Heard a guy at the lumber yard say 'concrete doesn't shrink'

This old timer next to me buying bags of portland cement just blurts out that concrete doesn't shrink, only the water evaporating makes it look smaller. I've been fighting with hairline cracks on flatwork for two years and now I'm wondering if my whole approach to curing is backwards. Anyone else ever had a random comment completely change how you prep a pour?

2 comments

2 Comments

the_joel14d ago

Whoa wait hold on. That old timer basically just told you the water is the only thing that's actually moving in the mix? That blew my mind a little bit man. So the concrete itself stays the same size but all the tiny spaces between the particles get smaller when the water leaves? That would explain why my curing blankets and constant misting never stopped the cracks from showing up a week later. I always thought the concrete was physically pulling apart as it dried out. Now I'm wondering if I should stop babying the surface so much and focus on keeping the whole slab from losing moisture too fast from the bottom and edges too. This changes how I look at mixing slump and where I put my control joints for sure.

ivangrant14d ago

Honestly @the_joel I get where you're coming from but I think the old timer was oversimplifying it. The water leaving does pull the particles closer together, but the concrete also shrinks as a whole because the cement paste itself shrinks. The aggregate stays the same size, so the paste around it has to pull apart to fit, which is where the micro-cracks start. I've found that if you don't seal the edges and bottom real good with plastic or wet burlap, the slab loses moisture way faster from those spots and cracks show up there first. The surface misting helps a little but you're right that controlling all sides is the real deal.