Pouring a Slab in Hot Weather? Some Items to Consider

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Placing a slab on grade foundation during hot weather can be challenging at the very least.  Some of the challenges hot temperatures during concrete placement include faster concrete setting times and decreased workability, an increase in visible shrinkage cracks, and potential loss of strength due to high water demand.

On the other hand, concrete gets placed in hot weather all the time, especially in the Houston area.  Here are some recommendations by the Portland Cement Association (PCA) to minimize the impact that high temperatures have on concrete.

  1. Moisten subgrade, steel reinforcement, and form work prior to concrete placement.
  2. Erect temporary wind breaks to limit wind velocities and sunshades to reduce concrete surface temperatures.
  3. Cool aggregates and mixing water added to the concrete mixture to reduce its initial temperature. The effect of hot cement on concrete temperature is only minimal. 
  4. Use a concrete consistency that allows rapid placement and consolidation.
  5. Protect the concrete surface during placement with plastic sheeting or evaporation retarders to maintain the initial moisture in the concrete mixture.
  6. Provide sufficient labor to minimize the time required to place and finish the concrete, as hot weather conditions substantially shorted the times to initial and final set.
  7. Consider fogging the area above the concrete placement to raise the relative humidity and satisfy moisture demand of the ambient air.
  8. Provide appropriate curing methods as soon as possible after the concrete finishing processes have been completed.
  9. In extreme conditions consider adjusting the time of concrete placement to take advantage of cooler temperatures, such as early morning or night time placement.

Item 6 above is perhaps the most important to me.  I've seen too many slabs that were improperly finished simply because the placement and finishing crew was too small.

For more information from the PCA on hot weather concreting, check out their website.

Concrete Cracks at a Location Near You

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Concrete cracks.  It's a fact of life; it's a material property of concrete.

Some cracks are significant and can indicate a structural deficiency (the new Allen, Texas, high school football stadium comes to mind).  But many cracks that I see on a daily basis are related to normal concrete shrinkage and are not structurally significant.

This past weekend I was out and about in town and, in two separate stores, noticed lots of concrete cracks in the floor.

The first store I noticed the cracking at was at Ikea off the Katy Freeway.  The cracks I took pictures of are actually from the second floor (so it's an elevated concrete slab).  

Then I went into Bassett Furniture, also off the Katy Freeway.  Same thing, except the cracks were on the main floor (slab on grade, not elevated).

When I rubbed my finger over the cracks, there was no faulting that I could tell (one side of the crack higher than the other).  Cracks with significant faulting get my attention; these cracks appeared to just be normal shrinkage cracks.

At any rate, if you look around, you'll see concrete crack just about everywhere you see concrete: streets, guardrails, floors, walls, etc.

Here's a slideshow of the pics that I took; just click on the pictures to move to the next one.

Texas General Soils Map

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Here's a really cool map of the different soils found across the state of Texas.  This map is provided by the US Department of Agriculture Natural Resources Conservation Service.

It's fascinating to me how the soils change across the state.

Notice the second page that describes each soil classification. In particular, note that the Gulf Coast Prairie is dominated by "well-developed, clayey soils with high shrink-swell properties."

That's us, Houston.