High Slump Concrete
High Slump Concrete
(OP)
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!
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!






RE: High Slump Concrete
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/n...
RE: High Slump Concrete
RE: High Slump Concrete
RE: High Slump Concrete
Added: Slump test sampling should be as per usual...
Dik
RE: High Slump Concrete
RE: High Slump Concrete
RE: High Slump Concrete
The guys placing the concrete can probably call the slump when it drops from the chute. If it is suspect, have the sampler pull a slump then and send that truck down the road if it isn't in spec.
RE: High Slump Concrete
Dik
RE: High Slump Concrete
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: High Slump Concrete
Additionally, if the job is large enough, request that the concrete supplier's salesperson be on site for the first few trucks to make sure everything gets off to smooth start.
RE: High Slump Concrete
Something like that will put them on notice. There will be phone calls and emails but in the end they will step up by providing the right concrete.
Don't allow field modifications except those directed by the supplier (and done by the supplier!) at the time of issue. If their immediate fix doesn't correct the issue by batch-to-placement times then the truck is rejected. 90 mins don't reset/start again after they mess with mix... I've had that argument before :)
RE: High Slump Concrete
RE: High Slump Concrete
Any modifications of the mix design in the field should be done by the QC representative of the supplier.
If you are having low strengths you need to understand why. Is it because your finishers are adding water at the site? Is it because the supplier is not providing the specified slump at delivery?
Low strengths are usually a result of adulteration of the mix design. Adding cement without consideration of other mix properties will increase shrinkage and will likely present placement issues.
RE: High Slump Concrete
RE: High Slump Concrete
There are a lot of people saying that the trucks should be rejected. In my experience, it is inappropriate for an inspector to approve or reject anything. They are simply there to observe what it being built. So to the folks calling for rejected trucks - are you suggesting that the EOR or a representative from their office be physically present on site to observe the slump results and reject trucks? Is it appropriate for an EOR to reject a truck if the slump is 1/2" too high? Granted it wouldn't be nice, but I`m not sure I understand where there is language present to justify this course of action.
Honestly, it seems to me like they're going to pour the concrete they're going to pour. You can take a hard stance, make it well known to the supplier, contractor, and owner that they need to step up their game. After that, make sure everyone is aware that concrete that doesn't meet strength or is excessively cracked will need to be removed and replaced at the contractor's expense (including the additional testing, observations, any applicable design time/meetings, etc).
RE: High Slump Concrete
Yes, that may be the old fashioned way, but when I have rejected trucks, it worked. The modern trend of separating design from construction monitoring doesn't seem to be working very well.
It is incumbent on the designer to provide language that allows him to reject faulty material at any stage of construction, including before it is installed. Not an inspector, but certainly the engineer.
1/2" is not a big issue with slump. As I said before, slump is only a useful indicator of consistency. Nothing more. But if you want 4" slump, and the first truck arrives with 7", don't dilly around. Solve the problem before it gets out of hand.
RE: High Slump Concrete
RE: High Slump Concrete
Do you still use 3000 psi concrete? Where I am, that is not considered to be structural grade concrete. If they can't even provide that, your quality is severely compromised.
RE: High Slump Concrete
RE: High Slump Concrete
RE: High Slump Concrete
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: High Slump Concrete
RE: High Slump Concrete
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.
RE: High Slump Concrete
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: High Slump Concrete
RE: High Slump Concrete
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.
RE: High Slump Concrete
Dik
RE: High Slump Concrete
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.
RE: High Slump Concrete
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.
RE: High Slump Concrete
Item 2: Under the Canadian concrete code, CSA A23, the mix design is considered proprietary and does not have to be presented.
Dik
RE: High Slump Concrete
Fortunately not the case in the US. We always require a mix design submittal.
RE: High Slump Concrete
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.
RE: High Slump Concrete
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.
RE: High Slump Concrete
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.
RE: High Slump Concrete