Concrete Basics

Flatwork

Basics

In its simplest form, concrete is a mixture of paste and aggregates. The paste, a composed of portland cement and water, coats the surface of the fine and coarse aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete.

Within this process lies the key to a remarkable trait of concrete: it’s plastic and malleable when newly mixed, strong and durable when hardened.

Concrete’s durability, strength and relatively low cost make it the backbone of buildings and infrastructure worldwide—houses, schools and hospitals as well as airports, bridges, highways and rail systems. The most-produced material on Earth will only be more in demand as, for example, developing nations become increasingly urban, extreme weather events necessitate more durable building materials, and the price of other infrastructure materials continues to rise.

Even construction professionals sometimes incorrectly use the terms cement and concrete interchangeably. Cement is actually an ingredient of concrete. It is the fine powder that, when mixed with water, sand, and gravel or crushed stone (fine and coarse aggregate), forms the rock-like mass known as concrete.

The Forms of Concrete

Concrete is produced in four basic forms, each with unique applications and properties.

  1. Ready-mixed concrete, by far the most common form, accounts for nearly three-fourths of all concrete. It’s batched at local plants for delivery in the familiar trucks with revolving drums.
  2. Precast concrete products are cast in a factory setting. These products benefit from tight quality control achievable at production facilities. Precast products range from concrete bricks and paving stones to bridge girders, structural components, and wall panels. Concrete masonry, another type of manufactured concrete, may be best known for its conventional 8-by-8-by-16-inch block. Today’s masonry units can be molded into a wealth of shapes, configurations, colors, and textures to serve an infinite spectrum of building applications and architectural needs.
  3. Cement-based materials represent products that defy the label of “concrete,” yet share many of its qualities. Conventional materials in this category include mortar, grout, and terrazzo. Soil-cement and roller-compacted concrete —”cousins” of concrete—are used for pavements and dams. Other products in this category include flowable fill and cement-treated bases.
  4. A new generation of advanced products incorporates fibers and special aggregate to create roofing tiles, shake shingles, lap siding, and countertops.

Finishing

Concrete that will be visible, such as driveways, highways, or patios, often needs finishing. Slabs can be finished in many ways, depending on the intended service use. Options include various colors and textures, such as exposed aggregate or a patterned-stamped surface. Some surfaces may require only strikeoff and screeding to proper contour and elevation, while for other surfaces a broomed, floated, or troweled finish may be specified.

Screeding or strikeoff is the process of cutting off excess concrete to bring the top surface of the slab to proper grade. A straight edge is moved across the concrete with a sawing motion and advanced forward a short distance with each movement.

Bullfloating eliminates high and low spots and embeds large aggregate particles immediately after strikeoff. This looks like a long-handled straight edge pulled across the concrete.

Jointing is required to eliminate unsightly random cracks. Contraction joints are made with a hand groover or by inserting strips of plastic, wood, metal, or preformed joint material into the unhardened concrete. Sawcut joints can be made after the concrete is sufficiently hard or strong enough to prevent raveling.

After the jointing the concrete, it should be floated with a wood or metal hand float or with a finishing machine using float blades. This embeds aggregate particles just beneath the surface; removes slight imperfections, humps, and voids; and compacts the mortar at the surface in preparation for additional finishing operations.

Where a smooth, hard, dense surface is desired, floating should be followed by steel troweling. Troweling should not be done on a surface that has not been floated; troweling after only bullfloating is not an adequate finish procedure. A slip-resistant surface can be produced by brooming before the concrete has thoroughly hardened, but it should be sufficiently hard to retain the scoring impression.

Curing

Curing plays an important role in strength development and durability of concrete. Curing takes place immediately after concrete placing and finishing, and involves maintenance of desired moisture and temperature conditions, both at depth and near the surface, for extended periods of time. Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance.

The length of adequate curing time is dependent on the following factors:

  • Mixture proportions
  • Specified strength
  • Size and shape of concrete member
  • Ambient weather conditions
  • Future exposure conditions

Slabs on ground (e.g. pavements, sidewalks, parking lots, driveways, floors, canal linings) and structural concrete (e.g. bridge decks, piers, columns, beams, slabs, small footings, cast-in-place walls, retaining walls) require a minimum curing period of seven days for ambient temperatures above 40 degrees Fahrenheit.

