Cold-Weather Concreting

Since successful concrete pouring depends on outside temperatures being at a certain minimum, cold-weather concreting comes with its own challenges. However, you can pour, finish and cure your concrete structure at virtually any temperature. Visit this page for more information.

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Concrete Troubleshooting

Even in the best of weather, concrete is sometimes difficult to work with. Problems can occur, no matter how careful you are. Here are some of
the more common issues and tips to avoid them.

 

Avoid Air Loss During Concrete Pumping

• Keep pumping boom pointed upward or horizontally

• Reduce vertical concrete drop distance - run the last 3-5 metres of the pumping hose horizontally and insert multiple 90˚ elbows into the line

• Use a flow-restricting device at the end of the pump line

• With cabling, loop the end section of the rubber hose into a circle to avoid free-fall out of the hose

• Inspect air content at both the truck and the pumping location when changing boom configuration. Elevate the plastic air content above the spec limits at the truck location to account for actual air loss.

 

Avoid Blistering

• Don’t place concrete on cold subgrades

• Place concrete during periods of lower surface evaporation

• Use chemical accelerators to reduce set time

• Heat the building and subgrade prior to concrete placement

• Take precautions to not finish prematurely when placing concrete on top of vapour barriers

• Use proper finishing tools and keep troweling blades as flat as possible

• Avoid sealing the surface prematurely

 

Avoid Bugholes (Surface Air Voids)

• Uniformly consolidate concrete, keeping vibrator insertion pattern uniform and well-spaced

• Make air removal easier by reducing height of each concrete placement

• Move the vibrator as close to formwork as possible

• Properly apply the form release agent. Remove forms only when concrete is sufficiently strong to resist suction forces.

• Use specialty concrete products like self-consolidating concrete

 

Avoid Crazing

• Curing is extremely important and often overlooked

• Cure as soon as possible. Keep the surface continuously wet during the curing period

• Avoid intermittent curing procedures

• Don’t apply cement to the surface of the concrete to eliminate excessive bleeding. Order the required slump from us and use chemical admixtures like water reducers, instead

• Prevent the surface from drying out during the finishing period using protection methods like evaporation retardant chemicals or fog spraying

 

Avoid Crusting Surfaces (Wavy Surface)

• Keep temperatures of the subgrade and the concrete surface similar. Heat subgrade in winter and avoid working in summer months when
  air temperature is rising significantly

• Use initial curing protection strategies like fog-spraying and evaporation retardant compound to minimize moisture differentials between bottom and top of slab

• Use finishing tools that open the surface of the concrete and don’t result in sealing. Delay floating as long as possible

• Use all floating and troweling equipment in a flat position

• Use chemical admixtures to accelerate the setting rate of the concrete. Minimize variations between loads of concrete.
  Avoid unloading delays

• Delay power troweling as long as possible

• Use pans on power trowels to break open the concrete surface immediately after the bleed water period ends

 

Avoid Curling

• Use the best curing system and consider extending the curing period

• Vapour barriers may increase the potential for curing if care isn’t taken

• Use the lowest practical water slump and avoid re tempering with water

• Minimize concrete shrinkage by using large coarse aggregate size, minimizing cement content, etc.

• Use smaller joint spacing to reduce size of concrete panels

• Increase thickness of concrete slab and or use slab reinforcement

• Use specialty chemical admixtures to reduce concrete shrinkage

 

Avoid Discolouration​

• Conduct finishing operations at the same time; don’t finish early or late

• Use a uniform curing system for the necessary time period

• Ensure that plastic sheeting does not come in direct contact with the concrete

• Use effective form release agents that prevent non-uniform loss of moisture from formed surface

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Avoid Dusting

• Don’t perform finishing operations on concrete while bleed water remains on the surface

• Initiate curing as soon as possible and cure for specified period

• Take extra care when placing concrete on vapour barriers

• Vent all heating devices and construction equipment outside work area

• Don’t add water to concrete surface to aid the finishing operation

 

Avoid Honeycombing

• Minimize restrictions due to reinforcing steel by resizing rebar and avoiding rebar lap splices in key structural areas

• Follow optimum concrete vibration practices: decrease height of lifts, reduce distance between vibrator insertions, etc.

• Re-engineer concrete element to allow for acceptable concrete placement methods / locations

• Select appropriate concrete slump for the placement technique being utilized

• Design the mix to optimize flowability and minimize the course aggregate size

• Use specialty concrete products like self-consolidating concrete

• Consider replacing reinforcing steel with equivalent steel or synthetic fibres

 

Avoid Low Concrete Strength Results

• Ensure that field testing is performed only by CCIL- or ACI-certified concrete technicians in strict accordance with CSA A23.1/A23.2

• Verify that proper facilities exist for site storage of test samples prior to the start of concrete placement; every job site should include a curing box that maintains samples between 15˚C and 25˚C

• Review concrete delivery ticket for every load of concrete and compare that to spec requirements for element being constructed

• Use the “Pre-Pour Meeting Form” to clearly designate the person with authority for job site water addition and define conditions when water addition will be allowed

 

Avoid Plastic Shrinkage Cracking

• Erect wind screens or sunshades to reduce surface evaporation rate

• Use fog-spraying equipment on the up-wind side of the slab to keep relative humidity at 100%

• Pre-dampen the subgrade during hot, dry weather to reduce moisture loss from concrete into subgrade

• Use synthetic fibres to resist tensile forces created during plastic shrinkage period

• Place concrete during off hours to avoid high temperature, high wind, high sun exposure

• Apply evaporation retardant chemicals between finishing operations to prevent rapid loss of moisture from surface of concrete

 

Avoid Scaling

• Use the proper concrete for the application (32 MPA, 0.45 W/C, air entrained concrete)

• Properly cure concrete

• Exterior concrete requires a minimum of 7 days’ curing

• Use de-icing chemicals sparingly on the concrete surface and never during the first winter; use only sparingly after that

• Allow at least 30 days of air drying of the concrete slab after curing period before exposing to freeze-thaw cycles

• Use concrete sealers to reduce the absorption of moisture and chlorides into the concrete

• Use wet curing methods during hot weather and severe drying conditions; fog spray the concrete surface between finishing operations

 

Avoid Uncontrolled Shrinkage Cracking

• Make sure subgrade is prepared based on exposure and loading conditions the slab will face. Use appropriate thickness of granular
  sub-base for the application and properly compact the sub-base

• Ensure positive drainage of both the subgrade and the concrete surface to avoid moisture build-up

• Use the proper concrete for the purpose

• Consider concrete jointing details like thickness, maximum joint spacing , fibre reinforcement, structural reinforcement and early
  installation of contraction joints

• Implement curing procedures as soon as possible

 

Avoid Mortar Flaking

• Follow proper curing procedures

• During periods of rapid surface drying, use initial curing procedures like evaporation retardants or plastic sheeting over the surface
  between finishing operations

• Use wet curing methods during severe drying and hot weather conditions

• Use fog-spraying systems to protect the surface of the concrete from drying out until wet curing can be initiated

 

Avoid Popouts

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• Ensure that overall design minimizes build-up of excess moisture in the concrete through proper surface grading and draining of the surrounding subgrade

• Use proper concrete curing and protection practices

• Use sealers to limit the inflow of moisture into the concrete