A National Labor Relations Board judge has ruled that six major concrete companies in the Washington area violated labor law by refusing to open their books to the Teamsters Union to back up the companies' claims that they had to reduce wages and benefits because of financial hardship.

NLRB Administrative Law Judge Walter H. Maloney Jr. also ruled that this refusal "caused or prolonged" a May 1982 strike by the Teamsters that lasted seven weeks and led to the loss of some workers' jobs.

In a 32-page ruling, Maloney ordered the concrete companies to turn over financial data to Teamsters Local 639 and to rehire and pay retroactive wages to any striking workers who were not reinstated.

William H. Willcox, an attorney for the companies, said yesterday they will appeal Maloney's decision to the five-member labor board. "With all due respect," he said, "we think the judge is wrong."

"I think it is a great decision," said Phillip Feaster, president of Local 639, which represents some 5,200 local construction workers and drivers. "It proves that an employer is obligated to provide information to workers when they are claiming poverty or the need for relief."

The Teamsters strike and the NLRB appeal arise from recent problems faced by the region's unionized construction firms, which, in the last decade, have lost substantial ground to nonunion companies that pay lower wages and can underbid union firms on many projects.

Industry sources indicate the unionized sector, which formerly won about 75 percent of Washington-area building contracts, now wins considerably less than half.

In the Teamsters case, the six firms are the only unionized companies in the District and nearby Maryland that handle the "ready-mix" concrete used in major buildings and Metrorail construction.

The companies involved in the case are Buffalo Concrete, District Concrete Co., Howat Concrete Co., Maloney Concrete Co., Silver Hill Concrete Corp. and Super Concrete Corp.

In bargaining with some 160 Teamster drivers and laborers, the firms made "repeated assertions that they needed 'take-aways' from wages and fringe benefits" so they could reduce labor costs and compete with nonunion firms, the judge noted.

Citing a 1956 Supreme Court labor-law decision, Maloney said, "It is well-established that, when an employer objects to a union's bargaining demands on the basis that it is unable to afford the cost of the proposal, the company is under a duty to let the union see its books and records so that the union can verify the truthfulness of the employer's contention."

In this case, the firms told Teamster negotiators that the problem was not their inability to pay, but their inability to compete with nonunion firms if they did not reduce costs, the decision noted.

But Maloney said such a claim was essentially no different from claiming inability to pay--a conclusion that Willcox contended was incorrect.

Kenneth Henley, the Teamsters' attorney, said the decision is potentially important because "many employers are jumping on the concessionary bandwagon" and demanding "takebacks" in the form of reduced wages and benefits.

Companies in many instances "will have to justify" their demands, he said.

The strike ended when the Teamsters offered to return to work in July 1982, but not all of the workers got their jobs back. Because the judge found the firms guilty of violating labor law, the strikers are all entitled to their old jobs.

Henley said the firms needlessly provoked the strike by refusing to surrender financial data.

But Willcox said the Teamsters were at fault for starting "a senseless, outrageous and extremely destructive strike" because it threatened the health of the union firms.

Willcox said "the big issue here is whether we are to be bashed because we were trying to react to nonunion competition" by lowering costs.

Willcox said virtually all the strikers already have gotten their jobs back, except about 10 who were discharged for alleged misconduct.