American Concrete Institute (ACI) Committee 301 recommends a minimum curing period corresponding to concrete attaining 70 percent of the specified compressive strength. The often specified seven-day curing commonly corresponds to approximately 70 percent of the specified compressive strengths. The 70 percent strength level can be reached sooner when concrete cures at higher temperatures or when certain cement/admixture combinations are used. Similarly, longer time may be needed for different material combinations and/or lower curing temperatures. For this reason, ACI Committee 308 recommends the following minimum curing periods:

  • ASTM C 150 Type I cement seven days
  • ASTM C 150 Type II cement ten days
  • ASTM C 150 Type III cement three days
  • ASTM C 150 Type IV or V cement 14 days
  • ASTM C 595, C 845, C 1157 cements variable

Decorative

It wasn’t long ago when deciding on a driveway material was easy: asphalt or concrete. Today, the “concrete” choice has expanded to include an astounding array of decorative options. Sometimes referred to as a cement driveway or as painted concrete, decorative concrete is one of the most reasonable ways to spruce up the entrance to your home.

Although plain gray concrete is still installed most often, more people are catching on to the dazzling effects possible with decorative concrete, and seeing the instant curb appeal a decorative driveway can give to any home, no matter what the style. If you already have a concrete driveway, you can still give it a decorative makeover. The concrete industry has introduced many products and methods that can rejuvenate or resurface existing plain-gray concrete projects.

Of course, to ensure that your concrete driveway will look good for many years to come, there are important steps your driveway contractor should follow during installation. How well your driveway looks and performs long-term is largely related to the quality of workmanship and materials that go into it.

Stamped

Stamped concrete, often called textured or imprinted concrete, is concrete that replicates stones such as slate and flagstone, tile, brick and even wood. Ideal for beautifying pool decks, driveways, entries, courtyards, and patios, stamped concrete is the perfect outdoor paving choice.

Recently, stamped concrete has become a popular choice for many homeowners because it offers a wide array of options when it comes to concrete patterns and colors. Another factor contributing to its popularity is its price. The cost of stamped or imprinted concrete is often considerably lower than the materials it is a substitute for.

Concrete is the perfect canvas for creating a cost-effective replica of more expensive materials, without giving up a natural, authentic look. When choosing colors and patterns for your stamped cement, make sure they blend with other stone, tile or textured concrete elements at your residence. Even in complex designs with steps and fountains, patterns can be still be pressed into the concrete. Stamped concrete can also be used in conjunction with other decorative concrete elements such as exposed aggregate or acid staining. Popular patterns include running bond brick, hexagonal tile, worn rock or stone.

Coloring/Staining

For many admirers of decorative concrete, the best attribute is that each installation is totally unique. Concrete can assume nearly any shape, design, pattern or texture. But the one characteristic that most distinguishes decorative concrete is color, whether used subtly to blend with nature or boldly to make a dramatic design statement.

The number of different products for coloring concrete has never been greater, and many manufacturers offer an extensive palette of shades to choose from for colored concrete. Although you’ll pay slightly more for colored concrete, the amazing transformation will be well worth the investment. In the hands of a creative contractor, these coloring mediums permit an endless array of decorative effects, from rustic earth-toned sidewalks and patios that harmonize with the surrounding landscape to vibrant multicolored floors that double as works of art.

Staining imparts a luxurious richness that can’t be achieved by any other coloring medium. Rather than produce a solid, opaque effect like paint, stains permeate the concrete to infuse it with luminous, translucent tones that vary depending on the surface they are applied to and the application techniques used. The results can mimic everything from polished marble to tanned leather to natural stone or even stained wood.

Stains for concrete come in two general categories: acid-based chemical stains and water-based acrylics. Most acid stains are a mixture of water, hydrochloric acid and acid-soluble metallic salts. They work by penetrating the surface and reacting chemically with the hydrated lime (calcium hydroxide) in the concrete. The acid in the stain lightly etches the surface, allowing the metallic salts to penetrate more easily. Once the stain reacts, it becomes a permanent part of the concrete and won’t fade, chip off or peel away. The palette for acid-etch staining is generally limited to earthy tones, such as tans, browns, terra cottas and soft blue-greens.

If you want to go beyond the subtle drama and subdued earth-toned palette of acid staining, consider using water-based stains, which come in a much broader spectrum of hues. Most manufacturers offer dozens of standard colors, including black and white and even metallic tints. Like acid stains, water-based stains (typically a blend of acrylic polymers and pigments) penetrate the concrete to produce permanent color, ranging from translucent to opaque depending on the product.

