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Using Concrete


Personal Safety Tips
Tools
Preparation
Placing and Finishing Techniques
Proper Curing
Practical Tips to prevent concrete problems

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









The Fundamentals of Using Concrete

The following are suggestions and helpful information to assist you in ensuring that your concrete project goes well:

Personal Safety Tips

It is important to ensure that you are protected from harming your skin from freshly mixed concrete. The materials in concrete can irritate and create severe injury to the skin so wear long sleeved shirts, pants and gloves. Wear goggles to prevent eye injuries from splashing materials and use a dust mask to filter out concrete dust. If you will be wading or kneeling in concrete, it is important to wear rubber hip boots or knee pads when floating or troweling. Wear waterproof shoes to protect your feet when standing in concrete.

Tools

The tools needed for concrete work include woodworking tools for building forms such as a hammer, hand cross-cut saw, portable circular saw, measuring tape, square, level and string level along with a small sledge and a maul. A pry bar is needed for disassembling the forms when the concrete dries. A builder's laser level can make it easy to level or place a form to grade.

For concrete preparations, the tools needed depend on the job you will be doing. Shovels, rakes, and hoes are required for moving the liquid concrete around. Some of the concrete finishing tools include a screeding board that is longer than the pour to pull the concrete off and level it with the form edges. A tamper or vibrator can also be helpful in settling concrete in the forms. Other essential finishing tools include: an edger, groover, magnesium or wooden float, finishing trowel, pointing trowel, cement brook and a water hose. For larger projects such as floors or slabs, you will need a bull float with a bracket.

Preparation

The success of your project is important to you and the most you can do in preparation will ensure success. If doing a slab, all topsoil and soft spots should be removed. The soil beneath the slab should be compacted soil or granular fill such as sand, well compacted by rolling or tamping. The slab (or sub grade) should be sloped for proper drainage. Smooth, level sub grades help prevent cracking. All formwork must be constructed and braced properly so that it can withstand the pressure of the concrete. See below for more information on preventing problems such as cracking and scaling from occurring.


A concrete sidewalk below is a simple project. Shown is a typical form before the gravel or sand base.

A driveway or patio (see below) may have a "stiffener" edge to add strength. Note the use of gravel and steel reinforcing materials.

Concrete steps (see below) are another common concrete project.



Footings and foundations should also be reinforced according to local municipal code regulations. Driveways and garage floors are usually poured 4 to 6 inches thick. Sidewalks and other works are usually poured 3 to 4 inches thick. Foundations and footings are poured to municipal code.

Placing and Finishing Techniques

Concrete should not be overworked and should be spread evenly and quickly once the pour begins. Make sure to overfill the forms slightly. If the pour is overworked, too much water will be floated to the surface, which can cause scaling after the concrete dries.

Once the concrete is spread well over the area and into all corners, gaps, and crevices, use a screed board to drag off the excess. This usually is a two person operation and is fairly hard work, especially on larger pours. The screed board should extend past the form edges at least 3 inches on either side. Beginning on one end of the form, place the screed board over the form boards and then, using a side-to-side motion and at the same time pulling the board, inch it across the form boards to the opposite end. Screeding levels the concrete with the tops of the form boards, pulling excess concrete off. Any low spots will be visible and should be immediately filled and the area re-screeded.



The next step is to float the surface. Small projects can be floated with a wooden or magnesium float. This helps fill any small voids and works the aggregate slightly below the surface. On larger pours a bull float is used. Push the float away from you across the surface with the front edge slightly raised to prevent the blade from digging in. Then pull the float back at an almost flat angle.



Use a wooden or magnesium float to smooth up the concrete and work the aggregate down. A bull float is used on larger pours.

Concrete finishing consists of several steps. Some steps should be done regardless of the desired roughness of the surface. Some concrete finishing results in either a roughened or a smooth surface. Regardless, the first step is to use a pointing or margin trowel to separate the edge of the concrete from the form.



Use a pointing trowel to "cut" the concrete edge from the form board.


Then use an edger around the top edge of the form. This creates a rounded edge that won't chip off when the form is removed. The edger should be held fairly flat, but keep the front tilted up slightly when moving forward and the rear tilted up slightly when moving backward.



Use an edger to slightly round the edge of the concrete.


