More than a dozen Burnco employees began picketing outside the company's southeast office in Calgary Monday morning after receiving a 72-hour lockout notice on Friday.
The move affects plants that produce aggregates, paving asphalt and ready-mix concrete in Airdrie, Springbank, Okotoks, Cochrane and Calgary, and it could affect dozens of major construction sites around the city.
A union representative says members planned to vote on a new contract on Thursday, and the company should have waited until then to give lockout notice.
Shop steward Doug Dodd says the biggest sticking point in negotiations is seniority.
"To be perfectly blunt they're trying to break the union. We got guys that have worked here over 30 years and they want to be able to sit them at home, bring in whoever they want, whenever they want," Dodd said.
Burnco CEO Michael Powell says the union voted for the right to strike on July 30, but the company is committed to finding solutions in a timely manner.
"As the current economic environment in Calgary is more challenging, we believe that this is a time when all parties need to be working together to find solutions," he said in a statement.