Common Concrete Problems

Common Concrete Problems

Spalling: Deterioration of the top surface. Appears to be pitted. May have the aggregate exposed. Pits may range from barely visible to deeper than 1". Accelerated over time if not treated by increased surface area exposure to future causes.

Probable causes: Commonly found in cold climates where sodium chloride (salt) is used to melt ice and snow. Coupled with a penetrable surface.

What we do to repair it: We fill the spalls up to 1/2" with skim coat, depending upon the desired finish, we may use 2 coats of skim coat. In the event of spalls deeper than 1/2" we initially fill with high density mortar (hdm), then skim coat to a smooth surface.

Dusting: Chalking or powdering of the slab surface, its actually a very weak surface, weak enough for foot traffic or sometimes even sweeping to erode the surface.

Probable causes: Finishing with bleed water on the surface which causes a high water to cement ratio at the surface. Carbonation, which is common for indoor commercial slabs where carbon dioxide from gas engines contaminated the surrounding air, then causes a chemical reaction which reduces surface strength and hardness of concrete. Inadequate protection from rain or snow on newly poured concrete.

What we do to repair it: Mechanically remove the weak surface, then our normal application will offer a new surface.

Blistering: The surface actually lifts off in a thin "sheet". Usually about 1"-2" diameter, often times as much a 12" diameter where it appeared only as a fine crack. Future blistering locations can be detected with a hollow sound.

Probable causes: Rapid surface bleed water loss, while directly under the surface still contains bleed water. The mix has a high sand content causing a lean mix making the surface appear ready to finish before its time. Using entrained air or higher oxygen % to make the surface ready to cure faster.

What we do to repair it: Mechanically remove the weak surface then our normal application will offer a new surface.

Crazing: A close network of fine random cracks or fissures. They are usually only unsightly not a structural defect in the concrete.

Probable causes: Finish troweling with bleed water present. Excessive floating, which draws water to the surface and works aggregate down. Water content too high either from intial mix or in the field to make the concrete more workable.

What we do to repair it: Since it is not a structural defect, our normal prep and skim coat will cover them.

Cracking: No definition needed. Things you should know about cracks. They don't usually affect strength and integrity, they are unsightly and most of all, they allow water to flow freely through the cracks and further erode the substrate. (See below)

Probable causes: Expansion and contraction of the concrete and substrate which causes excessive shearing forces. Erosion of the slabs substrate, causing excessive shearing forces. High moisture content in the substrate coupled with inadequate or improper jointing causes the substrate to move more than the flexibilty of the concrete and exceeding the shear strength of the concrete.

What we do to repair it: If possible reinforce the substrate first, usually not possible. V-grind the crack open to accept crack welding material, then properly joint with tension control cuts (properly retro joint). Our normal prep and application cover them. In some instances the irregularity adjacent slabs, need to be ground level and/or filled level. In extreme cases, we actually fill the crack with high density mortar (hdm).

Why we cut joints in concrete?

Substrate concerns: We know the local ground conditions and know where potential trouble areas are likely to occur. We can see where a slab should/could have been jointed.

So, why cut?: We are enhancing your existing concrete surface in strength, durability and certainly, appearance. We want to take every safeguard ensuring the enhancements are not compromised, take measures to try to control of whatever possible. There is much about concrete and its substrate that is far beyond our control.

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References

"Recommended Practice for Concrete Floor and Slab Construction", ACI 302-69
"Causes of Floor Failures", by A.T. Hersey, ACI Journal, June 1973
"Slab Construction Practices Compared by Wear Test", by L. Blake Fentreas, ACI Journal, July 1973
"Cement Mason's Manual for Residential Construction", Portland Cement Association, Skokie, IL
"The Effect of Various Surface Treatements, Using Zinc and Magnesium Flurosillicate Crystals on Abrasion Resistance of Concrete Surfaces", concrete Laboratory Report No.c-819, U.S. Bureau of Reclamation
"Durability of Concrete In Service", ACI 201 Manual of Concrete Practice, Part 1
"Scaled Concrete", by Fred F. Bartel, Tews Lime and Cement Company
"Problems of Ice Removal from Pavements", by William E. Dickinson, Calcium Chloride Institute, NRMCA Publications No.98
"Protective Coatings to Prevent Deterioration of Concrete by Deicing Chemicals", National cooperative Highway Research Program Report No.16
"An Unusual Case of Surface Deterioration on a Concrete Bridge Deck", by John Ryelll, ACI Journal, April 1965
"Recommended Practice for Concrete Floor and Slab Construction" ACI 302 Manual of Concrete Practice Part1
"How to Prevent Concrete Slab Surface Defects", Portland Cement Association (IS 777.01T)Skokie, IL
"Solutions to the Problems of Scaling, Crazing, Dusting of Concrete Slabs", Modern Concrete, November 1963
"ACI Standard Recommended Practice for Concrete Floor and Slab Construction", ACI 302, ACI Manual of Concrete Practice, Part 1, 1978
"Causes of Floor Failures", by A.T. Hersey, ACI Journal, June 1973
"Cracks in Concrete:Causes, Prevention, Repair", A collection of articles from Concrete Construction Magazine, June 1973
"Why and How: Joints for Floors on Ground", Report No. RP026.01B, Portland Cement Association, Skokie, IL