The hydraulic pressing process consists basically of filling a mould with a wet mix concrete, with a cement content of 400 kgs per cubic meter of concrete, then applying pressure in the order of 1800 to 2000 psi. The excess water is expelled and compaction is complete.
Careful preparation of the aggregates is essential in achieving quality products while obtaining optimum production rates. The aggregates and cement are batch weighed and dispensed into a pan mixer. Water is added to the mix until a slurry is formed, with an approximate water cement ratio of 1:1. Unlike conventional precasting, the excessive quantity of water is not critical, as the free water is forced out during the pressing process. Stone dust or “fines” are added to the mix to obtain a close texture and smooth surface on the finished product. To prevent the escape of cement and “fines” as the water is pressed out of the mix, a sheet of porous paper or a special filter cloth is placed or fixed to the bottom of mould former. A measured amount of the wet mix is then dispensed into the mould. This runs freely and fills the mould without requiring any vibration. A second filter cloth is attached to the die head. The press is then actuated and the hydraulic ram presses down into the mould squeezing out the excess water. The drainage of this excess water is an important part of the process. As the 400 ton ram presses the product, the extraction of the water is assisted by a vacuumed top drainage system. The duration of the pressing ranges from 10 to 30 seconds, depending on the thickness of the product, and quality the grading and characteristics of the aggregates. Immediately after pressing, having reduced the water cement ratio to 0.3, the product is strong enough to be handled, is removed from the press and stacked on racks within the plant, where the temperature and humidity are controlled. Within approximately 24 hours the product is removed from the racks, bundled and strapped, and carried to the storage yard, where further curing takes place over several days. Due to the extremely dry climatic conditions, the product is kept moist for a period of about 7 days. The kerbstones are now ready for shipment. Shipment is made with a fleet of flatbed trucks and trailers, all equipped with on-board cranes and grabs, hence making deliveries to the job sites efficient with no damage to the product.