Concrete 101 (Part Two)
It can be a patio, a house-walk, a porch, a retaining wall or even a set of steps. Regardless of the project, when it comes to pouring concrete there are a couple of simple and basic rules that apply to every pour. And if you keep them in mind, the finished product will usually look better and last longer. By the way, you don’t have a cement patio — you have a concrete patio. Cement does not contain concrete. However, concrete does contain – among other things – cement, sand and gravel or rock.
Last time we discussed grading, forms and form oil and preparing the soil. This week we will discuss the other important steps that make for a successful concrete pour:
- The Mix
- Pre-pour Soil Preparation
- Proper Curing
Concrete alone isn’t much stronger than plain old dirt. It’s true. Pour a slab of concrete over a bed of adobe soil and the adobe will soon crack the concrete to bits if it doesn’t contain gobs of steel bracing. Steel rods known as “reinforcing bars” or “rebar” are used to substantially enhance the strength of concrete. The bars are placed perpendicular to each other creating a kind of a tic-tac-toe board matt. The grid created really does add strength to concrete. The amount of added strength depends in part on the diameter of the reinforcing rod used and in part on the distance between the bars used in the grid. The most common configuration uses half-inch rebar placed two-feet on center in both directions. Each intersection of the rebar is tied together with heavy wire known as “tie wire”. The steel bars are lifted to and held within the center of the proposed slab with small square concrete spacers known as “dobys”. Each intersection of the rebar matt is supported by a doby acting as a spacer between the rebar matt and the ground. A steel matt laid on the ground beneath the concrete provides no support at all. To properly strengthen the slab the rebar matt must be as close to the center of the slab as possible. Keep in mind that adding steel is a very inexpensive way of substantially increasing the strength of concrete.
There are many different kinds of concrete. We won’t go into the various types except to say that various combinations of rock, or gravel and sand and cement can result in a myriad of concrete strengths. Concrete should not be purchased by the “number of sacks of cement” that it contains. Five sack concrete mix (5 sacks of cement per cubic yard) is not always better than a four sack mix. More cement in a given mix does not necessarily mean more strength. You want to purchase concrete based on its “compressive strength” – that is – how much pressure it will take to crush it. The higher the compressive strength – the stronger the concrete. 2500 PSI concrete is pretty much standard. We like to up it to 3000 PSI for the stuff we do at home. If you run a ratio between the increase in cost as related to the increase in strength you will find that you end up a winner.
Pre-pour Soil Preparation
If you have ever watched a commercial concrete pour you may have noticed someone watering the area with a garden hose. This is not done to keep the dust down or to smooth the grade or to clean the area before the pour. It is done to saturate the soil with water so that the soil will not draw the water out of the concrete before the water and the cement in the concrete mix have time to properly cure (harden). This is why it is so very important not to pour concrete on an extremely hot day. If it cures to rapidly the concrete will be weak and the lasting quality will diminish.
As we just noted, concrete that dries out rapidly will not have time to properly cure. Wetting the ground helps to reduce the escape of water through the bottom of the concrete. But believe it or not, the water also can escape from the top in the form of evaporation. This is prevented by applying a light layer of oil to the top of the concrete known as “curing compound” or “curing oil”. The oil holds the moisture in the concrete from the top in the same way that clear plastic wrap prevents food from drying out. The clear oil will itself eventually evaporate. But it remains long enough to slow down and stabilize the curing process.
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