A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
Level One (Core FAQs)
Part IV: Tests on Concrete
1. Some concrete specification requires the testing of compressive
strength for both 7 days and 28 days. Why? (T1)
7-day compressive strength test results are usually not used for
acceptance purpose but for information only. Instead, 28-day compressive
strength test results are commonly adopted for acceptance purpose.
7-day compressive strength test results are often used to monitor the gain
of early strength and they are estimated to be about 64% to 70% of the
28-day strength. As such, it serves as a warning signal to both concrete
producers and contractors should the 7-day compressive strength test
results are far less than 75% of the 28-day strength. Nowadays, most
concrete placement schedule are very tight and it is of paramount
importance for contractors to get to know as soon as possible the
occurrence of low 7-day compressive strength test results. As such, the
contractor could implement suitable measures promptly to get better
quality control procedures at construction site and to monitor closely on
sampling, molding, and testing of the test cubes so as to avoid the
recurrence of the production of low-strength concrete in the coming
concrete batches.
2. Is Schmidt hammer test a standard test for testing concrete
strength? (T2)
The Schmidt hammer test involves hitting the in-situ concrete with a
spring-driven pin at a defined energy, and then the rebound is measured.
The rebound depends on the surface hardness of the concrete and is
measured by test equipment. By referring to the some conversion tables,
the rebound result of the test can be used to determine the compressive
strength of the concrete. Although past investigations showed that there is
a general relationship between compressive strength of concrete and the
rebound number, there is a wide range of disagreement among various
research workers regarding the accuracy of estimation of strength from
Schmidt hammer. In fact, there is about a variation of 15-20% in concrete
strength measured by the method.
Schmidt hammer is in not a standard test for acceptance testing of
concrete strength. It is only a test used for estimating the strength of
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