Bridge Deck testing results

[OLDSOUL22]

Dear Fellow Engineers,
As a part of a concrete deck inspection and evaluation we've done some core sampling and testing for compressive strength and water soluble Chloride content, we got the attached results. the concrete is 60 years old, the mx design strength is 5ksi and the rebar is 60Ksi not coated. I'm just curious how everyone would go through the evaluation process. do you think the chloride content at 1" & 4" is enough? or a full profile would be more advantageous given the additional cost.

MSc in Structural Engineering, PE

 

[Ingenuity]

What is the depth of rebar (i.e. concrete cover)? I usually call for a chloride profile that has a Cl- content at the rebar depth.

 

[MotorCity]

If you find the chloride content is excessive, what's the fix?

 

[Ingenuity]

If you find the chloride content is excessive, what's the fix?

Sometimes chloride extraction is used, depending on the severity of Cl- content, but for a bridge deck that can be problematic.

 

[OLDSOUL22]

Design drawings shows 2 in clear cover, core samples shows cover as less as 3/4" at some locations.



MSc in Structural Engineering, PE

 

[SlideRuleEra]

...how everyone would go through the evaluation process.
do you think the chloride content at 1" & 4" is enough?
or a full profile would be more advantageous given the additional cost.

It would be a big help if you told us bridge deck thickness and what you are trying to do.
I'll assume you are trying to determine what thickness of the bridge deck to remove (hydrodemolition?) / replace because of chloride contamination.

With that assumption, I would graph the data to get some understanding of what it indicates:

ACI appears to considers 0.9 Lb CL/Yd[sup]3[/sup] to be acceptable.

Draw a graph for each core. Then you can easily visualize what is happening. I've done Core Sample #1:

See what the trend is for the 14 samples. From this one, looks like a sample from a little deeper would be a good idea. Pick one depth that works best (or decide that more testing is not needed).

 

[OLDSOUL22]

SlideRuleEra : this is very helpful! thank you so much.
The deck thickness is 9" with horrific spalling on both the top and bottom rebars! Personally, I don't think any portion of the deck is salvageable, but due to the size of the project, I want to showthe client with no doubt that full replacement is needed. what are your thoughts?
I've asked for a full profile for the Chloride content and that's what I got, now I have to wait another 3 weeks to do two additional points @2.5" & 6.5" which are the steel effective depths.


MSc in Structural Engineering, PE

 

[SlideRuleEra]

The deck thickness is 9"...

It's time to look at how much concrete will remain. Tests performed to a depth of 4" (5" concrete remaining), and the contamination is "high".

So tests will be be performed to a depth of 6.5" (2.5" concrete remaining). Even if all tests are acceptable at the 6.5" depth, IMHO, trying to "save" 2.5 inches of concrete is ridiculous. As a former bridge contractor, I would be careful (see below) bidding on partial deck replacement for that thickness. I never used hydrodemolition, but considering normal tolerance for "traditional" bridge deck removal this partial deck replacement would be a safety hazard... having the 2.5" thick slab "crumble away" while workers are on it.

I understand your need for "proof" to convince the client, but, if your client insists on a construction contract for partial deck replacement, for 2.5", or so, remaining concrete, be sure to tell the client to expect to pay for full deck replacement (using the contract's change order prices, not competitive bidding).

Contractors are not clueless, expect the change order prices to be "sky high" to (legally and ethically) take advantage of an Owner who made a very bad decision to proceed on a doomed approach to a project.

 

[Settingsun]

Do you mean partial replacement with the retained portion serving as a work platform, or remaining as part of the primary structure? If the latter, remember to present the chloride levels ss present day levels. The concrete is proven not to prevent chloride ingress si the levels will continue to increase.

You could consider presenting Option A and Option B to your client. Option A is full replacement, long life, low maintenance. Option B is partial replacement, high ongoing maintenance, little upfront saving-if any-with high risk (per SRE).

Re change orders: depends on the client's appetite for conflict. As SRE says, contractors aren't clueless. They would need to demonstrate some latent condition that wasn't reasonably foreseeable. SRE has foreseen it from a couple of paragraphs. If they bid, it means that they had a methodology in mind.

 

[dik]

Did you do any carbonation testing? Ron should hop in here...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[OLDSOUL22]

Although I agree that the Chloride content at 4" almost rule out the possibility of partial depth repairs, the testing lab is already paid to do the testing at 6.5" so we might as well have the data.
No carbonation testing has been done, we were planning to do half cell potential testing but the cost came back high and I am not sure if we need it anymore.
I think we will end up recommending Full scale replacement or to preform a cost comparison study between full replacement and a combination of partial, full depth repairs and resurfacing.


MSc in Structural Engineering, PE

 

[BridgeSmith]

From a brief survey of the chloride tests and your description of the condition of the top & bottom of the deck, I'd be very surprised that anything short of a full deck replacement would be cost-effective. Even if you can identify some areas where partial depth repairs could be done, those areas would still have a shorter life than the rest of the deck. Repair areas inevitably end up being bigger what is initially identified in a survey of the deck.

If we had that information in hand for one of our bridges, it would be a clear-cut full deck replacement; no question.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[BridgeSmith]

IMHO, trying to "save" 2.5 inches of concrete is ridiculous. As a former bridge contractor, I would be careful (see below) bidding on partial deck replacement for that thickness. I never used hydrodemolition, but considering normal tolerance for "traditional" bridge deck removal this partial deck replacement would be a safety hazard... having the 2.5" thick slab "crumble away" while workers are on it.

We require full-depth removal for any repairs that go deeper than half the depth of the slab. We assume the chipping hammers will micro-fracture the remaining concrete if it's less than 3 1/2" thick. We have considered revising that for hydrodemolition, since it doesn't produce the impact that a chipping hammer does, but so far, we haven't.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[SlideRuleEra]

...or to preform a cost comparison study between full replacement and a combination of partial, full depth repairs and resurfacing.

So you may decide to spend your Client's money studying the cost of a technically unacceptable solution (repairs)... I can hear the conversation now:

Engineer: "Here are the results of the cost study for full deck replacement vs. deck repairs."

Owner: "Good! Repairs are significantly less expensive than full replacement, we will take that option."

Engineer: "Sorry, repairs are not satisfactory considering the results of field tests."

Owner: "Then why did you waste our money making a worthless study?"

This is not hypothetical, as an Owner, I've had similar conversations with Consultants... thankfully before the "study" was authorized.

Since full deck replacement is costly, there may be a better way to spend your Client's money.
Make use of the data you have... concrete compressive strength variation is all over the map:

1) Half of the tests (7 out of 14) show compressive strength below design, several significantly low.

2)You have observed: "Design drawings shows 2 in clear cover, core samples shows cover as less as 3/4" at some locations."
and

3) ..."horrific spalling on both the top and bottom rebars! Personally, I don't think any portion of the deck is salvageable..."

"Reading the tea leaves" tells me this bridge was "poorly" constructed to start with (60 years ago).

Before proceeding will full deck replacement, I would recommend the Owner to at least do a preliminary evaluation of the substructure.
Good chance similar "shoddy" construction happened there, too.