Concrete Basics
In the preparation of planning for concrete work to be done at your residence or place of business, it is important to understand some concrete basics. Having a better understanding of its ingredients or makeup, how it is mixed, forming, finishing, and caring for concrete is important to ensure that your concrete work lasts for years and decades to come.
Concrete Basics #1: Three Basic Ingredients
Although there are differences, the three primary components of every concrete combination are Portland cement, water, and aggregate. When combined with water, Portland cement creates a binder that binds the aggregate—which can be sand, gravel, or a combination of the two—into a rigid mass with compressive strength comparable to that of steel.
Portland (hydraulic) cement is made up of minerals found in nature, including limestone, sandstone, granite, iron, clay, and fly ash. These raw materials are poured into cylindrical concrete kilns and heated to temperatures of up to 3,000 degrees Fahrenheit. This process removes water and carbon dioxide and produces new substances collectively referred to as clinker. Clinker is ground into a fine powder and then mixed with gypsum to regulate the pace at which it hardens when combined with water. Portland cement refers to this powdered substance.
Bags of concrete mix are available in a variety of shapes and sizes to fit a variety of applications. The aggregate of traditional concrete is a combination of sands and tiny gravels. The term “sand mix” refers to a combination that contains only smaller sands and no larger gravels. It’s excellent for patching and craft ventures that need concrete. Mortar blends, which are used to join bricks and stones, use much finer aggregates.
Additionally, a variety of chemicals may be applied to concrete to alter its properties. Epoxies, for example, are added to anchoring concrete blends to improve their staying strength. Fiber-reinforced concrete incorporates fiberglass filaments to help prevent cracks in areas such as patios and stucco repairs. Additional additives will either accelerate or retard the hardening of the concrete.
Concrete Basics #2: Mixing Concrete
Everybody is aware that when you apply water to concrete, it hardens. The chemical reaction is referred to as hydration, and the mechanism is somewhat complex because the cement comprises five distinct compounds that undergo this reaction. Calcium silicates, which account for 75% of the mixture, are the primary component that contributes to its hardness. When they hydrate, they emit sodium hydroxide and generate heat, which is why gloves should always be worn while mixing concrete. Wet concrete’s pH can reach 12 or higher, rendering the mixture highly corrosive. A job with concrete sometimes results in skin burns.
To stay fully hydrated, you need a particular volume of water. If you add less than the recommended amount—approximately 3 quarts per 80 lb. bag of premix—the concrete would not be solid enough, but adding so much water still weakens the mixture. Since concrete does not need drying out to harden, concrete may be poured underwater. However, after the hydration reaction is complete, any remaining water in the concrete mixture is absorbed by the pores, resulting in a weakened overall structure. Although most of this water gradually evaporates, it will take years for all surplus water to evaporate completely. When mixing concrete, only enough water can be added to make it pourable and workable but not so much that the mixture becomes soupy.
Small quantities of concrete may be combined in a wheelbarrow or plastic masonry trough by using 60- or 80-pound sacks of dry premixed concrete, a garden hose, and a hoe. For larger amounts, you can rent an electric mixer to mix up bags of concrete mix yourself, or you can buy ready-mix concrete from a distributor that can bring it in a big mixer truck and dump it on site.
Concrete Basics #3: Forming Concrete
Fresh concrete is essentially an extremely viscous material, and like any liquid, if not contained, it will level out into a puddle. Concrete must be contained as it is formed into slabs and other forms. The method of containment is called forming, and it is usually accomplished using dimensioned lumber and plywood. Since forms are not permanent, they are not designed for aesthetic purposes; therefore, they must be solid enough to withstand the weight of the concrete and cannot have any openings that could allow wet concrete to seep through.
When building a transparent surface, such as a wall, it is critical to use smooth plywood since the grain pattern of rough plywood would be permanently imprinted on the hardened concrete. Quality builders also take the extra step of taping the plywood to ensure a smooth surface after the shape is removed. Additionally, the tape guarantees a rapid escape.
It is preferable to build concrete forms with screws rather than nails. Any effect that fresh concrete is subjected to as nails are pulled to release the forms can cause it to break. You may not require high-quality screws for this; many builders construct forms using cheap drywall screws.
Concrete Basics #4: Reinforcing Concrete Structures
Although concrete has high compressive strength, it lacks the tensile strength of steel, and materials such as slabs and walls can break when loaded. To avoid this, architects integrate a steel reinforcing network into all massive structures. When pouring slabs, they usually incorporate a frame of reinforcing bars (rebars) into the walls and pillars or lay a ground cover of reinforcing mesh—referred to as remesh or strong fabric.
Constructing the steel reinforcement can be as time-consuming as constructing the shapes. Rebar and remesh are available in a variety of diameters, and the diameter you select is determined by the tensile strength needed by your project. Overdoing it is almost never a failure. You can bend rebar to make curved forms using a rented bender or get parts bent from any concrete supply dealer.
Concrete Basics #5: Finishing and Coloring Concrete
Finishing concrete can be a one- or multi-step operation, depending on the desired look. Typically, the finishing process starts with a straightedge or trowel screeding the surface while the concrete is already damp. Sceeding may be sufficient for certain uses, such as a utility pad. If you’re paving a patio or basement pad, you’ll want to smooth the surface again until the concrete has stiffened and then buff it with a polishing machine once it has hardened. Floating is the term used to describe the method of smoothing the floor, and it is an art that takes tremendous expertise and craftsmanship. Although floating aims to get finer aggregate particles and cement powders to the surface, excessive floating will undermine the concrete and produce a flaky surface.
It is possible to paint concrete before pouring it or to purchase pre-colored concrete. When the concrete has hardened, you can paint or tint it with an acid stain or a dye. Acid stains react with the additives in asphalt, resulting in a variety of earthy hues. If your preference is for more vivid hues, use a dye.
Concrete Basics #7: Caring for Concrete
If you’re using concrete to create an interior function, such as a countertop or a bathroom floor, you’ll want to seal it to keep dirt out of the pores. Most concrete sealers are composed of polyurethane or acrylic resin suspended in a liquid, most commonly water. Before use, the concrete should be completely dry, which requires waiting several weeks before sealing a freshly poured base. Apply the sealer using a sprayer, brush, or roller and allow it to dry before using the surface. Re-seal every few years or so. Clean the concrete well with soap and water or, if necessary, a pressure washer before each resealing.