Source: https://www.constructiondive.com, August 1, 2018
By: Kim Slowey
In the second of three reports, Nelson Forensics has determined that the cracks in the McKinney High School football stadium, according to The Dallas Morning News, were caused by: too much water in the concrete mix, which caused the concrete to shrink more than it should have while it dried; not enough concrete in the cross-section along pier lines; and too little steel reinforcing to control shrinkage cracking.
Nelson said that the stadium’s design was not at issue but rather that construction was not carried out in accordance to the project documents.
The company reiterated its position that the cracks did not pose safety concerns, only durability concerns. However, Nelson said that while cracking at the visitor concourse did not affect the structural integrity of the slabs, if the cracks are left unrepaired, they could widen and become a tripping hazard. Water intrusion could also corrode reinforcing steel.
McKinney Independent School District told NBC 5 that it is withholding payment from Manhattan Construction and Stantec Architecture until repairs are complete. An upcoming third report from Nelson will focus on the lower bowl of the stadium.
While cracks in concrete can signal problems with structural integrity, they can also sometimes be simply cosmetic issues that do not undermine safety.
When crews were performing repair work on the failed Oroville Dam spillway in Oroville, California, last November, the Federal Energy Regulatory Commission expressed concern over small cracks that appeared in new concrete slabs. The agency, however, and the California Department of Water Resources, came to agree that the cracks did not affect the integrity of the concrete, and both the existing work and concrete mix for future slabs were left unchanged.
And then there are cracks that could indicate a more serious issue.
During an investigation to determine the cause of a deadly pedestrian bridge collapse at Florida International University in Miami, Florida, the National Transportation Safety Board discovered, according to the Miami Herald, that cracks appeared in the bridge’s span as early as February 24, about 19 days before it fell, and then in a diagonal support prior to installation. The bridge was built offsite then installed over the course of a weekend, raising questions as to whether the stress of installation was a factor in the bridge’s failure or post-installation tightening of steel rods was to blame. The investigation is still ongoing, and no cause has yet been officially determined.