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oneintheeye (Structural) (OP)
11 Dec 07 5:15
Hi, looking into striking times of vertical formwork normally in water retaining applications. We typically use ggbs replacement mix. Apart from strength considerations what is minimum striking time used by others here? the ggbs will slow the strength gain and prolong the heat gain (although lower temps than non replacement mix). we have various 'guidance' here in UK and have advised site on times.
DTGT2002 (Structural)
11 Dec 07 10:13
When final finish is critical for architectural work (I don't do much work with water tanks), we will try to use a performance based stripping criteria (I am an engineer for a formwork and shoring company). From the commentary to ACI371-98 Guide for the Analysis, Design, and Construction of Concrete-Pedestal Water Towers, a minimum compressive strength of 800psi (5.5 MPa) is suggested for most situations.

A prior search of ACI documents provided a minimum strength of 1000 psi for stripping/striking of vertical formed surfaces for architectural finishes.

In a recent support structure for a government project, we did not strip until 1500 psi was attained and had little if any damage due to form removal.

The added cost of using a perrformance based spec is additional cylinders and additional testing, but the ability to actually know when things should be done is certainly worth it in our eyes.

Hope this helps,

Daniel Toon
oneintheeye (Structural) (OP)
12 Dec 07 4:54
similar spec here in UK though dictates strength must be 10 N/mm2 or twice subjected stress whichever greater. By conversion 10 is slightly higher than your psi figure of 1000. Slightly different with water retaining in that crack limits are obviously more onerous and shrinkage/thermal cracking consideration usually dictate. Many reports and guides but was looking for any 'real' examples other than our sites.

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