About 6:20 a.m., the driver of a vehicle towing a trailer loaded with broken pieces of concrete lost control in the westbound lanes just west of 4th Street, according to the California Highway Patrol.
The trailer disconnected from the vehicle, CHP Officer Jeff Briggs said, and got loose on the freeway.
"The concrete pieces came out and fell onto both sides of the freeway," Briggs said.
Three to four vehicles were involved in the collision, he said. There were no reported injuries.
While the collision happened near the guardrail, Briggs said there was no damage to the center divider.
Traffic on both sides of the freeway slowed while officials closed the westbound car pool lane and issued a Sig Alert, which lasted about an hour and a half.
Caltrans workers were called to the scene to clean up the concrete.