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Restoring the Alamo

The AlamoThe Alamo has been maintained as a beloved historical landmark, thanks to the Daughters of the Republic of Texas (DRT), since the early 1900s. In fact, it was during the first decade of the twentieth century that the current roof was placed. In 2010, a section of that century-old roof, located in an area known as the Monk’s Burial Chamber and Sacristy, was nearing failure. Collapse would be certain if repairs were not made, but it would not be an easy fix.

“There was nothing routine about this project,” said Fred Belfort, a senior technical advisor with more than 30 years of experience in construction materials testing from Terracon’s San Antonio office, who was part of the Alamo roof-repair team. “There were no records or information available from the original construction to guide engineers and give them even basic information about the types and quantities of materials used, or even about the basic structural elements. The team who worked on this roof would need a synergy – an ability to think creatively and quickly – with a level of skill that only experience could bring.”

Given that there was no information from the original construction, to discover how to save the Alamo’s roof, the DRT needed a team that combined design, architectural, and engineering and construction materials expertise with expert problem-solving abilities, and the experience to help devise a unique solution. Datum Engineers was the structural engineer of record and Ford, Powell & Carson Architects served as the registered design professional in responsible charge (RDPIRC). These firms worked diligently together to come up with the most plausible solution. They selected Terracon to monitor the installation of the design and to provide construction materials testing and observations during the repair.

When Terracon joined the team, the first task was to evaluate the proposed solution. The solution called for the installation of dowels for additional reinforcement of the roof. To determine whether this would work, Terracon needed to extract cores from the existing roof slab to find the compressive strength of the in-place roof slab concrete.

Ferroscan equipment, a non-invasive method for gaining information about reinforcement bars, was used to locate embedded rebar. This allowed Terracon to select core locations that would minimize the impact on the existing structure.

The coring operation was successful but difficult, as it had to be performed overhead and at a significant elevation above the floor. It was risky because the coring operation itself could impact the existing lead-copper roof if the core depth exceeded the thickness of the roof, either by over-penetration of the coring bit or by the thickness of the concrete being less than anticipated.

Terracon’s tests revealed flaws in the proposed solution, and the team began to explore new ideas.

In July 2011, the team was ready to evaluate a second solution, but again, because the internal structure of the original roof was still largely unknown, further investigation was needed. We needed to determine the continuity of the bars and the extent of overlap to make a judgment regarding the anchorage of the reinforcement that appeared to be acting as a catenary suspension member supporting the existing slab. To make this judgment, construction contractors had to take apart a portion of the inside face of the northwest parapet wall and a portion of the lead-copper roofing located atop the Monk’s Burial Room. The deconstruction and the use of Ferroscan equipment allowed engineers to visually observe the reinforcing steel.

Terracon drilled nine pilot holes within three feet of the exterior wall to determine the actual thickness of the concrete roof. Additional scans were taken of the western and eastern outer-facing walls at the slab interface. These inspections showed that the original roof did not utilize continuous rebar from the slab to the supporting walls, and the second proposal was shelved. The team went back to the drawing board.

In March 2012, team members from Datum Engineers and Ford, Powell & Carson Architects proposed a new plan, requiring crews to install two steel beams across the top of the roof. After Terracon’s evaluation, the team decided on this solution in order to transfer the load of the roof to the main support walls. With a viable solution ready, everyone was excited and anxious to begin the final processes of the reconstruction. Terracon conducted full-time observations during the installation.

Additionally, the solution required the construction of a new reinforced concrete structure along the northwest parapet wall, which helped stabilize the failing roof above the Monk’s Burial Chamber. Terracon provided observations and concrete testing to locate rebar in the existing roof slab for wedge anchor and all-thread installation. Also, we sampled fresh concrete, conducted reinforcing steel inspections, measured the amount of spalling on the underside of the roof slab, and observed the torqueing procedures as the roof was being lifted to the beams.

Design, testing, and construction teams came to expect the unexpected throughout the reconstruction of the Alamo roof. Frequent complications required in-field modifications with no room for error. “Having a vast knowledge of non-destructive testing methods and concrete was absolutely necessary while dealing with this historical landmark,” said Nathan Gunn, a senior engineering technician from Terracon and Alamo project team member.

The Alamo’s roof repair was completed with accolades from the client in July 2012. The risk of collapse has been eliminated, and the outside of the Alamo looks exactly as it always has. Per the client’s wishes, passers-by see the same Alamo that they have always seen.