. Apply vibration until the concrete is fully compacted (air bubbles cease rising, a glazed surface appears). Avoid over-vibration which can cause segregation.
Finishing: After compaction, strike off excess concrete flush with the top edges of the mould using a trowel or straight edge. Cover immediately with a non-absorbent plate or plastic sheet to prevent moisture loss.
2. Initial Curing:
Store filled moulds on a level surface in an environment preventing moisture loss and damage (temperature between 16-27°C / 60-80°F is typical for initial setting).
Keep covered for approximately 16 to 24 hours after casting.
3. Demoulding:
Carefully remove the hardened cubes from their moulds within 24 ± 8 hours after casting. Avoid damaging edges or corners during demoulding as these are critical stress points during testing.
Immediately identify each specimen with unique markings indicating batch, casting date, etc
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