Cracks on Basement Wall 1

[ATHAN2]

I have been supervising two building projects with basement and I have noticed that cracks on the basement walls are always occurring. This problem always give us a headache because of the delays being incurred during rectification works. We are using PU Grout to sealed it off and prevent the water from seeping through the cracks. We are usually using a waterproof concrete with the strength of 40 N/mm^2, thickness of the wall is ranging from 250 mm to 400 mm. Can you give me an idea on how to prevent this problem? Thank you in advance.

 

[BRIS]

Factors controlling cracking include:
1) The percenatge reinforcement and the diameter and spacing of the bars. concrete always cracks. What you want to do is to distribute the cracks into a large number of small cracks rather than a few large cracks.
The percenatge of reinforcement must be sufficient to prevent the cracks opening. Small diameter bars at small spacing will give better distribution of cracks.
2) spacing of construction/contraction joints.
2) Concrete quality - water cement ratio.
3)temperature and Curing.

using, small daimeter closely spaced reinforcement, reducing water cement ratios, reducing joint spacing, controlling mixing and placing temperature and providing adequate curing will all reduce cracking.

If you can provide a bit more information on the mix, the design and the site conditions it should be possible to be more specific.

brian

 

[dik]

Lots of questions...

Is the wall reinforced? How long is the wall? What is the concrete slump? What sort of aggregate? Do you need concrete at 40MPa? Are you stripping too soon? Why is the wall so thick?

Can you go to less cement, ie a weaker mix or more aggregate? For the normal flexure encountered in a wall, concrete strength has little bearing on flexural strength or is the higher strength due to other reasons?

Rapid drying also increases the shrinkage; could this be a problem?

 

[ATHAN2]

Our design mix per cubic meter are as follows:
40 N/mm^2 @ 28 days; waterproof concrete
Cement (OPC) 430 kg.
Water 167 liters
Fine Aggregates 735 kg.
Coarse Aggregates 1000 kg.
Admixture (S'pruf B): 2.58-3.23 litre/m^3
Slump : 100 +/- 25mm

Design of Basement WallBased on BS Code)
For 650 mm thk:

Main Vertical Bars: T32-100mm spacing
Hor. Distribution bar: T16-150mm spacing with T10 hook @ 600mm c/c

For 350 mm thk:

Main Vertical Bars: T20-150mm spacing
Hor. Distribution Bars: T13-150mm spacing w/ T10 hook @ 600mm c/c

Site Condition:

STP value = 100

 

[Maury]

AS others have said, you can't stop concrete from cracking. The best you can do is limit the width of the cracks by using reinforcing. If water infiltration is the concern, add a membrane on the outside of the wall before backfilling. If the aesthetics are the concern, use a cosmetic treatment to cover the wall on the inside to hide the cracks.

 

[ATHAN2]

The building is a two level basement. It is a perimeter basement wall with the lenght of one side of 93 meters. Coarse Aggregate Size 20mm. The designer assume that the wall (free standing) will support the external horizontal forces (soil, water, etc.).

 

[dik]

Your mix doesn't look odd...

Is the cracking vertical (assumed), or horizontal? Can you introduce vertical control joints at 10-15m centres; these can be caulked? Is the wall properly drained? This can significantly reduce design forces and remove water from the outside face of the wall so the cracks are less significant.

I'm not familiar with the T13, T16, T20, T32 bar sizes; is this the equivalent metric diameter? T32@100 seems like an awful pile of reinforcing. Is only the one face reinforced? Our code generally requires that anything more than 250 be reinforced in both faces.

What cover? Could the large vertical rebar telegraph a crack to the outside surface?

 

[BRIS]

cement content is high - BS 8007 for water retaining concrete allows a aximum of 400kg/m3 for RC.

 

[Bbird]

I had a misfortune of looking at a new large palace basement wall, which has a series vertical cracks (0.1 to 1mm) at regular 2m intervals. Horizontal cracks if exist are not significant since the wall is constantly under compression. Cracks in basement are not often a major structural issue but the ingress of water can be an embarrassment and the corrosion of rebar then becomes a problem.

It is my belief that the building superstructure can act as a restraint preventing a free thermal movement in the wall. To cure the problem the horizontal reinforcement has to be increased, especially for the 93m length. Providing crack control joints at close interval can also cure the problem but this may not be feasible for water tightness. In the palace basement the crack control joints were specified at 7.5m intervals but were installed between 15-20m instead.

The T32 @100 bars are designed to resist the vertical load and do little to the vertical cracks. The horizontal distribution steel in this case should be designed for resisting thermal stress from maximum temperature difference experienced by the basement. The thermal movement

 

[ATHAN2]

Dik,

T13,T16 is referring to the size of rebars based on BS Code. "T" denotes high tensile reinforcing bars with the specified characteristic tensile strenght of 460 N/mm^2. 13,16,etc are diameter of rebars in mm. Yes, its a double face reinforcement with concrete cover of 40mm. Cracks are running vertically (.5mm-1mm) with the average spacing of every 5 meters.

 

[BRIS]

I assume the horizontal T16 @ 150 ccs are in each face. The percentage is thus 0.42%. This is probably just ok for the high cement ratio you have.

Max Crack spacing for T16 dia bars should be about 1.5m but you have 5.0m. At 5.0m soacing the maximum crack width will be about 1mm which is what you have got.


I suggest you look at the mix - water cement ratio is low but slump of 75 to 125 mm is high. Aggregate cement ratio is low. Cement content is high - and I suspect that what you have got is not what you say it is. Consider reducing the cement content.

I am not sure what length of wall you are pouring as one pour. But with the rebar and concrete strengths you have you need to limit lengths to 15.0m. You could change the t16 @150 to T12 & 80 ccs.

If you really are trying to obtain a waterproof basement - it is unlikely that you will achieve it with this design. Either redesign to BS 8007 (assuming that you are in UK) or provide tanking and forget the cracks- Tanking as suggested by Maury is the obvious solution.


best of luck - Brian

 

[ATHAN2]

Thank you very much to all of you for giving me wonderful ideas. I can use it for my next project.