Concrete Problems and Causes

Posted by Dennis Yurconis on Nov 3, 2008 in Concrete Knowledge Base |

Concrete is an excellent material, but it is not perfect.  You don’t have to look very far to identify the real life concrete problems listed below.  However, most of these problems are avoidable, or fixable.  There are way to many variables in the production, mixing, placing, and finishing of concrete for me to discuss in detail.  Unfortunately, most concrete problems are the result of error with the finisher you or the builder chooses.  And builders, to maximize profit, take the lowest bidder.  And in my experience, so do many homeowners.  The lowest bidder uses the least amount of manpower to get the job done, and uses cheaper, lower quality concrete with low compressive strengths and inexpensive admixture fillers (fly-ash).  Most problems associated with concrete arises from the inability of the finisher to finish the concrete slab correctly, because of production rates and low manpower.  I have examined warranties offered by many homebuilders, and short of the concrete self-destructing, most concrete defects are not covered.  Therefore, builders get away with their practices, and you are left dealing with concrete problems long after your home has been built. 

These are the most common problems associated with concrete.  Every one of these problems is either corrected, prevented, or reduced with the installation of our polymer modified cement overlay.

Surface Scaling/Spalling

Surface scaling is when the surface of hardened concrete breaks off to a depth of 1.5mm to 5mm, generally during the first year of placement.  This occurs because of application of calcium or sodium chloride deicing salts on concrete with inadequate strength, air entrainment, or curing.  Unfortunately, as an end user, you may have no control over these factors.  Concrete that is subjected to use of deicing salts combined with freeze-thaw conditions are prone to scaling.  The National Research Council’s Strategic Highway Research Program tested deicing salts to see how they would etch and destroy concrete. The tests were interesting. It appears that magnesium chloride did the least amount of damage. Calcium chloride caused 26 times more damage to the concrete than magnesium chloride. Regular rock salt, sodium chloride, caused an astonishing 63 times more damage. If the tests were accurate, it appears that it may be worth the extra money to purchase and use magnesium chloride. Even still, your driveway will track rock salt from the roads, and it will concentrate in your garage where the snow/salt slurry collects and evaporates.  Chemicals such as ammonium nitrate and ammonium sulfate, which are components of most fertilizers can cause scaling as well.  Scaling is most common in concrete with poor surface strength, caused by finishing a slab while bleed water is on the surface, or overworking the surface resulting in a lower air content.  Air entrainment is vital for concrete slabs placed where freeze/thaw conditions exist.  When scaling occurs, so does the blame game.  The homeowner blames the concrete finisher, the finisher blames the homeowner or the redi-mix company.  The mix company blames the finisher

Cracking

This one needs little explanation.  Cracking is breaks that occur in areas other than those placed intentionally.  Almost everyone has cracks in their concrete, and because there are so many reasons why concrete cracks, it is often impossible to know the exact cause.  The good news is, cracks seldom result in structural problems. Some of the many reasons concrete cracks include:

Excess water in the mix.

A lot of water is not needed to allow concrete to cure.  However, builders add extra water to make it easier to finish out the concrete before it dries, because there are not enough people available to do the job correctly.  As concrete dries the slab will shrink as excess water evaporates.  This shrinkage literally pulls the slab apart.

Rapid drying of the concrete

The chemical reaction, which causes concrete to go from the liquid or plastic state to a solid state, requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete.  You can make sure that the necessary water is available for this reaction by adequately curing the slab.

Improper strength of concrete for the job.

Unknown to many people is that concrete comes in different strengths, and can have fiber mixed with the concrete to add to its strength.  The Roman Coliseum was built with fiber reinforced concrete and is still standing today. A pueblo house built in 1540 with straw reinforced adobe brick is believed to be the oldest house in the US.

Improperly placed tension control joints.

