Jan 23 2014

Hot Weather Concreting

Hot weather can also be damaging to concrete. The fresh mix will require more water than usual to achieve the required slump and workability, will set faster and have reduced working time, will more likely experience plastic shrinkage cracking on the surface, and will suffer variations in air content. The hardened concrete will have lower strength, more drying shrinkage and tendency to crack, less durability in freeze thaw exposures, and less uniform surface appearance. The adverse effects of hot weather increase as temperatures rise, relative humidity falls, and wind increases, and the damage can never be completely undone. There are, however, a number of recommendations which can help avoid problems. The following protective measures should be taken when temperatures are 90°F or above, especially when accompanied by windy conditions or relative humidity below 25%.

To decrease the possibility of plastic shrinkage cracking, use the largest size and amount of coarse aggregate compatible with the job requirements and, if ordering from a ready mix supplier, specify a water reducing admixture.
Locate control joints at slightly closer intervals than when concreting in milder temperatures, and plan the locations of construction joints ahead of time with smaller working areas in mind.
Use sunshades or windbreaks as appropriate, and avoid working during the hot afternoon.
Have enough workers on hand to keep the job running smoothly and quickly.
If you are mixing concrete on site, sprinkle aggregate stockpiles ahead of time for evaporative cooling and use ice as part of the mixing water.
Reduce the time between mixing and placing as much as possible and avoid excessive mixing. Do not add water to ready mixed concrete at the job site.
Moisten the forms and reinforcement and moisten soil sub grades before placing the concrete.
Cure the concrete for at least three days, but preferably for one week. When forms are removed, provide a wet cover for newly exposed surfaces.

The primary concern of hot weather concreting is the rapid loss of mixing water to evaporation. All of the protective measures outlined are aimed at preserving the moisture needed for cement hydration and curing. If adequate moisture can be maintained in the concrete for at least three and preferably seven days, there will be no decrease in the quality of the concrete compared to that placed and cured in milder weather.

Jan 22 2014

Cold Weather Concreting

Cold weather can have damaging effects on freshly placed concrete. Both setting time and rate of strength gain are slower in cold weather, and if the concrete freezes during the first few days of curing, it will suffer reduced strength and weather resistance, and increased moisture permeability. When it is necessary to work in cold weather, certain precautions must be taken to assure the quality of the finished concrete.

Cold weather is defined as a period when the mean daily temperature drops below 40°F for more than three consecutive days. On commercial projects, heated enclosures are often provided to protect concrete and masonry work during cold weather. Although this is not usually done on residential work because of the expense, the following protective measures can and should be taken.

For slabs and other flatwork such as driveways, sidewalks, and patios, reduce the amount of mixing water so that the concrete has a slump of 4 in. or less. This will minimize bleeding of mix water to the surface and decrease the time until initial set.
Use air-entrained cement or an air-entraining admixture even if the concrete will not be exposed to freeze-thaw cycling in service.
Use either an extra bag of cement per cubic yard of concrete, a high-early-strength cement (Type III), or a non chloride set accelerator to develop strength faster.
If you are ordering from a ready-mix supplier, specify heated concrete with a minimum temperature.
Remove ice and snow from inside forms and thaw frozen sub grade before concrete placement.
If you are mixing concrete on site, store ingredients in a heated area if possible, and use heated water for mixing.
Reduce the time between mixing and placing as much as possible to reduce heat loss. Work with smaller batches if necessary.
Delay form removal as long as possible to minimize evaporation and to reduce damage to formed surfaces caused by premature form stripping.
Wrap protruding reinforcing bars with insulation to prevent heat drain.

Whenever you can schedule concrete pours during milder weather, it is best to do so, but in some climates this is impractical. When cold weather concreting cannot be avoided, quality does not have to be sacrificed if proper precaution is exercised.

Jan 21 2014

Curing Concrete

Concrete must be kept moist for several days after it is placed to allow the portland cement in the mix to cure and harden properly. The most common methods of curing concrete are as follows:

Cover the surface completely with large sheets of plastic. Be sure to keep the plastic flat on the surface of the concrete, or it will cause uneven coloring. Weight down edges and joints with pieces of lumber.
Cover the surface with roofing felt. Tape the joints and edges or weight them down with pieces of lumber to help seal moisture in and retard evaporation.
Cover the surface with burlap bags, using a garden hose to keep the bags wet.
Sprinkle or fog the concrete with a garden hose or sprinkler.
Apply a chemical curing compound.

Plastic sheeting and roofing felt can cause uneven discoloration of the concrete surface if they are not kept flat. On large surfaces, it is difficult to smooth out all of the wrinkles in a covering, so if the concrete will be exposed to view and its appearance is important, use another method for curing. Wet burlap curing should not be used on colored concrete surfaces because it can cause the color to become splotchy. Keep the concrete moist for seven days.

