What causes wood stains on concrete

Stains on Concrete

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Stains & colors or discoloration on concrete surfaces:

This article provides a catalog of types of stains found on concrete surfaces: walls, floors, ceilings, including un-wanted stains from various causes & deliberate concrete coloring processes, stains, or acid treatments.

Methods for removing un-wanted discoloration or stains from concrete. Catalog of research on causes & cures of concrete staining. Concrete stain products & product sources are also provided.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Concrete Surface Stains, Diagnosis, Causes, Cures & Concrete Coloring Products

Catalog Of Sources of Concrete Colors, Marks, Stains & Staining or Discoloration

[Click to enlarge any image]
Stains on concrete sidewalks, porches, or interior concrete floors may be ascribed to a variety of sources that we list here. We categorize concrete stains and discolorations in these groups:

  • Biological stains on concrete: caused by algal growth, fungal growth, and other sources.

  • Concrete discoloration associated with the concrete mix, pour, or pour conditions. Examples include
  • Concrete stains or colour variations associated with finishing details or variations such as extent of trowel work or hard finishing. See this photo of black mottled stains around a heating system oil line set.

  • Concrete stains associated with spills or leaks, such as a sewage backup or oil spill, even spilling of a cleaner or other chemical or solvent.

    The stains shown in our photo above (a restaurant in Poughkeepsie, NY, USA) are characteristic of vinyl-asbestos or asphalt asbestos floor tile mastic or adhesive that was removed during building renovation.

    The concrete was cleaned, polished, and coated with a clear sealant, leaving the telltale but accidental staining that marks the pattern of original floor tiles.
  • Concrete stains associated with bleed-through of substances from other sources such as contaminants outside a foundation wall, insulating or expansion joint materials at the perimeter of a slab,oil leaks from an outdoor oil storage tank or vehicle.

  • Concrete Stains and Cracking associated with rusting iron or steel or slag steel inclusions in the concrete material, either by deliberate inclusion as reinforcement, or by accident such as stones or scrap containing iron that are later exposed to water and possibly freezing cycles.

    Our red rust stains on the concrete sidewalk shown above were found on the Vassar College campus in Poughkeepsie NY.

    Watch out: rusting and exfoliation of iron inclusions in poured concretre structures can lead to very costly damage.

    In the U.K. in Cornwall and Devon between 1900 and 1950 and more-recently in the U.S. in homes built in the 1980s, pyrrhotite-contaminated concrete in the Connecticut has been blamed on a significant number of concrete cracking failure.


  • Concrete stains on painted concrete surfaces, such as mold growth on painted concrete; mold may also grow even on bare concrete where organic dust and debris have accumulated (dogs living in the un-finished basement).

  • Deliberate stains or colors applied to concrete floor slabs as a finish treatment using acid treatments or paint-on stain colours. At below left the variations in gray color of the interior hard-finished floor are within normal occurrence on a hard-finished concrete floor slab of this Tivoli, New York home.


Below: the concrete floor was "stained" using an acid treatment. The light patches in the left of the photo indicate foot-traffic wear through the acid-treated concrete surface . (Rigby 2006).

Wear on a stained concrete floor will be increased by any of the following

    • Foot traffic, especially if there is grit tracked onto or present on the stained concrete floor surface
    • Wear caused by hard wheels on office chairs, library carts, trolleys, or other rolling equipment
    • Moisture penetrating the concrete floor from below
    • Moisture left atop the sealed floor
    • Spills of solvents or acids that damage the protective coating
    • Poor original concrete mix or placement or excessive water in the pour that yields a soft, spalling surface

  • Concrete stains associated with moisture such as foundation leaks or moisture below floor slabs.

    Even if we do not see actual water on the concrete floor or slab surface, variations in moisture sources below the slab can create dark areas of higher moisture.

    The white and gray stains shown on the poured concrete basement wall at left are due to water and moisture seeping through the wall at concrete form ties that were not adequately sealed on the wall exterior after initial construction.

See these diagnostic articles

Reader Question: what are these black stains on our concrete porch?

4/1/2014 AUTHOR:Rhonda (no email)

COMMENT: I have a problem with my front porch, it runs the whole length of the porch, tried pressure washing it, it didn't come off and seemed to make it worse.


Rhonda, the black marks in your photo (shown here so that others may comment) may be due to moisture variations in the slab, moisture from below, or moisture entering the house-slab juncture, possibly bearing staining contaminants. But most likely we're looking at a combination of variation in concrete hardness and moisture level.

