Concrete contraction joints for pumped concrete?

by Tom
(Central Florida)

Q. I heard hose pumped concrete shrinks more than that with large aggregate due to the use of pea gravel. I am pouring a 20x25 5 inch slab with 4/8 20 inch spaced rebar with 12x16 footers and a gravel base. I am concerned about shrinkage cracks and curling in the Florida heat should I use contraction joints?

A. Yes, you should saw contraction joints. Saw two joints on the 25' side and one on the 20 foot side giving you approximately 10 foot by 8 1/2 foot sections.

Wet cure the concrete for 7 days to minimize curling.

Smaller aggregate concrete does tend to shrink more than large aggregate concrete. The smaller the aggregate, the more cement is used to to achieve a specified psi. More cement in the mix usually means adding more mixing water to make the concrete workable, causing shrinkage cracking.

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