CONCORD, N.C. (AP) â€” A compound introduced into a filler used in a pedestrian bridge at Lowe's Motor Speedway ate away at the supporting steel, causing the walkway to give way under hundreds of race fans.
Tests conducted by investigators hired by the speedway found high levels of calcium chloride in the concrete slabs that made up the 320-foot-long bridge that collapsed Saturday.
An 80-foot section collapsed Saturday night, spilling race fans leaving the speedway onto U.S. 29 17 feet below. More than 100 people were injured. Three remained in critical condition on Thursday.
The concrete slabs were made using a process called prestressing, which was supposed to make the bridge particularly strong. The technique involves pouring concrete around steel reinforcing cables running horizontally through the slabs. A grout was used to fill gaps around the cables.
The calcium chloride was somehow mixed into the grout used by the Spartanburg, S.C., concrete manufacturer, said Charles Manning of Accident Reconstruction Analysts in Raleigh.
The calcium chloride caused the steel cables to corrode. It buckled as fans were leaving the speedway following The Winston, a NASCAR all-star race.
It remains unclear how the calcium chloride got in the grout. Industry standards discourage the use of chlorides in prestressed concrete structures because the substance permits rust to form more easily when moisture is present.
The concrete company's president and chief executive officer, William Lowndes IV of Tindall Corp., agreed with the consultant's conclusion of why the bridge failed.
``The preliminary results from investigators of the grout material shows unexpected, high concentrations of calcium chloride,'' Lowndes said. ``We are diligently investigating how this compound was introduced into the grout material.''
Lowndes declined to answer questions and left a news conference before it ended.
Each 80-foot section of the bridge was reinforced by 11 steel cables. Each of those cables was made up of four steel strands. Of the 44 steel strands in the section of the bridge that collapsed, 33 were significantly eaten away.
``I have never seen this, and I have been in the business 50 years,'' said Manning, a retired North Carolina State University civil engineering professor.
Manning and Benton Payne, district engineer for the state Department of Transportation, agreed that the only way to detect the corrosion would have been to X-ray the bridge or to chip out the grout and check the cables inside.
Track president H.A. ``Humpy'' Wheeler said he would consider using Tindall if the walkway were rebuilt. But he deflected responsibility for the collapse to the manufacturer.
Tests show that a second, four-year-old walkway at the track, also made by Tindall, is not contaminated and can be reopened, Manning said.
Wheeler said he didn't know if the track would use the bridge for Sunday's Coca-Cola 600, which is expected to draw more than 180,000 fans. Fans used the footbridges to cross U.S. 29 from the parking lots to the track.
Since both bridges are privately owned, they are not subject to inspections that state- and federal-owned bridges must undergo every two years. But the state DOT earlier this week ordered all nine remaining privately owned walkways over North Carolina highways to undergo inspections at the soonest opportunity.