Slump reduction with time 2

[pevans]

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???

 

[Ginger]

I don't know the limits of the ACI documents (I work in the UK) but this should really have been rejected. Any concrete mix can be rotated in the drum and allow hydration to progress sufficiently such that the appropriate slump is achieved. the result you get is actually a weaker concret that is designed for (assuming the slump is a result of excessive water or insufficient cement).

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

 

[emmgjld]

pevans

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.

 

[dai]

Should you be taking slump samples from concrete that hase been pored onto the ground, i have been tort to take samples only from concrete that has been poured into into a dumper or digers bucket. I think that the samples should not be taken from emediate first pore, thay usualy have a higher water content than the rest of the mix, maybe an idea would be to fill a bucket and take from the top of the bucket.

 

[bb29510]

This is a common sense question not a spec question. 6.5 to 4.5 with 30 minute of revolutions is pretty common. You are not lowering or increasing the water cement ratio but just mixing to a more uniform mix. The extra 30 minute of mixing is increasing the hydration of the mix by heat to lower the slump. The slump test is not an accurate test for water/ cement ratio but an INDICATOR of the condition of the mix. Different mixes will perform different by slumps with the same w/c ratio. 6.5 is not a wet mix. It is perfectly good to use, depending on the past history of the mix. Most spec will have a limit on slumps that been pulled out of the air many years ago and that one designer been quoting five inch slumps for years. this is a common sense question. What is the history of the mix. What is the product being poured? Is cold joint a factor? What are all the other trucks slumping at?

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.

 

[soilman2]

I hope brownbagg is far, far, away from any concrete placements of any consequence. The first clue is the use of the word 'pour'. Concrete is to be placed, if you can pour it, it better be plasticized. The second clue is the referencing to psi as the controlling specification. The psi attained yields no real information as to the overall quality of the concrete. Durability, permeability, shrink and curl potential, passive protection for reinforcement are some of the qualities that cannot be measured by psi alone. It is the responsibility of the supplier to supply the concrete as promised. It is the responsibility of the testing agency to verify that the concrete as delivered is as promised, or send it on its merry way. Simple. No value judgements are required or necessary, unless, of course, the testing agency is also the owner. The testing agency is respnsible to make assurances that the owners are getting value for their investment. Under the best circumstances, accidents happen. There is no need to invite any.

 

[GThomas1]

I firmly agree with soilman2 on this. A testing agency is the quality control of a project; ensuring that the specification of the design are met. I strongly disagree with brownbagg's statement that specifications are "pulled out of thin air" or simply used repeatedly. However, pne thing he said is in fact true. As long as the concrete is placed within designated time (usually less than 90 minutes from the batch plant to completion of the placement) there really is no need for concern. And yes, if the truck arrived shortly after batching and did not adequately mix the batch then there would be no problem with allowing more time in the truck; just ensure that the rev's guage doesn't exceed 80 (on most mixes). ACI and ASTM standards are clear on these points. About sampling from the ground, yes it is an accepted practice (but usually only done for paving jobs and curb-mix). That's because to do this properly the entire load would need to be poured and the sample taken from 5 different locations of the pile and consolidated/mixed in a wheelbarrow. Which is why it is usually only done with slipform or paver jobs. So, to have a truck dump a small portion from the top of the truck is obviously not IAW ASTM or ACI for a multitude of reasons.

 

[DirtDelight]

You didnt state who okd the concrete to be placed? As a QC person not QA you are there to observe, document, and record your findings. If I am in a situation like that I will bring it to the attention of the contractor first, explaining that the concrete is what it is. It is on that person then as to if they want to be liable for the batch being placed. Also you state 1 truck. I have never been on a job that a truck would sit there for 70 minutes while a decision was made as to place it or not?
Also, what kind of structure were you pouring?

 

[HectorB]

First, the ACI states that the sample should be taken between 15% and 85% of the discharge of the total load. The ASTM standard states "In any case do not obtain samples from the very first or last portions of the batch discharge."

 

[gatorn10ac]

This is a common acurrence. As stated earlier the heat of hydration caused the problem.

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.

 

[dirtdoctor]

Why not simply require that batch sheets be delivered with each concrete load? Simply compare the batch weights to the approved mix design. It is a very common practice for producers to assume moisture contents of aggregates and sands during batching operations; hence, if the moisture on the sand (most likely source) is higher than actually figured, then you get a higher slump.

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.