The strength of concrete is generally measured by testing concrete cubes which are prepared from the concrete used in that particular construction. The cube strength of concrete is specifi ed by a structural engineer and is assessed by crushing concrete cubes at a specifi ed age (28 days after casting). Early strength may also be assessed at 3 or 7 days after casting. The preparation, handling, curing and testing of concrete cubes are done according to SANS 5861-3: 2006 Concrete Tests: Making and Curing of Test Specimens and SANS 5860: 2006 Concrete Tests: Dimensions, Tolerances and Uses of Cast Test Specimens.
Apparatus Required
- Cube Moulds: 100x100x100mm or 150x150x150mm;
- Compaction equipment: Tamping rod or Vibrating table;
- Steel float
Cube Preparation
- One set of three cubes should be prepared for every test age (3, 7 or 28 days) as required and should be sampled from at least every 50m³ of each concrete grade placed.
- Ensure that the test moulds are clean.
- Ensure that all the mould sides are at 90° angles with the base plate
If moulds are out of square, the area of the concrete cube is changed, and this will negatively affect the compressive strength that is obtained from the test. The results will therefore not be a true representation of the concrete cast on site.
- Apply a thin layer of release agent or mould oil on the inside of the assembled mould.
- Place the moulds on a smooth, level surface.
- Fill the moulds with concrete in layers of depth 50mm.
- Compact each concrete layer in the mould either by:
- Using a tamping or compaction rod to tamp the concrete. For 150x150x150mm cubes, 45 evenly distributed strokes are required, and for 100x100x100mm cubes, 20 evenly distributed strokes are required. The tamping rod should not penetrate the previous layer by more than 10mm and it should not forcibly strike the bottom of the mould. Attention should be given to the mould corners when tamping the cube; or
- Using a vibrating table. Ensure that the mould is fi rmly clamped to the vibrating table. The duration of the vibration depends on the workability of the concrete and the effectiveness of the vibrator. Vibration should be stopped when there are no more large bubbles being released from the concrete and the surface of the concrete has a smooth appearance.
Concrete cubes which are inadequately compacted will have an excessive amount of air voids in them, which will reduce their strength. Concrete cubes which are over compacted undergo segregation of concrete constituents, and will give unreliable test results.
- Use a steel float to remove excess concrete from the cube and strike off the surface.
- Note the following information on a paper label:
- Company name and site;
- Reference of the cast element (beam, slab, column);
- The date on which the concrete was cast and on which the cube was made; and
- Testing age ( 3, 7, or 28 days)
- Lightly press the label onto the top of the cube while concrete is still wet
Handling and demoulding of cubes
- After fi lling the moulds, cover them with a damp cloth or polythene sheet.
- Store cubes in a cool place for 24 hours.
- Store cubes indoors, out of direct sunlight, wind or extreme temperatures.
- The storage area should be free of vibration, with a relative humidity of at least 90% and a temperature of 22°C – 25°C.
- Demould/strip cubes 24 hours after manufacturing.
- Ensure cube sides are not scratched or damaged during demoulding.
- Transfer the information from the paper label onto the cube byusing a lumber crayon or marker.
