CHALLENGE: Preventing Staining in Concrete Tilt-Wall
It has become increasingly common for new school construction to feature architectural tilt-wall panels, where the exterior concrete surface doubles as the finished façade. This approach aligns well with modern aesthetics while also offering a sustainable, cost-efficient solution.
American Constructors specializes in self-performing tilt-wall construction, bringing its expertise to Bastrop ISD’s newest school, Adelton Elementary. Located near the Colorado River, the team initially selected locally sourced river rock as the aggregate for the tilt-wall concrete, an economical and practical material.
However, this river rock introduced a hidden risk. It contained marcasite, an iron sulfide mineral that oxidizes when exposed to oxygen and moisture. Over time, this reaction can produce rust stains on the concrete surface. While largely cosmetic, even minor discoloration can be highly visible on an exposed, uncoated façade. Unlike typical surface stains, this staining cannot be removed through power washing as it is embedded into the concrete mixture. Marcasite stains can also develop gradually over the course of several years, creating the potential for costly remediation down the line.
SOLUTION: On-Site R&D to Find a Durable Alternative
To address the issue, American Constructors collaborated closely with material suppliers, the architecture team and engineers to identify an alternative aggregate that would eliminate the risk of staining from the outset.
We considered a common substitute, crushed limestone. However, depending on the source, it can have higher absorption potential, increasing water demand and making it more challenging to maintain consistent workability through batching, transport and final placement without careful mix design adjustments.
Ultimately, the team selected dolomitic limestone, which can be a denser and more uniform material depending on the source. While this option carried a higher upfront cost and required transportation from a greater distance, it provided the optimal balance of performance, durability and aesthetics for the needs of this project.
RESULT: Maintaining Design Integrity and Delivering Long-Term Value
The dolomitic limestone maintained the required concrete workability (slump) while eliminating the oxidation risk associated with contaminates found in river gravel like marcasite, ensuring long-term architectural and design integrity for the new campus.
Because this issue was identified early, our team was able to find a solution through on-site research and development and with the help of third-party laboratory testing to confirm the final mixture before pouring began. This proactive, thoughtful approach ultimately delivered a finished campus to Bastrop ISD as scheduled.
Although the switch to dolomitic limestone increased material costs on the front end, it significantly reduced the risk of staining and future maintenance expenses for the district.
The insights gained from this project are informing our work across Central Texas, where similar aggregate source conditions are common. By applying these lessons early in the design process, our project teams are better equipped to mitigate risk and deliver more resilient, cost-effective solutions for our clients.