Bond Testing and Evaluation at EBMUD Clarifier
Overview
In 2021, East Bay Municipal Utility District (EBMUD), a leading public utility in the San Francisco Bay Area, undertook repairs to a clarifier within its Main Water Treatment Plant in Oakland, CA. As part of this effort, Concrete Science, Inc. (CSI) was retained by MatriScope Engineering Laboratories, Inc. to evaluate the tensile strength of the concrete substrate and the bond strength of newly applied repair mortar at a repaired tunnel entrance section.
The tunnel entrance, located at the west perimeter of the concrete tank, had undergone repair work involving surface preparation and application of a cementitious mortar. The performance and durability of this repair were crucial due to the structural and operational importance of the clarifier system. CSI's role was to provide objective data that would confirm the adequacy of the repair in maintaining long-term serviceability under expected operational loads and environmental conditions.
Scope of Work
CSI performed a total of eight tensile pull-off tests in two distinct phases:
Phase 1: Evaluation of the in-situ tensile strength of the original concrete substrate.
Phase 2: Evaluation of the bond strength between the repair mortar (SikaTop 123 Plus) and the prepared concrete substrate.
All testing followed the ICRI Guideline 210.3R-2013 (updated to -2022 at the time of this case study writing), which governs the use of in-situ tensile pull-off tests to assess bond strength. The test area included an 80 sq. ft section located at the tunnel entrance along the west perimeter wall of the clarifier tank.
Key Findings
Concrete Surface Preparation: The substrate was roughened to an ICRI Concrete Surface Profile (CSP) No. 10, exceeding the specified CSP 6.
Phase 1 Results:
Average tensile strength of the concrete: 362 psi.
All failures occurred within the concrete substrate, with failure depths ranging from 0.06 to 1.5 inches.
The International Concrete Repair Institute (ICRI) defines Concrete Surface Profile (CSP) ratings to standardize the texture of prepared concrete surfaces. CSP levels 6 through 10 represent increasingly aggressive surface textures, typically created through methods like heavy shotblasting, scabbling, or milling.
CSP 6–7: Medium-rough profiles for high-build coatings or self-leveling overlays.
CSP 8–10: Very rough profiles ideal for thick repair mortars and structural overlays where maximum mechanical bond is critical.
Phase 2 Results:
Average tensile bond strength of repair mortar: 272 psi.
All test results exceeded the project minimum requirement of 200 psi.
Failures were a mix of cohesive failure in the repair mortar and substrate, indicating effective bonding.
Solutions & Recommendations
Based on the test results, CSI concluded that the repair mortar was effectively bonded to the substrate and met or exceeded industry and project specifications. While no specific remediation was required, CSI remained available to assist in preparing detailed repair procedures or specifications if needed for future work. The findings also helped confirm the adequacy of the surface preparation methods used, despite the more aggressive CSP profile.
Outcome
The testing verified the integrity of both the original concrete and the repair system. The results provided assurance to the project team that the repair approach was technically sound, with no concerns regarding adhesion or surface preparation. The successful outcome supports continued use of the repair strategy on similar infrastructure within EBMUD's facilities, reinforcing confidence in the current maintenance protocols and materials being utilized.
