What causes one part of a concrete slab pour to set slower than the rest of the pour? The amount of concrete involved is less than a truckload. The finishers have to wait longer for these spots to set up or settle for a slab that's wavy because some parts of the concrete are softer than other parts.
Guy Detwiler of Material Service Corp. in Chicago says that spot retardation can often be traced to one of three causes: A wet spot in the subgrade, often caused by a low spot that collects water used to dampen the base course. On pumped concrete jobs, concrete that's been wetted in the pump hopper. The last portion of a truckload of concrete is often rocky or drier than the rest of the load. Water is sometimes added to improve pumpability, but the added water also means the concrete will take longer to lose enough water to be ready for finishing. Spilled soft drinks or coffee that contains sugar. Workers sometimes toss the remains of a cup or can into the fresh concrete. Sugar is a potent retarder, so the affected area won't set as fast. Dick Gaynor of the National Ready Mixed Concrete Association says changes in the interaction between the cement and the water-reducing admixture may be the cause. To solve the problem, assuming the mix has worked in the past without causing problems, he advises the producer to: change cements, remove the water-reducer or retarder, or cut the admixture dosage in half. Gaynor also says water-reducing admixtures and retarding admixtures are more effective if the cement is wetted and the early hydration reactions have a few minutes to start before the admixture is introduced.