National operates mainly in Ohio and West Virginia and had a turnover of £72m last year, making an operating profit of £2.7m.
Balfour chief executive Mike Welton said the acquisition would add overall strength and market coverage to its US civil and specialist engineering businesses. The firm recently announced new contracts in the USA totalling £138m, principally in road and rail work on the west coast.
At Balfour's results meeting last week, the firm said it would continue to look at acquisitions, following several purchases last year of companies with railway and security interests.