In April 2005, my boss, Victor Lattimore, asked me to join him in the second floor conference room at our corporate office in McKinney, Texas.
The president of Lattimore Materials Co. said rather innocently, “See this table,” as he pointed to our 15-foot-long, 5-foot, 8-inch-wide, wooden conference table. “Can you make this out of concrete?”
We have a culture that “if you can't do it, you can't work here.” So, of course, I said yes. I hadn't really thought much about that conversation until April 2007, when he asked, “How's the conference table coming?”
I had no idea where to start, but I knew that I was now employed on borrowed time until I finished that table. The size was imposing enough, but the logistics of constructing and transporting the new table would factor heavily into the design.
The table was on a second floor room with access to stairs and an elevator that had a weight limit of 3000 pounds. The existing tabletop was 85 square feet. If the new concrete tabletop was 2 inches thick, the top would weigh about 2100 pounds. Then there was the weight of the legs and support structure to consider. So, I estimated the weight of the table would be about 4000 pounds.
Because this was on the second floor, we had to go back to the original design drawings for the building to determine if the floor of the building could support the table. Plus, with 15 people surrounding the table for a meeting, the total design weight would be at least 6000 pounds.
The slab for the second floor is a 5-inch-thick, post-tensioned deck with beams running parallel to the table's anticipated position in the room. The design engineers ran some additional calculations and concluded that the floor could support the table and people safely.
So, from a purely theoretical standpoint, the table could be built and installed on the second floor. Because the conference room could not be shut down for construction, the table would have to be built offsite, transported, and reassembled in the conference room.
That decision, along with the elevator and stairwell restrictions, led to two major design considerations. How many pieces would compose the top, and how would those pieces join together?
Five pieces
We decided to make the top out of five pieces—two end pieces and three middle sections. The end sections were 42 inches long and 70 inches wide. The middle sections were 97 inches long and would vary from 22 to 24 inches wide.
We didn't choose these sizes arbitrarily. By using 150 pounds per yard as the estimated unit weight for the concrete, we designed each piece not to exceed 500 pounds in total weight. Exceeding 500 pounds would prove too difficult to move and could either cause personal injury or result in cracking during transportation.
While weight and friction would hold the pieces down on the steel frame, it was still necessary to tie all five pieces together. When assembled, all five pieces could be drawn tight, minimizing movement and helping with the leveling process.
Next came building the forms, which took several weeks. The forms were constructed from ¾-inch melamine and stiffened with an underlying 2x4 frame. The edge forms were expanded polystyrene.
The pieces would be cast upside down on the melamine. Once cured, the concrete would be flipped over and the finishing process would begin. We decided on “reverse casting” because none of those involved in the actual construction trusted their abilities to use a hand trowel to make the top surface perfectly flat. Casting the top upside down on a flat surface eliminated the possibility of the top not being flat. Reverse casting also allowed us to imbed a company logo into the concrete.
Batching plans
Once all of the forms were prepared, it was time for casting. Based on the size of the forms and the legs, we would need about 1.5 yards of concrete. Batching out of a laboratory mixer was not an option, so the concrete was going to have to be batched out of a 10-yard mixer.
The mix design was a combination of pink granite, black limestone, manufactured sand, and natural sand. While the aggregate combination was complex enough, the paste portion was more complicated.
