introduCtion Offbearing and Curing Removal of the pipe from the machine is called offbearing and is accomplished in a variety of ways ranging from manual to fully automated. Fork lifts and overhead cranes are used to lift pipe from the machine. The freshly made pipe is transported to the curing kiln by fork lifts, hand trucks, overhead cranes or moving floors. Moving floors are commonly found in modern plants providing efficient transport of the pipe into the curing area. A section of moving floor, referred to as a kiln car, usually consists of a concrete slab supported on steel trucks. It runs on rails installed in a trench in the floor of the plant. The moving floor passes adjacent to the pipe machine and into the kiln. Depending on the production method, the pipe is either cured while in the form or immediately removed from the form and cured. Curing is accomplished by a variety of procedures. In some cases, the pipe is placed in a permanent kiln, and in other cases, the pipe is covered by canvas, plastic, or other material which functions as a kiln. As soon as the concrete pipe is formed, the curing process begins. Curing is optimized by control of kiln conditions and thus, the rate of hydration of the cement. There are three basic methods for curing: steam, water and sealing membranes. In the concrete pipe industry, low pressure steam predominates as a curing method. The principle of low pressure steam curing is that an accelerated rate of hydration produces concrete pipe of required strength in shorter time than is possible when curing at ambient temperatures. mixtures, materials, and conditions and is determined by experience. The three essential factors in all known methods of properly curing concrete are time, temperature and moisture. For equivalent strengths, an increase in temperature usually permits a shorter curing period. The time-temperature relationship is not the same for all mixtures, materials, and conditions and is determined by experience. In low pressure steam curing, it is essential that the relative humidity surrounding the pipe be high, as near as possible to saturation. High humidity, in excess of 80 percent, is provided when the curing temperature is obtained by direct injection of saturated steam into the kiln. With systems combining moisture and hot air, radiant heat, or other forms of dry heat it is necessary to maintain a closer check on the humidity in the kiln. Drying of the concrete can cause damage to the surface. A concrete pipe is sensitive to moisture loss because of the large surface area for the volumes of concrete. A moist atmosphere is not necessary when the concrete is entirely encased in a pipe mold. In such cases the water in the concrete mixture is sealed inside the forms. Concrete pipe can be cured in the open air provided temperatures are high and constant. It is necessary under these conditions to maintain the pipe in a moist condition. A sprinkler system is most commonly used to provide such an environment. Pipe has been efficiently cured in a chamber by maintaining a constant warm temperature achieved by the addition of heat or by the heat generated from the hydration of the cement. Moisture is usually provided in the form of a warm spray. ConCrete pipe stormwater treatment teChnologies box seCtions manhole and CatCh basin material LAFARGE PIPE eMANUAL PAGE 19