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I am building a patio in my backyard. How thick should it be?
Q:I am building a patio in my backyard. Since it will only be used to support the weight of people and standard patio furniture and possibly a portable backyard BBQ grill, can I pour a 3 inch fiber re-enforced slab instead of the standard 4 inch slab? My slab dimensions are approx. 30' X 15'. It will also feature exposed aggregate. I do plan on making the outer 4 inch perimeter 6 inches thick to prevent washout. I estimate 4.16 cubic yards of concrete for this job based on a 3 inch slab. Is this Correct?
A: I would not recommend a 3 inch thickness, even with fibers. While there are some exceptions, I recommend a minimum four inch thickness for any concrete placed on the ground. From a practical standpoint, you will most likely be using finished 2x4's for form material - and they are 3 1/2 inches wide. Grade the base of your patio to be no higher than the bottom of the 2x4, and keep in mind that the extra thickness will provide not only more load-carrying capacity, but will also reduce the slab's tendency to crack (note that I said reduce - not eliminate). Definitely include the thickened edge that you described, and make sure that your concrete supplier puts at least 1 pound of fibers in every cubic yard of concrete (I personally use a dosage of 1 1/2 pound per cubic yard - this may cost a few dollars more, but I think the benefit is worth the cost) Remember that fibers will not prevent cracks either. Your 30 x 15 patio needs to be jointed by sawing joints at least one inch deep. I would recommend sawing one joint to divide the 15 foot direction of the slab in half, and two joints to divide the 30 foot direction in thirds. This will result in 6 panels, each one about 7 1/2 ' x 10'. If you do not have experience in producing the exposed aggregate finish, I would recommend hiring a professional to help you complete this step. Keep in mind that you will be living with the results for many years. With regard to your calculations, you have figured correctly. At a four inch thickness, your slab will take about 6 cubic yards, allowing for waste and overage. And yes, you do need to make an allowance for overage - for your 30 x 15 slab a 1/2 inch difference in depth makes a difference of 3/4 of a cubic yard. Don't try to cut your quantity too close - a better approach is to form a small slab somewhere else on your property in case you have extra material. By the way, there is a quantity calculator on our website at the following URL: http://www.trmca.org/solutions/calculators.htm