Both types of stain can be applied to new or old and plain or integrally colored concrete. They are especially effective for revitalizing dull, lackluster surfaces. Because they penetrate the concrete surface, most stains have excellent UV stability and wear resistance, permitting their use on interior or exterior concrete. Applications range from walkways and patios to family room floors and kitchen countertops. They can also be applied to vertical surfaces such as walls or fireplace surrounds.

Like stains for wood, concrete stains are semi-transparent and are intended to enhance rather than disguise the surface. They will not hide cracks, blemishes or other flaws in existing concrete. Nor will they completely mask an underlying color or conceal the texture of the surface. An existing concrete slab with major cracks or spalling is usually not a good candidate for staining because any patchwork is likely to show right through the stain.

Because stains must be able to soak into the concrete to achieve full color saturation, they shouldn’t be applied to surfaces covered by anything that can inhibit stain penetration, such as dirt, grease, glues, coatings, curing membranes and sealers.


Restoration

Resurfacing

Conventional wisdom holds that old concrete, with cracks, surface discoloration, or surface imperfections, must be removed and replaced if improving the look of the concrete is the goal. But there are many options available for transforming that drab concrete patio, driveway, or floor into a new, decorative, colored concrete surface. Plus, you’ll save money, conserve resources, and eliminate unnecessary disposal.

Completely resurfacing concrete with a polymer-modified overlay is one way to upgrade the look, and you can choose from a wide variety of color and pattern options. If your concrete is in good condition but just needs a facelift, you can also stain, stencil or engrave it to improve the appearance.

Overlay

Want to permanently cover up surface imperfections in existing concrete? Or turn a plain-jane slab from drab to fab? With today’s decorative overlays, it’s easy to give almost any concrete surface, indoors or out, a complete face-lift and at a much lower cost than removal and replacement. The real challenge is choosing from among the many resurfacing products available and the diverse array of decorative finishes possible. There are multiple options available for achieving any look imaginable with a concrete overlay.

Although cement-based overlays have been around for decades, many of today’s systems blend polymer resins with cement, sand, and other additives to improve performance, wear resistance, and aesthetic qualities. Polymer-modified overlays can be applied in layers as thin as a credit card or up to several inches thick without delamination or failure. They adhere well to existing concrete and resist damage from salt, chemicals, UV exposure, freeze-thaw conditions, and abrasion.

While most types of polymer-modified overlays offer similar performance benefits, each system has its own unique characteristics. Overlay manufacturers use different types of polymer resins, often blending them to produce proprietary products with distinct physical attributes. Many of today’s decorative overlays use acrylics or acrylic blends because these resins provide excellent bond strength and UV resistance.

It’s important to be aware, however, that not all existing concrete can be resurfaced. The underlying base for an overlay must be sound. If your concrete is heaving, has severe cracks, is spalling due to damage from deicing salts and freeze-thaw cycles, or resting on unstable soil, resurfacing will not solve your problems. This is when total replacement will be your best option.

Polishing

Polished concrete is fast becoming the ultimate no-wax flooring material. Thanks to recent advances in polishing equipment and techniques, contractors are now grinding concrete floor surfaces, whether new or old, to a high-gloss finish that never needs waxes or coatings. Factor in the superior durability and performance of concrete, and it’s no wonder why more retail, warehouse, and office facilities are opting for polished concrete flooring as an alternative to marble, granite, tile, linoleum, or coated concrete. Even homeowners are catching on to the appeal of these smooth, high-luster floors, which can be stained to replicate the look of polished stone.

Because polishing is a multistep process, you can choose the level of sheen — from satin to high-gloss — that meets your maintenance and aesthetic requirements. This versatility makes polished concrete an ideal flooring material for a variety of applications

Be aware that the process of polishing concrete floors requires a great deal of expertise and the use of specialized heavy-duty polishing machines equipped with diamond-impregnated disks that gradually grind down surfaces to the desired degree of shine and smoothness. Considering the investment in equipment and the skill required, it’s definitely not a project for the do-it-yourselfer. You’ll want to hire a professional concrete polishing contractor to do the work.

Factor in the superior durability and performance of concrete, and it’s no wonder why more retail, warehouse, and office facilities are opting for polished concrete flooring as an alternative to marble, granite, tile, linoleum, or coated concrete. Even homeowners are catching on to the appeal of these smooth, high-luster floors, which can be stained to replicate the look of polished stone.

Content courtesy of ConcreteNetwork.com, and concrete.org Taken 4/17/14.