Hot-Weather Slump Loss and a Request to Add Water at the Site
At 14:00, with an ambient temperature of 38°C, a 6 m³ ready-mix truck arrives with a specified slump of 100 ± 25 mm and a specified maximum concrete temperature at placement of 35°C. The truck has been in transit for 50 minutes. The slump is 65 mm and the concrete temperature is 33°C. The batch ticket shows 380 kg/m³ cementitious content (including 25% fly ash), design w/cm 0.42, and batch water 150 L/m³. The contractor wants to add 20 L of water to the drum, retemper to about 90 mm, and place the load in a 250 mm thick slab. The foreman says the mix 'always stiffens' and that the added water is only about 3 L/m³. The inspector must decide whether to permit the addition and what records to check. The placement area has no windbreak, and the surface is dry and sunny. The specification invokes ACI 301 hot-weather provisions and prohibits water addition at the site unless approved in writing.
1. Evaluate the contractor's request to add 20 L of water. Can the inspector permit it, and does the water-cementitious ratio still comply?
Read the worked answer
Twenty litres in a 6 m³ load is 20/6 = 3.33 L/m³. The batch water is 150 L/m³, so the adjusted water content would be 153.33 L/m³. With 380 kg/m³ cementitious material, the adjusted w/cm would be 153.33/380 = 0.404, which is still below the design maximum of 0.42. So the arithmetic does not by itself show a w/cm violation.
However, the specification prohibits water addition at the site unless approved in writing. The inspector cannot approve a specification deviation. The contractor must obtain written approval from the EOR or owner's representative before any water is added. The inspector should also check the truck time and revolution limit, the actual slump, and the concrete temperature at placement. Adding water may raise the slump, but it can also increase segregation and plastic-shrinkage risk in the hot, sunny, windy conditions. If written approval is not obtained, the load should not be placed as proposed; the contractor should return it or obtain an approved alternative.
2. What tests and records should be collected before deciding whether the load can be placed?
Read the worked answer
Perform and record slump per ASTM C143, concrete temperature per ASTM C1064, air content per ASTM C231, and unit weight as required. Check the batch ticket for batch time, cementitious content, design w/cm, batch water, admixtures, and truck number. Record the time of arrival, elapsed time since batching, and drum revolutions. Verify that the concrete temperature at placement will not exceed 35°C; the current 33°C is within the limit, but the hot ambient conditions and sunny placement area could raise it before discharge.
Confirm the specified slump tolerance of 100 ± 25 mm, which gives a range of 75–125 mm. The measured 65 mm is below the lower limit, so the load is already nonconforming on slump before any addition. If water is added with written approval, add it gradually, remix, and retest slump and temperature; record the exact amount and the approval. Also check the project's time and revolution limits for placing concrete. Do not allow the load to be placed solely on the foreman's verbal assurance.
3. Recommend measures for the remaining loads to avoid repeating the problem.
Read the worked answer
The mix is stiffening because of the 38°C ambient temperature, the 50-minute transit, and likely insufficient retarding or set control for the conditions. For subsequent loads, require the batch plant to adjust the mix within the approved design, such as using a retarder or approved water-reducing admixture, cooling the mix with chilled water or ice, or lowering the batch temperature. Consider scheduling placement at night or early morning to avoid the hottest part of the day.
At the site, provide shade or a windbreak, dampen the subgrade and forms, and use fog spraying where appropriate. Place and finish promptly, then begin curing immediately to reduce plastic-shrinkage cracking; use wet coverings, curing compound, or continuous moist curing as specified. Monitor slump, temperature, and time for every truck, and reject loads that exceed the temperature or time limits. Do not use site water addition as a routine slump remedy; if it is ever allowed, it must be approved in writing and recorded.