Concrete cracking

[Ringoo]

Hi,

I am in the process of checking the data supplied by the contractor on the temperature measurement from the thermocouples installed at the core, left face, right face and top face of 12 ft high wall which is poured in one lift along with other walls perpendicular to it. The wall is 36 ft long and 4 ft thick. The outside face of the wall is poured against the rock. The data I am processing shows that the contractor is maintaining the temperature difference of 19 deg. C from the core to the face as specified in the specification. The contactor is also not exceeding the concrete temperature at core by not more than 57 deg C. The air temperature was 4 deg. C at the time of the pour. The whole exercise we are doing is because vertical cracks that appears on the wall.
I also checked the minimum temeperature and shrinkage reinforcement per ACI 350 which is ok.
I am not involved in the design of these walls but the conclusion I am picturing is why these cracks appears is because of:

1) No provision has been made for the contraction joints which are often required to dissipate shrinkage stresses and to control cracking. As ACI 224.3R-95 recommends a contraction joint at a spacing equal to the height of the wall for high walls and three times the height of the walls for short walls. Short walls are usually considered to be less than 2.4m and high walls taller than 3.6m.

2) Provision of too much restraint to the walls to shrinkage and temperature variations other than the foundation itself.

Do you experts think above listed points are valid or I should be looking into more information? Should I also check how much temperature difference is maintained at all the surfaces for the first five days after placing and not just only the inside concrete temperature difference.

Thanks

 

[miningman]

I was involved in something similiar a few years ago where we applied heat to the exterior form during curing ( as well as large quantities of water because we were concerned about the delta T between the interior of the pour and the exterior surface. Something else to think about.

 

[hokie66]

At what spacing are the vertical cracks occurring? How much horizontal reinforcement do you have? As you have surmised, the most common reason for vertical cracks is restraint, usually by the footing itself.

 

[JedClampett]

A couple of things:
Minimum shrinkage reinforcing per ACI 350 is quite a bit. We usually top out at #8's at 12 inches each face, no matter what the thickness. But that has nothing to do with your cracks.
A 36 foot long wall does not need contraction joints. It's on the long side (and kind of thick), but not out of range by any means.
I don't understand the statement:
A sketch would be helpful. But if you're pouring against a rigid surface, including already cured concrete, the rigid surface tends to restrain the shrinkage natural to concrete. The result is very often vertical cracks. I see it most at slab to wall joints, but I'm guessing it could happen at other places.

 

[GeoPaveTraffic]

If the concrete is indeed placed against a rock face, then it is restrained along the entire length. I would expect a lot of vertical cracks due to the restraint.

Mike Lambert

 

[Ringoo]

hokie66/Jedclampett,

My apologies for the delay. See attached crack pattern. Reinforcement is 25M @ 250mm c/c both faces and both direction. its Amin(r)=0.0067>0.005 per ACI350.The wall is pored against the rock surface. See the sketch the wall outer face is against the rock.

 

[Ringoo]

SEE ATTACHED.

 

[hokie66]

No sketch or crack pattern attached. As the wall is heavily restrained by rock, it will crack, but the reinforcement should control the width of the cracks appropriately.

 

[oldestguy]

Please give some concrete data, such as slump, cement content (bags/yd), aggregate max size. Timing pour and first cracks.

 

[hokie66]

I think those cracks are entirely as expected in a wall as described, and don't think it has much to do with differential temperature. Just normal drying shrinkage, restrained at one face and at the base. Not sure what type contraction joints you would propose, but there is no assurance that the cracks will occur where you want them.