Expert: Bruce E. Johnson - 1/30/2011

Question
QUESTION: We have an existing 'jetty' structure build 30+ years ago. The jetty is in tidal salt water. In the original construction, Steel uprights were 'piled' into the seabed, then encircled with concrete sewer pipes which were then filled with concrete.

Time has taken its toll and some of the old concrete surrounds have started to collapse and crumble. Our plan is to place new (larger diameter) sewer pipes over the existing 30 year old version and refill with fresh concrete.

A few questions....

Is the plan sound? or is it a bad idea to try to pour new concrete over very old?

Should we attempt to re-inforce the fresh concrete?

Many thanks

David

ANSWER: Hi David, this is a common practice for pilings provided the steel uprights are still in decent shape and provided that you use a large enough pipe to give you room enough between the old and the new pipe.  You don't mention how large of a diameter pipe we are talking about and how bad off the old pipe is.  Of course you would want to get as much of the old crumbles pieces as possible removed and clean the area with a fresh water pressure washer if possible to remove as much salt and slime from old piling as you can.  You might want to consider PVC pipe for the new cover it will last longer than the old concrete pipe and be easier to handle. There are even "slip" forms made that can clamp around the old piling, be filled with new concrete and then removed and reused on a different piling.  These can be rented from companies that supply formwork to contractors.  For this type of work you might want to reinforce the new concrete by drilling into the old and underpinning it to the new with epoxy coated reinforcing steel pins. I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: Thanks for the answer, I am most relieved to hear the words 'Common practice' Current pipe Diameter is 5 feet. We were considering a new internal diameter of approx 6 feet (giving 6 inches all round). Still sounding OK?

Answer
Hi again David, at that diameter you should probably stick with the concrete pipe or a formed and poured encasement and 6" cover should be okay but I would definately pin the new concrete to the old using salt water resistant rebar pins to help hold it all together an "L" shaped pin would even work better put one leg into the old concrete and allow the other leg to hang down and tie into the new concrete.  The concrete pipe will have reinforcement built into it as well.  It might not hurt to have that reinforcement coated against salt intrusion as well if you are going to have the new pipe built by a concrete pipe company you can possibly order it that way.  I would venture a guess that this is what happened to your existing concrete pipe, the salt water eventually got to the iron reinforcement and caused it to deteriorate.  This is another good reason for the underpinning.  By pinning to the old interior concrete you are getting a better tie in than just a friction tie in between the old concrete pipe and the new concrete.  You never said how you were going to get the new pipe around the old.  I assume you will do them in halves and then band them together like a clamshell?  If this is the case your banding should be salt resistant as well. I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com