Epoxy Systems for Concrete Protection

Comparison of Flexible Epoxy Systems versus Rigid Epoxy Systems for Concrete Protection
By: Gary French, Vice President of Technical Service, General Polymers (Published in Concrete Repair Bulletin January/February 1999)

Concrete is a relatively weak and porous surface that will wear rapidly in high use areas. Coatings applied to concrete, steel or wood may function to provide skid resistance, increased cleanability, weatherproofing, anti-microbial protection or improved aesthetics. While the above factors may be important, the purpose of this paper is to examine three basic factors that should be considered when choosing a protective flooring system. These factors are wear resistance, impact resistance and thermal shock resistance.

Coatings range in thickness from a few mils to several hundred mils and are applied by a variety of methods. These coatings consist of organic or inorganic materials. This paper will compare the performance characteristics of a typical hard epoxy resurfacing system composed of a bisphenol-A cured with a modified aliphatic amine, and a flexible epoxy flooring system composed of a bisphenol-A cured with a highly flexible aliphatic amine curing agent. It is important to note that this flexibilized coating is internally flexibilized and has no external plasticizers or solvents that would migrate from the system over time. Plasticizers exhibit this migration causing the material to embrittle and lose physical properties over time.

Wear resistance is the ability of a coating to maintain thickness and protection when exposed to an abrasive force. The wear resistance can be measured by two popular methods. The Taber Abraser method (ASTM D4060) measures a weight loss after being exposed to a given rolling load for a specified number of cycles. For the standard epoxy, the weight loss for 1000 cycles and 1000 gram weight is 87.4mg. and for the flexible epoxy the weight loss is 74mg. The flexible epoxy exhibits superior wear resistance by losing 15% less than the standard system. The second test method is the falling sand test (ASTM D968). This test measures the volume of sand required to erode one mil of coating. Coated panels are exposed to falling sand directed through a funnel to a specific point on the panel. The more sand required, the higher the abrasion resistance. For standard epoxy the volume of sand is 104.5 liters per mil and for the flexible epoxy the value is 181 liters per mil. The flexible epoxy requires approximately 80% more sand to erode one mil of coating. Both of the tests indicate that the flexible epoxy has better resistance to abrasion and therefore better wear resistance.

The second factor to examine is impact resistance. Impact is the force exerted by a falling object that might disrupt the bond. Impact failures could result in a cohesive failure - failure within the coating, or adhesive failure - failure of the bond to the substrate. The bond of a material is only as good as the tensile strength of the weakest material and the ability of a coating to wet the substrate to achieve adhesion. Since the epoxy material has the ability to wet the substrate and achieve a good bond, the bond strength is higher than the tensile strength of the concrete. In other words, failure will occur in the concrete and not at the interface of the concrete and the epoxy. For these reasons, testing for impact resistance is done by using a metal panel in the test method ASTM D2794 “Resistance of Organic Coatings to the Effects of Rapid Deformation”. A coated panel is deformed by a weighted steel ball as it is dropped from a given height. The force of impact (inch pounds) is increased until the deformation in the steel panel causes the coating to crack and/or disbond. The standard epoxy had impact resistance values of 20 inch pounds on forward test and less than 4 inch pounds on a reverse impact. The flexible epoxy does not lose bond due to impact from either the forward or the reverse impact using the maximum force of 160 inch pounds. While the reverse impact is not important on concrete, the results show the ability of the flexible epoxy to absorb impact and to provide a material that has much better resistance to failure either in a cohesive or adhesive mode.

Thermal shock resistance is the ability of a coating or resin system to stay bonded to a substrate when exposed to rapid changes in the environment as related to temperature. Depending on formulation, epoxies can have a coefficient of thermal expansion of two to ten times that of concrete. In typical situations, aggregate filled epoxies are used to minimize the differential in coefficient of expansion between the concrete and the coating. These systems are applied in thick cross sections (minimum ¼ inch) that act as a heat sink to prevent the rapid temperature change from reaching the concrete resin interface. The flexible epoxy can better handle the temperature change because of its ability to absorb the differential movement between the surface and the concrete. Testing has proven that the flexible material when filled with aggregate can handle live steam when applied at 1/8 inch thickness without disbondment. (Table 1.)

