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.
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
30m long tunnel walls providing LOTS of restraint ==> cracking.
How much longitudinal reinforcing (parallel to tunnel walls) was used?
RE: Horizontal Cracks on Slab
The placement temperature was relatively high. Was there any wind?
What is the spacing of control joints?
RE: Horizontal Cracks on Slab
Dik
RE: Horizontal Cracks on Slab
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
RE: Horizontal Cracks on Slab
What horizontal reinforcing did you have?
Dik
RE: Horizontal Cracks on Slab
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
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
RE: Horizontal Cracks on Slab
Dik
RE: Horizontal Cracks on Slab
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
Dik