After concrete slabs have cured for 24 hours, and concrete walls and footings for three days, remove the forms, but do not pry or hammer against the concrete itself. The concrete will continue to cure slowly for another month until it reaches full strength, but slabs are safe to use for foot traffic after the first day and for light rubber-tired vehicles after the first week. Heavy traffic areas should be protected with plywood. Foundation walls and footings should cure for at least two weeks before substantial framing loads are added.

Jan 20 2014

Concrete Finishing

The tops of concrete slabs, driveways, patios, and sidewalks must be leveled and finished to apply an appropriate surface texture. Surface finishes may be simple and utilitarian or more elaborate and decorative.

Floating, Troweling, and Brooming

The tops of footings and walls are left unfinished after the concrete has been vibrated, but flat concrete elements such as slabs, driveways, sidewalks, and patios must be leveled on top and an appropriate finish applied.

Special Finishes

An exposed aggregate finish will add color and texture to a driveway, sidewalk or patio, as well as a non slip finish. The concrete should be poured in small, manageable areas so that the aggregate can be seeded into the surface before the concrete becomes too hard. The seeding method of creating an exposed aggregate finish takes about three times longer than normal finishing so it is usually done in smaller sections.

Jan 19 2014

Pouring Concrete

There are several rules to follow when pouring concrete. The first is that concrete should be placed in the forms as soon as possible after it is mixed. Delays result in evaporation of moisture from the mix and a loss of both workability and strength. If the concrete is not placed within 11 ⁄ 2 hours and shows signs of stiffening, it should be discarded. Do not add water to a mix that has begun to stiffen. Even if you succeed in restoring some workability, the concrete will be of poor quality. To avoid delays, it’s important to make sure that all the necessary preparations have been made before the ready mix truck arrives or before you begin mixing. Preparations should include wetting the inside surfaces of plywood and kiln dried lumber forms and the soil sub grade to keep them from absorbing too much water from the concrete mix. Linseed oil or commercial form release oil can be used. Oiled forms will also make form removal easier without damage to the concrete surfaces. Oiling or wetting the forms and soil is especially important on a warm and windy day when moisture evaporation is at its highest.

The second rule in pouring concrete is to place the mix as near to its final location as possible. A ready-mix truck is equipped with metal chutes which can be extended a moderate distance to deliver concrete directly into the forms, and concrete pumps are often used on large commercial projects. On residential projects, it is more common to use wheel barrows or buggies to move the concrete from the mixer to the forms. You can build ramps and runways over the forms to keep from bumping the boards or reinforcing out of place. Deep forms may require pouring in more than one lift or layer.

The third rule in pouring concrete is to effectively compact or consolidate the fresh mix immediately after it is placed and before it begins to stiffen. Concrete must be consolidated to eliminate air pockets and voids and to get the concrete to flow around reinforcement and anchorages. In very small applications, adequate consolidation can be achieved by rodding or puddling by hand with shovels, metal rods, or tampers, but mechanical vibration is preferred on most applications.

Jan 18 2014

Mixing Concrete on Site

Maintaining Your Concrete Driveway

If you’re mixing concrete on site, the mixing area should be close to the pour area if possible, and your ingredients stockpiled nearby. Store bags of cement off the ground and cover them with plastic to keep them dry. Small quantities of sand can usually be purchased in bags at building supply yards. Larger quantities of both sand and gravel are sold by the ton or half ton by aggregate suppliers and delivered to the site in dump trucks. Spread tarps on the ground before the sand is dumped so that the moisture content of the sand is not affected by the moisture content of the soil, and so that rocks or soil are not accidentally shoveled up with the sand.

One of the most important things in mixing concrete is consistency from batch to batch. The ingredient weights and proportions are based on “wet” sand. Most sand that is sold for construction uses is “wet” sand, and the moisture that it contains has been accounted for in the recommended amounts of mixing water. If the sand you are using is “damp” rather than “wet,” and you are mixing ingredients by weight, reduce the quantity of sand in Table A by one pound, and increase the quantity of water by one pound. If your sand is “very wet,” increase the quantity of sand by one pound and decrease the water by one pound. The moisture content of sand is more difficult to adjust for when proportioning mixes by volume rather than weight.

Quick Tip

Damp sand falls apart when you try to squeeze it into a ball in your hand.
Wet sand forms a ball when squeezed in your hand, but leaves no noticeable moisture on the palm.
Very wet sand, such as sand exposed to a recent rain, forms a ball if squeezed in your hand, and leaves moisture on the palm.

Jan 16 2014

Estimating Total Concrete Volume

To estimate the cubic yardage of concrete needed, first calculate the area in square feet, If the area is larger than 300 sq. ft., first find the volume for 300 sq. ft., then find the volume for the remainder of the square footage, and add the two together. To allow for slight irregularities in concrete thickness and for some spillage and waste, round up at least to the next whole or half cubic yard measure, allowing a mini-mum of 5–10% extra.

