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How to Pour a Small Concrete Slab

When my new garage door wouldn't seal against the old, cracked concrete of the floor, I poured and finished my own concrete slab.

A DIY Concrete Slab

A DIY Concrete Slab

When I bought a new garage door, I thought it would create an airtight seal and keep the cold winter air out of my garage. The new door did seal up some of the holes that were starting to form in my old door. However, the garage door installers noticed that the concrete beneath the door was so cracked and uneven that a new door would never sit completely level on the ground when it was closed, and there would still be gaps where air could come in. The area that needed to be repaired was too small a job for most professionals to take on. So, I decided that I would try to repair it myself.

Having never worked with concrete before, I decided to do some research before tackling this project. Online articles helped, but I found that amateur YouTube videos from other first-time cement workers were the greatest help. I was able to see how an inexperienced person handled the cement, and I used their tips and mistakes to help me with my job. It worked, and I was able to lay a small, even piece of concrete that sealed the gaps in my garage door. This is how I did it.

Materials Used

  • one 40-pound of pea gravel
  • four 50-pound bags of concrete
  • several liters of water
  • a mixing tub or mixer
  • metal shovel
  • a long, flat piece of wood (longer than the cemented area)
  • trowel
  • wood float
  • disposable gloves
  • safety goggles
  • box cutter
  • hand tamper
  • broom

Prepping the Area

The area being poured was not very large, about 10x1 and about two inches deep. I used the online calculator on Lowes.com to determine how many bags of concrete I would need for my job. It came up with about six 40-pound bags. They do stress that their estimation is approximate. So, I ended up buying six 50-pound bags. I ultimately used a little over three bags of concrete for this job and additional bags for some other small spaces.

I don't recommend ordering concrete online because the shipping can turn a $4.00 bag into a $20.00 bag. So, if your job requires 10 bags or less, even if it takes two trips, pick it up yourself. Be careful handling the concrete, though. The bags are very fragile and can break easily.

First, I removed all of the old, cracked concrete and weeds from the area, using a shovel to lift the broken pieces out of the space and move them aside. Then, I shoveled some loose dirt in the hole until it was level and used a hand tamper to flatten the space.

Next, I poured pea gravel into the space. I needed only one 40-pound bag of gravel to cover the bottom of the hole I had made. Then, I hand tamped it flat again. Next, I wet the gravel with a few sprays from my garden hose. Then, it was time to pour.

How to Mix Concrete

Mixing Concrete by Hand

My job was too small to justify renting a cement mixer, like most of the YouTube videos recommended, but they were pouring large concrete slabs for sheds and patios. So, a cement mixer made sense for them. For me, it would have been overkill.

I researched ahead of time how to mix a small amount of concrete by hand. The cashier who rang me up at the store was leery of me buying quick-drying cement, but I needed it to dry in a few hours so that I could close my garage door at the end of the day. So, I proceeded to buy the quick-set concrete. However, based on what he said, I knew that I had to work fast. Even the videos said not to mix more concrete than you can pour in 10 minutes.

I placed my materials near the work area and kept my garden hose on and nearby for easy access. I threw my first bag of concrete into the tub, sliced it down the center with the box cutter, and poured it out into my mixing tub. The result was very dusty. I suggest wearing goggles, gloves, and even a face mask.

My cement instructions called for mixing each bag with 2.5 quarts of water. The video instructions recommended starting with just pouring in 2/3 of the water at first. Before adding the water, I took a shovel and made a crater in the center of the concrete mix. Then, I poured the first batch of water into that crater and mixed it around in small scoops with the shovel.

The powdery substance got really heavy once the water was added. Like mixing a cake batter, it's important to reach the dry mixture stuck to the bottom of the tub. So, I had to start shoveling deep scoops to get the entire tub mixed, adding the rest of the water once I had exposed the powdery concrete hiding in the corners and at the bottom of the tub.

After a few minutes, the concrete was mixed pretty well. The videos said you could tell it was ready when it looked like thick oatmeal and that if you hold it in your hand, it will hold its shape. I tested this in a gloved hand before I shoveled the mixture over the layer of gravel. Then, I proceeded to (quickly) mix the next bag.

How to Pour a Small Concrete Slab

The Screeding Process

Screeding

Screeding

Screeding

It’s important to overfill your space with concrete. After three bags, the concrete had filled in the hole I had made, but it wasn’t enough. So, I mixed one more bag and scooped almost all of it into the hole. Then, it was time to screed.

Taking a small, flat piece of wood that I had found in the lumber department, I moved the wood across the slab in a sawing motion, flattening down the concrete, smoothing it out, and filling in the cracks. The excess concrete spilled over the edges of my slab which is what you want to happen. I scooped this excess aside and used it to fill in some cracks in the corners of my driveway. Then, I went over the area with the board three more times until it was smooth. The concrete was surprisingly easy to work with, giving me time to complete this step without hardening too quickly.

Smoothing tools: wood float and trowel

Smoothing tools: wood float and trowel

Smoothing

The videos were vague about how to handle the next two steps. Most said to wait for the sheen to disappear from the concrete before smoothing it out with a wood float. My concrete never was shiny or very wet, just a very dark gray. So, after about 20-30 minutes, I took my wood float and began to smooth out the surface, going around the edges and trying to cover any gaps or rough areas. My work area was not completely flat like a slab for a shed would be, and with my driveway being old, the concrete that borders the area is uneven, making the left side of the outside edge of the hole higher than the other. So, my goal was just to make sure that the cement covered the entire border and didn’t leave any gaps once the garage door was closed.

finishing the concrete slab with a broom

finishing the concrete slab with a broom

Finishing My Concrete Slab

The videos all say to put a broom finish on your concrete in order to texture the area and make it less slippery and help water to run off of it. Again, it doesn’t say when in the drying process you should do this. So, I waited another few minutes after smoothing out the surface before taking out an old push broom (though any type of broom will work) to complete this step. The easiest technique is to start with the broom away from you. Then, pull it toward you across the surface of the concrete, lift, and then keep going all the way down the slab. You can make different patterns in the concrete with the broom, but this straight line technique is recommended for beginners.

The Completed Slab!

Drying concrete.

Drying concrete.

no gaps

no gaps

When I was finished, I had a concrete slab that was safe to drive over and filled in the gaps under my brand new garage door. Doing it myself saved me money, kept me from having to track down a professional willing to take on such a small project, and gave me the confidence to work with concrete in the future. This project is simple if you do your research ahead of time, work quickly, and follow the directions on your cement bags.

Have you ever poured concrete? Leave your tips and stories in the comments below!

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2017 Laura Smith

Comments

Dora Weithers from The Caribbean on June 26, 2017:

Laura, I don't think that I would try, but I admire your work--both the concrete and your article about it. You could only get better! Thanks for sharing.

Eugene Brennan from Ireland on June 25, 2017:

Your concrete was probably a bit dry, I tend to have it sludgy, with a slight excess of water. That's why there was no sheen or wetness on the surface. Alternatively because it was quick setting, the water may have started to bond with the cement (it doesn't evaporate, it's locked in a chemical bond forever) while you were laying it, so any excess disappeared fast.

It's a good idea if possible to cover concrete in dry weather to stop it drying too quick and cracking. It should be protected also from frost because unbound water freezes, expands and cracks the concrete, or leaves the concrete porous when it melts. Another tip, wet adjoining concrete before pouring because if you don't, it acts as a sponge and sucks water out of the new stuff.

Still though, you got a reasonably good finish!