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Pouring the shop slab

April 27, 2008

As an update from my last post, I poured concrete for my new workshop last Thursday.   My prep work of the footings, forms, rebar and remesh all passed the county inspection.  I had pumped the water out of the hole for the last time,  lined up the concrete finishers and ordered the concrete.   I was all set.

The first two truck loads of concrete were to arrive at 8:00 am, and the first members of the crew arrived at 7:20.   They stretched a string across my forms and had me cut pieces of rebar which they drove into the ground, setting the tops of the pins as interim screed points.  This old school method with the string quickly uncovered a problem.   The slab was supposed to be 4″ thick, and I had planned to double that, setting the forms 8″ above grade.   I had checked individual points inside with a laser level, but the string showed that several areas would only be about 3″ thick.   We had to fix this, and fast.

Option 1: Raise all the forms and relevel them.  This would take 1/2 a day.  No good

Option 2: take up all the remesh and plastic and remove some of the gravel to lower the middle.  Again, likely to take 1/2 a day.  We had 30 mins.

Option 3: Dash to the store and bring back a load of 2 X 2’s and nail them around the perimeter.  32 mins.  Done.

8:02 am, the first trucks arrive and the crew begins to pour the footings first, bringing the concrete just above the bottom of the form boards.  We spaced the subsequent trucks 30 mins apart, each emptying it’s 9 yard payload.

36 yards into the pour and the footers and about 1/4 of the slab are done.

The stiffness of concrete is measured by slump on a 10 point scale.   The scale is rooted In a lab test where concrete is poured into a cone and then inverted on a surface and the cone removed.  The amount the mounded cone of concrete sinks when it spreads out is measured in inches of fall, or slump.  Nearly dry concrete wouldn’t slump at all so it would be a zero, while very wet soup would just collapse into a puddle and would be a slump of 10.  The trucks arrive with a slump of about 4-5 which is the most liquid state that it’s safe to travel with.   We added water and remixed each batch to a very wet 6 ” slump so that the concrete would easily flow and the crew could rake it around.  Note the light colored 2 X 2 strips on top of the forms to increase the finished depth.


72 yards into the pour and we were down to one  last quarter to go.  I had estimated the job at 65 yards, but the extra 2 inches I added to the depth, just added 15 more yards of concrete to the job.  A quick phone call to increase my job site credit and call for more trucks.  The crew took advantage of the break to drink some water and take a quick rest.

The 9th truck arrived and it was back to work, filling in the corner and working their way out.  Those not still engaged with work on the main slab, filed down to the house and began to form up an entrance to the garage for the house.   I had previously leveled the gravel there setting a slope away from the garage and laid out more steel mesh and form boards.   This second, much smaller job found us ordering a tenth truck, lightly loaded with 3 more yards to finish up, bringing the total to 84 yards, or about 336,000 lbs of concrete.

After floating out the driveway with a bull float, and the concrete began to set, the crew grabbed scraps of boards to use as knee boards and worked to hand finish the slab, first with magnesium hand floats, and then with rectangular steel trowels to ensure a smooth finish.  I elected not to apply a “light broom” finish as it tends to trap dirt and quickly collects stains.

While the crew was troweling on the driveway, the foreman was hard at work with a power trowel, polishing out the 2400 square foot slab for my workshop.

By the end of the day, things were shaping up nicely and by early evening, both surfaces easily supported foot traffic.  Early Saturday,  I snapped chalk lines and used my concrete saw to cut expansion joints to relieve both slabs and further reduce the likelihood and severity of cracking over time as the concrete continues to dry and harden.

I’ll tell the manufacturer that I’m finally ready or the shipment of steel girders and beams and in about 4-6 weeks, I’ll be craning the first girders into place.   That should provide time to strip the forms, put in drains and gravel to backfill around the slab, and allow it to harden to reach maximum strength in 28 days.

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