Ask The Builder: How to mix a stronger batch of concrete

Q. This summer I’ve got a few outdoor projects that involve pouring small amounts of concrete. I looked into having a ready-mix truck do it, but the cost is so expensive. I’ve seen the bags of concrete in stores. Is it any good? Have you used it? Can I modify the mix to increase the strength? If so, what do you use and how? – Malcolm McC., Albany, Georgia.

A. I can clearly remember my first experience working with this magic material. I formed and poured a large set of steps that led up to the front door of the first house I remodeled. Looking back, it was too big a project for a rookie. That said, the job turned out not too bad!

The good news is the bagged concrete you can buy at local hardware stores is a fantastic product. The man who invented it built his first plant just a few miles from my childhood home in Cincinnati.

It’s important to realize that basic concrete contains just four ingredients: small rocks, sand, Portland cement and water. The cement is the glue or binder that holds the sand and rocks together. When water is added to the other three ingredients, it starts a chemical reaction. Countless invisible Portland cement crystals start to form and interlock all the sand and rock together.

I’ve used bagged concrete for years and had great success with it. My two most recent uses were for the in-ground piers that support a massive deck and the piers for my giant two-story shed. The piers are going to last for many decades because I used the right amount of water to mix the concrete. Add too much water and you’ll ruin the concrete.

You can add more Portland cement to bagged concrete to make it stronger. You can also add some hydrated lime. To make the strongest concrete, the sand should be sourced from volcanic lava that has a high silica content. This is not easy to locate, and I’d not worry about it. But you should get a bag of pure Portland cement and a bag of lime and add some of those two things. Both ingredients are inexpensive.

Most bagged concrete comes in standard-sized bags. If I wanted to make it stronger, I’d take an old kitchen measuring cup and add 16 ounces of Portland cement and 8 ounces dry measure of hydrated lime to each bag of concrete.

You want to blend the extra cement and lime with the bagged concrete in dry form. Use a newer plastic mixing bag and roll around the dry ingredients before adding the needed water. The concrete should have the consistency of stiff applesauce when you add the correct amount of water.

I’ve got many other bagged concrete tips and videos showing the magic plastic mixing bag at my AsktheBuilder.com website. Just go to: http://go.askthebuilder.com/concreteinabag