Common Problems with Concrete

Concrete is a relatively easy material to manage. However, you can run into big problems if it is not properly laid. Concrete problems could include discolorations, shrinkage and scaling among others. Here’s a list of common concrete problems and a guide on how to solve them:

1.  Discoloration

Concrete discoloration could occur if any or all of these factors are altered. Concrete discoloration could occur if water is being added at the jobsite, by poor workmanship, different cement material and even if calcium chloride is added to the mix. All concrete ingredients must comply with American Society for Testing and Materials (ASTM) standards.

To solve concrete discoloration problem you must:

  • Specify with the ready mix supplier your tolerance levels
  • Prepare a uniform subgrade
  • Wait until all water has evaporated to start finishing concrete
  • Avoid hard troweling exterior concrete
  • Use concrete forms that are in good shape
  • Cure the concrete uniformly over the entire surface
  • Use the same form release agent on your concrete forms
  • Use liquid calcium instead of calcium chloride

2.  Scaling

Another common concrete problem is scaling. Scaling happens when the concrete surface breaks off to 2 inches and keeps peeling away. This normally occurs due to inadequate concrete strength, or inadequate curing procedure. Concrete scaling may also occur when water seeps into porous, non-air entrained concrete exposed to freeze-thaw cycles.

Concrete scaling could be avoided when:

  • You specify a low slump-air (6 to 7 percent) entrained mix design on exterior flat work
  • Concrete is finished at the right time, once all water has been evaporated
  • Curing concrete adequately without using salt or other chemicals during winter
  • Avoid using a vibrating screed on high slump concrete

3.  Crazing

One of the most repeated problems in concrete is crazing. A crazed concrete occurs when the surface shows several interconnected fine cracks. Although the concrete strength probably is not affected, cracks will show up when the slab is damp. Crazing of concrete occurs when the cement paste comes up to the surface and it shrinks.

To avoid crazed concrete:

  • Use a moderate slump concrete without bleeding and segregation
  • Do not finish concrete until all water has evaporated
  • Do not dust cement on the surface while water is present at the surface
  • Do not sprinkle water over the concrete while finishing it
  • If the weather could produce high evaporation rates, spray some water onto the subgrade, so it will not absorb the water from the concrete mix
  • Use a broom finish instead of using a steel trowel
  • Cure adequately concrete so it can retain the necessary moisture for the hydration process

4.  Cracking

Concrete can crack easily. Concrete could shrink and sometimes cannot be prevented but it can be controlled. It is probably the main problem that concrete faces. Cracking can be the result of one or a combination of factors, such as drying shrinkage, thermal contraction, subgrade settlement, and applied loads. In the case of a wall, if a crack is not structural, is not too wide (the acceptable crack width is subjective but it could range from 1/16” to 1/4”) and is not leaking water, it should be considered acceptable.

To reduce cracking and shrinking of concrete:

  • Remove topsoil, soft spots and organic material in the subgrade
  • Compact all loose soil underneath the concrete slab
  • Slope the subgrade for proper drainage
  • Design a flexible concrete pavement that could accommodate load and movements
  • Install concrete joints accordingly, by sawing, forming or tooling a groove
  • Place, finish and cure concrete accordingly depending on weather conditions
  • Do not finish concrete if it has not finished bleeding
  • Do not overwork concrete surface
  • Avoid rapid drying conditions or use a set retardant admixture
  • Use wind breaks, fog sprays and cover the concrete with wet burlap
  • Minimize the mix water content by maximizing the size and amount of coarse aggregate and use low-shrinkage aggregate
  • Use synthetic fibers to help control plastic shrinkage

5.  Curling

Concrete curling occurs when there is more shrinkage at the bottom once the concrete has hardened. It is primarily due to differences in moisture and or temperature between the top and bottom surfaces.

To avoid concrete curling be sure to:

  • Use proper curing techniques
  • Place concrete joints accordingly
  • Use low water content concrete or use water reducing admixtures
  • Use the largest possible aggregate size
  • Ensure proper bonding when applying thin topping mixes
  • Use enough, not excessive, amount of steel reinforcement in the slab
  • Place concrete on a damp but absorptive sub grade so that all the bleed water is not forced to the top of the slab
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