Question: I placed a concrete slab for a store. The concrete surface was to be the finished floor and the specification called for a wet cure, where one of the acceptable options was using polyethylene. But when the plastic was removed, the floor was discolored.
While the specifications were followed without exception, the owner was unhappy with the appearance of the floor and withheld a considerable amount of money. What happened?
Answer: This discoloration is likely associated with the wrinkling of the polyethylene cover and is often referred to as the “greenhouse effect.” Moisture evaporates and condenses beneath plastic sheeting. The condensed water runs down the plastic until it contacts the slab surface. This creates the variable curing conditions, which result in the discoloration.
Question: How do we avoid this?
Answer: Where the concrete slab is the finished floor surface, you can use a curing compound to avoid the greenhouse effect. If moist curing is required, an absorbent curing cover (sometimes called felt-backed poly) can be rolled and squeegeed nearly wrinkle-free. When applied flat, these materials provide a more consistent cure across the slab surface, resulting in less discoloration. Some covers lay flatter and retain moisture better than others. Generally, cellulose-based curing covers perform the best when wetted upon rolling onto the surface.
Question: What other factors can cause discoloration?
Answer: Using cements with different alkali contents exacerbates this impact on color. The location of light and dark spots often depends on the formation of alkali chlorides and alkali carbonates and on the degree of hydration of the ferrite phase of the cement.
The use and relative percentage of admixtures can also affect the color. For example, mineral admixtures such as silica fume can darken concrete. Pozzolans such as flyash and ground granulated blast furnace slag (GGBFS) also contribute to the color. Consistency in type, amount, and mixing results in color consistency.
Question: Do finishing techniques affect color?
Answer: Yes. Another mixture factor that contributes to color is the water-cement ratio. The method of finishing can alter the w/c. You commonly see this color difference in the pour-back strips at the exterior of tilt-wall building floors. As these areas are finished by lighter equipment, they are typically lighter in color.
Greater finishing effort typically results in a lower w/c and corresponding darker surface appearance. You sometimes see this along hand-finished construction joints as opposed to the machine-troweled slab interior. The darker color (denser) of machine troweling is often called trowel burn.
Question: What about dark spots on a light surface?
Answer: Dark spots on a lighter surface are typically aggregate shadowing.
Question: What about light spots on a dark surface?
Answer: Light spots on a dark surface are often another risk of using calcium chloride in the concrete mixture. Bleed-water transports calcium chloride to the surface. As discussed previously, this has a darkening affect. But in some cases, larger aggregate particles near the surface block the upward migration of bleedwater containing calcium chloride, resulting in lighter spots above these aggregate particles.
Question: What can we do about discoloration once it is noticed after concrete placement?
Answer: You should expect some discoloration with typical variations in trowel effort, ambient conditions, and concrete setting. You should not expect concrete slabs to be perfectly uniform in color, as they do not have manufacturing controls like floor covering materials.
When excessive, discoloration can also be very difficult to treat once you see it. Often, discoloration changes with time, as the cement continues to hydrate and the slab dries. Changes in ambient humidity can result in changes in the mottled slab appearance. Porous aggregate near the surface can absorb moisture and darken the surrounding paste.
If you see discoloration when moist curing covers are removed, flush the surface with warm water. Several cycles of flushing and drying can correct some discoloration issues. If objectionable discoloration is noticed later, the first thing to do is monitor changes in the appearance. Dark spots generally fade and blend with time and drying. Once the discoloration has stabilized, scrubbers and typical cleaning solutions can reduce some discoloration. The effectiveness of the treatment depends on the cause of the discoloration. Removing salt solutions from the surface will reduce the variation in moisture-related discoloration due to variations in ambient conditions. If regular cleaning products are not effective, you can test other solutions. But the older the slab, the less effective chemical treatments generally are.
A solution of weak acid, such as 3% acetic acid (vinegar), has been used effectively to remove surface efflorescence and lighten dark areas. The slab should be pre-moistened so the solution is not absorbed into the concrete.
The solution is scrubbed into the surface and flushed. The amount of contact time required depends on the magnitude of the discoloration. Repeat the process if the discoloration is still present after drying. When lighter areas need to be blended into a dark background, you can apply a 10% solution of caustic soda (sodium hydroxide) to a dry slab surface. This is successful if you apply the solution to a slab for a day or two and then wash it. You typically see blending during the subsequent drying period.
The most effective chemical treatment is a 25% solution of diammonium citrate. This is expensive in small quantities; attempt other treatments first.
When you apply it to a dry slab, the solution penetrates the pores and slowly dissolves calcium carbonate and calcium hydroxide. This increases the porosity and enables moisture to penetrate, resulting in more complete hydration and a lighter color.
After about five minutes of exposure to the solution, a white gel begins to form. The gel should be diluted, lightly brushed for about 15 minutes, and rinsed off. The gel must be completely removed before hardening, as it will leave a deposit. After treatment, the slab should be moist-cured for three days to complete the hydration process. Repeat the process, if necessary.
Question: Will stronger acids work better?
Answer: No. Harsh acids will dissolve the cement paste (hard-troweled surface) and expose large aggregate. Strong acids have not been found to remedy discoloration issues.
Question: We have heard about using diluted muriatic acid. Does this work well?
Answer: Diluted muriatic acid (1% to 10%) has been recommended for removing certain stains and efflorescence. Muriatic acid can have the same result, as diammonium citrate on mottled discoloration, but is typically less effective. It is also difficult to treat localized areas.
What these chemicals do is dissolve the lighter calcium carbonate deposits that occur from carbonation of soluble calcium hydroxide. As a result, treated areas often have a darker appearance. Chemical remedies are typically most effective when large areas are treated. But the overall blend depends on the specific cause of the discoloration. And, similar to carpet cleaning recommendations, it is always a good idea to treat an inconspicuous area first to evaluate the results before treating the entire slab surface.
Question: What about using dyes and bleaches to try to blend the areas?
Answer: Using bleaches and dyes specifically applied to localized areas is a very expensive process. The slab may be artistically blended to a more uniform color, but the effectiveness of this approach highly depends on the specific slab (concrete mixture, surface density, ambient conditions, etc.). Also, as the slab ages and becomes worn by normal usage, the colored areas may not maintain the uniform appearance initially created by the artist.
— Scott Tarr is a partner with Concrete Engineering Specialists (CES). Before joining CES, he was a principal engineer in the Structural Evaluation Section of CTLGroup, Skokie, Ill., for 16 years. E-mailStarr@ConcreteES.com.