The observation of those round-ish blot stains on the concrete slab surface in front of the entry door suggest that a door mat was placed there previously, holding moisture in the slab (possibly during curing). Does the stain pattern vary with dry weather?

This comment was originally posted at MOLD APPEARANCE - STUFF THAT IS NOT MOLD

Research References on Stains on Concrete Surfaces

Causes of stains on concrete; diagnosis & removal of concrete stains & discoloration

  • Anastasiou, E., and I. Papayianni. "Criteria for the use of steel slag aggregates in concrete." In Measuring, Monitoring and Modeling Concrete Properties, pp. 419-426. Springer Netherlands, 2006.
  • Broomfield, John P. Corrosion of steel in concrete: understanding, investigation and repair. CRC Press, 2002.
  • Chew, Michael YL, and Phay Ping Tan. Staining of facades. Singapore: World Scientific, 2003.
  • De Muynck, Willem, Anibal Maury Ramirez, Nele De Belie, and Willy Verstraete. "Evaluation of strategies to prevent algal fouling on white architectural and cellular concrete." International Biodeterioration & Biodegradation 63, no. 6 (2009): 679-689.
  • Dubosc, A., G. Escadeillas, and P. J. Blanc. "Characterization of biological stains on external concrete walls and influence of concrete as underlying material." Cement and Concrete Research 31, no. 11 (2001): 1613-1617.
  • Escadeillas, Gilles, Alexandra Bertron, Erick Ringot, Philippe J. Blanc, and Arnaud Dubosc. "Accelerated testing of biological stain growth on external concrete walls. Part 2: Quantification of growths." Materials and structures 42, no. 7 (2009): 937-945.
  • Escadeillas, Gilles, Alexandra Bertron, Philippe Blanc, and Arnaud Dubosc. "Accelerated testing of biological stain growth on external concrete walls. Part 1: Development of the growth tests." Materials and structures 40, no. 10 (2007): 1061-1071.
  • Harrison, D. "The Removal of Stains from Concrete." Ontario Hydro Research News (1959): 19-24.
  • Higgins, D. D. Removal of stains and growths from concrete. No. Monograph. 1982.
  • Hurd, M. K. "Cleaning concrete." Concrete Construction 37, no. 11 (1992): 791.
  • Kosmatka, Steve. Discoloration of Concrete, Causes and Remedies. Concrete Products, April, 1987.
  • Kuenning, WH, Removing stains from concrete. Aberdeen Group, 1993.
  • Kuenning, W. "Methods of removing some specific stains from concrete: Oil to Wood." [PDF]
  • Mindess, Sidney, and James C. Gilley. "The staining of concrete by an alkali—Aggregate reaction." Cement and Concrete Research 3, no. 6 (1973): 821-828.
  • PYRRHOTITE INCLUSION CRACKING - iron and sulfur damage to foundations may include stains or may show up as extensive cracking in poured concrete
  • Ramachandran, Vangipuram Seshachar, and J. Beaudoin. "Removal of stains from concrete surfaces." Canadian Building Digest (1973): 1-4.
  • de Rincón, O. TROCONIS, A. ROMERO de Carruyo, D. Romero, and E. Cuica. "Evaluation of the effect of oxidation products of aluminum sacrificial anodes in reinforced concrete structures." Corrosion 48, no. 11 (1992): 960-967.
  • Uemoto, T. "Maintenance of concrete structure and application of non-destructive inspection in Japan." Proc. Non Destructive testing in Civil Eng (2000): 1-11.

Research on Concrete Stains & on Air Entrainment in concrete & freeze-thaw effects

