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Horizontal Cracks on Slab
4

Horizontal Cracks on Slab

Horizontal Cracks on Slab

(OP)
Hi,

There are horizontal cracks (0.1 to 0.5mm wide)running horizontally every 3-4 meters on the top of a metro cut and cover tunnel slab which is 0.6 meters thick. The cracks run full depth of the slab with water leaking at the bottom.
Concrete Information:
Mix is OPC+FA+MS C35 ASTM Slump 200mm
Curing & Placement:
Water curing and placement as per standard practices.
Temperature of Fresh Concrete:
Between 25-30 Deg C during placement.
Finishing:
Initial screeding and final finishing by helicopter (mechanized trowelling) to give a smooth surface.

The structure is slab (simply supported on tunnel walls) 30 m long x 6m wide x 0.6m thick

Please share your opinions on the nature of cracks and possible reasons.

RE: Horizontal Cracks on Slab

Restraint-to-shrinkage cracking.

30m long tunnel walls providing LOTS of restraint ==> cracking.

How much longitudinal reinforcing (parallel to tunnel walls) was used?

RE: Horizontal Cracks on Slab

At least one small section was indicative of plastic shrinkage cracking that was spread by further drying shrinkage.

The placement temperature was relatively high. Was there any wind?

What is the spacing of control joints?

RE: Horizontal Cracks on Slab

Thanks Ron, Ingenuity pretty much summed it up. I was going to add: what was the slump prior to adding Super P, that the temperature is a lot higher than we use, and what was the weather at the time of placement? For 90m you should have 5 control joints for the length.

Dik

RE: Horizontal Cracks on Slab

(OP)
Thank you all for your comments.
Ambient temperature at the time of placement = 36 Deg C
There were no windy conditions in general.
Each tunnel section is 30 meters long (without any joints). The joints are between each section. So the spacing is 30 meters.
Superplasticizer was added at the plant in wet mixing. Before addition of SP the control slump was 75-100mm. W/C = 0.38

The pattern of these straight cracks is almost in every section. I view this as having a structural design issue. Please see the photo of crack (typical) which is full depth (0.6m) in my above thread.

RE: Horizontal Cracks on Slab

I would not consider this to be a crack with structural implications. This looks like a typical drying shrinkage crack. Considering you have no control joints, I am not surprised at this. Had you put control joints in, you could have sealed them with a flexible sealant or even a semi-rigid epoxy. Now you have to route each crack and fill it with either sealant or epoxy.

RE: Horizontal Cracks on Slab

Urethane injection for the cracks and a good waterproofing on the outside... cracks appear to be too wide for Xypex or Vandex...

What horizontal reinforcing did you have?

Dik

RE: Horizontal Cracks on Slab

(OP)
Hello All,

Please see the crack mapping report for a section of tunnel slab. The pattern is typical cutting the length perpendicular. The red color cracks are full depth (60 cm)
Kindly put some more light on the causes and reasons for these cracks. Will be grateful if supported by some technical literature or standards.

Thanks in advance

RE: Horizontal Cracks on Slab

Quote (sarabia05)

Kindly put some more light on the causes and reasons for these cracks.

sarabia05: I think all responses thus far as still appropriate causes/reasons, based upon the crack maps you show. What is/was the longitudinal reinforcing quantity?

RE: Horizontal Cracks on Slab

Agreed shrinkage restraint. Minimum reinforcement parallel to the walls should have been .6% with probably closer to 1% for good crack control (.1 - .2mm), half each face.

RE: Horizontal Cracks on Slab

rapt... I'd go for proper control joints and 0.2% or maybe 0.3% rfg... likely

Dik

RE: Horizontal Cracks on Slab

Dik,

I do not think that control joints at 15m spacing will help if it is tied continuously into thick walls on both sides!

RE: Horizontal Cracks on Slab

My error... the joints should be at 5m spacing... should be 17 joints or 18 segments.

Dik

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