To repair the concrete structure supporting three nitric acid tanks at a chemical fertilizer plant, this contractor planned three 12- to 14-day shutdowns. But then fertilizer demand skyrocketed and the owner decided not to take the tanks out of service. To meet these changing needs, the contractor had to devise an on-the-run repair. One obstacle to doing the repair with the plant up and running was working in tight conditions. Another was providing the temporary supports necessary for the nitric acid tanks. Each tank, about 15.5 feet in diameter and 15 feet in height, was suspended through a hole in the concrete platform 18 feet above the ground. After almost 25 years, the aggressive attack of 70 percent nitric acid from the tanks had badly deteriorated the concrete platform.
The contractor devised a temporary overhead structural steel support to handle the large load of the tanks. With the task of suspending the heavy tanks taken care of, the crew could then restore the concrete platform. All the deteriorated concrete was removed and the rebar repaired. Reinforcing dowels were added to transfer the concrete shear stresses across the cold joints where old and new concrete met. Workers placed the concrete using a small trailer-mounted concrete pump. Problems with cracking were countered by cutting joints and filling them with asphalt to allow for shrinkage or expansion.