Concrete Tips
DUSTING
Causes
- Premature Finishing- This brings excess water, cement, and fines to the surface, which weakens the paste.
- Carbonation- Open salamanders, heaters, or engines give off carbon dioxide, inhibiting the normal hydration process.
- Condensation- Can occur when the temperature of concrete on an inside floor made with cool materials rises slower than surrounding, more humid air- common in spring and fall.
- Inadequate Curing- "Dryout" will occur where there is not sufficient moisture to continue the hydration process of the Portland cement paste, producing a soft, weakened material.
Prevention
- Do not begin finishing operation until all free water has evaporated or been removed from the surface, so that the concrete will support the weight of finisher.
- Vent exhaust and/or fumes to the outside, provide adequate ventilation.
- When high humidity is anticipated, use heated concrete. Open windows for ventilation and use blower-type heaters or fans to increase air circulation. Delay finishing operations until free water on the surface has been removed, but do not allow surface to dry out.
- PLAN AHEAD- Provisions should be made by the contractor prior to the placing of concrete to assure adequate time for proper placing, finishing, and curing based on the existing conditions at the job site.
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POPOUTS
Causes
- Porous aggregate pieces with high rates of absorption which fail in freezing conditions.
- Deleterious aggregates, like: chert, coal, soft limestone, weak shales, hard burned dolomite, pyrite.
- Aggregates that swell with moisture.
- Chemical reactions between reactive aggregates and the alkalis in the cement.
Prevention
- Use durable aggregates.
- Use concretes with low water cement ratios.
- Use proper curing methods.
- Slope exterior work for proper drainage.
- Popouts can be repaired by drilling out the spalled particle and patching the hole.
- Reduce alkali aggregate reaction with proper mix designs, aggregates, or mineral additives (like type F fly ash or slag).
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PLASTIC SHRINKAGE
Causes
High Rate of evaporation of water from the concrete surface. Affected by:
- Ambient temperature.
- Relative Humidity.
- Wind velocity.
- Temperature of Concrete.
Prevention
- Dampen subgrade and forms.
- Erect windbreaks to reduce wind velocity.
- Minimize placing and finishing time.
- Begin curing as soon as possible after finishing, using membrane curing compound, curing paper, wet burlap, sand or other acceptable methods.
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CRAZING
Causes
-Rapid drying of the concrete surface due to high temperatures, low humidity, drying winds, or any combination of these.
-Overuse of placing tools, such as the jitterbug vibrator, darby, or bull float, will 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 this moisture may cause crazing.
-Use of a dry cement or cement and sand mix as a mop coat to absorb excess water.
Prevention
-Use monomolecular films (evaporation retardants) or a fog spray immediately after the screeding to maintain the water-to-cement ratio at the surface.
-Use a fog spray or cover with wet burlap or canvas as soon as possible after strike off and darbying. Maintain spray or keep covers in place until surface is nearly ready for finishing.
-Begin curing immediately. Where possible, continue mist curing to lower temperature by evaporation.
-Use placing tools only to bring the surface to its proper plane, in preparation for finishing. Do not overwork surface paste.
-Do not begin finishing operations until all free water has been evaporated or removed and the slab can support the finisher with only slight indentation.
-Use low-slump, air-entrained concrete and finish at the proper time.
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CRACKS
Causes
- Shrinkage after hardening (drying shrinkage).
- Poor subgrade preparation and drainage.
- Uneven settlement.
- Hydrostatic pressure or structural loads (premature back-filling).
- Thermal cracking (in thick sections).
Prevention
- Place or cut control joints at recommended distances.
- Put expansion joints where concrete meets other sections or slabs, over grade beams, and other recommended locations.
- If existing material cannot be properly compacted and drained, add 8 to 15 centimeters of sound granular fill.
- Prepare subgrade properly and build construction joints where required.
- Provide for adequate drainage.
- Use adequate control and expansion joints.
- Lower the water-to-cement ration.
- Use the largest aggregate size allowed.
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DISCOLORATION
Causes
- Variations in any placing or finishing operations.
- Delayed finishing or early finishing can affect color.
- Finishing in bleed water will lighten and weaken the surface.
- Delayed finishing/hard troweling almost always darkens the surface.
- Job-site added calcium chloride flakes are very difficult to dissolve in cold concrete and can result in dark spots.
- Calcium chloride retards the ferrite phase of the cement. The ferrite phase tends to remain dark in the presence of calcium chloride. Cold weather increases this effect.
- Plastic sheeting will cause light and dark spots in areas of contact and non contact (greenhouse effect).
Prevention
- Avoiding the use of field-added flaked or pellet calcium chloride.
- Maintain constant subgrade, placing, finishing, and curing conditions.
- Finishing time is controlled by the concrete set time not bleed water. Surface water is controlled by wind speed, humidity, mix design, subgrade, admixtures, and temperature.
- Products are available to treat minor discoloration and stain removal.
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Scaling
Causes
- Freezing and thawing of the water in the porous non-air-entrained concrete.
- De-icing salts especially those containing ammonium sulfate or nitrate.
- Faulty workmanship.
- Excessive and early drying out of the mortar.
Prevention
- Use low-slump, air-entrained concrete; cure properly.
- Avoid the use of salt the first winter.
- Delay finishing operations until all free water has evaporated or been removed. Never use dry cement as a "mop coat".
- Avoid overworking the surface, a broom finish is recommended on exterior work.
- Allow a minimum of 30 days for the concrete to air dry before it is expose to freezing.
- Use caution when applying curing compounds and sealers late in the season.
- Continuous wet curing of concrete is best (7 days minimum).
- Specify a proper mix design for exterior flat work.