Jointing is the next step on projects such as sidewalks and driveways. This prevents cracking the slabs. Control joints are normally spaced at intervals equal to the width of the pour. It is recommended, however, not to exceed 10 feet in any direction without a joint. The joint should be cut at least one-fourth the depth of the slab. A jointer tool is used for this step. Place a straight-edge across the surface and run the jointer along the straight edge to create a nice straight line. As with the edger, hold the front up slightly when pushing forward. Control joints in large slabs can also be cut after the concrete cures, using a masonry blade in a circular saw or concrete saw.



Expansion joints must be cut in large slabs. A jointer held against a straight edge produces a nice, straight-line groove.

Next, use a float to smooth and level the surface. This will also help remove any marks left by the edger or jointer. For rough or textured surfaces, use a wooden float. For projects requiring a smoother finish, use a magnesium or aluminum float. Hold the float flat on the surface and move it in an arc, overlapping the arcs as you proceed. Don't overwork the surface.

The final finishing step is troweling. Small projects can be hand-troweled. The first troweling should be done with the blade held flat down on the surface. Again use the trowel in an arc, overlapping each previous arc by about 1/2 inch. The surface should be well troweled several times to produce a hard, durable surface. Allow the concrete to set up slightly for the additional trowelings. The proper time is when the sheen of water disappears and a footprint leaves less than 1/4 inch of an indentation. These trowelings should be done fairly vigorously and with the trowel tilted up slightly, pressing down on the "rear" edge.



The final step is to smooth the surface using a trowel.

Proper Curing

Curing is an important step to ensure durable crack-resistant concrete. Start curing as soon as possible by maintaining the proper temperature (neither too hot nor too cold) and the dampness for at least seven days or until concrete reaches 70% of its specified strength. If you skip the curing process, it will have a major impact on the quality of your finished work. While curing is important for all concrete, the problems that arise from not curing are most obvious with horizontal surfaces. An uncured slab, whether decorative or plain gray, is likely to develop a pattern of fine cracks (called crazing) and once it's in use the surface will have low strength that can result in a dusting surface that has little resistance to abrasion.

When most people think of curing, they think only of maintaining moisture on the surface of the concrete. But curing is more than that it is giving the concrete what it needs to gain strength properly. Concrete strength depends on the growth of crystals within the matrix of the concrete. These crystals grow from a reaction between Portland cement and water a reaction known as hydration. If there is not enough water, the crystals cannot grow and the concrete does not develop the strength it should.

The other important aspect of curing is temperature the concrete cannot be too cold or too hot. As fresh concrete gets cooler, the hydration reaction slows down. The temperature of the concrete is what is very important, not necessarily the air temperature. Below about 50 F, hydration slows down a lot; below about 40 F, it almost stops.

After final finishing, the concrete surface must be kept continuously wet or sealed to prevent evaporation. The following can be used to keep the concrete wet:
  • Burlap or cotton mats and rugs used with a soaker hose or sprinkler. Care must be taken not to let the covering dry out and absorb water from the concrete.
  • Sprinkling on a continuous basis is suitable provided the air temperature is well above freezing. The concrete should not be allowed to dry out between soakings.
  • Liquid membrane-forming curing compounds which should be applied to the concrete surface about one hour after finishing. Do not apply to concrete that is still bleeding or has a visible water sheen on the surface. A single coat may be adequate but a second application of the curing compound the next day is a good quality assurance step.

Follow these Practical Tips to help prevent some concrete problems from occurring.

1. Concrete Blisters (mortar blisters appear in the concrete surface)

Under traffic these blisters crack and the mortar breaks away, exposing the concrete underneath.
  • Do not seal surface before air or bleed water from below has had a chance to escape
  • Prevent surface drying or crusting by use of a fog spray to avoid exposure of the surface to rapid drying before the concrete underneath has stiffened.
  • Avoid dry shakes on air-entrained concrete
  • Use heated or accelerated concrete to promote even setting throughout the depth of the slab in cooler weather
  • Do not place slabs directly on vapor retarders. If vapor retarders are essential take steps to avoid premature finishing.
  • Protect surface from premature drying and evaporation
  • Do not use excessive vibration on slumps over 5 inches (125 mm)
  • Air entrained concrete should not be steel troweled. If required by specifications, extreme caution should be exercised when timing the finishing operation

