Ready mix concrete is concrete that's mixed in a factory or batching plant and delivered in an elastic, plastic condition, via agitating trucks or truck-mounted, rotating drum mixers known as cement mixers to your construction site. It's a much more precise and efficient way of getting the concrete you need for your project, without the hassle of mixing it on-site. Each set of ready mix concrete is unique, and batches are tailor-made according to the specific ratios and volumes determined by the contractor.
At its core, concrete is a mixture of paste and aggregates (coarse to medium grained particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete, etc). The paste, composed of cement and water, coats the surface of the fine (sand) and coarse aggregates (rocks) and binds them together into a rock-like mass known as concrete, which in turn is used to build everything from foundations to sidewalks, skyscrapers, and these days, even furnitures and home decor. Strong concrete is the result of carefully proportioning and mixing ingredients - 10 to 15 percent cement, 60 to 75 percent aggregates, 15 to 20 percent water, and 5 to 8 percent air bubbles. Properly proportioned and mixed concrete is workable and plastic when fresh and strong and durable when set.
While it may seem like a recent innovation, ready mix concrete actually goes as far back as 1909, when concrete was delivered by a horse-drawn mixer that used paddles turned by the cart's wheels to churn the mixture as it made its way to the job site. Then in 1916, Stephen Stepanian of Columbus, Ohio invented the first self-discharging motorised transit mixer, that in turn paved the way for the ready-mixed concrete trucks that we know today. Ready mix concrete became a fast favourite in the 1940s, where its popularity grew to meet the high demand for concrete caused by World War II and spurred by the advent of heavier trucks and better engines allowed mixing drum capacities to increase.
Today, there are several main types of ready mixed concrete:
Transit-mixed: Also known as truck-mixed concrete, this method involves materials being batched at a central plant and then completely mixed in the truck in transit. Most time, the concrete is partially mixed in transit and completed at the site. Transit-mixing keeps the water separate from the cement and aggregates and allows the concrete to be mixed immediately before placement at the construction site. This eliminates problems associated with premature hardening and slump loss that usually arises from delays in transportation or placement of central-mixed concrete. This method of mixing concrete also allows for materials to be hauled to construction sites further away from the plant. Shrink-mixed: Shrink-mixed concrete is used to increase the truck's load capacity and retain the advantages of transit-mixed concrete. In shrink-mixed concrete, concrete is partially mixed at the plant to reduce or shrink the volume of the mixture and mixing is completed in transit or at the jobsite. Central-mixed: This method involves concrete being mixed in a stationary, plant-mounted mixer before being discharged into a truck mixer. Central-mix plants are sometimes referred to as wet batch or pre-mix plants. The truck mixer is used primarily as an agitating haul unit at a central mix operation. Central-mixing offers several advantages including faster production capabilities than a transit-mixer, better quality control and consistency, and less wear on equipment like truck mixer drums. Ready-mixed: This type of concrete is often remixed once it arrives at the job site to ensure that the proper slump is obtained. However, concrete that has been remixed tends to set more rapidly than concrete mixed only once. Materials, such as water and some varieties of admixtures, are often added to the concrete on-site after it has been batched to ensure that the specified properties are attained before placement. Mobile Volumetric Proportioning Plants : These "Mix Mobiles," as they're colloquially known, are truck-mounted, volumetric batching and continuous mixing units that supply small-volume or specialty pours and offer the convenience of freshly mixed concrete in precise quantities. They're essentially a hybrid between centralised batch plants and on-site mixing, and consists of a truck with bins of sand, coarse aggregate, cement, water, fiber, colour, and admixtures. These mobile miniaturized versions of the large stationary batch plant can mix as much or as little amount of concrete as needed, effectively eliminating the travel time hydration that often causes transit mixed concrete to become unusable. These trucks can mix a precise amount of concrete, when and as needed, with the same precision and attention to quality as a centralised batch plant system. Materials are mixed in an attached cement mixer while flow meters control the introduction of water and admixtures.
Ready-mixed concrete offers a host of benefits over site-mixed concrete. It's great when only small quantities of concrete are needed or when materials are needed infrequently as it reduces waste and overheads. When you add up the cost and time of hiring mixing equipment, labour, plus purchase and storage for the ingredients of concrete, added to environmental concerns (cement dust is a particular airborne health hazard and nuisance), ready-mixed concrete emerges a clear victor, ensuring finished, on demand, and specifically mixed concrete that allows the customer to pay for what they use. Ready-mixed concrete is also ideal for large jobs where space is limited and there is little room for a mixing plant and piles of unmixed cement and aggregate, or where companies are looking to delegate the quality control and testing, material logistics and supply chain issues, and mix design, to specialists who are already set up for those tasks. Ready-mixed concrete is also much more consistent and of higher quality than hand-mixed concrete because it's manufactured in factories with the help of proper technology and equipment. It takes up less space on site and material waste and loss becomes negligible because of the precise volumes of concrete mixed ahead of time.
On the downside, ready-mixed concrete is on average 20% more expensive than regular concrete, companies stand to lose the control and immediacy of on-site mixing, and there's an increased dependency on external agencies for concrete-building projects.
In Doha, there are numerous companies that offer premium ready-mixed concrete, with batching plants in Lusail, Messaied, the Pearl Qatar, the Industrial Area, and near the Hamad International Airport. These companies come with strict quality control procedures that ensure that raw materials received and concrete produced meet the highest international standards, namely QCS, ACI, ASTM, BS and NRMCA.