Plain and simple, concrete cracks because there is a stress on the concrete that exceeds the tensile strength of concrete at any given point in time.  The use of tension control joints are placed to help alleviate those stresses.  Many finishers have a lack of understanding about where control joints should be placed.  Improperly placed, or too few control joints will mean the slab will crack to alleviate those stresses.  The saw cut, or tooled control joints are placed to provide an area for a controlled crack, because once they are in place, the concrete will eventually crack in those places.  This prevents unsightly cracks elsewhere. 

There is a popular misconception that the use of steel reinforcement bar (rebar) will prevent cracks.  It will not.  However, rebar will hold a slab together that has cracked and reduce shifting and heaving.

There isn’t a single concrete contractor that has never had to deal with their concrete cracking.  Sometimes, no matter what you do, problems will arise that were not seen before the job started. 

Dusting

Dusting is the presence of a powdery material at the surface of a hardened slab. A concrete floor dusts under traffic because the wearing surface is weak. This weakness can be caused by the finishing operation performed over bleed water on the surface. Finishing or working this bleed water back into the top of the slab produces a low strength layer right at the surface. Placement of concrete over poly or some non absorbent surface, increases bleeding and as a result the risk of surface dusting.

It is caused by insufficient or no curing of the surface. This omission of curing often results in a soft concrete surface, which will easily dust under traffic.  In cold weather the concrete sets slowly, particularly cold concrete in below grade placements. If relative humidity is high, water will condense on the freshly placed concrete. This water condensation, if troweled into the surface, will cause dusting.

Popouts

A popout is a conical shaped fragment that breaks out of the surface of concrete.  Popouts are usually caused by the expansion of porous aggregate particles having a high rate of absorption. As the offending aggregate absorbs moisture or freezes under moist conditions, its swelling creates internal pressures sufficient to scale the concrete surface. Ironstone, coal, shale and soft fine grained limestones are the commonly observed causes of popouts.

Most popouts occur within the first year of concrete placement. Moisture induced swelling may occur shortly after placement due to moisture absorption from the plastic concrete, or they may not occur until after prolonged rainy weather or the first winter. Popouts are generally considered a cosmetic flaw primarily affecting the concrete appearance and usually do not affect the service life of the concrete.

Efflorescence

Efflorescence is a crystaline deposit on surfaces of concrete. It is whitish in appearance, and is sometimes referred to as “whiskers”. Efflorescence has been a problem for many years, and is a topic of much controversy. The formation of these salt deposits are not mysteries. They are, for the most part, water-soluble salts that come from many possible sources to mar and detract from an otherwise beautiful and serviceable structure. First of all, there must be water present to dissolve and transport the salts. Groundwater is often a source of efflorescence. For water to carry or move the salts to the surface there must be channels through which to move and migrate. The more dense the material, whether it be brick, stone, stucco or concrete, the more difficult for the water to transport salts to the surface. Conversely, the more porous the material, the greater the ease with which salts are transported and deposited. Salt-bearing water, on reaching the surface of a structure, air evaporates to deposit the salt.

Crazing

Crazing is the development of a fine network of random cracks on the surface of concrete caused by the shrinkage of the surface layer. Generally, these cracks develop at an early stage and are evident the day after placement or within the first week. Crazing does not affect the structural integrity of the concrete and rarely affect the durability or wear resistance.

Crazing is caused by poor or inadequate curing, an excessive concentration of cement paste and fines at the surface caused by an overly wet mix, which allows coarse aggregate to settle, bullfloating or finishing while there is bleed water on the surface or the use of a steel trowel sealing the surface and diluting the cement paste.  Sprinkling cement on the surface to dry up the bleed water is a frequent cause of crazing surfaces. This concentrates fines on the surface. 

Blistering

Blisters are hollow, low profile bumps on the concrete surface typically ranging from the size of a dime up to an inch, but occasionally 2 or 3 inches in diameter.  The most common cause is when a dense troweled skin of mortar about 1/8 inch thick covers an underlying void which moves around under the surface during troweling. Blistering can also be caused by troweling too soon; resulting in the surface being sealed too early while the underlying concrete is plastic and bleeding or able to release entrapped air.

