Concrete Q & A: Penetration Resistance versus Concrete Compressive Strength and Minimum Time before Stripping Vertical Forms

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Title: Concrete Q & A: Penetration Resistance versus Concrete Compressive Strength and Minimum Time before Stripping Vertical Forms

Author(s):

Publication: Concrete International

Volume: 32

Issue: 9

Appears on pages(s): 67-68

Keywords:

Date: 9/1/2010

Abstract:
1. When I told a contractor that concrete could resist a single cycle of freezing after its compressive strength reached 500 psi (3.5 MPa), he surprised me by saying that the testing laboratory found that his concrete had reached 500 psi (3.5 MPa) prior to the completion of finishing. He then wondered if that meant no protection against freezing was needed. What was the testing laboratory measuring and is it related to the resistance to frost damage of the concrete? 2. We’re placing columns and walls using a concrete mixture containing portland cement, fly ash, and slag cement. Our vertical forms don’t support formwork for slab or beam soffits, and we want to cycle them as quickly as possible. Because the concrete has a relatively slow rate of strength gain, however, the engineer is concerned that stripping the forms too early will result in damage to the concrete surface. We can estimate the strength using the maturity method. What strength is needed to prevent mechanical damage to the concrete as a result of form stripping?