Concrete Basics

What is Concrete?

It is created by mixing aggregate (granular material such as sand and gravel), cement (see Difference Between Cement and Concrete), and water. The water and cement form a past that coats the pieces of aggregate and fills the spaces between them. The water triggers a chemical reaction in the cement that causes it to dry out and set or harden, bonding itself and the aggregate into the hard mass called concrete. In structural concrete, Portland cement (a mix of lime, silica, aluminum, iron, oxide and gypsum) is used with gravel, crushed stone, or sand.

The characteristics of a concrete vary according to the nature of its aggregate and cement and the proportions in which they are mixed with each other and with water.

Concrete is valued for its ability to withstand great compression, such as when a large amount of weight is placed on it. The strength improves as the density of the concrete increases.

Concrete is relatively inexpensive and durable and can be molded to any shape. It can be made porous or watertight, heavy or light and will even harden under water. These and other variable characteristics of concrete make it ideal for many uses. Concrete is the recognized material in driveways, patios, basements, and a host of other household items. It also is the world's most widely used building material.

There are as many different concrete mixes as there are applications for concrete, and the design mixture used for a parking ramp will be essentially different from the mix used for a basement floor. Also, the material and procedures used to construct a concrete driveway in a warm climate are completely different from the requirements necessary to construct a durable driveway, pool deck, walkway or basement floor in a harsher environment, such as the Lower Mainland. Freeze and thaw cycles, together with exposure to chemical de-icers can cause the wrong mix of concrete to crack, shrink or scale.

How Concrete is Made

To make concrete, appropriate amounts of dry ingredients, chemical admixtures (see Chemical Admixtures) , cement powder are thoroughly blended together. Next, enough water is added to make a stiff but workable mixture.

Concrete is produced with many grades of fine and coarse aggregate (see Aggregate) available to meet standard and special mix needs. Concrete can be delivered ready to use in a ready mixed-concrete truck. It can come in many forms and applications each of which, when properly prepared, handled, and placed, can provided decades of service.

Yard-At-A-Time Concrete makes different kinds of ready-mix concrete and purchases most of its aggregate and other raw materials from local suppliers. We have built strong partnerships with many leading construction suppliers.

The Difference between Cement & Concrete

Cement and concrete might be one and the same as household terms, but by nature are different. Cement, an ultra-fine gray powder, binds sand and rocks into a mass or matrix of concrete. Cement is the key ingredient of concrete.

Cement's natural chemistry comes to life in the presence of water, sand, and gravel or crushed stone-known as fine and coarse aggregate. Upon mixing with water, cement's calcium compounds hydrate to form new agents that bind the aggregates into concrete.

CONCRETE IS

Cost effective: Concrete lasts longer and requires less maintenance than other paving materials, which results in lower lifecycle costs.

Energy efficient: Buildings and homes with concrete walls use less energy to heat and cool.

Sustainable: The making of cement, the key ingredient of concrete, uses many materials recycled from other industries that would otherwise be wasted.

Air-Entrained Concrete

Air entrainment is recommended for nearly all to concrete mixture which in principal is to improve resistance to freezing when exposed to water and deicing chemicals. However, there are other important benefits of entrained air in both freshly mixed and hardened concrete. Air-entrained concrete contains billions of microscopic air cells. These relieve internal pressure on the concrete by providing tiny chambers for the expansion of water when it freezes.

Air-entrained concrete is produced by introducing air-entraining as the concrete is mixed at the plant. The amount of entrained air is usually between 5 percent and 8 percent of the volume of the concrete, but may be varied as required by special conditions. The use of air-entraining agents results in concrete that is highly resistant to severe frost action and cycles of wetting and drying or freezing and thawing and has a high degree of workability and durability.

Chemical Admixtures

Chemical admixtures are the ingredients in concrete other than Portland cement, water, and aggregate that are added to the mix immediately before or during mixing. Yard-At-A-Time uses admixtures primarily to modify the properties of hardened concrete; to ensure the quality of concrete during mixing, transporting, placing, and curing; and to overcome certain emergencies during concrete operations.

Successful use of admixtures depends on the use of appropriate methods of batching and concrete mixture. Most admixtures are supplied in ready-to-use liquid form and are added to the concrete at the plant or at the jobsite. Certain admixtures, such as pigments, expansive agents, and pumping aids are used only in extremely small amounts and are usually put in at the job site from pre-measured containers.

The effectiveness of an admixture depends on several factors including: type and amount of cement, water content, mixing time, slump, and temperatures of the concrete and air. Sometimes, effects similar to those achieved through the addition of admixtures can be achieved by altering the concrete mixture-reducing the water-cement ratio, adding additional cement, or changing the aggregate and aggregate gradation.