Given the advantages over standard, rigid epoxies, there are several environments where a flexible epoxy material can be used. One application is decking systems for parking garages (see photo #1). Traditionally these applications have utilized urethanes due to their ability to provide waterproofing regardless of movement. Urethanes serve as good waterproofing materials because they lose bond from the concrete surface. Their cohesive strength is greater than their adhesive strength. Typical urethane coating failures results in large sections disbonding in sheets. The flexible epoxy parking deck system consists of a neat layer of epoxy to provide waterproofing, impact resistance and crack bridging. The flexible epoxy membrane exhibits tremendous adhesion to the concrete deck as compared with urethanes. After the application of a neat coat of flexible epoxy, aggregate is broadcast into another layer of flexible epoxy to serve as the wear and the skid resistant layer. The aggregate is more tightly bonded in the wear coarse using epoxy as compared to urethanes. This is why typical urethane systems contain multiple layers of wear coarse versus a single layer using epoxy. The ability to absorb differential movement caused by temperature changes or structural vibrations combined with the outstanding adhesion to both the concrete and the aggregate leads to a successful application of flexible epoxies in the parking deck market.

Mechanical equipment rooms require similar waterproofing and durability as parking decks. The flexible epoxy installed as a neat membrane will, not only provide added protection against reflective cracking, but also will be much less prone to chipping that results from the dropping of tools during maintenance. (See photo 2)

Industrial aisle ways are ideal areas to use a flexible epoxy system. Traditional epoxies have been used for many years in this environment and resist the wear quite well. However, chipping and the resulting delamination make constant repairs necessary. Flexible epoxies provide the wear resistance and the impact resistance to overcome the constant need for maintenance.

Other areas that flexible epoxies would be beneficial include applications as a crack bridging membrane under decorative flooring. In this application, the flexible epoxy bonds to both the concrete surface and the flooring finish cushioning the movement associated with the concrete, thus suppressing reflective cracks. In secondary containment areas, chemical resistant coatings are typically required. These highly cross linked polymers are extremely rigid and can be protected from underlying concrete movement by using a flexible epoxy membrane. The flexibility of the epoxy material also provides resilience to a flooring system when exposed to impact. Industrial flooring systems built with these flexible products provide superior crack and impact resistance. Food processing areas require flooring systems that can withstand the chemical exposure of organic acids from food products as well as caustic cleaning solutions. These floors must be seamless to allow for wash down conditions and the ability to withstand hot water or steam cleaning. A flexible membrane under this flooring system provides crack bridging, waterproofing and thermal shock protection to the chemical resistant floor system. Other areas which would benefit from the use of flexible epoxy are plaza decks or balconies where waterproofing and wear protection is required. In any of these applications a decorative aggregate may be used to provide colored or patterned finishes.

In conclusion, flexible epoxies offer advantages such as wear, impact and thermal shock resistance that exceed the properties of standard epoxies. When properly formulated without the use extenders or plasticizers, flexible epoxies will perform to a higher standard than traditional rigid epoxies.

Table 1.

                                                        Rigid Epoxy                          Flexible Epoxy

Wear Resistance
     Taber Abraser                    87.4mg loss                               74 mg loss          Falling                                  104.5 liters per mil                    181 liters per mil

Impact Resistance
     Forward Impact                   20 inch pounds                         no loss of bond
     Reverse Impact                   4 inch pounds                           no loss of bond              

Thermal Shock                    1/4 inch required                         1/8 inch required

Gary French has been associated with the flooring business for over 25 years. His wealth of experience makes him a valuable resource for architects, engineers, sales representatives and contractors. Gary has a degree in Chemistry and understands the why and how of polymer interactions as well as the practical applications.