Concrete and Steel Tips
Concrete Strength
Not all concrete is the same. The design strength of concrete is a function of the quantity of cement used when it is mixed. The more cement that is used, the higher the strength of the finished product, as long as you place, finish, reinforce and cure the concrete properly. Thus, the final strength of the concrete really depends upon what is done after the concrete is unloaded from the truck! Many people are unaware of this responsibility. However, certain applications require different minimum strengths. For example,exterior slabs (driveways, patios, sidewalks, steps, etc.) should be a minimum 4,000 PSI strength. The PSI refers to pounds per square inch, or its compression strength. As the number gets larger the concrete becomes stronger.
Thickness
The performance of a slab is directly related to its thickness. The thicker you pour a slab the stronger it will be. In fact, increasing the thickness of a slab from four to five inches increases the strength dramatically.Thicker slabs can handle more weight and are better able to bridge depressions which may form due to soil settlement. A thicker driveway slab will possibly be able to handle that once a year heavy delivery truck. When you consider the long term benefits, the slight extra cost to increase the thickness is well worth it.
Cracking
Most homeowners are shocked when I tell them that concrete is supposed to crack. In fact, I guarantee that it will crack in a majority of instances. The reasons are really very simple. Concrete cracks can usually be blamed on one or more of the following: cracks caused by shrinkage related to the curing or drying process, cracks related to expansion and contraction induced by temperature changes, cracks related to concentrated heavy loads, cracks related to poor, uncompacted sub-grade conditions, cracks due to rapid water loss prior to the final finishing stage. These all might sound complicated, but in reality they are not.
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Concrete shrinks when it drys, just like many things. Clothes shrink, wood shrinks, and concrete shrinks. Concrete is similar to wood with regards to shrinkage. As a piece of wood dries, it shrinks. The wood fibers pull closer to one another in response to the loss of water. Eventually a weakness develops between a 'row' of fibers, and a crack or a split develops. This is exactly what happens to concrete. The crystals of concrete are actually pulling away from one another. This pulling or tensile force is enormous and will crack the slab. The rate of shrinkage is very close to 1/16" per ten feet. So, if you poured a 100 foot long slab it will shrink nearly 5/8"! That's a lot! The trick is to anticipate this shrinkage and attempt to force the concrete to crack where it will look the least noticeable.
The installation of control joints every 10 feet in large slabs will generally account for this shrinkage. A control joint is an induced crack point. You may notice them as tooled lines that occur on a regular basis in sidewalks and driveways. These tooled lines should be deep enough (1/4 of the thickness of the slab) so as to create a natural weak spot in the slab. Isolation joints are points where concrete meets up against something that for all intents and purposes will not move. These joints are made of a material that will react to the expansion and contraction of the slab. These materials are sold under different names such as expansion paper, homasote, fiberboard, etc. It should be installed in strips the same thickness as the slab which is being poured.
Reinforcing Steel
Steel is an inexpensive and user friendly material. Concrete and steel work very well together. The addition of steel to concrete increases concrete's strength enormously. Concrete has very little tensile strength - that is, it can be pulled apart easily. Steel, on the other hand, has tremendous tensile strength. So, if you add enough steel to concrete, it is virtually impossible to pull it apart. Cracks commonly occur in concrete due to its low tensile strength. So, adding steel helps to minimize cracks that might develop as a result of tension. Cracks that do develop often will remain as hairline cracks. If the steel is large enough (1/2"reinforcing rods), frequently there will be no vertical displacement between the two cracked sections.
Steel is very inexpensive. I can purchase 1/2" rods locally for $0.15 a linear foot. Using this number, you can purchase enough steel for a 10 x 20 foot slab for less than $40.00!! I calculated that sum by creating a mat of steel rods, two feet on center both directions, one foot in from each edge. That matrix creates a very strong slab. Would you be willing to pay an additional $40 in materials to significantly increase the strength of your slab? I thought so. An experienced mason with a helper can install this steel in just several hours. It is well worth the price.
You can also install welded wire mesh in slabs. This material is usually available in two sizes, 21# and 42#. The mesh is generally five feet wide and is comprised of squares six inches on a side. The 21# mesh is constructed of steel wire approximately 1/16" thick. The 42# mesh wire is close to 1/8" thick. Mesh is very effective at strengthening slabs. Although I personally prefer steel rods, mesh is a good alternative for those who feel they cannot install the heavier steel rods.
Please take some time and do additional reading concerning concrete. I guarantee that you will be amazed at how much science and technology is involved. This added knowledge will enable you to spot the informed professional that you intend to hire for your next job. Ask him or her some direct, pointed questions. Ask them if they know what the PCA is. See how they respond. Hopefully, you will be able to locate an individual who can answer a large majority of your questions correctly. Good reading and good luck!
