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Concrete targets

Concrete targets

Concrete targets

We are making a batch of concrete panels for some testing. It is outside of the range of what we normally do, and we are far from experts. The goal is a square 24" panel at 1.8" thick. We made some forms out of pine and plywood and had a local concrete place deliver a yard of 5000psi grout with a 4" slump. Typical 10ga remesh from a big box store was suspended about halfway through the thickness. We were concerned with using a mix with large aggregate because of the shallowness of the pour, and we were recommended to go with 5k grout by the concrete guys.

The temperature outside has been in the upper 20s and dry. We poured inside our shop space with the truck backed up to our open garage door. When the concrete truck started to dump, the batch looked like meatballs coming down the chute. They were hard clumps of cement and would crumble when squeezed. The truck driver added some water and churned the mix until it smoothed out, and we poured our panels.

Cracks began to form during the curing process. The cracks are strongly correlated with the lines of the remesh. The mix was pretty soupy and easy to pour, so I have some reason to believe that too much water was added in the truck. I suspect cure temperatures in the shop were never much below 45 and up to 60 shortly after we finished.

We are trying to decide whether we should simply chalk our experience up to too much water or do the next batch without the remesh. Also, we planned to leave the concrete in our forms for our testing, so we did not put any kind of release on them. Is it possible that the concrete sticking to the sides of the form caused it to crack as it cured and shrunk? Any thoughts would be appreciated as we try to decide what to do differently moving forward. Many thanks.

RE: Concrete targets

Some pictures of the cracks might help to elicit input from others on this forum.

How was the mesh installed? Did you put down about 0.9" of grout, place the mesh, then cover it with the remaining grout needed to get to 1.8" thick?

The "meatballs" of cement are a concern - were they 100% cement with no sand in them?

Structures Consulting
Northeast USA

RE: Concrete targets

Did you cure the concrete with wet burlap, plastic sheets or spray-on compound?

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RE: Concrete targets

The NRMCA prepared an article on the topic named Cement Balls or Lumps in Freshly Mixed Concrete “Causes and Cures” I have attached it here. This will only solve part of the problem. I recommend you use a UHPC mix that has about 2% fibers. There are some commercial sources, ITW Engineered Polymers, Lafarge Ductal® and Steelike™ Concrete System. You could go even thinner with these systems.

Larry from Lehigh White Cement Company

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