Classification of crack

[Contech2011]

Hi All,

Please classify the crack shown in the uploaded pic. Other data is as below,

1. Concrete grade 45 Mpa (490 Kgs cement)
2. Pumped @ 100 meters
3. Initial water curing not done. Curing membrane was applied late.
4. Slab thickness 40cm & 70cm.

Also recommend measures to control such cracks.

Thanks

 

[Ron]

Tough to accurately characterize such from a single photo, but here are my comments....

Assuming the concrete in the core was newly placed within the last few weeks.....

1. Since the crack occurs only from the top and does not extend through the cross section, it is likely a plastic shrinkage crack.

2. The crack is quite wide at the top, thus also indicating plastic shrinkage.

3. The crack does not show tensile failure of the coarse aggregate, so it occurred prior to the development of adequate bond strength between the paste and aggregate...another indication of plastic shrinkage; however, this can also occur in typical early-occurring drying shrinkage cracks and will occur in weak concrete mixes as well. Also occurs when coarse aggregates are contaminated.

All in all, my guess is plastic shrinkage crack.

How to stop? Use proper concrete mix, designed for the application, control the placement techniques, prevent drying at the surface....in short, use good concrete technology controls.

 

[hokie66]

Is this a core through one of the cracks from your other post? Agree with Ron that it is a plastic shrinkage crack. Amplifying on his recommended measures about placement techniques, inadequate/incomplete compaction and inattention to finishing have left these cracks which could have been closed while they were still plastic.

 

[Contech2011]

Thank you for your valuable comments. The pic is not from my other recent post. This concrete was casted a month ago using static pump. The slump ex-pump was 100-120mm. You are absolutely right that these cracks could have been closed while they are still plastic, but masons keep complaining that they cannot work on the crack (finishing operations) unless the concrete is still wet...The problem is they are waiting enough time with the concrete surface uncovered for the concrete to achieve initial set, but during this more cracks are developing making situation worse...By the time all the surface moisture is lost (top dry condition), the concrete beneath the surface still is wet (foot can be pressed inside)....

We recommended the contractor to immediately after intial finishing, cover the concrete with wet burlap and plastic sheets...wait for 1-2 hrs till the concrete can take some load...then remove the coverings and late finish the concrete using power float, hand trowels as required...upon late finishing re-cover the burlap...continue sprinkling cold water and cover with plastic sheets.. for a minimum of 3 days...Is that sufficient??

 

[BigH]

Plastic shrinkage cracks were common on the job I was on in India - there was little if any attention paid to hot weather concreting and curing was not immediately carried out. On one contract when the contractor cured with both the curing compound initially and immediately reinforced the curing with wet curing (burlap) we had none. Should look at the placement factors (temperature, wind, etc) and what measures (e.g., misting) was used/not used.

 

[Contech2011]

Mr. Ron and others your comments plz on my last post...

 

[BigH]

When I worked in India, HCC had one of the most experienced and knowledgable experts in concrete in Mumbai. Why not discuss this with him?

 

[Ron]

hccindia....I would expand the wet curing to 7 days instead of 3 days.

With as much evaporation as you describe, you're going to have to provide ancillary means of moisture protection. As I mentioned before, set up a fogging system and employ it immediately after screeding.

 

[hokie66]

Pumping concrete 100 metres can add to the problem. I've never seen it pumped that far.

I think the additives have been overdone, so that the retarding effect is too much. Waiting 2 hours to finish is extraodinary in hot weather conditions.

Spraying aliphatic alcohol on the surface immediately behind the screeding and bullfloating will help to prevent premature surface drying.