2. Cracks on Concrete Walls

Since the performance of concrete is affected by climate conditions, unusual loads, material quality and workmanship, care should always be exercised in their design and construction
  • Uniform soil support is provided. Soil investigation should be thorough enough to ensure design and construction of foundations suited to the building site.
  • All formwork must be constructed and braced so that it can withstand the pressure of the concrete. Observe local codes and guidelines for wall thickness and reinforcement.
  • Ready-mix concrete is placed at a moderate slump (up to 5 inches or 125 mm) and excessive water is not added at the jobsite prior to placement
  • Place concrete in a continuous operation to avoid cold joints.
  • Curing should start immediately after finishing. Forms should be left in place five to seven days or as long as possible.
  • Ensure adequate curing practices are followed
  • Proper construction practices are followed
  • Steps need to be taken to control the location of the cracks by providing control joints every 20 to 30 feet (6 to 9 meters).
  • Back filling is to be done carefully to avoid damaging the walls.

3. Cracking on Concrete Surfaces

Shrinkage during drying and cooling causes concrete to crack. Some shrinkage cracking can be prevented however, cracking that cannot be prevented, can be controlled.
  • In slab-on-grade work, prepare a stable sub grade. All topsoil and soft sports should be removed. The soil beneath the slab should be compacted soil or granular fill. The sub grade should be sloped for proper drainage.
  • Design the slab to accommodate movements, load and support conditions.
  • Provide proper contraction and isolation joints. This can be constructed by sawing, forming or tooling a groove about 1/4 to 1/3 the thickness of the slab with a spacing between 24 to 36 times the thickness. A maximum 10 to 15 feet spacing for contraction joints is often recommended. Isolation joints should be provided whenever restriction to freedom of either vertical or horizontal movement is anticipated such as where floors meet walls, columns or footings. These are full-depth joints that are constructed by inserting a barrier of some type to prevent bond between the slab and the other element.
  • Place and finish according to recommended and established practices. Initial screeding must be promptly followed by bull floating. Do not perform finishing operations with water present on the surface or before the concrete has completed bleeding. Do not overwork or over-finish the surface.
  • Protect and cure the concrete properly. Curing is an important step to ensure durable crack-resistant concrete. Start curing as soon as possible. When ambient conditions are conducive to a high evaporation rate, use means to avoid rapid drying. Associated plastic shrinkage cracking can be avoided by using wind breaks, fog sprays and covering the concrete with wet burlap or polyethylene sheets between finishing operations.

4. Scaling on Concrete Surfaces

Scaling, flaking or peeling of a finished surface of hardened concrete may result from the destructive effect of freezing and thawing however, properly constructed concrete will resist scaling.
  • In cold weather, concrete temperature should be at least 10C (or 50F), contain an accelerating admixture and be placed at a lower slump
  • Concrete exposed to freezing and thawing cycles must be air entrained
  • Allow at least 30 days of drying after curing and do not use deicers the first winter. Only use clean sand for traction and hose off accumulation of salt deposited by cars.
  • Do not use harmful chemicals such as ammonium nitrate or ammonium sulfate which are fertilizing materials. They attack concrete chemically and should never be used.
  • Seal the surface with a commercial sealer
  • Overworking the surface during finishing will reduce the air content in the surface layer, making it susceptible to scaling in freezing conditions. Finishing while bleed water is on the surface will be worked back into the top surface of the slab, creating a high water-cement ratio and a low-strength surface layer is produced.
  • Use correct timing for all finishing operations and avoid the use of steel trowels for exterior slabs.

5. Curling of Concrete Slabs

Curling occurs around the slab perimeter or at joints and this happens where there is more shrinkage at the top than at the bottom after the concrete has hardened. It is primarily due to differences in moisture and or temperature between the top and bottom surfaces.
  • Proper curing and drying techniques are essential including joints and edges.
  • Use the lowest practical water content in the concrete and can generally be accomplished by using water-reducing admixtures.
  • Take precautions to avoid excessive bleeding. If conditions permit, place concrete on a damp but absorptive sub grade so that all the bleed water is not forced to the top of the slab.
  • Avoid a higher than necessary cement content and use the largest practical maximum size aggregate.
  • For thin topping mixes, clean the base slab to ensure bonding. Use of properly designed and placed slab reinforcement may help reduce or eliminate curling.

