Concrete 101 (Part One)
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.
As with any home improvement project, when it comes to pouring concrete — there are several important steps that are key to a good job:
- Excavation and Grading
- Form Release Oil
- Proper Sub-grade
- The Mix
- Pre-pour Soil Preparation
- Proper Curing
Grading With Concrete Is A No-No
A concrete patio or walk can be used to level out the existing grade by simply pouring it thicker in some places and thinner in others. And although this reduces the amount of earth movement it creates a long term problem that can literally reduce the life of your concrete by half. Here’s why. Nature can be devastating to everything left outside – including concrete. This is especially true with concrete in areas where there are extreme temperature changes between mid-day and midnight. Like all elements in nature, concrete expands when it gets hot and shrinks when things cool back down. Concrete is more likely to crack if its thickness is irregular. The thicker parts will not heat up (or cool down) as quickly as thinner areas. That’s why it is important to prepare the grade before the pour. In fact, the only difference between the surface of the graded earth and the finished surface of the concrete should be – you got it – the thickness of the concrete. Granted, there are other influences on cracking – and we will discuss those – however, keep in mind that concrete slabs that go from thick to thin to thick are more prone to cracking than slabs that have uniform thickness.
Many of us who attempt to pour concrete for the first time don’t seem to have any difficulty at all with the concept of perimeter forms. Forms seem to makes sense to everyone. They hold the concrete into a given perimeter and they help us to manage the thickness of the finished product. 2×4 forms for 4 inch thick concrete, 2×6 forms for 6 inch thick concrete and so on. When the project is a walk path the forms are close enough together to be bridged with a short board that can be used to rough-in an initial surface. This roughing in process is known as “screeding”. Actually, a poor job of screeding can leave the finished surface with highs and lows that ultimately can result in a surface that ponds and puddles or otherwise doesn’t drain properly. And although this isn’t a major problem on smaller projects it can be a serious issue when pouring a large patio. That’s why redwood dividers used to be so popular. A large area of concrete could be broken down into easy to screed four-foot squares. Achieving a proper surface can be accomplished without placing the dividers “into” the concrete. All you have to do is place the dividers above the surface. That is the bottom of each divider is positioned to align with the top of the concrete. These guides are placed on wooden or steel stakes and after the troweled concrete begins to firm up the screeds (and the stakes that support them) are removed and the remaining holes are quickly patched. Keep in mind that perimeter forms should always be designed so that the finished surface will actively shed water. The rule of thumb is to align the forms so that the finished concrete surface will slope at least one-quarter inch for each foot of travel. In other words, an eight foot long patio should be at least two-inches higher on one end than the other.
Form Release Oil
Form release oil or “form oil” is applied to all form surfaces that will be in contact with concrete. This is sort of like “greasing a cake pan”. When the ingredients have finished baking – or in our case – curing, the forms can be easily removed without causing damage to the concrete. By the way, if you can’t find form release oil you can substitute with used motor oil. Yes, new motor oil would be OK, but it might be a bit expensive.
In some instances it is OK to pour concrete directly onto the ground. In other situations doing so can result in a short lived project. Some ground – like sand is extremely stable and is a super base for a concrete slab. On the other hand, adobe soil is highly expansive (expands when wet) and can crack concrete after one season. Always check with a soils engineer to determine what kind of soul you have and find out how to properly prepare it. Sometimes sand is used over adobe, sometimes loose gravel is best. Other times crushed rock must be compacted over the natural grade to stabilize the area. The variables here are endless and the specific method should be determined by a pro. By the way, this is the one are most overlooked by the novice and most considered by the professional concrete contactor.
Next week in Part Two of Concrete 101 we will discuss the importance of steel in concrete, the trick to getting the strongest concrete mix, how to prepare the ground before the pour and how to get your concrete to cure so that it lasts forever.
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