Concrete Shrinkage Calculation
Concrete Shrinkage Calculation
(OP)
I have a project where, in his infinite wisdom, the contractor decided not to follow the plans and install contraction joints in the exterior wall. Now a year and half later we are getting calls about cracks that have formed in the exterior wall. The cracks are 1mm or less in width and occur about every 6'-8' o.c. The wall is 8" thick and reinforced with one layer of #5 bars at 18"o.c. in each direction. How can I calculate the amount of shrinkage I would expect per foot of wall length? I have seen some numbers of around 0.04% but am wondering if this number is correct.
Also, is there a product that you can apply to the outside of the wall that will cover the cracks and not allow the existing cracks to propagate through?
Also, is there a product that you can apply to the outside of the wall that will cover the cracks and not allow the existing cracks to propagate through?






RE: Concrete Shrinkage Calculation
For your application, that works out to be about 1.5 to 2mm. If you have a "hard aggregate" then a little less shrinkage...a porous aggregate, a little more.
Check A.M. Neville's book on concrete properties. It will give you some good info on this.
RE: Concrete Shrinkage Calculation
As far as your second question, there is no product that seals a crack and stops cracks from propogating. Believe me, I've looked. Once concrete has made up its mind, it's going to crack. You could epoxy inject the existing cracks and enforce joints by making 2 inch saw cuts.
But the good news is that the shrinkage is a reverse exponential function and will slow down. So if you chase the current cracks with epoxy, there will be less new ones.
RE: Concrete Shrinkage Calculation
RE: Concrete Shrinkage Calculation
I found a reference that placed the amount of concrete shrinkage at approx 400x10^-6 to 800x^-6 in/in in a textbook I had a university. Taking the average and multiplying by the crack spacing yielded the exact width that I observed (who would have thought). I guess next time the contractor will put the contraction joints shown on the construction documents.
RE: Concrete Shrinkage Calculation
Are the cracks structurally signicant or just a visual problem? You could coat the exterior of the wall with a flexible membrane or use a sheet membrane or some other cladding system.
RE: Concrete Shrinkage Calculation
I have found that many contractors have very short memories. When it comes to bidding a project, it's all about the cost and trying to beat the competition. Obviously, with greater placement size, the fewer placements will be required. Adding the detailed control joint(s) requires more labor. The trials & tribulations resulting from a previous "oops" often get forgotten unless they turned out to be extremely costly...
Ralph
Structures Consulting
Northeast USA
RE: Concrete Shrinkage Calculation
Intended spacing was to be at 20'o.c.
That's the beauty of not putting in the contraction joints. I get to say "maybe if you put in the contraction joints the wall the wall wouldn't have cracked as much as it did".
The cracks are in a frost wall that extends up beyond the SOG to form a knee wall. The cracks are in the visible knee wall. The knee wall only takes a small portion of wind load. From what I can tell, since the cracks are vertical they are not structural and therefore just causing some visual discomfort.
RE: Concrete Shrinkage Calculation
In this example the wall reinforcing of 0.2% is a minimum for unrestrained concrete by ACI. For continuous wall it will be three times as much. So, it's a trade off. There're plenty of structures built without contraction joints. Unfortunately ACI does not easy number for amount of reinforcing in restrained walls or slabs, the other codes like British Standard and AS3600 do.
RE: Concrete Shrinkage Calculation
I know ACI 350 has a requirement of rho = 0.0050 when movement joints are not provided.
RE: Concrete Shrinkage Calculation
11.6.2 Horizontal reinforcement for crack control
Where a wall is restrained from expanding or contracting horizontally due to shrinkage or
temperature, the horizontal reinforcement ratio shall be not less than the following, as appropriate:
(a) For exposure classifications A1 and A2—
(i) where a minor degree of control over cracking is required: 0.0025;
(ii) where a moderate degree of control over cracking is required: 0.0035;
(iii) where a strong degree of control over cracking is required: 0.006.
(b) For exposure classifications B1, B2 and C: 0.006.
RE: Concrete Shrinkage Calculation
For tight cracks, I would hand-route and cover with a buttering grade of sealant such as Vulkem 116, then cover the entire surface with a compatible elastomeric coating.
These will likely always be "working" cracks, so don't use a rigid repair method.
RE: Concrete Shrinkage Calculation
Attached is an example of one of the cracks.
RE: Concrete Shrinkage Calculation
RE: Concrete Shrinkage Calculation
RE: Concrete Shrinkage Calculation
Perhaps the commissioning parties should be classified on if they are going to accept these cracks as something that happens, or want to provide the neccesary means to deal with these issues; something that is not usually talked about at commission time.