Five Functions

Admixtures are classed according to function. There are five distinct classes of chemical admixtures: air-entraining (see Air Entrained Concrete), water-reducing, retarding, accelerating, and plasticizers (super plasticizers). All other varieties of admixtures fall into the specialty category whose functions include shrinkage reduction, workability enhancement, and coloring. Air-entraining admixtures, which are used to purposely place microscopic air bubbles into the concrete, are discussed more fully in "Air-Entrained Concrete."

Water-reducing admixtures usually reduce the required water content for a concrete mixture by about 5 to 10 percent. Concrete containing a water-reducing admixture needs less water to reach a required slump than untreated concrete. The treated concrete can have a lower water-cement ratio. This usually indicates that a higher strength concrete can be produced without increasing the amount of cement.

Retarding admixtures, which slow the setting rate of concrete, are used to counteract the accelerating effect of hot weather on concrete setting. High temperatures often cause an increased rate of hardening which makes placing and finishing difficult. Retarders keep concrete workable during placement and delay the initial set of concrete.

Accelerating admixtures increase the rate of early strength development; reduce the time required for proper curing (see curing concrete) and protection; and speed up the start of finishing operations. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.

Super plasticizers, also known as plasticizers or high-range water reducers (HRWR), reduce water content by 12 to 30 percent and can be added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete. Flowing concrete is highly fluid but workable concrete that can be placed with little or no vibration or compaction. The effect of super plasticizers lasts only 30 to 60 minutes, depending on the brand and dosage rate, and is followed by a rapid loss in workability. As a result of the slump loss, super plasticizers are usually added to concrete at the jobsite.

^ top Fiber Reinforcement

The use of a fiber reinforcement system, good concrete, and good concreting practices a quality structure that actually reduces the formation of cracking (see Why Concrete Cracks).

The use of polypropylene fibers with a decade of use in the concrete industry has reduced intrinsic cracking; provide toughness to the concrete; lower permeability; and increase resistance to dynamic loads by providing impact and abrasion resistance. When used according to specifications, these synthetic fibers inhibit 80-100 percent of the cracking typical in freshly placed concrete and provide toughness in the hardened concrete, helping contain any cracks should they occur.

The disbursement of millions of these tiny plastic fibers actually holds the concrete together and prevents microscopic fissures from becoming large cracks visible at the surface. The result is an easily implemented, cost-effective and uniformly distributed reinforcement system that is placed correctly, fights cracking and provides wear protection for years to come.

Aggregate

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and Portland cement, are an essential ingredient in concrete. For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete. Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories-fines and coarse. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch (9.5-mm) sieve. Coarse aggregates are any particles greater than 0.19 inch (4.75 mm), but generally range between 3/8 and 1.5 inches (9.5 mm to 37.5 mm) in diameter. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder.

Placing & Finishing Concrete

Mixing, transporting, and handling of concrete should be carefully coordinated with placing and finishing operations. Concrete should not be deposited more rapidly than it can be spread, struck off, consolidated, and bull-floated. Concrete should be deposited continuously as near as possible to its final position. In slab construction, placing should be started along the perimeter at one end of the work with each batch placed against previously dispatched concrete. Concrete should not be dumped in separate piles and then leveled and worked together; nor should the concrete be deposited in large piles and moved horizontally into final position. (For further information refer to The Fundamentals of Using Concrete)

Consolidation

In some types of construction, the concrete is placed in forms and then consolidated. Consolidation compacts fresh concrete to mold it within the forms and around embedded items and reinforcement and to eliminate stone pockets, honeycomb, and entrapped air. It should not remove significant amounts of intentionally entrained air. Vibration, either internal or external, is the most widely used method for consolidating concrete. When concrete is vibrated, the internal friction between the aggregate particles is temporarily destroyed and the concrete behaves like a liquid; it settles in the forms under the action of gravity and the large entrapped air voids rise more easily to the surface. Internal friction is reestablished as soon as vibration stops.

Finishing
Concrete that will be visible, such as slabs like driveways, highways, or patios, often needs finishing. Concrete 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 strike-off and screeding to proper contour and elevation, while for other surfaces a broomed, floated, or troweled finish may be specified. In slab construction, screeding or strike-off 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.

Bull-floating eliminates high and low spots and embeds large aggregate particles immediately after strike-off. 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 concrete has been jointed, 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, 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 bull-floating 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.

Why Does Concrete Crack?

Concrete is a brittle material and cracks due to inability to flex under stress. Cracks easily spread through plain concrete because there are no fibers present to hold the surrounding substance together.

