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.
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 metres).
- Back filling is to be done carefully to avoid damaging the walls.
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.
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 10°C (or 50°F), 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 de-icers 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.
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.
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.
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 three to seven 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.
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.
Placing and Finishing Concrete in Hot Weather
Temperature is an important factor in proper curing. Generally concrete temperature should be maintained above 50°F (10°C) for adequate rate of strength development. For concrete that is exposed to the elements, such as high temperatures, humidity, and wind conditions, the elements 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
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 10°C (20°F) will approximately double the setting time. Cold weather is defined as a period when the average daily temperature falls below 4°C (40°F) 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.