Concrete Cubes Strength If 28 days result less than 7 Days Strength results, what was the solution?

Before the Metrication movement started in 1956-1960 , the cube size used to be 6" x 6"x 6" . Even today , certain laboratories have both the British moulds (6" x 6"x 6") and the Metric mould ( 150 mm x 150 mm x 150 mm ).

So first logic is that the size of 6" x 6"x 6" which in mm means 152 mm x 152 mm x 152 mm was adapted to 150 mm x 150 mm x 150 mm as a step in the conversion from British to metric mode of measurements.

3 reasons why this size is standardized?

1. The cubes shall represent the concrete member cast and shall also be in proportion to the size of the aggregates used.

2. The crushing load shall shall be such that the compression testing machine used to develop such load shall still remain compact and easy to install ( also economical )

3. The cubes shall be easy to carry , cure and store and handle. ( weight shall be reasonable )

If the cubes have a failed test result at 28 days, then the spares will be used to test for either 42 days or 56 days, depending on what test age you prefer beyond 28 days.

Two cubes will be tested again with the same cube ID number, but these will be spares.

But I think the case you mentioned rarely happened, so you can follow the above action to verify the concrete strength.

Investigation of Low Strength Test Results:

Following engineering parameters are the minimum requirements for investigating the low strength of concrete.

1.       Identify the procedures of testing apply on sample whether is comply with standards or not.

2.       Check it out total required strength of the particular area or structures (not to worry, if the strength is  more than applied weigh)

3.       If the strength is vary on large scale than following NDT (Non destructive test shall be performed by discussing with your clients or consultant)

·         smith hammer test

4.       Core Testing

5.       Load Testing

6.       Other corrective measures by mutual discussion with your clients and consultant

Solution of the Occurred Problem (Your Main Concern)

Two thing are very important that need to be understand

1.       The average strength of the three core (Concrete Core taking from particular effected concrete structure) is equal or greater than85% of f’c

2.       No single core test result should less than75 % f’c

Need to do:

Core sample should be tested in a period b/w48 hours and7 days after they were drilled.

Hope the above mentioned details shall comply with your requirements.