Ready Mixed Concrete – Code of Practice

3460
Ready Mixed Concrete

As per Indian Standard code of practice (IS 4926) Ready Mixed Concrete (RMC) is defined as the concrete delivered in tailor- made condition and requiring no further treatment before being placed in position in which it is to set and harden. Instead of being batched and mixed on site, concrete is delivered for placing from central batching plant. Provisions for compliance for requisite quality of concrete made using fly ash and slag have been duly covered for the manufacturers of ready-mixed concrete in the Indian Standard Code of practice for RMC (IS 4926).

What is the time interval between production of concrete and its placing as per BIS 4926?

  • The general requirement is that concrete shall be discharged from the truck-mixer within 2 hours of the time of loading.
  • The time of loading shall start from adding the mixing water to the dry mix of cement and aggregate or of adding the cement to the wet aggregate, whichever is applicable.
  • However, a longer period may be permitted if admixtures are used or in cool humid weather or when chilled concrete is produced.
  • RMC’s have transported and placed concrete successfully even after 6 hours by selecting the admixtures carefully for extra slump retention / and by redosing for exigency. Hence, the limiting factor of 2 hours is only notional and can be stretched with the mutual consent of the Engineer-In-Charge.

What is the recommended time of discharge after the transit mixer is received at the site?

  • The contractor should plan his arrangements so as to enable a full load of concrete to be discharged within 30 min of arrival of the transit mixer on site.
  • If discharge from the RMC’s vehicle is delayed on site due to lack of preparedness on behalf of the contractor, then the responsibility passes to the contractor after a delay of more than 30 min.

What are the permissible limits on workability as per IS 4926?

Slump should be within the specified value with a tolerance of +- 25 mm or +- 1/3 of the specified value, whichever is less.

What is the relevant clause on sampling of concrete from a transit mixer?

  • After the truck-mixer has re-mixed its delivery on site, allow at least the first one-third of a m3 of concrete to be discharged prior to taking any samples.
  • Take at least 4 incremental samples from the remainder of the load avoiding sampling the last cubic metre of concrete.
  • Thoroughly re-mix this composite sample either on a mixing tray or in the sampling bucket and proceed with the required sampling.

What is the range of slump required for pumping the concrete?

80 to 130 mm is the range of slump required at the site for pumping the concrete without causing choking of the pump.

How long will the concrete be able to retain the slump after its production by opting for standard water reducing admixtures / superplasticisers?

Normally, concrete will be able to retain the slump for a period of 1 hour to 1 hour and 30 minutes, if standard water reducing admixtures / superplasticisers are used.

Can a concrete mix designed for RMC be used to produce concrete at the site?

  • Yes. The mix designed for RMC can be used in the production of concrete at the site, provided water reducing admixture/superplasticiser specified for the RMC design mix is available at the site.
  • If water reducing admixture/superplasticiser that has been specified in the RMC design mix is not available at the site, then the water content should be increased to approximately 200 Kg/m3 and keeping the same water to cement ratio, cement content should be increased.
  • In case fly ash or any other mineral admixture that has been specified in the RMC design mix is not available at the site, then the design mix has to be suitably modified with the approval of the PMC / Client.
  • The following table gives a rough estimate of the design mix to be adopted in the case of production of concrete at a site where superplasticiser is not available for a grade of M25.
Grade of concrete- M25 RMC Design Mix Tentative design mix for production at site, where superplasticiser is not available
Cement in Kg/m3 340 400
Water in Kg/m3 170 200
W/c ratio 0.5 0.5
Admixture (%) by weight of cement) 0.6 to 0.8 Nil

Is it proper to submit design mixes obtained from a third party to the client without conducting trial mixes at the site?

  • No. It is not proper to submit the design mixes obtained from a third party to the client without trials at the site.
  • The design mixes submitted by third party may sometimes contain sand contents, which may be lower than that required from cohesiveness point of view.
  • Such under sanded mixes will invariably lead to choking of the pump causing delays in the execution.
  • Trial mixes will also help in checking the yield of the concrete. The concrete produced can be tested for its yield by filling a cylindrical metallic yield container of 15 or 30 litres capacity (refer page no. 27 of IS 1199) and weighing it. The fresh concrete density measured should be greater than the theoretical fresh density assumed in the design or else the issue should be referred back to the third party, who issued the design mix

What are the probable causes for the failure of the cubes?

  • Moisture correction may not have been carried out precisely.
  • Concrete could have been delayed at the site inordinately beyond 2 hours leading to loss of slump and as a consequence, water being added to maintain a workable slump from the pumping point of view.
  • The cubes being made by inexperienced technicians.
  • Wrong practices being followed in the demoulding, identification and transportation of the cubes.
  • Faulty cube moulds and wrong batch quantity can also lead to failure of cubes.

Name the steps that could be undertaken in case there is a sudden loss of slump at the site due to any unforeseen events?

In case of a sudden decrease in the slump, which may eventually lead to choking of the pump, Redosing the mix with Superplasticiser to the tune of 0.1% by weight of cement can be done at the site with the approval of the PMC/Client. However, redosing should not be resorted to as a routine in the case of every load of transit mixer

What are the important information one can obtain from reading the batching sheet?

  • Grade of concrete
  • Cement content
  • Time of batching
  • Slump of concrete at the plant
  • Water content(to assess whether moisture correction has been carried out)
  • Superplaticiser dosage

Summary

  • The general requirement as per BIS code 4926 for RMC is that concrete shall be discharged from the truck-mixer within 2 hours of the time of loading.
  • However, a longer period may be permitted if admixtures are used or in cool humid weather or when chilled concrete is produced.
  • 80 to 130 mm is the range of slump required at the site for pumping the concrete without causing choking of the pump.
  • Normally, concrete will be able to retain the slump for a period of 1 hour to 1 hour and 30 minutes, if standard water reducing admixtures/superplasticisers are used.
  • The mix designed for RMC can be used in the production of concrete at the site, provided water reducing admixture/superplasticiser specified for the RMC design mix is available at the site.
  • The design mixes obtained from a third party may be submitted to the client after conducting trials at the site to verify the pumpability and yield of the mix