Dusting
The presence of a powdery dusty material on the surface of a new hardened concrete slab.
Popouts
A hole in a concrete surface
Plastic Shrinkage
Small cracks in the surface of the concrete which can be combined with drying shrinkage cracks.
Crazing
Blistering
Causes
- Premature finishing will bring excess water, cement and fines to the surface and will weaken the paste.
- Carbonation can occur when open salamander heaters/engines give off carbon dioxide and inhibits the normal hydration process.
- Condensation may occur when the temperature of the concrete on a floor inside made with cool materials rises slower that the surrounding, more humid air. This is common in the spring and fall.
- Dryout, or inadequate curing, will occur where there is not enough moisture to continue the hydration process of the cement paste. This produces a soft, weakened material.
Prevention
- Be sure the water has evaporated or been removed from the concrete surface before the finishing process begins.
- Provide adequate ventilation for vent exhaust and fumes.
- When there is high humidity, use heated concrete and open windows to provide ventilation. Also, using blower-type heaters and fans to increase air circulation is recommended.
- Planning ahead by the contractor is recommended to provide adequate time for the placement, finishing and curing based on the current conditions on the job site.
Cracks
Breaks in the concrete in areas other than those that were placed intentionally.
- Shrinkage after hardening.
- Poor subgrade preparation and drainage on job site.
- Uneven settlement.
- Premature back-filling, hydrostatic pressure or structural loads.
- Thermal cracking in thick sections of concrete
Causes
Prevention
- Be sure to place or cut control joints at recommended distances.
- Be sure to place or cut control joints where concrete meets other sections or slabs, where there are grade beams present or other recommended locations.
- Add the proper amount of granular fill in areas where proper compacting or drainage is an issue.
- Properly prepared subgrade and building construction joints where required is necessary.
- Adequate drainage needs to be provided.
- Water-to-cement ratio needs to be lowered. Be sure to use the largest aggregate size allowed.
Causes
- In freezing conditions, porous aggregate pieces with high rates of absorption will cause popouts.
- Chert, coal, weak shales, hard burned dolomite, pyrite and other deleterious aggregates can cause popouts.
- Aggregates that swell with moisture.
- Reactive aggregates and the alkalis in the concrete will sometimes cause a chemical reaction which will cause a popout.
Prevention
- Use durable aggregates.
- Lower water-to-cement ratios.
- Follow proper curing procedures.
- We sure to slope exterior work for proper drainage.
- A repair of a popout can be done by drilling out the spalled particle and patching the hole.
Discoloration
Spots, light or dark patches or change in the color of the concrete from pour to pour.
Causes
- Delayed finishing or early finishing can affect the color of the concrete.
- Hard troweling or delayed finishing typically darkens the concrete surface color.
- Changes in any placing or finishing procedures.
- Finishing in a bleed water will affect the color of the concrete and will weaken the surface.
- Calcium chloride added on the job site to cold concrete is very difficult to dissolve and can result in dark spots.
- If plastic sheeting is used, light and dark spots will occur in areas of contact.
- Calcium chloride can retard the ferrite phace causing the concrete to remain dark in color, especially during cold weather.
Prevention
- Avoid, when possible, using calcium chloride in flake or pellet form.
- Maintain subgrade, placing, finishing and curing conditions.
- Make sure finishing time is controlled by the concrete set time and not bleed water. Surface water is controlled by wind speed, mix design, humidity, subgrade, admixtures and temperature.
- Products are available to treat and remove minor discoloration and stains.
- Recommend using wet burlap instead of plastic sheeting to maintain curing and prevent discoloration.
Concrete Surface Defects
Crazing
A large number of small connected cracks in the surface of the concrete.
Causes
- Rapid drying of the concrete surface due to high evaporation rates such as low humidity, high winds or temperatures.
- Overuse of placement tools can bring excess mortar to the surface.
- Premature finishing with a float and trowel will bring excess moisture and paste to the surface. Rapid loss of moisture may cause crazing.
- Crazing can be caused by using a dry cement or cement and sand mix as a mop coat to soak up excess water.
Prevention
- Immediately use an evaporation retardent or fog spray after screeding to maintain the correct water-to-cement ratio on the surface.
- Cover concrete with a wet burlap or canvas as soon as possible after striking off and darbying. Keep cover in place until the surface is ready for finishing.
- Begin curing immediately and when possible continue mist curing to lower the temperature by evaporation.
- Only use placing tools to bring the concrete surface to its proper plane. Do not overwork surface paste.
- Do not begin the finishing process until all visable water has evaporated or has been removed and the slab can support the finisher with only a slight indentation.
- Using low-slump, air-entrained concrete and finishing at the proper time prevents crazing.
Scaling
When a thin layer of the hardened concrete surface breaks away. This usually happens within the first year after placement.
Causes
- High rate of evaporation of the water from the concrete surface due to ambient temperature, temperature of concrete, relative humidity and/or wind velocity.
Prevention
- Dampen subgrade and forms.
- Erect windbreaks to reduce wind velocity.
- Begin the curing process as soon as possible after finishing, using membrane curing compound, wet burlap, sand, curing paper or other acceptable methods.
- Minimize placing and finishing times.
- Use evaporation retardants or a fog spray immediately after the screeding to maintain the water-to-cement ratio at the surface.
Causes
- Freezing and thawing of the water in porous, non-air-entrained conrete.
- Faulty workmanship.
- Excessive and early drying out of the mortar.
- De-icing salts, especially those containing ammonium sulfate or nitrate.
Prevention
- Use air-entrained concrete and cure properly.
- Delay finishing operations until all free water has evaporated or been removed. Never use dry cement as a mop coat.
- Allow a minimum of 30 days for the concrete to cure before exposing it to freezing weather.
- Avoid overworking the surface. Recommend a broom finish on exterior slabs.
- Use curing compounds and sealers cautiously late in the season.
- A minimum of 7 days of continuous wet curing is best.
- Avoid using de-icing salts the first winter.
- Use ACI recommended mix design for exterior flat work.
Blistering
Small bumps in the surface of the concrete that appear during the finishing process.
Causes
- Blisters are caused by air and/or water getting trapped under the concrete surface.
- Improper or premature finishing is the leading cause of blistering.
- Troweling tends to move water and air bubbles trapped under the sealed surface together.
- Dry and windy weather conditions cause the concrete surface to dry out and crust over.
Prevention
- Avoid premature finishing of the surface of the concrete. Using wood or magnesium floats on the first pass.
- Avoid overworking the concrete surface with jitterbugs, screeds and bullfloats.
- Beware of placing concrete directly over plastic sheeting. Place a layer of sand over the plastic before placing to concrete.
- Use proper compacting techniques.
- Fog sprays or evaporation-retardant films will facilitate finishing in dry and windy weather conditions.
- Refloating with a wood float can reduce the blisters in a concrete surface that was prematurely closed.
- Avoid using concrete with excessive air contents and fines for hard troweled finishes.