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Epoxy Solutions / Concrete 101 / Concrete Vulnerability September 2014
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VULNERABILITY OF CONCRETE

Concrete is an alkaline material with a pH of 12-23, and as such is susceptible to attack by chemicals with a pH of less than 7, known as acids.  In acidic environments, the degree of degradation and vulnerability of concrete is directly related to permeability (porosity), and reactivity. Reactivity is a dependent variable which affects the degree to which various chemicals attack concrete. The components of reactivity are as follows:

  • Moisture
  • Temperatures
  • Concentration

Pozzolans are defined as anything other than cement, aggregate, and water that are added to the concrete mix.  The more common types of pozzolans are:

  • Temperature
  • Concrete Mix Design
  • Aggregate Gradation
  • Chemical Admixtures
  • Air Voids

The permeability of concrete leads to two very distinct and significant problems for the concrete industry and polymers coating and topping industry. The first is the passage of water, harmful chemicals and contaminates into the concrete. These spawned the need for protective coatings.  The second is the passage of moisture and contaminate out of the concrete once protective coatings or toppings are applied, The following discussion will be limited to the latter.

MOISTURE RELATED PROBLEMS

It has been reported that 80-90% of all construction claims involve some form of moisture related damage.  If these figures are true, it is important that we understand the sources of moisture within our industry.

One obvious source is the presence of visible free moisture on the surface of concrete prior to application of polymer-based products.  Regardless of whether or not a particular product has the ability to cure in the presence of moisture, the ability to bond to the surface will be impaired, relative to the bond strength that could have been achieved on a dry surface.  Because of concrete’s porosity, penetrating primers are recommended for their ability to soak into the pores of the concrete. Water saturated concrete has its pores filled with water thus the polymer primer must have the ability to displace the water or its ability to penetrate will decrease substantially.

Another source of moisture related problems is surface moisture as a result of the dew point cross over. Dew point is the temperature at which moisture will condensate on the surface.  When high relative humidity and warm ambient air come in contact with a cooler concrete, moisture will collect on the surface of the concrete, and may be difficult to detect until after a problem is experience. It is always preferable to calculate the dew point and note the readings in a logbook for future referencing. In other words, treat concrete like steel with regards to dew point. Do not apply a polymer coating if the temperature is within 3 C or 5 F of the dew point and falling.

Moisture related problems with polymers on concrete may be categorized as follows:

  • HYDROSTATIC PRESSURE
  • CAPILLARITY
  • MOISTURE VAPOR TRANSMISSION
  • GASSING
  • HYDROSTATIC PRESSURE
    • A distinct head of water exerting pressure against a concrete structure.  The weight of the water creates the pressure and is dependent on how the height or column depth is. An example of this would be a below grade structure that experiences moisture intrusion problems during a rainstorm.  Another example would be a high water table exerting pressure on the underside of a slab on grade. CAPILLARITY – Moisture pulled through the concrete by the attraction created when a distinct moisture source comes in contact with the fine hair like openings in the porous concrete surface. The action may go up, down, or vertical and is attracted by warmth and dryness.
  • MOISTURE VAPOR TRANSMISSION
    • MOISTURE VAPOR TRANSMISSION – Water in the vapor or gaseous state is a natural occurrence, and may not originate from a distinct water source. This is the means by which all concrete breathes and releases moisture. GASSING – A temporary condition usually occurring during installation of coatings, and usually with urethanes and methacrylate, when components are incompatible with moisture in or on the concrete. A chemical reaction takes place where carbon dioxide is formed in a gaseous state and rises to the top of the uncured liquid resin.  A phenomenon often incorrectly referred to as out gassing, is the formation of air bubbles in a primer or coating, cause by displacement of air in the concrete or the release of entrapped air created during the mixing of the polymer product. The aforementioned moisture related problems present themselves as blister in the surface of the coating or topping.  If severe enough these blister may cause delamination of the surface, which is often progressive.  Chemicals, which have penetrated the concrete prior to application, may cause adhesion problems immediately or work their way to the surface weeks or months after application.