Handling bulk cement in RMC and Project sites.
In major projects and in RMC plants cement is received in bulkers as the consumption of cement is very high on a day to day basis. In such cases the following precautions need to be taken to ensure the quality of cement received before it is used in concrete production
• Temperature of cement received to be checked before loading into silos. Temperature of 45o C to 50o C is acceptable. If the temperature is high then store the cement in silo for a day or two.
• Collect sample of cement as soon as the bulker reaches site and check the consistency of cement. The test result should match with the previous batch test results. If any large variations found in consistency then do not use the cement until one day compressive strength results were obtained. This again should match with previous one day results.
• Check the fineness of cement by dry sieving and compare with the previous results.
• Ensure the brand and type of earlier stored cement in silo is the same before pumping.
DO NOT MIX DIFFERENT CEMENT TYPES AND BRANDS IN SAME SILO. This may lead to problems in future.
Test Certificates : Sampling requirements.
a) When the cement has ISI Certification mark, or when it comes with manufactures certificate, no further testing is usually required.
b) If clients still insist, one sample for every 250 tonne or batch of cement may be tested for physical properties or sent for testing to an external agency.
c) Take the samples within one week from receipt of cement and arrange to test within one week from the date of sampling.
d) Send the sample for testing at least 10 days before the cement is to be used.
e) A sample should consist of twelve roughly equal portions, each portion taken from different bag. The total weight of the sample should be 4 to 5 kg.
Problems of setting and hardening .
a) False set : Concrete may stiffen too early say with 5 to 20 minutes after mixing. This problem arises because of inadequate cooling during grinding of cement or because cement is fresh and hot from the mill. If false set occurs remix the concrete without adding water. Plasticity will return and concrete will set in the normal manner.(IS 4031 part 14 – Method of test for determination of false set)
b) Flash set : If concrete stiffens too early and does not regain plasticity on re-mixing, flash set might have occurred. The cause may be ,
1) Use of hot water ( 60 o C to 80 o C) for mixing.
2) Presence of calcium chloride, especially in admixture.
3) Contamination with high alumina cement or calcium chloride , possibly stored for emergency repairs.
c) Pozzolana and Slag cement : Concrete with pozzolana cement and slag cement hardens at slower rate. Therefore, prolonged curing is necessary.
In major projects and in RMC plants cement is received in bulkers as the consumption of cement is very high on a day to day basis. In such cases the following precautions need to be taken to ensure the quality of cement received before it is used in concrete production
• Temperature of cement received to be checked before loading into silos. Temperature of 45o C to 50o C is acceptable. If the temperature is high then store the cement in silo for a day or two.
• Collect sample of cement as soon as the bulker reaches site and check the consistency of cement. The test result should match with the previous batch test results. If any large variations found in consistency then do not use the cement until one day compressive strength results were obtained. This again should match with previous one day results.
• Check the fineness of cement by dry sieving and compare with the previous results.
• Ensure the brand and type of earlier stored cement in silo is the same before pumping.
DO NOT MIX DIFFERENT CEMENT TYPES AND BRANDS IN SAME SILO. This may lead to problems in future.
Test Certificates : Sampling requirements.
a) When the cement has ISI Certification mark, or when it comes with manufactures certificate, no further testing is usually required.
b) If clients still insist, one sample for every 250 tonne or batch of cement may be tested for physical properties or sent for testing to an external agency.
c) Take the samples within one week from receipt of cement and arrange to test within one week from the date of sampling.
d) Send the sample for testing at least 10 days before the cement is to be used.
e) A sample should consist of twelve roughly equal portions, each portion taken from different bag. The total weight of the sample should be 4 to 5 kg.
Problems of setting and hardening .
a) False set : Concrete may stiffen too early say with 5 to 20 minutes after mixing. This problem arises because of inadequate cooling during grinding of cement or because cement is fresh and hot from the mill. If false set occurs remix the concrete without adding water. Plasticity will return and concrete will set in the normal manner.(IS 4031 part 14 – Method of test for determination of false set)
b) Flash set : If concrete stiffens too early and does not regain plasticity on re-mixing, flash set might have occurred. The cause may be ,
1) Use of hot water ( 60 o C to 80 o C) for mixing.
2) Presence of calcium chloride, especially in admixture.
3) Contamination with high alumina cement or calcium chloride , possibly stored for emergency repairs.
c) Pozzolana and Slag cement : Concrete with pozzolana cement and slag cement hardens at slower rate. Therefore, prolonged curing is necessary.
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