Idea Transcript
Discoloration of Concrete-Causes _ and Remedies especially difficult to remove and may by Steven H. Kosmatka Concrete Engineer Portland Cement Association Concrete visible to the public eye should have a uniform color and texture to be esthetically pleasing. Of course, concrete can be specifically designed with a combination of colors and textures to make an attractive concrete surface, but many factors affect concrete color, resulting in occasional discrepancies in color uniformity. A departure from the normal or desired color of concrete is called discoloration and, as discussed here, does not include post-construction stains caused by spillage or application of foreign substances. This article describes some of the common causes of surface discoloration and suggests some methods to remove or reduce discoloration.
Causes of Discoloration The discoloration of concrete cast in forms or in slabs on ground is usually the result of a change in either the concrete composition or a concrete construction practice. Following are some of the main causes.
Water-Cement Ratio. A change in the water-cement ratio of a concrete mix can significantly affect color. Such a change may result from localized changes in construction practices or from a batch-to-batch variation in the concrete’s water or cement content. A high water-cement ratio will usually produce a light-colored concrete, a low ratio a darker color (see Fig. 1). Subgradesshould also be uniformly moistened before concrete slab-on-ground placement to avoid discoloration due to a localized loss of water from the concrete to the subgrade.
Cement. Individual brands and types of cement may differ in color; therefore, changing the brand or type of cement in the middle of a job may noticeably change the color of concrete (see Fig.
2). Calcium Chloride. The addition to concrete of calcium chloride or admixtures containing calcium chloridecan darken a concrete surface (see Fig. 3). Light spot or dark spot (mottled) discoloration can occur depending on the curing method and the alkali content of the cement. Repeated washings or weathering can remove or reduce the discoloration; however, light spots are
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require a chemical wash. Calcium chloride accelerates the hydration of silicate compounds in the cement and retards the hydration of aluminate and iron compounds. The retarded and unhydrated iron compounds, namely tetracalcium aluminoferrite (C,AF), remain dark. Hard troweling can especially prevent the continued hydration of the iron compounds by reducing the water-cement ratio at the surface, resulting in indefinite delay in C,AF hydration and delay in lightening of thesurface (see Fig. 4). Alkalies also affect the calcium chloride and iron compound reactions. A high alkali content moderates chloride discoloration. Alkalies react with calcium chloride to form calcium hydroxide and sodium or potassium chloride; the latter two do not impede C,AF hydration. Light spots on a dark background are often produced by a low alkali-to-calcium chloride ratio while dark spots on light background are formed by a high alkali-to-calcium chloride ratio. Light spots often correspond to coarse aggregate particles located just below the surface. The aggregate particles interfere with the surface mortar’s chloride content by blocking the normal upward migration of chloride salts to the drying surface. Water ponding of slabs and use of membrane curing compounds will reduce the amount of discoloration caused by chlorides. Retarding admixtures can also discolor concrete.
Mineral Admixtures. Changes in the amount, source, and chemistry of a mineral admixture can affect concrete color. The extent of the discoloration will depend upon the color and the amount of admixture used. Some mineral admixtures resemble portland cement and have no effects on concrete color. Silica fume may give concrete a dark grey tint. Dark grey fly ashes may also give concrete a darker color, whereas tan or beige colored fly ashes can produce a tan color in concrete if used in large quantities.
reducing the water-cement ratio at the surface. Metal abraded from the trowel onto the surface is often given as the explanation for trowel burn. While some such discoloration is undoubtedly taking place, it is not significant. Trowel burnsarealmost impossibletoremove, so the best way to avoid them is to trowel the surface at the appropriate time-before the concrete becomes too stiff. Fig. 5shows theeffectsof delayed hard troweling on concrete color.
Curing. Discoloration caused by curing procedures may be related to (1) the particular curing method used, (2) changes in the curing method used, or (3) nonuniformity of curing. Some curing compounds can discolor concrete surfaces, and air-cured concrete (that is, no curing at all) can cause the worst discoloration. Figs. 4 and 6 illustrate the effect of some curing methods on discoloration. The “greenhouse effect” (see Fig. 6) is caused by the evaporation and condensation of water at different points under plastic film. Water condenses under folds in the film and runs down to collect in low spots and where the film is in contact with theconcrete. Thesurface is exposed to varying water contents (nonuniform curing) resulting in discoloration. This type of discoloration is primarily associated with concrete containing chlorides. To avoid the greenhouse effect, wet the slab with water and lay plastic film as flat as possible. Efflorescence. Efflorescence is a crystalline deposit, usually white in color, which occasionally develops on con-
Finishing. The type and time of finishing of slabs can significantly alter their color. Hard steel troweling significantly reduces the water-cement ratio of the surface concrete, resulting in a darker color. Localized areas of trowel burn (hard troweling of the surface after it has become too stiff to trowel properly) can cause extreme discoloration by
Fig. 7. Effect of wafer-cement ratio on the color of f-year-old paste (‘cement and water). All discs were made with the same cement.
Crete or masonry surfaces after construction iscomplete. Water in moist or wet hardened concrete dissolves soluble salts. This salt-water solution migrates to the surface by evaporation or hydraulic pressure where the water evaporates, leaving a deposit of salt (usually a carbonate) at the surface. If either the water, evaporation, or salts are not present, efflorescence will not occur.
Aggregate. Aggregate color may influ-
Fig. 2. Concrete color change in driveway caused by two brands of cement.
ence concrete color. Occasionally, dark areas occur over coarse aggregate particles near the surface. These dark spots lighten with drying. Some aggregates, such as iron oxides and sulfides, can cause unsightly rust stains if they are located near the surface.
