READY MIX CONCRETE DELIVERED AT SITE CAN BE REJECTED? 8

[redshot]

OUR MIX DESIGN HAS A SLUMP AT SITE 125 mm +/- 25mm with super plasticizer but usually, concrete arrived at site is less than the minimum or more than the maximum specified slump in the design mix. Is it right the QC inspector can reject the concrete?

Example Case: 1 truckmixer has slump at site 40mm only while the 2nd trucmixer has slump at site 240mm. Design mix is 28 MPa with super plasticizer; slump at site 125 +/- 25mm. These two trucload of concrete were rejected. Is it right for the inspector to reject the concrete?Pleae any expert on this may give his opinion is highly appreciated. Thanks a lot...

 

[bridgebuster]

Specs are specs. You follow the specs so you don't get into trouble - you might not make any friends but you stay out of trouble.

If your QC inspector rejected a 240 when 150 is the max, it sounds like a good call.

 

[bridgebuster]

One thing I forgot to ask: Do you add the superP at the plant or at the site? You have a 40 slump and a 240 slump; too much disparity. You need to identify the source of the problem in order to fix it.

 

[Qshake]

out spec Slump is not usually a reason for rejection. Adding water is most always allowed but documented and limited to amount and number of times.

Most typical reasons for rejecting ready mix concrete on site are: out of spec air content and concrete that is well past it's time limit.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Ron]

It sounds like your batch plant is having difficulty maintaining quality control. With such disparity in slump values, it is likely that the moisture content of the aggregates is not being properly controlled, or that the plant is using poor weighing/dosing devices.

Both BB and Qshake are correct; however, slump is a valid reason for rejection, since it is the first parameter checked at the site. Rejection for slump variation is valid because it implies variation in the consistency of their quality control.

 

[concretemasonry]

When I worked for the DOT as a bridge construction inspector, I rejected the first two loads of the day. The reason was 6" of rain the night before and the concrete was needed for pouring slope protection (Rows of 2x2x4" squares in an alternating pattern on a slope) between the abutments and the piers. The water collected in the bottom of the huge elevated aggregate storage bins and these were very early morning loads before traffic.

After I rejected the loads, the superintendent thanked me for doing it so he did not have to. His opinion is that in that context it would not be usable on a slope, but it might have been adequate in small squares on a level surface.

When the product has a specific use, the specs must be followed very closely if there is a minor question/variance. Since I had made many tests and cylinders, I knew how to alter the result even with the normal ASTM specifications.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[redshot]

bridgebuster,
Thanks a lot for your excellent comment. Just to ask further, slump less than minimum specified in design mix, say Slump @ site =40mm while the specified minimum is 100mm. Is it acceptable to reject the concrete?

 

[Ron]

redshot...when the slump at delivery is less than specified, you can let the Quality Control representative of the ready mix supplier bring the slump to within the specified tolerance. Do not allow the contractor, the placement crew, the truck driver or anyone other than the ready mix supplier's QC rep to do this. If you do, you just accepted the liability for the mix, good or bad.

 

[bridgebuster]

redshot,

It's a little tricky; what is the concrete being used for? Slump is a measure of workability but it is also related to the w/c ratio. If you increase w/c too much your strength could go down.

In my case, many of the construction projects I worked on followed NYSDOT specifications, which requires adding the last 10% of the water at the site to obtain the initial slump. During the placement, we could add water one time, based on a rule of thumb - 1" slump = 1 gallon of water/cubic yard in the truck.

Earlier, I said adding water could reduce the strength. This is because NYSDOT designs concrete strucutres based on a lower strength than their mixes produce at 28 days. I've never come across another agency or owner who did this.

 

[mbt22]

Redshot,

Reversing the question, is there anything that would suggest that the QC inspector should accept loads that do not meet the specification?

Matt

 

[redshot]

Bridgebuster,

Our design mix has superPlasticizer to be added at batching plant, but the slump at site was found to be 40mm only, sounds unusual and unacceptable. So the QC inspector reject the concrete.

 

[Teguci]

As a child, I always interpreted the response "because I said so" as a cop out. Out of spec concrete due to slump should be rejected but the reasons should be understood:
- Variations in slump are the quickest to test on site and indicate that there are other problems/inconsistencies with the mix design
- Low slump may have trouble being placed in congested areas and will allow significant voids in the final concrete.

A high slump should not in and of itself be an issue with today's technology and I have often considered removing it as a criteria.

Being a performance design criteria engineer, I've always considered rejected concrete as usable at the contractor's risk. If the contractor is placing foundations in an open excavation with a large storm coming in, the decisions required to maintain a safe site is his. If an out of spec high slump concrete is placed, my concerns are limited to final compressive strength and air content. The obvious risk is considerable use of the concrete eraser.

 

[MiketheEngineer]

Yes - you can reject - then let the lawyers figure out who pays.

My greatest concern after slump is usually time from plant. Over about 45 minutes - you can run into real problems!!

 

[Shadenem3]

I agree 100% with rejecting the 240mm slump.

In regard to the 40mm slump...it sounds like there was not enough initial mixing water in the batch to activate the plasticizer. If the losd was outside it's prescribed criteria for adding mixing water (eg. after 1 hour or whatever may be specified) then you are pretty well strapped as far as adjusting the load. The driver should have recognized this either when setting his load, or before arrival to the job site.

 

[cvg]

low slump after adding super p is unusual, what about the delivery time? low slump could mean the truck took a long time to deliver the load and the concrete is setting. adding water would be a bad idea

rejection or not depends partly on who the QC person works for

the term "QC inspector" is a bit mis-leading. Quality control is the responsibility of the contractor or subcontractor and quality assurance the responsibility of the owner. therefore the QC guy can reject anything he wants and the QA inspector might allow the contractor to use the out of spec concrete "at his own risk"