The concrete floor for my shop
was the easiest and best slab I have ever poured was not without difficulties. All ultimately ended well, but as always, there is a story.
For some time now, we have had a pile of cascajo (cas-ca-ho [the letter a has a soft sound in this word] — river run sand and gravel) taking up space in the driveway. Cascajo is great for foundations and large columns. The mix as it is dug from the river has a range of sizes from sand granules to six-inch rocks. But it is not so great for floors because of the larger rocks. The rule of thumb for pouring a slab is no gravel/stone larger in size than one-third the thickness of the slab. I know we should have sifted the pile, but Armando and his crew balked. They said that sifting just isn’t necessary. If you come across a rock that is grande and causing a problem, just pull it out of the slab and throw it aside.
Additionally, most of the pile had been sitting for months and it was permeated with grass, weeds, and roots. I could foresee a problem when it came time to trowel a nice finish, but again the guys said “es normal.”
Here is a photo of the root-infested pile of cascajo:
I’ve learned to pick my fights with the guys because I can really make a fool out of myself; they usually know things that I don’t. A case in point is when two men showed up to put an internet antenna on the roof of the house we rented when we moved to Panama. They arrived with:
- the antenna and connecting cable
- a six-foot length of half-inch electrical conduit
- a small coil of bailing wire
- a nail
- a hammer
- a pair of pliers
I scoffed to myself. Where was the electric drill? Where were the numerous brackets, bolts, and accessories that they would surely need to mount the mast to the roof? And where was the mast? In my opinion they were missing about $129.95 in essential pieces and parts. But I watched.
They found an old tree stump in the yard to use as an anvil. Then with the hammer they pounded flat about three-inches of one end of the electrical conduit. Next with the hammer and nail, they punched a hole in the flat section, then using the bumper on their small van they bent the flat section to a right angle. Next, back to the tree stump they switched ends of the conduit and punched a hole in the conduit about two inches from the end.
Then using the pliers up on the roof they slightly unscrewed three screws that were holding the metal roofing in place, plus a fourth screw all the way. They put the removed screw through the nail hole in the bent end of the conduit and put the screw back into the hole in the roof that it came from and tightened it down. Then they wrapped some bailing wire around one of the loosened screws, ran the wire up and through the hole at the top end of the conduit, and back down and around the second screw, back up and through the conduit, and down to the third screw. They tightened the screws. At this point the conduit mast was triangulated firmly in place. Then they mounted the antenna dish on the conduit and ran the cable to the router on my desk inside the house. D.O.N.E.
So I am careful. I learn a lot from these capable and resourceful men.
But Sr. Murphy often shows up on the job. In the case of the floor slab in my shop, it seems that my image of troweled-smooth concrete is very different from their version. I like it smooth. Very smooth. I don’t want a sandy surface. They like it flattened out so you don’t trip on any of the ridges. They don’t take smooth into account because it is a long day’s work and anally troweling the floor to a mirror finish is just not going to happen.
There is more to the story, but let’s see some photos:
I held my ground on preparing for the slab. I like a nice level couple-inch layer of gravel to pour the slab on. If you pour concrete on dirt, moisture in the dirt will capillary-action wick its way up into the slab. But capillary action can’t happen through the stones because the spaces between the stones are too large. The slab will stay dry. So even though they thought I was stupid, I had a few yards of one-inch stone delivered. Sammy and I spread it level. Jabo wasn’t sure he liked it:
On Thursday, Cynthia and I put welded wire mesh on the floor to keep the slab from cracking. Here is the floor ready for concrete. My laser level is set up to level the concrete, and in the corner is a wooden bull float that I made from scrap.
The guys mixed a big pile of concrete:
And Armando and I spread it out. Because of the large rocks in the mix, it was difficult to raise the wire mesh to the center of the slab, but I got it raised about a third of the way up. Close enough. I’m using the laser level to set the grade so Armando could more easily screed the concrete. You can see he has thrown out a few trouble maker rocks:
Here Erin and Pancho take a break after using 24 sacks of cement:
In the next photo Armando trowels the floor. After wooden floating the floor, you shouldn’t steel trowel the concrete until all the water has disappeared from the surface. This way you won’t be pushing water around, washing the cement away from the granules of sand. But it was pushing 5:00 and the guys (including yours truly) were tired. So Armando pushed the envelope slightly and troweled while there was still water on the surface. I knew I wouldn’t be happy with the finished product, but it is, after all, just a shop.
We finished, cleaned up the tools, and called it a day. The next morning, Saturday, Armando had work elsewhere, so it was just me arriving on the job. I took a look at the floor. I didn’t like it and gave the job a C-minus, maybe a D. It was rough and there wasn’t enough cement “cream” on the top. It was going to be difficult to sweep it clean in the future.
What to do? It is generally a bad idea to apply a skim coat of cement paste onto a concrete floor because in time flakes will spall off the floor. But our floor was still very green, still hydraulically pushing out water. I’ve read that you have a good chance of succeeding with a skim coat if you catch the slab while it is still only a few hours old. The most common reason for needing to do this is when a surprise rainstorm pops up and washes the cream away just after steel troweling a driveway or a patio slab.
So that’s what I did. I got a steel trowel, a bucket of dry cement powder, a bucket of water, and a sponge. I spent most of the day sprinkling cement on the slab, then adding a bit of water by wringing the sponge, then troweling the paste onto and into the floor. I had to take numerous breaks to stand up and straighten the old Arthur Itis knees. At the end of the day I gave the floor a B-plus. I can’t give it an A because the mesh didn’t get lifted as high as I wanted and also because there were a few dips just a bit deeper than I would have liked. Troweling the top coat was gruelingly difficult for my old body, but I am glad that I did it.
I’m spraying the floor with water a few times a day now, and will continue for a week. This will slow the rate of cure and will prevent a lot of surface cracks. Here’s my finished floor slab just after I sprayed it:
The next time I am in the city I plan to go to Discovery and buy a few gallons of garage floor epoxy. With the epoxy, a quick sweep of the broom will clean the floor “real nice.”
I need to let the slab sit and cure without foot traffic for at least a week, so depending on the condition of my knees, I should be back working on the house windows maybe tomorrow.
That’s all for now.