  • Cordon, William A. "Freezing and thawing of concrete-mechanisms and control." (1966).
  • A Symposium. "Entrained Air in Concrete." In ACI Journal Proceedings, vol. 42, no. 6. ACI, 1946.
  • Gokce, A., S. Nagataki, T. Saeki, and M. Hisada. "Freezing and thawing resistance of air-entrained concrete incorporating recycled coarse aggregate: The role of air content in demolished concrete." Cement and Concrete Research 34, no. 5 (2004): 799-806.
  • Pigeon, Michel, and Richard Pleau. Durability of concrete in cold climates. CRC Press, 2010.
  • Powers, Treval Clifford. "The properties of fresh concrete." (1969).
  • Powers, Treval Clifford. "Void space as a basis for producing air-entrained concrete." In ACI Journal proceedings, vol. 50, no. 5. ACI, 1954.
  • Powers, Treval Clifford, and R. A. Helmuth. "Theory of volume changes in hardened portland-cement paste during freezing." In Highway research board proceedings, vol. 32. 1953.
  • Powers, Treval Clifford, and T. F. Willis. "The air requirement of frost resistant concrete." In Highway Research Board Proceedings, vol. 29. 1950.
  • Verbeck, George J., and Paul Klieger. "Studies of'salt'scaling of concrete." Highway Research Board Bulletin 150 (1957).
  • Whiting, David, and David Stark. "Control of air content in concrete." NCHRP report 258 (1983).
  • Wright, Peter Joseph Frederick. "ENTRAINED AIR IN CONCRETE." In ICE Proceedings, vol. 2, no. 3, pp. 337-358. Thomas Telford, 1953.
  • Baragary, Chance, and Hani A. Salim. "Effects of cold joints in blast resistant structural concrete [abstract]." In 2007 Undergraduate Research and Creative Achievements Forum (MU). University of Missouri--Columbia. Office of Undergraduate Research, 2007.
  • Biel, Timothy D., and Hosin Lee. "Performance study of portland cement concrete pavement joint sealants." Journal of transportation engineering 123, no. 5 (1997): 398-404.
  • Bussell, M. N., and R. Cather. Design and construction of joints in concrete structures. Vol. 146. Thomas Telford, 1995.
  • Collepardi, M., R. Khurana, and M. Valente. "Construction of a dry dock using tremie superplasticized concrete." ACI Special Publication 119 (1989).
  • Den Entwurf, Neue Deutsche Richtlinie Für, Zur Rückhaltung Von Wassergefährdenden Von Betonbauten, Nouvelle Directive Allemande Sur La Conception, R. E. T. E. N. T. I. O. N. De Produits, And Dangereux Pour L'eau. "New German Guideline For Design Of Concrete Structures For The Containment Of Hazardous Materials." Otto-graf-journal 17 (2006): 9.
  • Erickson, Scott. "Pervious concrete durability testing." In Proceedings of the 2006 Concrete Technology Forum. 2006.
  • Fowler, T. J. "Lessons Learned from Refractory Concrete Failures." ACI Special Publication 57 (1978).
  • McCurrich, L. H., and N. R. Cook. "Airfield and road pavement use in the UK of high performance coal tar pitch PVC joint sealants, and a comparison of UK and ASTM standards." ACI Special Publication 70 (1981).
  • Odum-Ewuakye, Brigitte, and Nii Attoh-Okine. "Sealing system selection for jointed concrete pavements–a review." Construction and Building Materials 20, no. 8 (2006): 591-602.
  • Ribeiro, AC Bettencourt, J. Díez-Cascón, and A. F. Gonçalves. "Roller compacted concrete-tensile strength of horizontal joints." Materials and Structures 34, no. 7 (2001): 413-417.
  • Sarsam, K. F., and M. E. Phipps. "The shear design of in situ reinforced concrete beam–column joints subjected to monotonic loading." Magazine of Concrete Research 37, no. 130 (1985): 16-28.
  • Soliman, Haithem, Ahmed Shalaby, and Leonnie Kavanagh. "Performance evaluation of joint and crack sealants in cold climates using DSR and BBR tests." Journal of Materials in Civil Engineering 20, no. 7 (2008): 470-477.
  • Yan, Shangyao, and Weishen Lai. "An optimal scheduling model for ready mixed concrete supply with overtime considerations." Automation in Construction 16, no. 6 (2007): 734-744.
  • Yildirim, Yetkin, Yusuf Yurttas, and Ilker Boz. "Service Life of Crack Sealants." In First International Conference on Pavement Preservation. 2010.

Reader Comments & Q&A

Tomie thanks for a helpful question.

Painting a slab means applying a surface coat of some color and texture that adheres to the surface of the slab - as a sort of "skin" while staining a concrete slab means applying a liquid that carries pigment, but that actually penetrates the concrete surface. There's no skin to peel off.

The article above on this page discusses un-wanted stains or discolorations or contamination on concrete surfaces while

deliberately "staining concrete" to add a color or hue as you phrased it in your question is discussed separately:


what is the difference of painting or staining concrete slab that is inside the hone.

Excellent thanks for clarifying that. It's certainly a convincing stamp;

More likely an algae than mold.

Watch out: if I'm right and that's algae, even though the stamped concrete has a textured surface it may still be very slippery when wet.

Details about just how slippery algae can be are found


See STAMPED CONCRETE CLEANING for some cleaning advice

Also see our algae removal advice at


It's concrete stamped with a stamp to look like wood.

Could you help identify these greenish stains on our patio?


This is actually pretty easy to resolve.

Collect a tape sample from a particularly dark blotch and have it analyzed by a forensic lab. I suspect you're going to see that it's an algae bloom. If you confirm that, I can make some suggestions on reducing the problem.