6. Crazing of Concrete Surface

A crazed concrete surface is a network of very shallow fine cracks. Although the surface is sound, the cracks are unsightly and show up most when the slab is damp. The cause is shrinkage of the cement paste on the surface.
  • Use moderate slump (3-5 inches or 80-125 mm) without excessive bleeding and or segregation
  • Follow recommend practices for placing and finishing operations such as:
    • Do not finish concrete before the concrete has completed bleeding
    • Do not dust any cement onto the surface to absorb bleed water
    • Do not sprinkle water on the surface while finishing concrete
    • If steel trowel is required, delay it until the water sheen has disappeared from the surface
  • When high evaporation rates are possible, lightly dampen the sub grade prior to concrete placement (do not allow the water to pool) to prevent it absorbing too much water from the concrete.
  • Use a broom finish wherever practical
  • Start curing the concrete properly as soon as possible. Curing retains the moisture required for proper reaction of cement with water called hydration.

7. Dusting of Concrete Surfaces

Dusting is the formation of loose powder resulting from disintegration of surface of hardened concrete. The slab dusts under traffic because the wearing surface is weak.
  • Use a moderate slump not exceeding 5 inches (125 mm)
  • Do not finish concrete before the concrete has completed bleeding
  • Initial screeding must be promptly followed by bull floating
  • Do not add water to the surface or sprinkle water prior to or during finishing operations
  • Use adequate curing measures to retain moisture in concrete for the first 3 to 7 days
  • In enclosed areas, ensure adequate venting from gas heaters. Some heaters give off high levels of carbon dioxide and it reacts chemically with fresh concrete. If a concentration of carbon dioxide is not prevented, a dusting floor will probably be the result.

8. Uneven Colour or Discolouration

Surfaces might have light or dark areas or is non-uniformity of colour or hue on the surface of a single concrete placement. It may be caused by excessive bleeding which can cause a light surface or low water-cement ratios, which darkens a surface. Finishing and curing procedures also influence surface colour. Time, wear and weather will lessen most colour differences. Timely finishing and curing will help prevent them.
  • Extended hard troweling darkens a surface. Eliminate trowel burning of the concrete since most common consequence is that metal fragments from the trowel are embedded in the surface of the concrete
  • Careful in laying plastic sheets which may partly be in contact with the surface as they may leave a mottled appearance
  • Type and condition of formwork can influence colour by the different rates of absorption. A change in the type or brand of a form release agent can also change concrete colour.
  • Concrete which is not properly or uniformly cured may develop discolouration and can be minimized and prevented by following proper curing procedures and adding proper protection from drying by the wind and sun

9. Placing and Finishing Concrete in Hot Weather

Temperature is an important factor in proper curing. Generally concrete temperature should be maintained above 50F (10C) for adequate rate of strength development. For concrete that is exposed to the elements such as high temperatures, humidity, and wind conditions, contribute to the rate of moisture loss and could result in cracking, poor surface quality and durability. Protective measures to control surfaces before it sets are essential.
  • Prepare for early slump loss and shorter finishing time
  • Request a retarder in the concrete mix
  • Schedule truck to reduce waiting time on the job and away from the heat of the day
  • Sprinkle the sub grade and forms before placement.
  • Concrete should be protected from losing moisture until final finishing is completed
  • After final finishing, the concrete surface must be kept continuously wet or sealed to prevent evaporation for a period of at least several days after finishing.

10. Placing and Finishing Concrete in Cold Weather

Low concrete temperature has a major effect on the rate of cement hydration which results in slower setting and rate of strength gain. A drop in concrete temperature of 10C (20F) will approximately double the setting time. Cold weather is defined as a period when the average daily temperature falls below 4C (40F) for more than three consecutive days. Slower setting time and strength gain of concrete during cold weather typically delays finishing operations and form removal. Chemical admixtures such as an accelerator can speed up the rate of setting and strength gain.
  • Do not place concrete on a frozen sub grade. Thaw the sub grade with steam or protect with insulation
  • Heat forms and reinforcing bars above freezing
  • Place and maintain concrete at the recommended temperature
  • Place concrete at the lowest practical slump
  • Protect newly placed concrete from freezing and thawing cycles until it has attained adequate strength
  • Provide insulation thickness at corners and edges of walls and slabs
  • Cure exposed surfaces in heated enclosures with plastic sheeting
  • Plan ahead and have insulation and heaters ready
  • Avoid setting heaters directly on concrete to prevent localized spalling.
  • Keep heaters attended at all times and avoid using unvented heaters since carbon dioxide from the heaters can cause soft, dusting floors
  • Do not expose concrete surfaces to a sudden temperature drop and gradually reduce insulation or enclosure temperature to control concrete cooling






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