Texans have earned a reputation for doing everything in a big way. Consider the San Jacinto Monument in Houston, which holds the Guinness record as the world's tallest monument column. This historic limestone structure, towering 570 feet high and crowned with a star spanning 34 feet, has just undergone a four-year, $6 million restoration.
The monument is impressive not just for its size, but for its design innovations such as the load-balancing and anchoring techniques used when it was built in 1936. As a result, replacing what nearly 65 years and several tropical storms had stolen from the beloved memorial proved complex.
Made of Texas cordova shell limestone quarried near Austin, the imposing obelisk rises from a 125-foot-square base, which houses a museum. Above the base, a 15-foot-tall frieze of carvings and lettering depicts historical events. The octagonal shaft, 218 courses high, tapers from 48 feet square at the bottom to 19 feet square at the observation deck 490 feet above ground. At the apex, symbolizing the Lone Star of Texas, stands a 220-ton star constructed of structural steel and concrete and faced with limestone with cementitious coating.
The Western Group's Houston and Dallas branches (Western Waterproofing Company of America) played a major role in the massive repair effort, along with project engineers Wiss, Janney, Elstner and Associates. They began by setting the stone three courses high. On the bottom part of the shaft, they set the outside stone, then the inside stone, then placed concrete between the two stone wythes. Thus, the inner wythe took the place of concrete forms and serves as the inside wall seen by visitors in the building. This method was repeated to the top of the frieze, about 45 feet high. Above that, where the elevator shaft rises to the observation deck, plywood forms were used to create an inner wall of smooth concrete.
The outer wall called for thousands of soft, porous limestone pieces alternating in size by one inch to offset the load. The stones were placed plumb on the outside, allowing the concrete to flow into the offset on the inside. The restoration team discovered that more than 500 stones in various areas of the base shaft ranged from 5-1/2 to 9 inches in depth, with some as deep as 16 inches.
The load-balancing placement of stone was just one innovative aspect of the original design. More creativity was evident in the anchoring. Normally on stone, you have the back-up structure and attach the anchors to it. In this case, flat-headed J-anchors were placed with the leg of the anchor extending through a 1/4-inch notch in the stone. They set the form for the inside surface of the concrete frame, tied the J-anchors to the rebar, and when the concrete was placed, everything was locked in.
Unfortunately, moisture from the Gulf of Mexico would cause those cold-processed steel anchors to suffer severe rust over time, leading to delamination and splitting of the stone. The deterioration would go largely unnoticed until the summer of 1989, when a 30-pound chunk of limestone tumbled from the side of the monument and crashed through a skylight into the museum below. The investigation that followed revealed much work to be done.
The resulting restoration plan called for removing the corroding anchors by coring through the stone, and then reanchoring every stone with a stainless steel rod. More than 9,300 circular 3-inch limestone plugs were used to cover core holes left after removing the old anchors. In addition, 3,500 Dutchman pieces (small, matching pieces of stone attached with a tight joint to larger stones) were installed to repair existing stones.
The reanchoring task involved installing some 110,000 rods, each requiring a hole drilled at a 30-degree angle into the stone and concrete substrate. The hole was then injected with latex-modified grout before the new anchor was inserted.