Concrete Strength / Concrete Removal Effort

[Thespr]

We set up a contract for a bridge rehabilitation that required the complete removal of the concrete deck from a precast box beam support system.

The concrete had some surface damage, but the core matrix was sound, but contaminated with chlorides.

A contractor was selected and after they started to the work, they realized their production rates were low and it was taking longer than they expected. They have requested additional money, largely, because the work was harder than they expected. The concrete was sound, and later tests indicated that the compressive strength was around 55 MPa, where concrete was sound.

I have not been able to find any definitive research on the effect of concrete compresive strength on concrete removals.

Typically, access to the concrete, whether you can stand above, ( versus overhead etc) the amount of reinforcement, the need to protect underlying components ( precast box beams) limitations on equipment and the soundness of the concrete are all considerations, but compressive strength specifically is not a consideration.

I have yet to have a contractor give me a credit when the concrete was weak, so not sure why we should pay more if it is hard.

However, to be fair, I am looking for any research or technical information that might link compressive strength to concrete removal effort.

Thanks

Thespr

 

[Ron]

Obviously concrete of higher strengths is more difficult to removed; however, its relative difficulty as compared to, say 45 MPa, is not easily quantifiable.

If the contractor is experienced in concrete removal, he should have either investigated the strength or asked the questions BEFORE bidding. Too late now. I would deny a claim for extra money.

 

[bridgebuster]

55 MPa is quite high for deck concrete, at least in the US. Is your bridge adjacent boxes or spread boxes since this would affect the demolition procedure?

The contractor could have a claim if he was led to believe the deck concrete had a lower strength. In any case, 55 MPa can be removed by waterblasting.

I have some old research about concrete removal. I'll take a look to see if there's anything helpful.

 

[Thespr]

Ron / Bridgebuster

Thanks for your comments.

The original plans for the bridge called for 30Mpa concrete or similar, so we are not sure why / how the strength ever got to 55Mpa either.

The box girders are not continuous below the deck, there are two sets of three box girders, a small space then exterior box girders. between the boxes, the deck was cut out.

The concrete was obviously sound from its appearance. We had one contractor call during the tender process to ask why we were removing it as it appeared sound. A valid question, but the client / bridge maintenance consultant had made the determination to remove based on chloride contamination. That contractor"s bid price was twice a high ( $200,000) as a result of his awareness that the work would be difficult.

My opinion is that the contractor interpreted that the deck would be removed from precast beams to be CPCI type girders, where much more deck could be cut out and the difficult area's limited to above the CPCI. I suspect the person in charge of the bid, had not been to the site or reviewed the drawings.

There are no guarantee's as to the nature or condition of the concrete and I think it is up to the contractor to assess the difficulty.

As a result I am not inclined to consider the claim, however, to be fair, I am still looking for some research on the topic. I have a copy of SHRP-S-336, which in its consideration of concrete breakers, indicates there is no definitive correlation between strength and effort, but I am looking for more, if possible.

Thanks again

 

[cvg]

of course it was sound, that is not the same as having high compressive strength.

the concrete is nearly twice the compressive strength as both you and the contractor thought. I think the contractor has a valid claim for changed conditions. Concrete is much stronger, schedule may be lengthened and potentially some methods / equipment for removal may not work successfully for this high strength concrete.

 

[bridgebuster]

I found the attached in my files but I don't think it's what you're looking for. It discusses various demo techniques but no mention of compressive strenght.

 

[cvg]

The following engineering manual also gives quite a bit of advice on concrete domolition and repairs.

http://140.194.76.129/publications/eng-manuals/em1110-2-2002/toc.htm

One quote from this manual is particularly relevant:

c. Contract work. If work is to be contracted, the information describing the condition and properties of the concrete must be made available at the time of invitation for bids to reduce the potential for claims by the contractor of “differing site conditions.” Information provided may include type and range of deterioration, nominal maximum size and type of coarse aggregate, percentage of reinforcing steel, compressive and splitting-tensile strengths of concrete, and other pertinent information. When uncertainties exist regarding the condition of the concrete or the performance of the removal technique(s), an onsite demonstration should be implemented to test production rates and ensure acceptable results before work is begun.

 

[bimr]

The first thing that you have to review is the Contract.

If you are using the typical DOT pay items, there is no provision for different materials or even thickness. It is the Contractor's responsibility. Attached is the description of a DOT pay item for bridge demolition.