Moisture Probe Error, Low Breaks, and Map Cracking on Steam-Cured Tees
A plant is casting 5,000 psi precast double tees with a 15% slag blend at 705 lb/yd³ cementitious, 260 lb/yd³ water (0.37 w/cm), and air entrainment. Over two days, 12 batches show slump 3.5–4.5 in and air 6.8%, while the ticket shows the sand moisture probe at 6.2%. A morning oven test on the same sand reads 8.1%; the coarse aggregate probe reads 0.4% versus an oven value of 0.3%. The first 28-day breaks are 4,150 and 4,320 psi. The top flange shows fine map cracking after a steam cure that reached 150°F in 4 hours and held 14 hours. The plant needs to ship 8 tees next week. The QC manager must quantify the batching error, evaluate the curing and cracking, and decide disposition and corrective action.
1. Quantify the batching error from the moisture probes and assess whether it explains the 28-day strengths.
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Assume the sand batch weight is about 1,250 lb/yd³. A probe reading of 6.2% versus an oven value of 8.1% means the batch underestimated sand water by 1.9%, or about 23.8 lb of water per cubic yard. The coarse aggregate error is 0.1% of roughly 1,900 lb, or about 1.9 lb. Total unaccounted water is about 25.7 lb, so actual water is about 286 lb and actual w/cm is about 286/705 = 0.406, not 0.37. That increase can reduce strength by roughly 8% to 12%, so a 5,000 psi mix might be expected around 4,400 to 4,600 psi. The observed 4,150 to 4,320 psi is lower than that estimate, so moisture error is a major contributor but not the only one. The high air content, cement lot variation, curing, sampling, and cylinder handling should also be checked. The calculation matters because it shows the batch was outside the approved w/cm and gives a defensible basis for holding product and correcting the probes.
2. Evaluate the steam cure and map cracking. What changes are needed?
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Reaching 150°F in 4 hours is aggressive for a 5,000 psi section and can create a steep temperature gradient between the hot surface and the cooler interior. The surface dries and contracts while the interior is restrained, producing fine map cracking. The 14-hour hold also matters less than the rate of rise and the cooling rate. The plant should compare the actual cycle to its written steam-cure procedure and to the approved project requirements. A typical improved cycle includes an initial delay before steam, a controlled rise, a maximum temperature verified by probes at the coldest and hottest locations, and a controlled cooling period. The enclosure should maintain moisture, and the top flange should not be exposed to direct steam or dry air. The cracking should be mapped and evaluated for depth; if it is only surface deep, it may be a durability concern rather than a strength failure, but it still requires engineering disposition. The cycle should be changed before the next production run.
3. Recommend disposition of the 8 tees and a corrective-action plan.
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Hold all 8 tees until the strength and cracking questions are resolved. Retest by core testing or another approved method, with cores taken from representative units and tested in accordance with the project requirements. If cores meet the required acceptance criteria and the cracking is judged cosmetic by the engineer, the units may be accepted with documented repair. If cores fail or the cracking is deep, reject or downgrade the units. Corrective action has three parts. First, moisture control: verify probes daily against oven tests, recalibrate or replace the sand probe, and require the batch operator to use the oven value when the probe and oven differ by more than 0.5%. Second, batching review: audit the 12 batch tickets for actual w/cm and air, and require a second-person check on the first batch of each day. Third, curing: revise the steam cycle, install temperature probes, and record rise, hold, and cooling. Verify effectiveness by reviewing the next 10 batches and the next 5 steam cycles.