Quality of Underwater Concrete 3

[graybeach]

I am working on a project that involves a mass placement (100 CY, about 5 feet thick) of underwater concrete. The purpose of the concrete is scour remediation. My question is: What are some options for verifying the quality of the concrete? The concrete is not load bearing, but we want to make sure the contractor places it properly. The water is 15 feet deep and very murky.

Is the only way to take cores? The budget is limited, and I am getting complaints from the project manager that cores are overkill for this application. Its my decision, but just for due diligence - are there other options?

 

[civilperson]

A specification 40% higher than required for your application, (checked by samples and cylinders taken from the truck chute), will give you plenty of cushion and excess safety for good sleep. Thus, if 2500 psi is needed ask for 3500 psi as F'c.

 

[graybeach]

Thanks Civilperson. That is a good point, and our specified f'c is at least 40% more than needed. Maybe we can do without cores. Taking a cores at a couple of spots would not tell the whole story anyway.

Should I be concerned that the contractor will not tremie properly - i.e., not keeping the pipe embedded in fresh concrete or moving it around horizontally through the concrete? We will have divers inspecting the work. Will such deficiencies be obvious to them visually or via tactile inspection?

 

[Zambo]

Are you using formwork, or how is the concrete retained?

You have a 100 CY at 5' thick so that gives a plan area of around 24' x 24' (I'm more used to SI units so about 7m x 7m)Assuming I'm correct then I would place four tremies evenly spaced, it is then easy to calculate the depth of penetration of the tremie into the concrete during casting.

If it's as easy as this I think no need for cores. But if my assumptions are wrong and no formwork or more complicated casting shape then could be more difficult to ensure the quality.

 

[StephenA]

If the answer to Zambo's question is yes (to formwork) can you check on the theoretical quantity required?

If some of the formwork is removable then divers may be able feel for bad concrete after stripping. If it is murky then they will not see a thing.

Another check is to always ensure that the concrete in the tremie hopper is visible. ie. do not let the level drop completely down the tube. If kept in the hopper it means that there is always back pressure and the bottom of the tube is always inside the fresh concrete.

Stephen Argles
Land & Marine

http://www.landandmarine.com

 

[graybeach]

Zambo's dimensions are correct. We are basically just filling in a hole with minimal formwork on one side to contain the concrete. I will look for multiple tremie locations when the contractor submits his work plan. We can monitor this from the topside, and we can make sure concrete in the tremie hopper is visible. I am starting to come around to deleting the requirement for cores. Thank you everyone!

 

[DRC1]

Cores may teellyou if you have soil inclusions. A few cores are not terribly expensive compared to the whole operation. Most importantly, be sure the contractor is expeienced, review the work plan with the contractor and be in the field when he is pouring.