READY MIX CONCRETE
Concrete Basics
At its simplest form, concrete is a mixture of paste and rocks, or aggregates. This paste is made from Portland cement and water, it coats the surface of the fine and coarse aggregates. Through a chemical reaction called Hydration, the paste hardens and continually gains strength, forming Concrete.
The concrete production process spotlights a unique trait: Concrete is plastic and malleable when newly mixed, but strong and durable when hardened. This specialized feature is how concrete is capable of building skyscrapers, bridges, sidewalks, superhighways, houses, and dams.
Ready Mixed Concrete is ideal for large jobs where space is limited and there is little room for a mixing plant and aggregate stockpiles. Ready Mixed Concrete is also preferable when smaller quantities of concrete or intermittent placing of concrete are required.
Various Types of Concrete
Transit Mixed or Truck Mixed Concrete materials are batched at a central plant and are fully mixed in transit. Or the concrete is partially mixed in transit and final mixing is completed at the jobsite.
Transit Mixing keeps the water separate from the cement and aggregates. It allows the concrete to be mixed immediately before placement at the construction site. This method avoids premature hardening and slump loss that result from delays in transportation or placement of Central Mixed Concrete. Transit Mixing also allows concrete to be hauled to construction sites further away from the plant.
On the downside, a ready mix truck has less capacity when carrying Transit Mixed Concrete as compared to the same truck hauling Central Mixed Concrete.
Shrink Mixed Concrete is used to increase the truck’s load capacity but also retain the advantages of Transit Mixed concrete. Shrink Mixed Concrete is partially mixed at the plant to reduce or “shrink” the volume of the mixture Final mixing is completed in transit or at the jobsite. Ready Mixed Concrete is often remixed after arriving at the jobsite. It’s important to ensure the proper slump is obtained. However, repeatedly remixed concrete sets more rapidly than concrete that has only been mixed once. Before concrete placement, specific slump properties need to be achieved; additional water or admixture varieties are often added at the jobsite.
Precise Proportioning
The key to strong and durable concrete is precise proportioning and thorough mixing of the ingredients.
Portland Cement and water form a paste that coats each particle of stone and sand, the aggregates. Through a chemical reaction called hydration, the cement paste hardens and gains strength over time.
If a mixture lacks paste, it cannot fill all the voids between aggregates. This concrete will be difficult to place and will produce rough surfaces and porous concrete.
Conversely, if a mixture has an excess amount of cement paste, it will be easy to place and will produce a smooth surface. However, the resulting concrete is not cost-effective and will crack more easily.
The quality of the paste determines the character of the concrete.
The strength of the paste depends on the ratio of water to cement. The water:cement ratio is the weight of the mixing water divided by the weight of the cement. High quality concrete is produced by lowering the water:cement ratio as much as possible, without sacrificing the ability to place, consolidate, and cure the fresh concrete. A typical mix is about 10-15% cement, 60-75% aggregate, and 15-20% water. Entrained air in some concrete mixes may also occupy another 5-8% of the mixture.
Additional Mixing Ingredients
Most drinkable water that has no pronounced taste or odor can be used as mixing water for concrete. Excessive impurities in mixing water can affect setting time and concrete strength, as well as; efflorescence, staining, corrosion of reinforcement, volume instability, and reduced durability. Concrete mixture specifications also set limits on chlorides, sulfates, alkalis, and solids found in the mixing water.
In contrast to mixing water, aggregates are chosen very carefully. Aggregates comprise 60-75% of the total volume of concrete. The type and size of aggregates used depend on the thickness and purpose of the final concrete product. Aggregates also need to be clean and free from any other matter that might affect the quality of the concrete. For efficient use of the paste, a continuous gradation of particle sizes is preferred.
Chemical Hydration
After the aggregates, water, and Portland cement are combined, the mixture starts to quickly harden. This chemical reaction is called Hydration. All Portland cements are hydraulic cements that set and harden through this reaction. During this process, a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles, or adheres to adjacent aggregates.
When the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff. During placement, the concrete is consolidated in order to compact it within the forms and to eliminate potential flaws, such as honeycombs and air pockets.
For concrete slabs, the concrete is left to stand until the surface moisture film disappears, then a wood or metal hand float is used to smooth off the concrete. Floating produces a relatively even, but slightly rough, texture that has good slip resistance and is frequently used as a final finish for exterior slabs. If a smooth, hard, dense surface is required, floating is followed by steel troweling.
Concrete Curing
Curing begins after the exposed surfaces of the concrete have hardened sufficiently to resist marring. Through the process of curing, it ensures the continued hydration of the cement; so the concrete continues to gain strength. Concrete surfaces are cured by sprinkling with water fog, or by using moisture-retaining fabrics such as burlap or cotton mats. Other curing methods prevent evaporation of the water by sealing the surface with plastic or by using special sprays called Curing Compounds.
Special techniques are used for curing concrete during extremely cold or hot weather. The longer the concrete is kept moist, the stronger and more durable it will become. The rate of hardening depends on the composition and fineness of the cement, the mix proportions, and the moisture and temperature conditions.
Concrete continues to strengthen as it gets older. Most of the hydration and strength gain take place in the first month of concrete’s life cycle, but hydration still continues at a slower rate for many years following.
@IRMCA
@IRMCA
@IRMCA
@IRMCA
