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Planning Weather

Weather conditions can affect the appearance of concrete.

  • Temperature effects the amount of water required to make the mix workable in cool temperatures, it takes less water to get a workable mix. But, always remember that variations in the water-to-cement ratio have a significant effect on concrete color. Maintain a consistent water/cement ration throughout the project and follow ACI guidelines for hot and cold weather concreting where applicable.
  • Schedule placing and finishing work to minimize exposure to hot sun before curing materials are applied.
  • Consider postponing color concreting until windy conditions pass, or create a wind barrier to avoid crusting.
  • Don’t pour concrete if rain, snow or frost is in the forecast.
  • Dark or black-colored concrete will be exposed to additional heat during sunny, dry conditions. Provide extra curing protection to prevent thermal cracking when this combination is anticipated.
  • Concrete made from the same supplier can cure to different colors if pouring or curing takes place under different climates. Place concrete in similar conditions if possible.
  • Maintain concrete temperature between 60F and 80F in most applications for proper curing to occur during the crucial first days.

Resources

Planning – Site Preparation:

Uniformly grade, compact and dampen the sub-grade.

  • Add a 2-3” layer of sand, gravel or crushed stone and compact with vibrating equipment to insure a consistent base.
  • Do not place concrete if the ground has standing water, hard or soft spots, ice, frost or muddy areas.
  • Follow American Concrete Institute (ACI) procedures for the installation of vapor barriers when applicable.

Planning – Forms and Reinforcement:

For slabs, place formwork to achieve consistent thickness throughout, to ensure even curing and even color.

  • In vertical applications, seal forms to ensure no leakage and staining of the surrounding area.
  • In applications where the formed surface is visible, use form liners or employ a resin coated high-density overlay, epoxy or urethane coated plywood.
  • Always ensure that the form release agents are compatible with colored concrete.

Planning – Mix Design:

Always ask your ready-mix supplier for recommendations on the specific components for your mix. Hear are a few guidelines:

  • The slump should be no greater than 4” unless a Grace mid-range or high-range water-reducing admixture is used.
  • Low water-to-cement ratios will minimize shrinking and cracking, maximize concrete strength and produce a richer, darker color.
  • Plastic shrinkage fibers will reduce shrinkage cracking, allowing your work to continue looking beautiful well into the future.
  • If freeze-thaw issues existing in your area, specify air contents in the range of 5% to 7%.

Planning – Placing:

Minimize water added to fresh concrete at the job site. Water added to the mixer, pumps or placing tools will cause the final color to pale.

  • Discharge the concrete as close as possible to the final location. Move the concrete with shovels rather than vibrators.
  • Protect areas near the pour with plastic sheets. Splatters con stain adjacent slabs and structures.

Planning – Finishing:

Time, attention to detail and consistency in finishing, will pay off in your final product allowing your customer to enjoy your craftsmanship.

  • Avoid adding water to your finishing tools. This will cause pale streaks discoloring the surface.
  • Bull float after striking off the slab. Magnesium bull floats can trap moisture in the surface of the concrete; use wood floats and derbies when possible.
  • As with regular concrete, wait for the bleed water to completely disappear before troweling as this can also trap moisture. Over troweling or late troweling leads to burns and dark spots. The potential for discoloration rises as troweling time increases. The easiest way to achieve color uniformity is to skip troweling and use a broom or swirl finish. Less time spent of finishing the concrete is better for consistent color.
  • For more information on creating finishes, see Finishing Concrete Slabs with Color and Texture by the Portland Cement Association.

Planning- Curing and Sealing:

Now that you have applied your skills to getting the proper finish, complete the job with a membrane-forming cure and seal formulated for colored concrete. A properly applied curing compound will assist in achieving consistent drying and a consistent final color. Lack of proper curing can lead to shrinkage cracks, dusting and surface deterioration.

  • Any saw cutting, either decorative or structural, should take place before sealing materials are applied. Thoroughly clean all cutting residue before applying sealing products.
  • Curing with water sprinkling, membranes, paper, sodium or fluro silicate-type hardeners and non-approved compounds can cause discoloration. If water is used to cure, a lighter color is likely.
  • Always use curing, sealing, and cure-and-seal products suitable for colored concrete and follow the manufacturer’s recommendations for application.

Planning – Efflorescence:

Efflorescence is a salt deposit that forms on concrete, causing unsightly white stains. Water, concrete permeability and soluble salt content determine how much and when efflorescence will occur.

  • Control efflorescence by mixing with a low water/cement ration, using a curing compound, and designing a well-drained sub-grade.
  • Seal concrete against water penetration and leaks.
  • Keep de-icing salts away from concrete which is not fully cured and sealed.
  • Efflorescence can be removed with a water wash and stiff bristle broom, if treated early. If not, it converts to calcium carbonate, which is removed with dilute acid wash that will affect the surface appearance

 

 

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