High Slump Concrete 6

[Stenbrook]

I have a contractor who has had some issues on a previous job with the concrete showing up at the site with a higher slump then specified and allowed the pour to continue anyways. On this previous job, the concrete strengths came out low consistently across many of the pours and caused a bunch of issues. We are now in the construction phase of a new job that is very similar to this one with all of the same players (engineer, architect, owner, contractor, and concrete provider). This contractor wants to ensure that the concrete poured is acceptable and has asked us (the EOR) to allow them to add a bag of cement to the concrete mix if they should encounter a high slump while pouring.

My first thought is that this is a bad idea as it changes the mix design that the concrete provider is providing. I would assume that once you alter the mix, the provider may no longer have any liability for the concrete and it would fall on the contractors (or engineers) shoulders. At the end of the day I believe that adding some cement would probably only help the mix, but I don't want to be the one responsible for the concrete when the provider should be providing concrete with the correct slump in the first place.

Does anyone have any thoughts on this? Is it acceptable to add cement to a concrete truck with a high slump? Has anyone run into this issue before? Any input would be helpful.

Thanks in advance!

 

[hokie66]

Maybe someone can explain for me why you would use a slump cone in the middle of the batch. Why not at the beginning, when it will perhaps do some good? Slump, concrete temperature, air content if applicable, at the beginning.

 

[SlideRuleEra]

hokie66 - Agree that is ridiculous, but can imagine that someone not familiar with construction would think that performing the slump test in the middle of a batch would be the "best" measurement of the "average" slump for that batch. Bet it does happen.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[hokie66]

Obviously it does, by comments here.

 

[Ron]

The slump for acceptance of the load should be performed when the truck arrives at the site as SRE and hokie66 have noted. To do otherwise is absurd. The slump for record purposes to conform to ASTM standards (in the US) is to be done in the middle third of the load. Horse is out of the barn...too late to close the barn door. Do both and report both.

I agree with hokie66 that there should be a provision in the specs to allow for rejection of the load at the time of delivery. I spent many years working for one of the top engineering testing firms in the US....our lawyers never wanted us to accept or reject anything and we followed that directive; however, if you have the responsibility to observe and report, that reporting can be oral and immediate.....example....
Tell the contractor that the concrete does not meet the specifications and if he places it he will be responsible for the cost of removal and replacement because it does not comply.....if that doesn't work, tell the contractor that you are required to inform the engineer of record about non-conforming materials and you will do so immediately by phone. If the EOR says it is not acceptable, he has rejected it and if placed, could result in a lawsuit.

I have "inspected", tested and rejected lots of concrete over the years. Now I investigate failures resulting from those who don't reject bad concrete and from those who place bad concrete.....both get sued.

Remember....good concrete is made from cement, water and aggregates. Bad concrete is made from the same materials.

 

[SlideRuleEra]

Thanks, Ron, learned something today.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[hokie66]

Ron, what, if you know, is the reason for the ASTM standard that calls for "record purposes" taking the slump in the middle of the load? The slump records tell you little about the characteristics of the hardened concrete. Is there a similar requirement for the test cylinders to be taken from the middle? Are these requirements typically in project specifications in the US, or do specifications default to ASTM, ACI, etc?

 

[BigH]

Ron correctly points out the problem with the ASTM test procedure (as well as BS or now the EN standards). The horse is out of the barn is a good analogy. Project specifications never seem to address this issue - do the slump before placing or do it the ASTM way. Also the point of "rejection" responsibility is highlighted. I've been on jobs where the full control of Quality Control is that of the Contractor - the Engineer has no "right" to stop the job . . . I find this outlandish . . . but this is one reason that an inspector needs to have a good relationship with the Contractor - so that they are on the same page and both wanting to get the job done right. In my case, many times, it is the temperature at delivery that has been a problem. I made several "stinks" in the initial placements sort of coercing the contractor leader to reject the truck - wanting to set the groundrules. It worked and the concrete supplier got his act together.

Perhaps much of this might be handled if a meeting of all parties prior to concrete work were to be had and all these "bugs" and "issues" addressed with all parties knowing what is expected and what will not be accepted. Supposedly this happens, but likely far too many times it doesn't.

hokie66 The concrete cylinders, too, in ASTM and BS/EN both require them to be composite samples from various parts of the transit mixer; i.e., 3 samples (or 4 if Britsh) to be all mixed together to form the material to be consolidated into the moulds.

So Ron . . . how should specifications really be written to address on this issue?? . . . I have seen occasions where the first bit of concrete out of the transit mixer is dry - and as the mixer discharges the concrete becomes more and more "wet" (so to speak). In such cases, I've forced a stop to the placement and had another slump taken (contractors, at least here, do not do the proper ASTM/BS/EN composite for slumps.

 

[dik]

See the top of page 3...

Dik

 

[hokie66]

BigInch,

Yes, temperature at delivery is a big issue in hot climates, like where I am, and where you are, I believe. It is really irritating to take all the time to write a complete specification for hot weather concreting, only to arrive at site on the first day and find that the technician has no concrete thermometer.