After being properly cured concrete will also shrink as it dries. An average concrete slab will shrink 3/4 of an inch for every 100 feet of length. The underside, bound to the sub-grade that prevents it from contracting over its entire length builds internal stresses resulting in shrinkage cracks generally spaced about 20 feet apart. Unfortunately, excess water is sometimes added to the concrete load at the job site. Making the concrete easier to handle by adding water is a short-term solution that will leave long lasting results - more cracks. The wetter the mix laid down, the more it will shrink and crack. Unplanned cracks are usually in the wrong place and never straight. We can design cracks that are pleasing to look at. They are called joints. By grooving the slab at predetermined intervals before it sets, or saw cutting it shortly after setting, we can force the cracks to appear where we want them. As an added bonus the crack is hidden beneath a surface feature that can very aesthetically attractive.

Concrete that has not been cured (see Curing Concrete) but allowed to dry out the next day will tend to have more unsightly cracks. It simply lacks the strength needed to hold itself together. Curing is essential for three days and recommended for seven to help unwanted shrinkage cracking.

Cracks that are visible on the surface of concrete are usually the end result of months or even years of interior micro-crack formation, likely from microscopic fractures in the concrete formed during the curing, or drying, process after the concrete was placed. These fractures or fissures developed as excess water came out of the concrete, creating stresses that pulled it apart. The crevices then grew into cracks because the welded wire reinforcing was either not properly placed and did not inhibit their formation or was left out altogether. For years, the tendency for concrete to develop shrinkage cracks during the curing process has been accepted as natural to its use. The problem has been the inability to effectively limit the formation of these stress cracks. Today, fiber-reinforced concrete is minimizing those problems (See Fiber Reinforcement). Secondary reinforcement is not an answer to concrete cracking due to sub-grade failure. When the soil beneath the slab gives way, secondary reinforcement cannot help. But homeowners and contractors with a well-prepared site, who use fibers in their concrete in conjunction with the good concreting practices can expect the slab they just installed to remain structurally sound and attractive well into the future.

^ top Curing Concrete

After concrete is placed, satisfactory moisture content and temperature (between 50 F and 75 F) must be maintained, a process called curing. Adequate curing is vital to quality concrete. Not curing the concrete can lead to: lower ultimate strength, lowered durability, more cracking, larger cracks forming, and higher chances of salt damage. Almost all concrete defects that can occur are more likely to occur to concrete that is not cured. Curing should be started immediately after the final finishing stage is completed.

Curing has a strong influence on the properties of hardened concrete such as durability, strength, water tightness, abrasion resistance, volume stability, and resistance to freezing and thawing and deicer salts. Exposed slab surfaces are especially sensitive to curing. Surface strength development can be reduced significantly when curing is not done properly. Curing the concrete aids the chemical reaction called hydration. Most freshly mixed concrete contains considerably more water than is required for complete hydration of the concrete; however, any considerable loss of water by evaporation or otherwise will delay or prevent hydration. If temperatures are favorable, hydration is relatively rapid the first few days after concrete is placed; retaining water during this period is important. Good curing means evaporation should be prevented or reduced.

To properly cure your concrete you need to keep the top surface continuously wet for at least 3 days and preferably 7 days. The surface can be wet in a number of different ways. Some of the most common methods of curing are:

• using sprinklers or soaker hoses to continuously apply water

• apply wet burlap over the surface and wet down periodically,

• cover with plastic to keep the surface water in.

• Liquid membrane-forming compounds sprayed onto the surface are effective, economical moisture barriers for moist-curing concrete.

• Straw or hay is still used to insulate fresh concrete in freezing weather.

Sealing Concrete

Sealing will protect your concrete from surface contamination. Sealing concrete involves applying a liquid concrete sealer to the finished surface. This sealer can be applied either with a roller or by using a simple garden sprayer. There are many sealers available on the market that will give different final finishes. Most hardware or concrete supply stores will have a selection of sealers. Your concrete finisher would likely be willing to do this job for you. With a few suggestions from a supplier of the sealers, it is an easy task for the homeowner to achieve. Concrete is inherently a porous material, which is susceptible to staining from liquids seeping into the surface. It can also be attacked by harsh chemicals such as oils, antifreeze, or even drinks seeping into the porous surface. Sealing your concrete does essentially what it sounds like, that is, it seals the surface. Thus by sealing, the concrete can be protected from attack or staining. Sealing should be performed several weeks after curing has finished and after several days of dry weather to allow the concrete to dry.

Through proper curing (see Curing Concrete) and sealing in conjunction with other correct concrete practices you can ensure that your beautiful concrete project stays that way for years to come.

Decorative Floors & Other Flatwork

When you interconnect an artist with a concrete finisher, you get a great decorative concrete surface such as floors, driveways, sidewalks, patios. Through the use of stains, stamps, dyes, colored pigments, textured patterns, ornate saw-cuts, epoxy overlay, and more, concrete floors are becoming increasingly attractive for home and business owners.

Concrete floors provide an alternative to moisture-sensitive flooring materials. Using concrete as the exposed finish offers several benefits. With fast construction schedules, concrete may not have time to dry adequately prior to placing an adhered finish. If that finish is impermeable or is applied with moisture-sensitive glue, there is a potential for failure: debonding, delamination, blistering, and expansion are a few problems associated with moisture in floors.