Forms. The type, kind, and condition of
Fig. 3. Driveway discoloration due to use of calcium chloride admixture. Concrete at top of photo does not contain the admixture; at bottom it does.
Air-Cured Slabs
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formwork can influence surface color. Forms with different rates of absorption produce surfaces with different shades of color. For example, unsealed wood forms will absorb moisture from the concrete, which causes dark-colored surfaces due to the reduced watercement ratio. Sealed, nonabsorbingforms, on the other hand, produce lighter surfaces. Absorption discoloration often occurs when form release agents are not uniformly applied to absorptive forms. A change in the type or brand of form release agent also can change concrete color. Leaky joints in forms can allow water loss from the concrete, producing a change in the water-cement ratio and thereby causing discoloration. Segregation of the concrete within the forms can cause discoloration and sand streaking, Forms that separate or pull away from the concrete surface, allowing air to come in contact with the surface, can cause a drying discoloration of the exposed area. A pink tint can occur on a formed surface if the forming material or release agent contains phenolics. The phenolics react with the alkali and hydroxide solutions in the fresh concrete to create the stain. The best way to prevent this is to use a nonstaining release agent on wood forms that have been properly sealed with a nonphenolic sealer. The pink stain generally disappears after a few weeks of air drying.
Remedies for Discoloration Some LOW-ALKALI
L--
CEMENT
HIGH-ALKALI
Slabs.
Wetted
CEMENT
No Calcium Chloride Admixture-
ond
Drh?d
5 Tames
LOW-ALKALI
CEMENT
HIGH-ALKALI CEMENT
L----- 2 % Calcium C h l o r i d e Admixture-
Fig. 4. Effect of calcium chloride admixture on hard steel troweled slabs. Note: Photo reproduction causes the slabs to appear somewhat darker than they actually were.
To remove most discoloration caused by chloride admixtures or finishing and curing methods, the first, and usually effective, remedy is an immediate, thorough flushing with water. The slab should be alternately flushsd and then dried overnight until the discoloration disappears. If possible, hot water
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2% hr
2hr
Z3/4
hr
3hr
Fig. 5. Concrete discoloration due to &lays in troweling time on concrete containing calcium chloride and a low-alkali cement.
Film-cured
slab
should be used. Scrub brushes c a n help remove surface deposits. If the water wash and scrubbing is unsuccessful, acids and other chemicals can be used. A dilute solution of hydrochloric acid (about 1% to 2Or6 concentration) can remove carbonate efflorescence. Harsh acids should not be used, as they can expose the aggregate. Acid washing using weakeracids, such as 3% acetic acid or 3% phosphoric acid, will remove efflorescence and lessen mottled discoloration. Before an acid is applied, the surface should be dampened with water to prevent the acid from being absorbed deep into the concrete. Treating a dry slab with a lO%solution of caustic soda (sodium hydroxide) gives some success in blending light spots with a dark background. The sodium hydroxide solution should be left on the surface 1 to 2 days, followed by a thorough rinsing with water. The best remedy to remove most discoloration is to treat a dry surface with a 20% to 30% water solution of diammonium citrate. Treatment consists of applying a solution to the dry surface for about 15 minutes. The white gel formed by the solution should be diluted with water and continuously agitated by brushing. The gel should be scrubbed off with water after the treatment. Water curing between or after treatments increases the treatment’s effectiveness. Two or three treatments should be adequate. Acid and other chemical washesshould be tested on a small, inconspicuous portion of the discolored concrete to detect possible detrimental effects of the method as well as to determine the method’s effectiveness in reducing the discoloration.
Remedial methods should be implemented as soon as possible after the discoloration is noticed.
Recommendations to Avoid Discoloration To avoid or minimize discoloration of concrete: (1) do not use calcium chloride admixtures; (2) use consistent concrete ingredients, uniformly proportioned from batch to batch; and (3) use proper and timely placing, finishing, and curing practices. Concreting practices should not be allowed to vary, as any disruption or change in the concrete mixture, formwork, finishing, or curing can result in significant and sometimes permanent discoloration. References 1. Greening, N. R., and Landgren, R., Surface Discoloration of Concrete Natwork, Research Department Bulletin RX203, Portland Cement Association, 1966,17pages. 2. How to Prevent Concrete Slab Surface Defects, IS177T, Portland Cement Association, 1977, 4 pages. 3. EfNorescence, IS020T, Portland Cement Association, 1968, 2 pages. 4. Removing Stains and Cleaning Concrete Surfaces, IS214T, Portland Cement Association, 1982, 9 pages.
The morning after
Fig. 6. “Greenhouse effect” discoloration of hard-troweled, highalkali-cement concrete slab containing 2% calcium chloride and cured with polyethylene film.
Concrete buildings of all types--castin-place, precast, concrete masonryin the United States and Canada are eligible to be entered in the 1986 Concrete Building Awards Competition. The competition invites owners, developers, architects, engineers, and contractors to submit buildings of all sizes that show creativity of design, construction economy, technical innovation, problemsolving design strategies, and positive environmental impact.
Entries must be submitted by September 15, 1986. Awards will be presented to winners on November 20during theThird International Forming Conference at the Westin Galleria Hotel, Dallas. Photographs and descriptionsof the winning buildings will be published in an early 1987 issue of Architectural Record. For rules and entry forms either write or call Glen Simon, Portland Cement Association, 5420 Old Orchard Road, Skokie, IL 60077, phone: 312/96&6200.
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