Ask the Builder: For professional finish, watch concrete ratio
DEAR TIM: I’m going to do an ambitious concrete project this summer pouring a long sidewalk. My initial results are disappointing. The finish I’m getting on my concrete is rough, and too many stones are at the surface. I wonder if I have my mix proportions wrong or what else may be the problem. Can you help me mix concrete like the pros do? Should I purchase an electric mixer? – Richard S., Lakeside, Calif.
DEAR RICHARD: I discovered at the ripe age of 13 how to mix concrete by hand and get it right. Please be aware that concrete is a very complex material, and you could fill a small library with the technical books and papers written about all the aspects of it. For our discussion, we’ll limit ourselves to very basic concrete.
Most concrete experts will probably agree on the following proportions of ingredients for a standard concrete mix: 3 parts stone; 2 parts medium or coarse, clean, washed sand; 1 part Portland cement and just enough water to make it a plastic mix.
The stones, or aggregate, in the concrete are what make concrete strong. The bigger the stones, the stronger the concrete, for the most part, with this critical proviso: The diameter of the stones in the mix should never exceed one-third the thickness of the slab you’re pouring.
This means that if your sidewalk will be 4 inches thick, the largest stone should be no more than 1 1/3 inches in diameter. It’s best that you have a blend of stones in your mix that range from say 1/2 inch to 1 1/4 inches in size.
When you decide to mix concrete by hand, I was taught to blend the stone, sand and Portland cement powder together DRY before you add water. You want the dry mixture to be uniform and the same color.
Understand that the Portland cement is the binder or glue that holds the sand and stone together once the concrete cures. You can add more cement to your blend if you want, just be sure you add the same exact amount to each batch. I would probably increase my ratios to: 3 parts stone, 2 parts sand and 1 1/2 parts Portland cement.
Water is both your friend and your foe when mixing and pouring concrete. If you add too much water to the mix, you dilute the amount of cement paste that coats the sand and gravel. If you add too little, you make it very hard to place and finish the concrete.
Professionals can place and finish concrete that’s got minimal water in it because they have special machines that can vibrate the concrete to produce an acceptable finish that’s almost impossible to achieve doing it by hand.
The proper amount of water to add to your dry ingredients is enough so that, once blended, the concrete is fully wet but not so wet that it won’t stay up in a pile. Professionals refer to this as slump. The higher the slump number, the more water you have in the mix.
If I were to order concrete from a ready-mix plant that came in a truck to the job site, I’d order a 4-inch slump for a sidewalk job. This mix flows down the chute of the concrete truck and can be placed in forms by an experienced crew.
You can mimic this 4-inch slump by making sure the concrete in your wheelbarrow, once mixed, will stay up on the sides of the wheel barrow and not fall back down into a flat level surface in the wheelbarrow. If the concrete mix acts like thick gravy, then you have far too much water in it.
Your disappointing finish results may have been a rookie error. It’s entirely possible you mixed the concrete correctly but didn’t place it right. You may have put the concrete mix in the forms and not worked the mix to drive the stones just below the surface while the mixture was still in the plastic state. Professionals use a tool called a bullfloat or a vibrating power screed to do this.
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