Concrete Shrinkage Calculation

[SteelPE]

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?

 

[Ron]

Depending on the type of coarse aggregate you have in the concrete, the shrinkage strain will be somewhere around 800 x 10^-6 in/in.

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.

 

[JedClampett]

Ron has good advice for calculating the width of cracks.
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.

 

[Ron]

JC is right about the slowdown in cracking and crack growth. About 75 percent of the ultimate shrinkage will occur in the first year of age of the concrete.

 

[SteelPE]

Thanks guys,

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.

 

[hokie66]

Just curious...what was the intended spacing of contraction joints? If a wall is restrained at the base, it will crack at about 10' centres no matter what. More reinforcement will control the crack width, but not prevent them.

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.

 

[RHTPE]

I guess next time the contractor will put the contraction joints shown on the construction documents.

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

 

[SteelPE]

hokie66

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.

 

[yakpol]

Alternative to contraction joints is more reinforcing, which will not completely prevent cracking but will reduce opening and spacing between the cracks.
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.

 

[slickdeals]

Just out of curiosity, what are the values for restrained walls/slabs in the BS and AS codes?

I know ACI 350 has a requirement of rho = 0.0050 when movement joints are not provided.

 

[yakpol]

Here's the relevent chapter from AS3600. Similar ratios, but not as simply given in BS.

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.

 

[Ron]

steelpe....agree they are not likely structural. Look at the cracks and you will see that they are likely to be wider at the top (less restraint) than at the bottom, though probably not much considering they are not very wide to begin.

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.

 

[SteelPE]

Ron,

Attached is an example of one of the cracks.

 

[Ron]

Yep...seen a bezillion of 'em! Butter the crack (no comments please!!) then coat with elastomeric.

 

[hokie66]

Ron is right that the cracks are wider at the top than at the bottom, but only if the crack goes all the way up. They actually initiate at the bottom, because that is where the restraining force is. Sometimes they stop before reaching the top. Depends on the geometry. Just saying...

 

[ishvaaag]

Both attached masonries and monolithic concrete are to show this kind of crack on structural and rheological movement if one does nothing about. For doubly reinforced walls 1 ft thick, I also have seen tons at about 5.5 meter spacing - and even then, of better behaviour than badly executed jointed walls with water stops.

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.