Plastic Shrinkage

Plastic shrinkage cracks appear on the surface of a freshly placed concrete slab during finishing operations or soon after.  Plastic cracks are usually parallel to each other, between 1 and 3 feet apart, and do not cross the perimeter.  High slump concrete increases shrinkage. Excess water can be expected to increase shrinkage approximately in proportion to its percentage of the total mix water.  Vapor barriers are a key contributor to plastic shrinkage cracking. All bleed water must migrate to the surface, which seriously affects timing, and surface set control methods. 

Stamped Concrete Issues

There are additional problems conventional stamped concrete contractors must have knowledge of in order to avoid failure of their finishes.  These problems are non-issues with polymer modified cement overlays.  They are:

* Air entrainment–Low air entrainment that results in spalling and scaling of the surface destroys decorative finishes. When dry-shake color is used, scales typically have color on one side and plain concrete bonded to the colored layer on the back side of the scale. This is because dry shake color hardeners provide a densified layer that effectively protects the colored layer from freeze-thaw damage. When air entrainment is excessively high, strength goes down, and there is virtually no bleed. It can also be difficult to properly “wet out” dry shake color hardeners.

* High water-cement ratios–Because of the more porous surfaces that result from too much water, colored finishes, including chemical stains, diffract more light, giving the impression of weaker coloration. Due to the weaker surface, traffic wear patterns can develop. In the case of chemical staining, wear can remove the colored layer.

* Lack of curing–These symptoms can be similar to high w/c ratio conditions. Un-hydrated cement does not develop calcium hydroxide, so there is less of it for some decorative products to react with. Colored surfaces will appear less intense than well-cured concrete with the same amount of color. Stained surfaces also appear less intense. Dusting and traffic wearing patterns problems can also result. However, most decorative finishes can’t tolerate the same curing methods used for plain concrete.

* Low strength–When there isn’t enough cement paste in a mix, integral color isn’t properly restrained in the paste, and color can be lost from the surface. Chemical stains may not have enough calcium hydroxide to react with, resulting in less coloration. Low strength in decorative finishes can cause traffic to wear into the finishes.

* High moisture levels in concrete–Chemical stains react differently in areas of a slab that have higher relative humidity. Decorative treatments, which do not have good moisture vapor transmission properties, can peel off the surface, blemish, turn cloudy white, or cause blisters to develop.

* Cold weather conditions–Long initial set times and excessive bleed water mean that more calcium hydroxide comes to the surface where it reacts with carbon dioxide from the air to form efflorescence. More laitance also comes to the surface from silica in the aggregates, causing hard white silicates to form. Also, slab finishes are often wet in appearance. In cold weather, concrete is usually covered with curing blankets or plastic, causing unsightly efflorescence markings.

* Hot weather conditions–When conditions are really hot, there is less time to perform all the added steps needed for some decorative finishes. In the case of stamped concrete it is more likely that impressions will be “mushy” at the beginning of the stamping process and too light, with little texture, at the end.

Abstract Concrete is well aware of these problems and how to prevent them.  If you are looking for quality in your next project, you can be confident in your choice to hire us.

3 Comments

Kylie BattName
Apr 11, 2010 at 7:34 pm

мне б такой…

  There are way to many variables in the production, mixing, placing, and finishing of concrete for me to discuss in […….


 
Kylie Batt
Apr 12, 2010 at 3:49 am

В этом что-то есть. Большое спасибо за информацию, теперь я буду знать….

  There are way to many variables in the production, mixing, placing, and finishing of concrete for me to discuss in […….


 
Kylie Batt
May 13, 2010 at 2:18 am

постоянно что-то горит…

Concrete is an excellent material, but it is not perfect.  You don’t have to look very far to identify the real life concrete problems listed below…..


 

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