A new nondestructive test for estimating the compressive strength of concrete and mortar at early ages is described in this paper. The pin penetration test was used to relate the early age strength of hardened concrete and mortar in the laboratory. It was found that the pin penetration tester can be used successfully, under laboratory conditions, to determine the compressive strength of concrete cylinders and mortar slabs at early age from 10 hours up to 28 days. For concrete and mortar cylinders, slabs, and cubes with a compressive strength of up to 27.6 MPa (4000 psi), the pin penetration readings were found to be linearly related to the compressive strength. The tester was also used successfully in the field to determine the compressive strength of a concrete floor slab in a new building in order to post-tension the strands and remove the forms. The building was constructed in winter under severe climatic conditions. Key words: early age, strength evaluation, concrete slabs, form removal, nondestructive tests, mortar testing, pin penetration test.
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