Driveway less than one year old. Whenever it rains black and dark spots come up from the concrete. The blotches take days to start to dry out while other driveways will be dry in a few hours.

Have had many concrete people look at it and have no explanation of how it could have occurred. Not mold or oil.

Plausible explanations - bad batch of concrete that had something in the mix, some type of chemical made its way onto the driveway and seeped into the porous concrete.

Everyone is unsure and have heard multiple time that they have never seen it before in all their years of concrete work.

IMAGE LOST by older version of Clark Van Oyen’s useful Comments code - now fixed. Please re-post the image if you can. Sorry. Mod.


That looks to me like oil stains, possibly from spills along the foundation wall; the lower white deposit may be effloresence - search InspectApedia.com for EFFLORESENCE to read details.

Bleach won't remove oily deposits but other cleaners such as a garage floor cleaner are often effective.

I have these stains on my concrete foundation. Bleach products have no effect. Do you have any idea what they are and how to get rid of them?

Thank you.

IMAGE LOST by older version of Clark Van Oyen’s useful Comments code - now fixed. Please re-post the image if you can. Sorry. Mod.

Reader Question: what are the different concrete stains & markings shown in these photographs?

Please offer me your expert opinion on what is going on with my structural walls. I am working on a project located on Eko Atlantic Lagos Nigeria. And despite all our efforts we keep getting bug holes as shown below. C.I., Nigeria, 7/2/2014

Reply: a catalog of concrete surface markings & stains

I'll be glad to try to assist, with the warning that I am not a concrete engineer, though I do have experience in the subject.

Commenting on your photos in order [Click to enlarge any image]

1. Your photograph of "bug holes" in the concrete wall photo above look to me like small voids due to air bubbles in the concrete pour, probably due to poor mixing;

On larger & commercial concrete placement projects concrete contractors use a vibrator to remove these bubbles and avoid the voids. In my photograph below, taken at the Vassar College building project in New York in 2014 you can see workers using the vibration tool as a large floor slab is being poured.

Only if these "bug holes" are in very great number would one expect there to be a structural concern. I can't evaluate that from a photo - you'd need an onsite expert. Usually for an important pour of structural concrete an expert checks the mix and the slump and the placement of reinforcing steel.

It's worth noting that entrained air has been intentionally included in some concrete placement operations for a very long time as a partial protection against freezing effects, salt scaling and other forces, as described in these research citations where proper air entrainment procedures are explained.

As you'll see in our citations given below, Powers (1969) has written extensively on this topic.

2.Photograph of holes & chip marks in a concrete wall (above) looks like chipping or spalling where concrete form ties were removed (cosmetic)

3. Photograph of the dark horizontal lines in a concrete wall (photo above) look like concrete form ties; the horizontal lines seem to mark boards used for the concrete forms

. I suspect that moisture variations during wall curing or possibly accumulation of water in irregularities in the form-wall caused the darkness.

The rippled surface in the lower right corner of the photo above (enlarged just below) suggests the mix contained excessive water or that plastic had been placed against the inner surface of the concrete form, but that's just an OPINION and we'll be interested to see what some of our readers who are concrete experts can offer.

4. & 5. Photographs of dark lines and blotchy areas in the concrete walls in the two photographs below look like cold pour joints in the concrete wall.

In your last photo (below left) we see both cold pour joints and rough surface areas or surface voids in the poured wall as well as holes (in both photos) from form ties.

Concrete Stains from Rusting Inclusions or Exfoliating Steel

The lower photo also includes reddish stains that may be rust from either metal forms or from iron slag inclusions in the concrete mix.

See details about cold pour joints found at CONCRETE COLD POUR JOINTS

If you give permission for me to publish these photos I may be able to solicit more expert opinions for you - I can keep your identity information private or can publish it with your contact information - which ever you prefer.- DF

Reader follow-up:

You reflected my thoughts as well. Thanks for the link on concrete cold pour joints it was insightful.
You have my permission to publish these photos though I think your observation is spot on.
And I look forward to reading what anyone else has to say.

Below are a concrete batch report and a sketch of the concrete wall plan for our project.

[Click to enlarge any image]


Continue reading at STAIN DIAGNOSIS on BUILDING EXTERIORS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see CONCRETE STAIN CAUSE & CURE FAQs - questions and answers posted originally on this page.

Or see these related articles

CONCRETE COLORING & POLISHING - how color is applied to concrete floors & other surfaces

STAINS on STONE, STUCCO DIAGNOSE & CURE for cleaning methods


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