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PLASTIC SHRINKAGE CRACKS AND PREVENTIVE MEASURES

A Field Exper ience Report

Structure Slab of a Building Date 05 Feb 2009Pour Duration 10.30 AM to 01.30 PM 3 hoursGrade M20 Volume Supplied 30 cum

Condition of Early Curing dur ing Inspection - NO

Photo 1 : The portion with age of 2 to 3 hours after casting, starting to have thincrack appearing

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Photo 2 : The portion with the age of 3 to 4 hours after casting, experiencing

wider crack or precisely saying crack widening

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Photo 3 : The crack appearing in Photo 2 got closed by gentle compaction with

wooden runn er The ramming oozes out water th at helps concrete to jo in again

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Photo 4 : Prevention better than curing?? ? Curing is the Prevention !!!

Other observation made:

Very uneven and loose surface due to lack of final compaction with traditionalLong & Thick wooden Runner after the process of vibrating needle compactionfollowed by levelling.

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Note :

Tamping with the Traditional Long & Thick Runner not only makesloosely levelled (after needle application) surface stronger through gentlecompactionbut alsobrings out little amount of water from concrete to the slab surface, whichforms a protective coat for the surface and reduces its vulnerability toplastic shrinkage.

Stepping over the stiffening surface will also lead to cracks as surface isput under tension by the foot steps.Hence sprinkling should go along with casting and not necessarily aftercompletion of the casting.

If there is any need to step over the stiffening surface, broad and thickwooden planks shall be laid over the surface to prevent any local settling& resultant cracks.

Handling Delay in setting :

If delay setting is encountered, more importance shall be given to sprinkling and

covering of surface.

It shall also be ensured that the surface is not disturbed at all in the form ofinspection.

Concrete that is having delay setting issue (extending beyond 16 to 20 hours)will have water that has not been utilised for stiffening and hardening(Hydration) process.

This water, available across the wider area, will evaporate if the surface is notcovered.

This evaporation of surface water will eventually lead to plastic shrinkage andcracking.

Curing .. C.o..nt.i.n..u..ou..s.:

Depending on the climatic conditions the appropriate frequency of (intermittentcuring in the form of) sprinkling shall be maintained.

Once the surface gets hardened, the process of continuous curing in the formof water ponding shall begin immediately.

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Adverse effect of Plast ic Shrinkage Cr acks :

Though these cracks appear invariably on the surface with the depth of about10mm or less, neglect of continuous curing @ hardened state will make thesevulnerable to drying shrinkage and restrained effects. And consequently these

may extend to full depth of the slab, especially when slab thickness is less than150mm.

Filling the cracks with Cement Slurry???????

As shrinkage is the inherent property of cement particles, it is not advisable to

fill the cracks with cement slurry in the event of report of cracks @ hardenedstate of concrete.

Remedial measure :Customers shall be advised to approach the APPLICATORS through firms likeFOSROC or BASF, who can carry out appropriate measures to make thestructure durable.

Responsibility of rmc manufacturer to prevent plastic shrinkage cracks:

Good workabilityMaintaining good workability in Concrete mix is essential for better handling &effective compaction of concrete. The slump shall not be less than 120mm @site for slab structures.

Retention of workabilityConcrete shall be designed to retain its workability for the process of pumping,compaction, levelling & Finishing loss of slump shall not be more than 20mmin half an hour duration.

Gradual setting of concreteConcrete shall be designed in such a way that it sets gradually after casting andnot @ a faster rate.

CommunicationCommunication on Handling of Fresh Concrete & control of Plastic shrinkagecracks shall be made @ all level and on more number of occasions, especiallyduring the execution of pours.