Cracks in HPC

[karma134]

Dear friends

I work in a Hydropower Construction company for the client. This is my problem.

We have a 90 mtrs *3 chambers surface desilter chamber.So now the struture has an inclined wall near the invert about 1.2 mtrs of ht and running both ways of the chamber.We started pouring M50A20 grade concrete along this patch say 30 mtrs at once since the construction joint is at after every 30 mtrs interval. We have seen cracks along the reinforcement bar cutting the whole width and depth of the concrete.
The crack appears on each shutter(MS shutter plate 1.2*2.5 mtrs. YOu can also view an image to see waht i am talking about.
cement: 480 kg(OPC 43)
water :156 kg
Micro SIlica fume : 38.0 kg (Elkem)
20 mm(crushed) : 560 kg
10 mm(crushed) : 560 kg
crushed sand : 600 kg
Super plasticiser : 3.0 kg(TAM cem)
SLump : initial collapse to 150 mm upto 2 hrs
Slump flow : 480- 550 mm

Possible causes: 1. Autogeneuos shrinkage????
2. Pouring Method: The concrete is poured through a concrete pump. The pipes rests directly on the main rebars. So I assume that the stoke of the pump disturbs the hardening of the concrete. ????

KINDLY HELP ME THIS PROBLEM.

 

[Ron]

Based on your description and your sketch, these appear to be drying shrinkage cracks. Reinforcement will not prevent drying shrinkage, it will just hold the cracks tightly together.

If the environment of the chamber is aggressive for steel corrosion, the rebar should be protected, either by coating the concrete or by epoxy injection of the cracks.

 

[JedClampett]

All concrete cracks, even HPC. Your CJ spacing of 30 meters seems pretty agressive. Repair the cracks with epoxy injection.

 

[karma134]

Thank you very much for the advice.We have already asked the contractor for epoxy treatment of the cracks.

But I also want to know how to ensure that these cracks doesn't form again. Is it in the mix design? The concrete is flow-able concrete as you can see from the slump flow. Do I need to change the construction technique. You see we opened the shutter as soon as the concrete got hardened and started curing it continuously with jute blanket for a week.

SO kindly help me in this area.

 

[karma134]

I also wanted to add that the cracks cut the concrete through the whole width as if somebody sliced it with a knife.We took out the cores to confirm it.

 

[hokie66]

The "flowable" concrete probably aggregates your problem, but Ron is correct...they are drying shrinkage cracks. You didn't indicate what the wall is cast on, but presumably it is restrained by a previously cast element which has already experienced shrinkage. When the wall shrinks, it has to crack. The degree of control of these cracks depends on the amount of reinforcement used.

 

[hokie66]

I meant aggravates, not "aggregates". As the shrinkage cracks follow the transverse reinforcement, they likely started as plastic settlement cracks, then these cracks served as stress risers for the tension shrinkage cracking which followed. How was the concrete consolidated? Relying totally on "self-consolidating" concrete is not going to work. You still have to compact, and to close plastic settlement cracks, compaction has to be repeated after a time lag.

 

[karma134]

Thank you guys for your valuable information. As to your questions:

1. the wall is cast as you said on a concrete structure both at the invert and one side.
2. the structure has only reinforcement on the cover spacing @ 6" both ways. Minimum cover provided is 60 mm.

3. We use needle vibrators and as well as shutter/plate vibrator.

4.the concrete as you said is flowable concrete since we are using PCE based admistures. We have also modified our mix design and made it just workable ie say with a normal flow.

 

[dik]

Your CJ spacing is too much, would have used 15m tops. I cannot tell from your mix design what the slump of the concrete is for non-superP mix. Also your aggregate size could have been much larger.

Dik

 

[Hoaokapohaku]

You might consider a Shrinkage Reducing Admixture (SRA). Since you mentioned you are using a TamCem super P, I looked at their website and didn't find one, but perhaps one or more of their many superplasticizers is formulated for shrinkage reduction. I use BASF products for this purpose, but I think most or all admixture vendors have a similar product. This isn't advertising, but as an example you can look at a SRA from BASF at

http://www.basf-cc.com.au/en/produc...aguardAS21/Documents/Tetraguard_AS21_TDS.pdf.

Also, I notice the amount of cementatious materials for your strength requirement seems very high. If I were to use those mix proportions using locally available materials, my compressive strength would likely be more than double your requirement. With that much cement, volume changes due to the heat of hydration may be contributing to your shrinkage cracks. A design that maintains your 0.30 water/cementatious (w/c) materials ratio but with reduced cement content might be helpful. If you use an SRA, you can probably increase your w/c ratio. This would vary with local materials, but as an example, in my location, a mix with 385 kg/m2 cement, a w/c ratio of 0.40 and using an SRA would probably be satisfactory.

 

[karma134]

This is the link to the super plasticiser we are using

http://www.tam.uk.com/media/file/product_files/S4-TamCem 23SSR V1UK-10.pdf

Well regarding the local materials we are using the quarry has a rock mass mixture of quartzite,sandstone and gneiss. We get the impact value, crushing and abrasion value < 32 %. As you can see we have considerable amount of mica content. However till now we haven't seen any defect in concrete due to alkali aggregate reactivity.

Thank you guys I will really consider reducing the cementious material content. And i think the shrinkage reducing admixture is the best solution which i can see rite now.

Thank you guys. Any comment is highly appreciated.

 

[Hoaokapohaku]

Hmmm...your type of aggregates tyically have high thermal coefficients of expansion. You might find the following useful:

https://docs.google.com/viewer?a=v&...HTyRYE&sig=AHIEtbRWvrtMi_7REHTbpJRn5kPJ9W34TA

 

[karma134]

Thank you @Hoaokapohaku I am reading the material you have sent.