 

[Ron]

As BigH correctly points out, the sample is to be a composite sample. That's another issue with the ASTM Standard. When concrete is being placed conventionally (not pumped for instance), the process can be relatively slow, sometimes taking 30 minutes or more to place. The compositing is to be done within a 15 minute time span, and volumetrically by thirds of the volume. That's difficult to do and often the technician will just pull from the middle of the batch.

Each time the technician queues up to take a bit of sample, the crew has to stop....even if only for a few seconds. Contractors don't like this annoyance.

Here are a few suggestions for specification inclusion:

1. Require a pre-placement conference with all parties involved. Lay out all the parameters of inspection, testing, acceptance/rejection, etc. Make it clear to all that these are the rules of concrete placement and rejection is the result if not followed. Have a sign-in sheet and take minutes for distribution. This should be done in advance of the expected start of placement, but only by a few days so it is still fresh in everyone's mind when the placement starts.

2. Require a mix design submittal and make sure the technician has a copy.

3. Make sure all the equipment on the transit mixers is working properly....site glass on the water tank, revolution counter, etc. Preferably do an NRMCA or similar plant audit prior to placement (National Ready Mix Concrete Association in the US).

4. When concrete arrives at the jobsite, check the trip ticket first. This does two things. It puts the driver on notice that someone is paying attention and it confirms the mix design and mix time. Pay attention to mix times, particularly in hot weather as hokie66 notes.

5. Have the driver mix with 40 to 60 mixing speed revolutions of the drum. Take a slump test. If within spec, proceed with placement and sample according to the standard. If not in spec, have the Ready Mix supplier's QA person adjust the mix to spec. They know their mix design better than anyone else on the job and it is their responsibility to deliver within spec. He/She might authorize some water addition at the site or might require more mixing with drum revolutions....keeping in mind that there should be a limit on total revolutions of somewhere in the 300 revolution range to prevent additional heat build-up in the mix.

6. If the mix cannot be brought to spec by the QA rep, then follow the rejection protocol established in the pre-placement conference.

7. If within spec, proceed with placement and sampling.

hokie66....agree....an unprepared technician does not need to be on the job. Thermometer, clean slump cone and base, rod, air meter, wheelbarrow, ice scoop, bucket, water....all necessary.

 

[dik]

Good list Ron:

Item 2: Under the Canadian concrete code, CSA A23, the mix design is considered proprietary and does not have to be presented.

Dik

 

[Ron]

dik...Thanks.

Fortunately not the case in the US. We always require a mix design submittal.

 

[Ron]

hokie66...you bring up why ASTM standards are frustrating at times. I've been on various committees in ASTM and as a voting member it is frustrating to get a standards provision in place. Often the committees have an imbalance of suppliers and design professionals, so the suppliers block the necessary changes such as what you have mentioned.

Yes, the sampling requirement is for compositing, which is most practical near the middle third of the load. It applies to both slump and compressive strength specimens.

The slump has two "consistency" components. The first is the consistency of viscosity....that's the one specifications cover. The second is consistency from batch to batch. Both are relatively important and yes, the slump does have some influence on the hardened concrete....mostly in its durability, finishing and shrinkage. Slump has little relevance to structural compliance as it is easy to meet a compressive strength requirement with almost any slump.

 

[darthsoilsguy2]

ASTM is a committee. Concrete producers have put the lockdown on all the tests so that delivered goods get placed and then you figure out acceptability after the fact. Measuring water content of fresh concrete by microwave has been in committee for the last 20 years i figure, although AASHTO does it... The last thing the industry wants is for people to have the ability to say that w/c is exceeded in the field by review of batch weights and field water testing.

But, the concrete industry looks like rookies when you see what the masonry and mortar cement producers have pulled off in their respective ASTM committees.

 

[Ron]

DSG....amen!!

I've been trying to get the word "nominal" removed from the stucco standard for 10 years and replaced with a tolerance. Gets blocked every time.

 

[epoxybot]

This is silly. The Concrete Slump Test is largely intended to check the Workability of the mix. What good does it do to test workability when only half the batch is left. The Minnesota DOT specifies that the Slump Test shall be taken after the first 1/3 of a yard has been discharged. Strength tests are to be taken in the middle of a batch. So maybe the contractor is just trying to consolidate interruptions to the work but a supplier that is having trouble delivering 3000psi concrete, both in slump & strength, sounds like Shenanigans. Honestly, if the concrete were being pumped and the Ready-Mix supplier had had jobs where the mix separated and plugged the hose, does anyone think the Concrete Pumping contractor would proceed without having assurances the slump was within spec beforehand? If concrete strength samples are to be taken from the middle of the load to simulate average strength and a slump test fails at the middle of the batch, doesn't it strongly suggest that the first half discharged is crap and the pour should stop? Even if a bag of cement were to remedy the slump what good does it do to place good concrete on top of bad. Why would a the contractor take on the problems of the concrete supplier and why would they both want the EOR to join the merry band of fools?