Do concrete overlays affect deterioration rates? A case study of two parking structures presents durability of overlay repair schemes

Concrete Construction, May, 2002 by Richard C. Reed

The State Street Capitol Ramp and the Government East Ramp, two Madison, Wis., parking garages, had suffered significant corrosion-induced delamination. Over several years we repaired much of the damage and installed concrete overlays in an effort to reduce the rate of continuing deterioration. The performance of specific repairs was periodically monitored. We found that concrete deterioration rates were greatly reduced in one garage, while repairs to the other parking structure were not uniformly effective in reducing the rate of surface delamination.

State Street Capitol Ramp

The State Street Capitol Ramp (originally called the Dayton Street Ramp) is rectangular (198x264 feet) and originally consisted of a slab on grade and three supported levels. After the repairs were complete, three more supported levels were added. Each supported parking level is divided into four sections (designated South, Mid-South, Mid-North, and North) by the ramp system and by east-west expansion joints. Most of the garage was built in 1962-63, with the east portions of the South and Mid-South sections (column lines A to E) completed in 1966.

The supported levels are constructed of 11-inch-thick reinforced concrete flat slabs with drop panels. The slab in each garage section is supported by two rows of columns spaced 36 feet on center in the east-west direction, spans 37.5 feet in the north south direction, and cantilevers about 14 feet beyond each column line.

Wiss, Janney, Elstner Associates (WJE) originally investigated the garage in 1978, 12 and 15 years after the original construction. Deck delamination areas varied from less than 25% in some sections to more than 50% of the concrete surface in others. We repaired the garage in five phases from 1979 to 1988. The repair program included deck shoring, deck patching, and the installation of a latex-modified concrete overlay. In all phases except the first, the deck shoring was removed before the overlay was cast. In the last two phases, the reinforcing steel in the patch areas was partially or entirely coated with epoxy. The condition of the garage was reassessed in 1984, 1993, and 1998. Table 1 (page 30) summarizes the repair history and the findings.

Government East Ramp

The Government East Ramp (originally called the Doty Street Ramp) is rectangular (about 264x197 feet). The parking garage consists of a slab on grade and three supported levels. Each supported level is divided into three sections, designated East, Middle, and West. The East and West sections are gently sloped, and the Middle section provides a ramped transition between levels. Most of the garage was built in 1957-58, with the north portion of the West sections (column lines A to E) completed in 1965.

The supported levels are constructed of reinforced concrete, beam-and-slab construction. Beams measure 8 feet wide by 16 inches deep and span in the north-south direction between columns. Each garage section has two rows of columns spaced about 36 feet on center in the north-south direction. The slab typically spans about 36 feet between columns in the east-west direction, and cantilevers about 14 feet beyond each column line. On the first and second supported levels, the slab is 9 inches thick, while the roof slab is 10 inches thick.

We first investigated this garage in 1978, 13 to 20 years after the original construction. Delaminated deck surface areas varied from less than 25 % in some sections to over 50% in others. We repaired the structure in four phases from 1980 to 1989. The repair program included deck shoring, deck patching, and the installation of a low w/cm overlay. In all of the phases except the last, the deck shoring was removed after the overlay was cast. In the last phase, the reinforcing steel in the patch areas was coated with epoxy. The condition of the garage was reassessed in 1984, 1993, and 1998. Table 2 summarizes the repair history and the findings of the various condition surveys.

What we found

In the State Street Capitol Ramp, all of the concrete deterioration rates decreased significantly--by 37% in phase 1 and by at least 63% in the other four phases. In the Government East Ramp, the concrete deterioration rates also generally decreased significantly, by at least 33%, with two exceptions. The exceptions were one area in both phase 1 (2 West) and phase 2 (3 East) where the concrete deterioration rates remained essentially the same or even increased. The condition surveys were based on sampling 10% to 35% of the various repair areas. However, we are confident that the sampled results are representative of total repair performance.

In general, the concrete overlays appear to be effective in decreasing the rate of concrete deterioration. Repair details, such as whether the deck shoring was removed before or after the overlay placement or whether the exposed reinforcing bars were coated, do not appear to significantly affect the durability of the repair. Rather, we found that areas that were experiencing high levels of concrete deterioration continued to experience rapid concrete deterioration even after the repair installation. These areas may have some durability flaws inherent in the original construction--such as adverse effects of particular admixtures, less than required concrete cover, or poor concrete quality--any of which can be factors in ongoing concrete deterioration.