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Discolored Concrete Flatwork

Standards for the surface appearance of flatwork have risen considerably. The non-uniform color of concrete slab surfaces can be a point of cosmetic issue, often to the point of owner rejection. The expectation is for the concrete to be attractive in color / shading uniformity.
Dan Green
BY DANIEL GREEN

Retired Technical Services Manager, National Cement

Unfortunately, concrete can become discolored due to conditions present within the concrete itself or as a result of external influences. Below are general comments on the subject. More detailed information can be found in the publications listed at the end of this article.

THE PORTLAND CEMENT ASSOCIATION MENTIONS SEVERAL FACTORS THAT INFLUENCE DISCOLORATION:
  • Calcium chloride additions
  • Hard troweled surfaces
  • Inadequate, inappropriate, and or lack of curing
  • Variations of the water cement ratio at the surface
  • Changes (of raw materials) in the concrete mix

Regarding calcium chloride: it accelerates the hydration process. It also retards the hydration of the ferrite compound in the Portland cement. This compound usually becomes lighter with hydration. But, in the presence of calcium chloride the retarded, unhydrated ferrite phase remains dark. In other words, and in my experience, increased contents of calcium chloride can darken the concrete. To make the explanation even more lengthy; in the absence of moist curing, the ratio of alkali content of the cement (all cement contains a percentage of alkali ) compared to the quantity of calcium chloride, dictates weather we see light sport on a dark background or dark spots on a light background.

Blackening of the surface is common when the concrete is “burned by late and vigorous troweling”. The more the troweling effort, the more compacted and dense the concrete surface becomes. The result is a reduction in the moisture content (water cement ratio) at the surface. Also, abrading of the metal from the trowel on the hardening concrete surface has a darkening effect.

Uneven curing of the surface will result in uneven color. When using plastic sheeting over concrete use care to either entirely “tent” the surface or entirely lay the plastic sheet directly and evenly over the surface. Both efforts are extremely difficult, time consuming, and costly. There must not be any occasional “tents, bubbles or ripples”. If this occurs, a “pronounced mottled” (color from light to dark, etc.) appearance will result. What happens is that on warm days, each ripple becomes a miniature greenhouse where the evaporation-condensation cycle is repeated several times. The buildup of condensate water around the ripple and in low spots causes the variation in curing and therefore variations in color. This is especially true in concrete containing calcium chloride.

Air cured concrete (which is to say… lack of curing) exhibits the greatest amount of discoloration. Thoroughly moist-cured concrete exhibits the least. Uneven curing will result in uneven coloring. In my opinion, the final color of the surface of the concrete slab is greatly dependent on 1) the moisture content (water-cement ratio) at the surface during the time of the final finishing effort, 2) the intensity of that effort, and 3) the curing conditions.

Lastly, avoid changes in raw materials in the concrete, such as cementitious material or sand sources. These individual materials may differ in color. Stay with the same concrete supplier and the same mix design. Avoid heavy or varying dosages of calcium chloride and varying dosage rates or the intermittent use of high range water reducers (superplasticizers) that alter the water-cement ratio. Minimize inconsistencies.

TO MINIMIZE OR AVOID DISCOLORATION:
  • Minimize or avoid the use of calcium chloride Use consistent concrete materials.
  • Maintain consistent water-cement ration in the concrete.
  • Use proper placing, finishing, timing of finishing, and curing practices.
  • Eliminate aggressive troweling and trowel burning.
  • Don’t overwork the concrete.
  • Cure the concrete uniformly either by moist cure or by applying an even coating of curing compound.
  • Do not allow the concrete to dry out.
TO REMOVE DISCOLORATION (A DIFFICULT TASK):
  • Flush w/ water, use hot water if possible. Allow to dry. Repeat. Repeat, etc.
  • Use 3% acetic acid (vinegar) or 3% phosphoric acid…this may lessen mottling.
  • Blending light into darker background: treat dry slab with 10% solution of sodium hydroxide (some success).
  • When different concrete raw materials are used, for example, from multiple suppliers: the best fix is an opaque coating such as paint or colored floor wax.
THIS FINAL TRUCKLOAD OF CONCRETE CONTAINED A HEAVY DOSE OF CALCIUM CHLORIDE. AGGRESSIVE TROWELING AND NO CURING AGGRAVATED THE DARKENING CONDITION.
THE CONCRETE TO THE RIGHT OF THE DISCOLORED AREA WAS DELAYED ARRIVING TO THIS POOL DECK PAVEMENT PROJECT. THE DELAY CAUSED THE CONCRETE TO THE LEFT TO SUBSTANTIALLY STIFFEN. IN TRYING TO MELD THE TWO CONCRETES TOGETHER THE OLDER CONCRETE NATURALLY WAS OVER TROWELED. LATE AND AGGRESSIVE TROWELING & LOWER WATER CEMENT RATIO IN THE OLD CONCRETE COMBINED WITH THE HIGHER RATIO IN NEW CONCRETE CAUSED VARYING COLOR.
THE ADDITIONAL BROOMING EFFORT CHANGES THE SLAB COLOR. THIS IS DUE TO THE DIFFERENCE IN SURFACE MOISTURE CONTENTS (WATER-CEMENT RATIO) FROM THE INITIAL TO FINAL BROOMINGS.
GARAGE FLOOR SLAB: AGGRESSIVE TROWELING AND AIR CURED.