Sags and cracks throughout the concrete floor in an $8 million vocational center for handicapped students have forced city officials to halt construction and may add up to $1.5 million to the project's cost.

Cyril Crandon, acting head of design, construction and engineering for the D.C. Department of Public Works, called the floor problem "serious" and a possible threat to "life and safety." He estimated that construction of the two-story center at H Street and New Jersey Avenue, NW would be delayed six to nine months.

Crandon said an independent consulting engineer hired by the Public Works Department determined that both the structural design and the concrete used for the second floor are flawed. He said the city hopes to recoup the cost of shoring up the floor from the contractor, A.A. Beiro Construction Co. Inc., and the architectural firm, Fry & Welch Associates.

Clay Murray, a vice president with A.A. Beiro, said his firm's analysis of the city's tests of the concrete used showed "the strength was adequate" and the firm's work was not at fault.

Monte Newman, project manager for Opportunity Concrete Corp., which supplied the concrete, said the tests showed the concrete was weak in some areas but suggested that an inadequate design could have "caused it to stretch apart." Newman also said the Public Works Department failed to perform all the concrete tests required when workers were pouring the concrete and had increased the cost of repairs by not addressing the problem when it first came to light last May.

Crandon said the department acted promptly. He estimated the cost of repairs, which he said would involve extra columns and reinforcements to the concrete, at$1 million to $1.5 million.

Louis E. Fry, president of the architectural firm, could not be reached for comment.

School officials characterized the problems at the center -- the city's first vocational school for handicapped children -- as the latest in a series of mistakes and delays that have hobbled school jobs handled by the Public Works Department. "I'm frustrated, obviously," said school board member Robert Boyd, who represents Ward 6. "We've had continuous problems with capital construction projects as well as repairs."

Boyd said the delay in construction of the center will prevent school officials from doubling the number of students who can participate in the program. He said school officials had expected to open the center in January and construction was already six months behind schedule.

Similar structural problems beset the city's new $41 million municipal center at 14th and U streets, where concrete beams supporting four elevated walkways cracked. The issue of whether the walkways were safe lingered for more than a year until last week, when project managers said the beams would support their loads.