What could be the reasons for such cracks? 1

[concreteworld]

Hello
A concrete tunnel wall (30m length x 6.5 m height x 1.2 m thick) cast using 35 Mpa triple blended concrete mix saw vertical cracks(please see attachment) from the kicker up to mid height and diminishing at top. The fresh concrete temperature at placement was kept under control at 28 Deg C. Proper workmanship measure were taken care during casting.
Please note that there is no expansion joint in the entire 30 m length of the concrete wall.
What could be the reasons for such cracks? Thermal gradient or restraint? or any other

Thanks in advance

 

[hokie66]

Drying shrinkage cracking due to restraint by the previously cast base. These cracks are inevitable, but can be controlled in width with an adequate amount of reinforcement, especially near the bottom where the cracks initiate.

 

[SlideRuleEra]

Proper workmanship measure were taken care during casting.

How about after the concrete placement? What steps were taken to cure the concrete?

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[concreteworld]

Concrete placement done by pumps and vibrated properly. Curing was done using curing compound.
The third party inspection agency concluded that the cracks are due to early thermal contraction. Are these not due to lack of expansion joint because the length of the wall is 30 meters ! Please see the attached photo of the wall

 

[hokie66]

What sort of curing compound? The PVA ones do no good, and are just a waste of time.

"Early age thermal contraction" is a European term, and is the same as drying shrinkage. Your reference to "expansion joints" is not logical, as only contraction joints would have been useful in this case. But these restraint cracks tend to form at about 3-4 metre intervals, so that would be a lot of joints, and you still might get cracking that doesn't follow the joints.

How much horizontal reinforcement do you have in the wall? It takes about 0.6% Ag to control cracks when you have heavy restraint such as provided by that base slab.

 

[epoxybot]

I agree with hokie66 that this looks like a drying shrinkage crack. Different cracks have different features and the photo is similar to what one can find if you search for examples of drying shrinkage cracks. The concrete had taken a hard set but wasn't so hard that there wasn't some strain relief, which is why the crack intermittently stops & jogs over. Drying shrinkage cracks usually look like a tear in the fabric of the concrete. They may or may not be full thickness cracks.

 

[rapt]

Actually early thermal cracking is an addition to normal drying shrinkage cracking. The concrete expands due to the heat of hydration while still plastic. So the concrete is in an expanded state as it sets and ties into connected concrete elements.

It then cools and shrinks due to the reduction in temperature plus the drying shrinkage effect.

Agree with Hokie that the minimum reinforcement in this sort of situation is at least .6% possibly up to 1% if the section is thick resulting in higher thermal effect.

If you have foll;owed standard BS8110 minimum of .13% in a wall, you have no chance.

 

[dik]

What was the specified slump and strength?

Dik

 

[hokie66]

dik,
He did say that it was 35 MPa, thus a normal strength concrete.
It is 30 metres long, with no attempt at crack control joints. And we don't know how much reinforcement. The cracking was inevitable, but for those cracks to be uncontrolled in width was not inevitable.

sarabia05,
Do the cracks affect the serviceability of the tunnel? In other words, does water leak through? Is there an external membrane?

 

[dik]

Thanks Hokie... I was wondering about the in-situ strength and slump... wondering if anything was out of whack... high water content, etc. maybe aggravate the shrinkage.

Dik

 

[dcarr82775]

30m long pour restrained by the base slab = restraint cracking

 

[dik]

The cracks should be at a uniform spacing of 5 or 6m o/c...

Dik

 

[JohnnyN]

Im also leaning toward restraint/thermal (restraint cracking is partly due to thermal change)

Was your mix design hot? by that I mean did you design it to reduce thermal issues? (slag,silica fume, cold water?)



Johnny at cast stone and precast concrete

http://premier-stoneworks.com