WeÆre just developing our mix designs for a project specifying lightweight aggregate. How long must we presoak the material to be sure that itÆs achieved full saturation? Since this project might continue into the late fall, when we commonly experience freezing temperatures, is it permissible to presoak lightweight aggregate in our mixer trucks?
The original question drew a wide range of interesting responses that began with a discussion of various presoaking procedures for lightweight aggregate. The proposal about soaking lightweight in a watertight tank drew several responses offering similar solutions. A Vermont producer indicated that he had experienced good success with the mixer drum presoak method, especially if the period was overnight, or for a day or two. For larger jobs, he offered another alternative. ôWe have used dump trucks and/or trailers and caulked the tailgates for soaking, depending on which truck could be idled for the long soaking periods. In the winter, we put the trucks in a heated garage.ö One producer offered an alternative to tying up mixers or dump trucks for the soaking period. ôWe have had good luck in the past in using garbage hauling bins,ö he wrote. They are watertight, and you can park them in a warm garage the day before the pour for soaking,ö he offered. The Vermont producer also offered an answer on the soaking duration question. ôIf you are pumping [the concrete], the longer you can soak [the lightweight aggregate], the better. In fact, if you are pumping concrete multiple stories, [the aggregate] should soak 3 to 5 days, or buy it vacuum soaked.ö Vacuum-saturated lightweight aggregate results from an additional process just after the material comes out of the kiln. While lightweight aggregate is still very warm and completely dry, itÆs placed in a special airtight pressure vessel. Once the vessel is filled and sealed, the manufacturer sucks the air out with a vacuum pump. Once the air has been removed, water is pumped into the vessel. As the aggregate cools and the vessel returns to atmospheric pressure, the water fills the pores within the aggregate particle. Using this process, producers can reduce large volumes of process water and increase product quality. In regard to the original question regarding about the required soaking period, vacuum-saturated lightweight aggregate may be more consistent. In his book Properties of Concrete, A.M. Neville suggests producers should expect a wide range of soaking times based upon aggregate types. ôThe 24-hour absorption of lightweight aggregate ranges from 5% to 20% by mass of dry aggregate, but, for good quality aggregate for use in structural concrete, it is usually not more than 15%,ö he writes. One contributor offered an interesting opinion about presoaking. ôYou may not have to presoak at all if the lightweight aggregateÆs absorption is less than 10%.ö He writes that during pumping, the line pressure will convert some [of the concreteÆs free] water into [aggregate] absorbed water. The amount of [the concreteÆs] leftover free water depends on the [aggregateÆs] degree of saturation. He also warns even with all the best presoaking plans in place, there could be still a problem with absorption. ôOne crazy attribute of lightweight aggregate is that the absorption value goes up under pressure,ö he warns.