Laboratory Mix Qualification: Which Stability Test Governs?
You are qualifying an SCC mix in the laboratory for a drilled shaft with a 60-ft free-fall drop. The mix shows slump flow 25 in., T50 2.5 s, VSI 1, and J-Ring flow difference 1.5 in. A static segregation column test gives a segregation index of 12% (specification limit 10%). A sieve segregation test gives 18% (specification limit 15%). The supplier proposes to drop the column test because 'the sieve test is faster and the J-Ring passed.' The project has no explicit statement about which segregation test governs, only that 'segregation shall be controlled.'
1. Evaluate the two segregation results and explain why they can disagree in direction or magnitude.
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Both results exceed their respective limits (column 12% vs. 10%; sieve 18% vs. 15%), so on the numbers the mix fails both. The tests probe different mechanisms: the column test measures static segregation under self-weight over time (relevant to a deep, slow-placed element), while the sieve test measures the tendency of mortar to separate from coarse aggregate under a standardized washing/draining action. A mix can be marginal in static stability yet show a different sieve result because the two stress the mix differently. Passing J-Ring only speaks to passing ability through bars, not to segregation resistance, so it cannot substitute for either segregation test. The disagreement in magnitude does not create a conflict in verdict here: both indicate insufficient stability.
2. The supplier wants to drop the column test. How do you decide which test governs, and what is your qualification recommendation for a 60-ft free-fall shaft?
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Since the specification only says 'segregation shall be controlled,' use engineering judgment tied to the application: a 60-ft free-fall drop is dominated by static and dynamic segregation under gravity, which the column test most directly represents. Dropping it would remove the most relevant indicator. Recommendation: retain the column test as the governing stability check for this application, retain the sieve test as a supplementary indicator, and reject the mix as submitted because it fails both limits. Require reformulation (increase stability via VMA/paste adjustments, reduce slump flow or coarse aggregate content) and retest both segregation tests plus J-Ring. Document the rationale for selecting the column test as governing, and seek written concurrence from the engineer of record so the basis is unambiguous for future lots.