Need some CONCRETE advice

[Structures33]

I have an existing bridge with pre-cast hollow core panels that has never been in service. As part of the bridge completion and renovation, I am specifying a 2" concrete topping slab and have the following questions:

1. Should I specify a bonding agent be applied to the existing precast panels prior to installing the 2" concrete topping slab?
2. What mix parameters should I place on the 2" topping slab? For example, max aggregate size of 3/4"? High slump / flowable mix?
3. What should the concrete finish be for a bridge driving surface? I read it should have a friction coefficient of .35 but don't know what this would mean in terms of a finish.

Thanks!

 

[DDBPE]

I assume the precast panels have a roughed surface already. If this is the case, then wetting down for an hour or two prior to concrete placement will usually be sufficient to bond the new concrete to the panels. This prevents the panels from pulling the moisture out of the concrete when it is placed. This is typically how i have seen them.

1)Bonding agents if not used properly they can also be a de-bonding agent. Some require the application to be just prior to the concrete while others do not (which is problematic when trying to pour a bridge deck). If it is an epoxy and you wait too long, you have just created a de-bonding surface and then you have to remove the epoxy and start all over.
2) A 4000 psi concrete should be sufficient, the aggregate top size should be 3/4". You do not want high slump or flowable mix, should have a slump around 3". High slump or flowable mix, if you have a crown or anything other than flat, when it is worked will flow to the low side.
3) For the driving surface, usually the finish will be a burlap drag (transverse)on the wet concrete and then typically states require grooving.

There are some latex modified concretes which may work better than the 4000 psi I suggested, I just do not have any experience with it and it may be worth checking out.

 

[jayrod12]

I'm only chiming in on the slump requirement and this may seem like a bit of a rant.

Specifying the slump, especially in today's world, is an absolute waste of time. If your slump test fails because it's too high all they need to do is let it run in the truck and test again in 15 minutes, repeat until your slump test passes. If your slump is too low they're just going to add water to make it pass which changes the w/c ratio and screws the concrete strength. In this world of admixtures and supplemental aggregates concrete can be an absolute slurry paste and still harden to specified strengths.

Concrete should meet the w/c, air entrainment, max aggregate and exposure(sulphate vs. non-sulphate) parameters. The workability of the concrete (i.e. slump) can be modified easily with plasticizers and etc without affecting the final product.

Rant complete.

On the question at hand, more often than not a bonding agent is detrimental to the work (although talk to the local pre-cast mfrs to confirm). There was a bridge deck here that had to be resurfaced 3 or 4 times because their bonding agent was not working correctly. probably due to what DDBPE said.

 

[Structures33]

Thanks for the information. What has your experience been with a bonding compound that would be added to the concrete mix?

 

[bridgebuster]

Bonding compound isn't added to the concrete mix; it's placed on the surface to receive the concrete. Check your state DOT specs for overlay requirements. If there aren't any, download NYSDOT standard specs and look at Section 584, which deals with overlays.

 

[Structures33]

bridgebuster - What about a product like SikaBond NV - see link

http://idn.sika.com/en/solutions_products/02/02a013/02a013sa11.html

It states that it is "added to the cement to increase bond strength..."

 

[bridgebuster]

Structures33

"added to the cement to increase bond strength..." doesn't mean add it to the concrete mix. A common bonding agent is slurry made from mixing water & cement. The Sika product is an admixture for your slurry. Look at the technical data sheet.

 

[bridgebuster]

Structures33

see attached; go to Section 584; everything you need to know is there.

 

[Ron]

jayrod12....gotta disagree with you on this one. The slump test, when used properly, is still relevant and a handy tool for catching issues with the mix design.

Slump testing tells two things....consistency (as in sameness from batch to batch) and consistency (as in the relative viscosity of the mix). Lack of batch to batch consistency reveals issues with the batching, mixing or both. Changes in consistency relative to the workability give indications of aggregate issues, mixing issues, admixture dwell time decay and others.

There is often a general misunderstanding of slump testing, its interpretation and its diagnostic capability.

 

[jayrod12]

Ron,

In order for the slump test to meet the requirements of consistency as you've outlined above then the tests have to be done within a small window of time since batching. 15 minutes of additional mixing in the truck, or each truck being delayed a different amount in traffic will affect the slump test. Should the results be within a reasonable range? Of course, but what is reasonable?. What if the contractor asked for a super-plasticizer to be added (and didn't inform the engineer) so that it flowed into the forms better? Does that detract from the quality or the final performance of the concrete, no, but it sure as hell would ruin slump tests.

 

[Ron]

Jayrod12...the time constraints are in sampling the concrete for compositing a sample, not from the batch time. You have 15 minutes for compositing a sample, 5 minutes before starting the slump test, then 2-1/2 minutes start to finish to complete the slump test. The time since batching has placement limitations under ASTM C94, but not for the sampling and testing except those noted in C172 and C143.

 

[jayrod12]

Ron,

I understand and know what the standards say. My whole point is that an additional 15 or more minutes between different trucks coming to site (which is well within the allowable time since batching requirements) will cause different slump test results even for the exact same concrete mix. The slump test seriously needs to go the way of the dodo bird as concrete is becoming a performance based material.

Engineers specifying things such as concrete type, amount of air entrainment, slump etc. should be changed to specifying what we want the concrete to be able to do, i.e. it must provide at least 30MPa of compressive strength, be able to perform under freeze/thaw, be resistant to sulfate attack, etc.

Let the concrete mix designers and suppliers figure out which additives, w/c ratios and so on are required to meet the performance specifications. That's what they get paid for.

And honestly, do you actually believe that the testing guys on site are the most reliable people (same for the contractors installing the concrete). I can't even count how many times I've read a concrete testing report that stated the concrete passed all of the tests but once the tests were complete water was added to the mix. It seems like a waste of time and effort to specify slump tests specs and others only to have it ruined by the guys on site.

 

[Ron]

My whole point is that an additional 15 or more minutes between different trucks coming to site (which is well within the allowable time since batching requirements) will cause different slump test results even for the exact same concrete mix.

Your comment illustrates my point. If there is a significant variation in slump test results, there is a reason. Time to look at the batch plant and see if their procedures are proper or if their materials are consistent.

I came up in very professional firm that was a Geotechnical and Materials ENGINEERING firm, first and foremost. Our technicians were pretty well trained, not only in the testing methods, but the background of the tests and their significance. They were taught to spot things like you noted and to report them. I can't tell you how many times I was called by contractors complaining and saying that our technicians were going to be kicked off the job because they reported everything.

I agree that the professionalism of "testing labs" has declined over the years. I see technicians who are not well trained and engineers who review their work that are often not well versed in the subjects. In my opinion, a testing lab should be an engineering entity that provides testing, not a testing lab that has an engineer on staff for looks and credibility.

For the past 30+ years, the construction industry has been pushing for and receiving performance based specifications. The liability to produce a performance spec is lower for the design professional and less troublesome for the contractor....it only has to be a sales job on the front end. Good deal for all around? Not for the end user. Assessing construction based on performance alone is akin to closing the barn door after the animals have escaped. It's too late to correct problems when performance doesn't meet expectations and the cost to repair is often multiples of the original cost to have done it correctly.

There's nothing wrong with telling a contractor what you want and how you want it done. Yes, it increases our liability, but if we know what we're doing, we only have determine if the contractor performs, not wait 5 years to see if the construction performs and then try to figure out why it failed. I realize I'm also cutting my own throat here if construction gets better....after all, I deal with construction defects and failures in my forensic practice on a daily basis. Unfortunately, I don't think I have to worry.