After Steve Zelnak retired from his 27-year position as CEO of Martin Marietta Materials, Inc. (NYSE: MLM), a Raleigh-based producer of sand, gravel and crushed rock used in construction, he formed a family enterprise.

Two years ago, Zelnak and his son Brent used their partnership, ZP Enterprises LLC, formed in 2008, to purchase a majority stake in four companies: Accu Tool in Apex, Advanced Machining in Salisbury, Quality Products in Charlotte and Plasma Processes in Huntsville, Alabama.

In a…