The top half of this typical cast-in-place concrete is covered with exposed bugholes. A contiguous foundation for topcoating is formed (bottom) after an epoxy cementitious thin overlay is applied.
Rehabilitating and protecting concrete on wastewater structures has consistently been a challenge to engineers and contractors. No hydraulic cement, regardless of its composition, will long withstand water of high acid concentration (pH 3 or lower) commonly found in these environments.
This challenge is even greater when the concentrations of hydrogen sulfide (H2S) rise beyond the levels protected by traditional protective barrier systems. It is common to see up to 1/8-inch concrete loss per year within collection systems averaging 20-50 ppm H2S. In extreme cases, 2 inches of concrete can be lost in a year.
To combat this extreme wear, engineers in the protective coatings industry have focused on developing concrete resurfacers to address defects of new or existing concrete in these environments. These pinholes, which are a typical condition in vertically cast-in-place and precast concrete surfaces, are caused by the release of entrapped air (bughole-induced outgassing) within the substrate when the protective coating system is applied. Resurfacing improves the film quality of a protective coating by eliminating possible pinholes and preventing corrosive gases and fluids to penetrate an otherwise impervious material. This ensures long-term barrier protection of the concrete structure.
In the last few years, manufacturers have introduced new cementitious resurfacing materials and repair methods. The rate of these new product introductions has been increasing, often without thorough testing.
Contractors installing these new materials often lack the understanding of cement/grout curing that is required to allow these products to perform at their design standards. Specifically, contractors need to be certain that the bond strength of the repair material is sufficient to provide a long service life.
Bond strength (adhesion) is the resistance of the repair material to separate from the concrete substrate, from reinforcing steel, or from other materials to which it is in contact (including the topcoat). Good bonding refers to the ability of the materials within the system to act as one.
Structural concrete has excellent compressive strength properties, typically designed with 3500-5000 psi for municipal wastewater structures. But for bond strength, contractors must focus on the concrete's tensile strength, generally assumed to be about 350-500 psi.
To effectively protect concrete, the cementitious repair mortar and protective coating must develop and maintain an adhesive bond strength greater than the tensile strength of the host concrete. If workers improperly install the cementitious thin patch resurfacer, it can weaken the concrete protection system, resulting in premature failure.
Determining bond strength
Researchers at Tnemec recently investigated the general tensile behavior of various cementitious resurfacing composites available on the market for use under high-performance protective coatings. The objective was to provide information on the influences and effects of external curing and surface preparation (of the repair mortar) to determine the bond strength characteristics when topcoated with high-performance protective coatings.
To begin the study, researchers reviewed 100 wastewater projects. They found the four most common cementitious repair composites specified for use as thin patch materials were epoxy-modified cementitious mortars, acrylic-modified cementitious mortars, portland-based cementitious mortars, and calcium aluminate-based cementitious mortars.
They then tested three commercially available products within each type of composite category. All tests were performed on a set of cast panels. The concrete was a high-strength, 5500 psi portland Type I design mix conforming to ASTM C387.
Bughole-induced outgassing develops when a high-performance protective lining is applied without a thin overlay.
The panels remained in the forms for seven days. After 28 days, the concrete panels, less one control, were mechanically prepared by dry-abrasive blasting the top face of the panels to an SSPC-SP13/NACE No. 6 surface condition, and achieving an ICRI-CSP5 surface profile.
Each of the 12 selected cementitious mortars was applied to the concrete panels at their respective minimum recommended thickness. The mortars were finished using a steel trowel to obtain a smooth, uniform finish. A testing matrix was developed to test the effects of proper curing (with and without external curing per ACI 308R), subsequent surface preparation (via dry-abrasive blasting to an SSPC-SP13/NACE 6, ICRI-CSP 3), and adhesion of a 100% solids epoxy coating. Each section of this matrix was tested for bond strength using ASTM D7234 adhesion tester using 2-inch-diameter dollies and an average value reported for the respective mortars.
The results, presented at the International Concrete Repair Institute's Fall 2007 convention, yielded some practical information for contractors and engineers involved in repairing wastewater collection and treatment structures.
Of most importance is curing. Like fresh concrete, most hydraulic cementitious resurfacing materials require proper curing. Based on this study, conventional cementitious mortars for repairs of concrete must be externally cured to the recommendations of ACI 308R or other guidelines governing proper hydration.
Common cementitious thin patch repair mortars, with the exception of the epoxy-modified compositions, require external curing to maximize their tensile properties. This is critical, and often overlooked, when topcoating with high-performance coating systems.
The findings are also consistent with industry guidelines. Previous studies have also reported bond strength development of cementitious resurfacers proceeds more slowly than compressive strength development. This often causes contractors to mistakenly abandon curing procedures prematurely when the traditional cementitious mortar “seems strong.”
But once the mortar dries, strength development stops, and bond failure of the mortar patch can result. Using liquid membrane-curing compounds is the most practical method of curing vertically placed mortars where job conditions are not favorable for wet-curing. The membranes prevent the loss of moisture from the mortar, allowing the strength to develop.
The testing also demonstrated the importance of proper surface preparation of the cementitious resurfacers before topcoating with high-performance protective coatings. When applied, the membrane-curing compound must be removed before the protective coating is applied.
Also, cementitious resurfacers develop a weak surface layer from using too high a water/cement ratio, overworking during finishing, exuding fines with bleed water, or the improper curing of the mortar. The unhydrated/poorly hydrated mortar forms a plane of weakness near the surface that causes significant reduction in tensile strength properties. This can lead to a cohesive failure of the mortar when topcoated with a high-performance protective coating.
Also, as cementitious repair materials become thinner, differential behavior with the concrete substrate becomes accentuated and enhanced repair material properties, such as tensile strength, become more important.
Contractors should pay attention to the parameters recommended by the thin patch manufacturers and to industry standards governing curing and preparing these materials before topcoating. Manufacturers of cementitious thin patch composites should provide contractors laboratory testing results to substantiate claims when applying these materials as thin patch resurfacers used under high-performance protective coatings.
Since adequate curing of repairs can be difficult and are sometimes neglected within the wastewater market, contractors should pay close attention to warnings and instructions for resurfacer curing and preparation to avoid premature coating failures.
Vaughn O'Dea is director of sales-waste and wastewater treatment at Tnemec Co. This article is based on a technical paper he presented in 2007. To read the entire paper, visitwww.tnemec.com.