Exposed Aggregate

One of the most popular and enduring decorative concrete finishes, exposed aggregate uses the texture of the rock in the concrete to embellish the surface. In this technique, concrete is placed and floated as normal (see Placing and Finishing). After placement of the horizontal retarder, the concrete is left to set and the surface paste is later removed by washing and/or brushing. Washing and brushing follow as before to remove the mortar from the surface, fully exposing the natural color and texture of the aggregate. Yard-At-A-Time Concrete offers two sizes of rocks for the exposed, B/E (5-8 mm) and -1/2” (10-15 mm).

Stamping

Stamping, which can be used for both interior and exterior concrete surfaces. Stamped surfaces are created by enhancing finishing operations on fresh concrete with patterned or textured mats and templates. Having started as simple shapes and minimal textures, stamping tools and techniques have continually evolved to an advanced stage. Truly realistic textures can be enhanced with color additions, impersonate natural stone, rock, wood, brick, and more. The only limitation is the designer's imagination.

Standard procedures for placing, finishing (see Placing and Finishing), and curing (see Curing Concrete) concrete flatwork apply's with a few added steps. Using a stamping tool or pattern, fresh concrete is imprinted after the initial float pass and application of a release agent. If colour (see Coloured Concrete) is to be used, a dry shake colour harder may be applied during finishing. The stamping procedure effectively replaces steel troweling, and the effect is a unique surface in both pattern and texture.

One way to reduce the cost of materials for coloured flatwork finishes is to place the colour only at or near the surface. There are two common approaches for doing this: Dry-shake finishes or the two-course method. Both are compatible with stamped finishing techniques.

Integral Colours, Staining, Tinting, & Dying

There are many ways to colour concrete before it has hardened or afterwards. Integral pigments are mixed into fresh concrete to create through-body colour. Alternately, stains, tints, or dyes can be applied to hardened surfaces to impart colour. Chemical stains react with hardened concrete to become an integral component of the floor surface. Pigmented tints or dyes deposit finely ground pigments into the substrate. Stains enhance the inherent irregularities in the concrete surface with colour to resemble a marble appearance. (Yard-At-A-Time Concrete does not offer colour for sale at our plant, however, call or email one of our customer service representative and we can give you one of our knowledgeable suppliers for the colour information that is required.)

Coloured Concrete

Today the use of colours from slate black to deep reds, beige to brick, and almost any shade between, are available at a reasonable cost. Concrete can be coloured by either adding a coloring agent to the surface layer, the "shake" method, or by adding colour throughout the entire batch of concrete. This is referred to as "integral' colour. Both methods have their advantages and disadvantages. Generally, deeper colours can be produced by the shake method, in which the colouring agent is sprinkled onto the wet surface and smoothed in during finishing. This is not a job for the amateur and requires considerable experience to obtain a proper bond between the surface and underlying layer to retain the concrete's freeze/thaw durability. The downside of the shake method is that over time the use of deicer chemicals, cracking (see Why Concrete Cracks) or even heavy traffic wear can cause the surface to break down or wear away exposing the grey concrete underneath. Integral colour, while tending to be less vibrant, is available in all the shades of the shake method. Surfaces wild with integral color concrete will continue to show their true colors forever regardless of wear or damage as the color is throughout the entire body of the pour.

Stained Concrete

Chemically reactive stains are water-based, acidic solutions that contain metallic salts. These metallic salts react chemically with calcium hydroxide compounds (hydrated lime) in hardened concrete. Such reactions form the insoluble colors that become a permanent fixture of the hardened concrete surface.

Partly due to the irregularity in finishing patterns and partly due to the inherent variation of the concrete materials, stain chemicals react in a very irregular pattern giving it a decorative effect. As a result, mix ingredients and finishing techniques will greatly influence how effective the staining can be. Controlling these factors are important for achieving the best effect from the stain.

Stains can be applied to old or new concrete; to colored or gray surfaces; and may be used in conjunction with other decorative techniques and creative joint patterns. Combining chemical stains with dyes presents an unlimited palette of colors to create floor patterns and designs. To protect and enhance the color of the floor, using a sealer (see Sealing Concrete) is an important step. However, just as material selection for the concrete is important, selection of the appropriate sealer is also important for the performance and durability of the floor.

Jointing/Joint Patterns

Sawcut joints are installed using either an early-entry saw after concrete finishing or a conventional saw after concrete setting. Alternately, jointing tools (rather than saws) can add pattern lines to fresh concrete surfaces. Since joints should be used to control concrete cracking (see Why Concrete Cracks) anyway, this is a simple and purposeful method for adding greater interest to concrete surfaces.