Ready mix suppliers will need to know minimum compressive strength and maximum aggregate size for the concrete mix, and any special requirements such as air entrainment for added freeze thaw durability. As a general guideline for ordering ready mix concrete. Local conditions can change within a very short distance, particularly in mountainous regions, and may be more or less severe than indicated by the regional classification. Severe exposures are those in which deicing salts are used because of significant snowfall combined with extended periods in which natural thawing does not occur. If you are in doubt about which classification applies, always use the more severe exposure.

Jan 14 2014

Control, Construction and Isolation Joints

Concrete shrinks irreversibly as it cures and dries out. After this initial shrinkage has occurred, concrete expands and contracts reversibly with changes in temperature and moisture content. This movement can cause concrete to crack uncontrollably unless it is reinforced with steel and built with special joints that are designed to control cracking locations.

The amount of expansion and contraction that concrete will experience is influenced by several things, including the water content of the mix, and the weather conditions during the curing period. Mixes made with a high water content are more prone to cracking from initial shrinkage than drier, stiffer mixes. Reinforcing steel increases the strength of concrete and absorbs the stress of expansion and contraction, but it cannot prevent cracking altogether it can only distribute the stresses so that there will be many minute cracks instead of a few big ones. While reinforcement limits the amount of expansion and contraction to some extent, cracking and movement can also be regulated by subdividing the concrete into smaller sections with control joints and construction joints, and separating it from adjacent construction with isolation joints.

Jan 13 2014

Concrete Mix Designs

For work requiring more than one cubic yard of material, concrete is usually ordered from a ready mix supplier for delivery to the job site. The supplier will need to know the minimum compressive strength, the maximum aggregate size, and any special requirements such as air entrainment for added freeze-thaw durability. The supplier will then select a mix design that is appropriate for your needs. If you are mix-ing small batches of concrete on site, you will need to understand the basic principles of concrete mix design yourself. The proportion of dry ingredients and the ratio of water to cement are the two most important factors.

Cement and aggregates provide strength, durability, and volume stability in concrete, but too much or too little of one in relation to the other reduces quality.

Lean or oversanded mixes with low cement content and high aggregate proportions are harsh and have poor workability.
Fat or undersanded mixes with high cement content and low aggregate proportions are sticky and expensive.

Within the range of normal concrete strengths, compressive strength is inversely related to water content. That is, the more water you use, the lower the concrete strength. But increasing water content increases fluidity and workability. Since water is required for workability, and since workability is required for high quality concrete, the low water requirements for strength and high water requirements for workability must be balanced. The ratio of water to cement is the weight of water divided by the weight of cement. Water cement ratio affects the consistency of a concrete mix. The consistency, in turn, affects how easily the concrete can be poured, moved around in the forms, compacted, and finished. Up to a point, a mix with more water is easier to work with than one that has less water and is therefore stiffer. Too much water, though, will cause the ingredients to separate during the pouring, placing, and handling and will destroy the integrity of the concrete. Too much water also lowers strength, increases the porosity and water permeability of the cured concrete, and makes it more prone to shrinkage cracking. The trick is to use enough water to make the fresh concrete workable, but not so much that it creates weak or porous structures.

Jan 12 2014

Concrete Ingredients

The basic ingredients in concrete are cement, aggregates, and water. The type, quality, and proportioning of these ingredients affect the curing rate, compressive strength, and durability of the concrete. Chemical admixtures can be used to enhance one or more properties of the concrete or to improve its handling and placing characteristics.

Cement

Cement is not the same thing as concrete. Many people mistakenly refer to “cement” sidewalks or “cement” driveways and the like, but cement is only one of the ingredients in concrete. It is also an ingredient in masonry mortar, stucco, and other materials.

Cement + water = cement paste
Cement + water + sand =cement mortar
Cement + water + sand + lime = masonry mortar
Cement + water + sand + coarse aggregate = concrete

Aggregates

The aggregates most commonly used in concrete are sand, gravel, crushed stone, crushed slag, and pumice. Cement and water are mixed with aggregates to produce concrete. Concrete contains both fine and coarse aggregates. When cement is mixed only with fine aggregate, it is called cement mortar, which is used typically for patching and small repairs, or for coating a concrete surface to provide a smooth, even finish. Masonry mortar is different from a simple cement mortar because it contains other ingredients as well.

Water

As a rule of thumb, water used for mixing concrete should be drinkable. Any water that is drinkable is generally free of harmful impurities. In urban areas where municipal water supplies are available, contaminated water is usually not a problem.

Admixtures

Admixtures are substances other than cement, water, or aggregates which are added to concrete mixes for the purpose of altering properties of the fresh or hardened concrete. Admixtures are not generally required to produce high quality, low cost concrete, but they may sometimes be necessary or desirable to alter specific properties of the concrete for specific conditions or circumstances.

Chemical Admixtures

Set accelerators speed up the setting time and early strength development of concrete. This can be helpful in winter weather to reduce the length of time required for curing and protection and to compensate for the effects of low temperatures on strength development.