Segregation at the Bottom of a Deep Wall Form
You are testing SCC being placed in a 14-ft-tall wall with heavy vertical reinforcement (clear spacing about 3 in.). The mix has a specified slump flow of 26 to 28 in. and a VSI requirement of 0 or 1. At the pump, slump flow averages 27 in. and VSI is judged 0. Placement proceeds in lifts. After the first lift cures enough to strip a window port near the base, you observe a 6-in. band of laitance and a porous, aggregate-poor zone at the bottom of the wall. The contractor claims the mix 'passed all the field tests' and blames the formwork.
1. The field tests passed. Explain how a mix can satisfy slump flow and VSI at the pump yet still segregate in a tall, heavily reinforced wall.
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Slump flow and VSI are measured at the point of discharge, before the concrete experiences the drop through a 14-ft wall and the tortuous path through 3-in. clear reinforcement. Static segregation and dynamic segregation can develop during free fall and while flowing through congestion even when initial spread and visual stability look acceptable. A high slump flow (27 in.) with a marginal stability margin can be stable in a wheelbarrow but unstable under the shear of a long drop. The porous zone at the base is consistent with dynamic segregation during placement plus static bleeding pooling at the lift line. Field tests at the pump do not capture the in-form behavior; that is why passing-ability and segregation-resistance testing (e.g., column segregation or sieve segregation) matter for this application.
2. What testing would you add for the next lift, and what placement changes would you recommend?
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Add a segregation-resistance test such as the column segregation test (or sieve segregation) on samples representative of the mix, and a passing-ability test (J-Ring) reflecting the 3-in. reinforcement spacing. Re-evaluate the mix design with the supplier: the 26-28 in. slump flow may be too high for this wall's congestion; consider reducing spread or increasing stability. Placement changes: reduce free-fall height using a tremie or drop pipe to keep the discharge near the rising surface, place in controlled lifts, and avoid discharging against the reinforcement. Document the observed defect, test data, and recommendations. The 'all tests passed' claim is true only for the tests run; the test plan was inadequate for the application.