Slump reduction with time
Slump reduction with time
(OP)
Recently, on a 50%R.H. day, Temp of 75 deg. F, concrete trucks arrived with susicious consistency. The maximum allowable slump for the prduct code as submitted for approval was 5". Approx. 1 cy of concrete was discharged onto the ground, and a sample was taken for a slump test. The tested slump was 6.5". A second sample was taken and tested immediately and it too was a 6.5" slump. Approx. 30 minutes later, and 70 minutes after batching, a slump was taken (again twice), and the slump was 4.5". There was no increase in concrete temp (72 deg), the truck had continuous drum revolutions the entire time, the only admixture to the mix was a low dose (3.0 oz/cwt)of low range water reducer (pozz 80). Is ACI min. 5% load discharge before testing suspect material an unreliable standard?? I believe the truck had been tampered with, but no proof. The concrete was allowed to be placed. Anyone have comments???





RE: Slump reduction with time
I acknowledge that there are cases whereby the mixer could have been stuck in a traffic jam or has driven a couple of times around the block before arriving on site, but you cannot be held to account for this. However, if you are aware that something untoward is happening and the test has failed then you now have a responsibility to reject the concrete.
Be aware that factors of safety exist in the design of the structures to allow for variation in mix strength, however, you are undermining this if you willfully allow concrete into the pour which is outside specification when it arrives on your site.
Regards
Andy Machon
RE: Slump reduction with time
Rejection of the load appears to have been justified. I have allowed a truck to undergo mixing, at the 'charging' rate of revolution for up to 70 revolutions if, and only if, I suspected that inadequate mixing had occurred. The final slump test should have been taken within 5 minutes of the start of additional mixing.
I am wondering if the correct additive was mentioned in your description. The addition of a water reducer should not have decreased the slump.
RE: Slump reduction with time
RE: Slump reduction with time
If you pouring a slab with dry mixes, You are killing the finisher and the finish will not be smooth. If you place a wet mix against a dry mix on the slab, that where it will crack. If you rotate for thirty minutes and then place the mix, a cold joint will occur. Holding a truck because of 1 1/2 increase of slump taken off the tail is not good practice. The concrete will still meet psi specs. The purpose of the slump is to provide a guide for uniformity of the mix. On a five inch spec, try to hold between a 4 and 6 do not sweat the small stuff and keep the placement moving.
RE: Slump reduction with time
RE: Slump reduction with time
RE: Slump reduction with time
Also, what kind of structure were you pouring?
RE: Slump reduction with time
RE: Slump reduction with time
Secondly as QC you are there to sample, record ,report the results. The contractor should be notified of the results and you should note that the super was notified of the result. Concrete can be placed after 90 mins as long the temp. does not exceed the specifications. As long as the temp is ok and the contractor does not add water it's ok. Always remember when in question make a couple of cylinders to add to the set you made.
RE: Slump reduction with time
Another point of note - if the slump was high on arrival, allow the producer to rapidly spin the drum to ensure that the concrete is well mixed (this shouldn't alter slump characteristics drastically if 50 to 60 revs set as maximum. Then, re-slump within five minutes. If slump doesn't change, then you know that concrete was well mixed to begin with and is simply wet - REJECT!! As far as sampling the front of the load for slump, if we (testing agencies) did not do this, a lot of bad concrete would be placed before you actually knew what the concrete consistency was. As far as HectorB's comments concerning obtaining a 'sample', ACI is more concerned with molding of strength specimens from the first or last portions of a load - YOU CAN RUN A SLUMP TEST ANYTIME YOU WANT. As a matter of fact, some agencies require us to obtain slump loss data over time - the only way to do this is to sample the first part of a load.
A note of caution to specifiers, structural engineers, et al - In the future, you will begin to see the use of new polycarboxalite {spelling?} mid-range water reducers. Trucks will begin to show up with higher slumps, as this product is a plant-added admixture that improves workability w/o loss of strength. So instead of adding super-p at the jobsite, you will begin to see higher arrival slumps. TO ALL SPECIFIERS - PLAN FOR HOW THIS WILL CHANGE YOUR SPECS.