Tuesday, December 30, 2014

Why did my concrete crack and other questions about concrete

Why is my Concrete Cracking?
Q & A about Concrete
Concrete is an incredibly strong and versatile building material and has been a standard of modern construction since Joseph Aspdin invented Portland Cement in 1824.  But a lot of questions arise when someone plans to pour a patio or replace a driveway.  Here is my attempt to answer the questions I hear the most.

A freshly poured slab, waiting to be stamped


Q:  What is the difference between concrete and cement?
A:  This is kind of like asking what the difference is between flour and a cake.  Cement is a component of concrete that causes it to harden.  Concrete is the product we pour in wet form and dries into a solid material.    So basically, Cement + Sand or gravel + Water = Concrete

Q:  Will my concrete crack?
A:  The sad but true answer is absolutely.  All concrete hardens, turns gray and cracks – this is just the nature of the stuff.  But the good news is that we can do a lot to minimize the cracking and keep the slab from heaving or shifting when it does crack.
Take a look at my video on stamped concrete patios to learn about good preparation by clicking the link below.

Q:  How long does it take for concrete to harden?
A:  The initial hardening of concrete takes about 3 hours depending on the exact mix and the weather.  It takes about 3 days to harden enough for heavy foot traffic and 7 days before vehicles should drive on it.  A near full cure occurs in about 28 days although technically it continues to harden to many years.

Q:  My contractor said he is using 2,500 psi.  What does this mean?
A:  Concrete’s projected strength is rated in psi or pounds per square inch.  This is rating given to a concrete recipe based on past tests with this mix.  A certain mix may have more cement or other additives in it that make it stronger or make it cure faster.  A hardened sample is pressed to see how much pressure it takes to fracture the sample.  This same recipe is then duplicated at the plant when someone asks for concrete of this strength.  I highly recommend using 3,000 or 3,500 psi concrete for residential slabs if you can afford it.  It will pay you back by lasting longer.

Q:  What is concrete’s Slump?
A:  The slump is a measurement in inches that is used to describe how thick the concrete is.  The wet concrete is placed in a small tub with a gate at one end.  The gate is opened and a measurement is taken of how far the concrete “slumps” out of the trough.  (It’s a little more complicated than this, but you get the idea.)  The higher the number, the runnier and wetter the concrete.  The lower the number, the stiffer the concrete.  Most concrete delivered to job sites is 4” slump.

Q:  How much concrete do I need for my job?
A:  Concrete is measured and ordered by the cubic yard.  This means measuring the area you intend to pour and doing the math.
            A 12’ x 16’ patio that is 4” thick would look like this
            12’ x 16’ x (4”/12”)
                        or
12’ x 16’ x 1/3 = 64 square feet
64 square feet divided by 27 square feet per square yard
            Or
64  ÷ 27 = 2.37 yards of cement

Q:  What quantities can I order?
A:  The smallest quantity that a plant will generally send out in a truck is 1 ½ cubic yards but you will have to call your local cement plant and ask them what their policy is.  You can also expect to pay an additional short delivery fee of up to $100 for any order less than 5 yards.

Did I miss anything in this list? 
Let me know and I’ll do my best to answer it.

3 comments:

  1. Dan, that patio looks great... interesting article.

    ReplyDelete
  2. Your patio seems like the kind of thing I want in my yard. I have the rectangular strip of patio under the deck. However, I want to bring it out further into the yard so my guests can have a place to congregate when they come over for dinner.
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  3. My driveway is so cracked and chipped. My husband and I were talking, and we think it would be a good idea to repour the concrete in our driveway. We want to know what to expect after the job is done, so this was a great article to find. It's good to know that it will crack no matter what, and that we should expect that.

    http://www.planitdirt.com/

    ReplyDelete