Precasting Methods and Materials
Wet-Cast versus Dry-Cast
Most decorative precasters use a wet-casting method, as opposed to dry-cast. Wet-cast means that the concrete has a water-cement ratio of about 0.4 or higher. Dry-cast concrete is very dry (w/c of 0.30 to 0.36), has zero slump, and the forms can be stripped as soon as the concrete has been consolidated. Dry-casting requires an expensive dry-casting machine and is typically used for mass production of products like concrete pipe or manholes and also in concrete block production. The big advantage of dry-casting is that you only need one set of forms to mass produce a specific part, since the form can be stripped immediately.
Materials for Precasting
There are two ways to get concrete for your precasting operation-have ready-mixed concrete delivered or make your own concrete in your shop. In general, most decorative precasters make their own concrete. That allows them to make only as much as they need and to more carefully control the characteristics and quality.
But if even you are only making small batches, you will need to maintain stock piles of the various ingredients, which include aggregate-in various sizes, cement and other cementitious materials, admixtures, and reinforcement. Controlling the quality of the materials is critical, both before they arrive and after.
Start with your aggregates which will make up the largest portion of the concrete and take up the most room for storage. Keep the different sizes of aggregate separate so you can blend them as needed for each casting job. Make sure the aggregate sizes don't segregate during storage. Bins are available for aggregate or you can just keep aggregate in piles. Either way, you need to be able to control the moisture content and prevent contamination-make sure dirt isn't dug up with the aggregate and that adjacent aggregate stockpiles don't cross contaminate one another. The moisture content of the aggregate can have a big impact on the workability of the concrete and its water-cement ratio. Ideally, aggregate is kept in a state of saturated surface dry (SSD) meaning the rock is wet on the inside but just barely dry on the surface. Load your bins with a front-end loader and maintain about a two-day supply. Larger precast operations use conveyors to fill their bins.
Cement, on the other hand, must be kept completely dry-for obvious reasons. Even moist air will allow some of the cement to hydrate, reducing its strength and leading to clogging of the bins. Fly ash isn't as sensitive to moisture, but still needs to be kept dry to flow easily. Both cement and fly ash can be purchased in bags or delivered by bulk and moved by fluidizing with air.
Admixtures come as liquids or powders and should be clearly marked. Liquid admixtures should not be allowed to freeze, since that can damage some materials. Powdered admixtures in water-soluble packets are a good approach for smaller operations. Remember that most liquid admixture dosages are specified in fluid ounces per hundred pounds of cement (fl oz/cwt) not on a cubic yard basis.
Reinforcing steel also needs to be protected from contamination. This usually means just not storing it on the ground. A little rust on the surface doesn't damage the steel-in fact some research suggests that slightly rusted surfaces provide better bond. Fiber reinforcement typically comes in toss-in degradable bags.
Self Consolidating Concrete
Self consolidating concrete (sometimes called self compacting), or SCC, is a dream material for precasters. Not only does it virtually eliminate the need for vibrators, it allows much higher pour rates and fewer workers, results in glassy smooth surfaces, and can provide the finest of details in the casting. SCC is a very high-slump, flowable concrete-but it's not as simple as just adding a dose of water and some high-range water reducer (superplasticizer). A true SCC must also be "stable," meaning that the mix doesn't segregate into paste and aggregate portions.
Sef-consolidating concrete is highly flowable and can spread as much as 30 inches. Note that there is no segregation or bleeding.
True SCC requires a mix with higher fines content, low water-cement ratio, a well-graded aggregate blend, and doses of a polycarboxylate high-range water reducer and, usually, a viscosity-modifying admixture (VMA). The mix must be carefully worked out, but once it has been established in a precast plant setting, you should be capable of achieving consistent results. Work with your materials suppliers to develop a mix design and do a lot of experimentation before going live with this.