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cracks in RMC

 
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ssv
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Joined: 17 Sep 2009
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PostPosted: Sat Sep 26, 2009 7:15 am    Post subject: cracks in RMC Reply with quote

Dear Sefians,
        
       First of all i want to thank sefi, for im learning a lot through sefi.

my ?
why cracks in RMC
how to prevent
how to rectify

please explain

thanks
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suraj
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Joined: 17 Apr 2008
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Location: NCR Faridabad, E mail suraj_engineer@yahoo.co.uk

PostPosted: Sat Sep 26, 2009 6:15 pm    Post subject: Cracks In Ready Mixed Concrete? Reply with quote

Dear All,

Cracks In Ready Mixed Concrete?

The originator has raised a blank question with no practical reference.
Cracks in either in situ mix or procured ready mix, are liable to occur that are acceptable within certain range. Cracks formation is a natural part of concreting procedures depending on the existing circumstances of environment & the product deliveries. This discussion is other than hair cracks formation that is caused by instant shrinkage.

Ready Mix Concrete production & its inclusion is a high profile technology not fully known to the ours working civil engineers. The involvement of scientific basis relating to the material management makes all the difference. The knowledge of in depth material science does play a significant role in connection with the performance practical behaviour of concrete that too never remains constant for long.

In addition to that, the mechanical aspect of the batching plant operations also adds to the problems generated on the procured concrete. Rush on the ready mix supplies is always intense that sometimes various mandatory activities of plant controls are played with which must be avoided but virtually, it is not in the control of the concrete executive engineers who depend on the outcome of the sample test on the soft as well as on the hard concrete.

GGBS or PFA or Microsilica adds to the flavour of producing a good mix for the structures, yet, it depends on the quality standards of these additives or replacers of cement. In the absence of microsilica, few additives are used to serve as plasticisers or self settlers for concrete. Sometimes, calcium nitrite is also used in concrete for serving as corrosion inhibitor.

On top of these, the degree & competency of the receiving concrete from the plant, transporting, placing & compacting make a lot of difference. The concrete control personnel & crew are required to be well trained to understand the pouring procedures which I think, is one of the significant factor leading to substandard work in placing the concrete.

The environment & the weather conditions too make an impact on the produced quality of concrete. Adverse weather may mar the concrete while too cold would not work for a concrete meant for hot weather condition. Application of right vibrator & its right use makes another difference but It is not understood in a right spirit by the crew that are ill trained. The pour batch after placing is pulled or pushed by the crew members using the vibrator & it is sometimes tedious job to control team keeping in view the speed of concrete supply & laying laying unless the crew is trained by a right instructor.

The important point from the mixing plant to the location of the placement, various steps are involved. Plant manager must be advised about the description of the element of pour weather it is raft, or it is footing, or it is columns, thin members or slabs etc. so that the required slump could be produced accordingly. The frequency of loading has to be instructed to the plant otherwise, there would be a queue of the trucks on the site awaiting the discharge.

Plant operator has to control all required aspect of the mix grading & ingredients quantities by weight though, the plant system is fully computerized, yet, there are the chances of erroneous work that should be audited from time to time.

All ingredients supplied to the plant needs regular testing under full range definition at frequent defined intervals. Source of the cement has to be fully under control since there could be any shortage of this item & the plant in commercial interest goes for an alternative without an approval.

The declaration on the truck document should be fully completed while loading indicating the production time, production slump, production temperature, production quantity, quantity of the additives etc all recordable parameters. The leaving time from the plant has to be invariably shown on the document slip,

On the receipt end, the truck arrival time, the slump, the air voids, the temperature all must be inspected properly before allowing the truck to station at the pump to supply the concrete on proper location.

The concrete cannot be place on the temperature outside the extreme specified limits. The weather condition & the winds & the sand storms etc must be taken care of. The weather temperature & the humidity should also be checked in. The member quantity & if so required differential rising & falling temperatures are to be ensured for the purpose of recording to ensure the permissible thermal gradients in the poured concrete mass.

Procedures to be employed for the curing purpose have to be decided before the pour commencement by all the involved parties. Instant protective curing as well as water full scale curing have to be decided accordingly. Use of Hessian & the plastic sheet are to be decided if so required by the existing weather conditions.

The concrete placement duration has to be planned weather in the early morning hours or within night hours or in the evening if the weather so dictates. For some special cases, the shedding of the proposed location area has to be installed prior to receive the trucks.

Sequence of the concreting is also too significant   on the planned member. The rotation of the placing has to be decided accordingly so that the cold joints are not allowed to form anywhere within the pour. To meet any emergency, preparatory forms should be available to be erected immediately in case of failure if the machinery. These forms are generally called stoppers. Flash setting of the concrete has to be avoided. Proper vibration of the concrete laid has to be done in sequence without mixing any portion of the concrete & over vibration has to be avoided.

Based on the experience & the site observation, the instant curing has to be applied so that the cracks are prevented as mush as possible. Though, even after resorting to all procedures, these formations are expected but should be not delayed to repair them the same day or the next day by grouting all portions wherever these are seen. Excessive cracks formations are not acceptable that may invite a thorough investigation of the production. In case the cracks are visible in the hardened concrete, resin injection using are recommended that is an expensive procedure.

Should the specification require an application of power float, the concrete after initial setting requires a few hours depending upon the mix quality that must be discussed with the plant management.

Regards

_________________
Thanks & Warm Regards
IntPE(India)Suraj Singh FIE Civil
Engineering & Arbitration

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mkalgal
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PostPosted: Sun Sep 27, 2009 10:03 am    Post subject: Reply with quote

Thanks to Er. Suraj Singh for a detailed reply covering almost all the aspects.
Just want to add that light sprinkling and covering with a plastic sheet/tarpaulin would go a long way in preventing cracks on the surface of slabs
Dr. Kalgal
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ssv
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Joined: 17 Sep 2009
Posts: 21

PostPosted: Sun Sep 27, 2009 1:52 pm    Post subject: Reply with quote

Dear Sirs,

my sincere thanks to Er. Suraj Singh & Dr. Kalgal.

ssv
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