Standard Ready-Mix Concrete - CEMEX USA
The Characteristics of Ready-Mix Concrete & Delivery
Ready-mix concrete is a mixture of paste and aggregates: the paste, composed of Portland cement and water, coats the surface of the fine and coarse aggregates. A chemical reaction, called hydration, causes this paste to harden and gain substantial strength, forming the rock-solid mass commonly known as concrete.
Within this process lies the key to concrete's remarkable character: it's plastic and pliable when freshly mixed, strong and permanent when hardened.
This unique character is the basis for why a single material can be ideal for building skyscrapers, bridges, sidewalks, superhighways, dams, and even houses. Furthermore, ready-mix concrete delivery makes this material easy to use wherever it’s needed.
Achieving a strong, durable concrete requires careful proportioning and mixing of ingredients. A mixture lacking enough paste to fill every void between the aggregates will prove difficult to place and will yield rough, honeycombed surfaces and porous concrete. While a mixture with excess paste will be easy to place and will produce a smooth surface, the result will be concrete that shrinks excessively, rendering it uneconomical. Therefore, it is important to partner with an experienced ready-mixed concrete supplier you can trust.
A properly designed concrete mixture will possess good workability when fresh and solid durability and strength when hardened. Such a mixture contains approximately 10%-15% cement, 60%-75% aggregate and 10%-15% water, with entrained air often ranging from 3%-5%.
The "magic" of Portland cement is that its chemistry comes to life in the presence of water. The ratio of water to cement determines the quality of the paste, the strength of which determines the character of the finished concrete. This water/cement ratio is the total weight of the 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 workability of the fresh concrete. Generally, using less water produces a higher quality concrete, as long as it is properly placed, consolidated and cured.
The Value of Other Ingredients
Although most any natural water that is drinkable and has no pronounced taste or odor can be used in a concrete mixture, some waters that are not fit for drinking may still be appropriate for making concrete. Excessive impurities in water can affect a concrete's setting time and strength and cause efflorescence, staining, corrosion of reinforcement, volume instability, and degraded durability.
Specifications usually set limits on the amounts of chlorides, sulfates, alkalis, and solids in concrete mixing water unless tests can be performed to determine the effect that an impurity will have on various properties.
The aggregates that comprise a concrete mixture must be chosen carefully, since the type and size of the aggregate mixture depends on the thickness and purpose of the final concrete product.
Relatively thin building sections require small, coarse aggregate, even though aggregates up to six inches (150 mm) in diameter have been used successfully in building large dams. A continuous gradation of aggregate particle size increases the efficiency of the paste. In addition, aggregates should be clean and free from any extraneous matter that may impact the final quality of the concrete.
The "Magic" of Hydration
Soon after the water, aggregates and the Portland cement are combined, the mixture starts to harden through a chemical reaction with water called hydration.
During this reaction, a node "magically" forms on the surface of each cement particle, growing and expanding until it links with nodes from other cement particles or adheres to nearby aggregates. This continuous linking process produces a progressively stiffer, harder and stronger concrete mixture that is ready for placement in forms while it is still pliable.
During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, such as honeycombs and air pockets that can degrade its durability over time.
For slabs, concrete is left to stand until the surface moisture film disappears, then is worked over with a wooden or metal hand float until it is smooth. Floating produces a relatively even, but slightly rough, texture that offers good traction, making it an ideal finish for exterior slabs. In situations where a very smooth and hard surface is the goal, floating is followed by steel troweling.
The concrete begins to cure after its exposed surfaces have hardened sufficiently to resist marring. Curing ensures that the cement continues to hydrate and the concrete continues to gain strength. Concrete surfaces can be cured by sprinkling them with water, fog, or covering them with moisture-retaining fabrics, such as burlap or cotton mats. Other curing methods prevent water evaporation by sealing the surface with plastic or special sprays called curing compounds. During extremely hot or cold weather, special curing techniques are used to protect the concrete.
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, as well as the moisture and temperature conditions. While most of the hydration and subsequent gain in strength occur within the first month of the concrete's life cycle, hydration continues at a slower rate for many years.
This is why concrete grows stronger over time.
Four Forms of Concrete
Concrete is produced in four basic forms, each with its own set of distinctive advantages.
- Ready-mix concrete - by far the most common form, accounts for nearly 75% of all concrete produced. Ready-mix concrete delivery begins with batching at local plants and delivery in the familiar trucks with revolving drums.
- Pre-cast concrete - used in products that are shaped in a factory where quality control is tightly monitored. Pre-cast products range from huge bridge girders and complex structural components to standardized panels for cladding and smaller concrete bricks and paving stones.
- Concrete masonry - although best known for the venerable 8”x8”x16” building block, today’s masonry units can be molded into a myriad of shapes, configurations, colors, and textures to satisfy a seemingly infinite spectrum of building applications and architectural designs.
- Cement-based materials - these products defy the label "concrete," yet share many of its desirable qualities. The more conventional include mortar, grout, and terrazzo. Soil-cement and roller-compacted concrete - "cousins" of concrete - are used for pavements and dams. Other products include flowable fill and cement treated bases. A new generation of advanced products incorporates fibers and special aggregates to create roofing tiles, shake shingles, lap siding, and countertops. Using cement to treat and stabilize waste is an emerging market.
The seemingly infinite flexibility of concrete virtually guarantees its continued use in futuristic construction applications yet to come, even as its permanence preserves the traditional structures that anchor us to our heritage