concrete beam repair

huangyhg

concrete beam repair
i need to repair an 18' long beam for a digester building built in the 1960s.  there is extensive water damage.
concrete has spalled, exposing the bottom of the stirrups, which are rusted considerably.  in some areas, the bottom corners have spalled, exposing deteriorated longitudinal reinforcing.  the bottom reinforcing is unknown, as the section shows 5 bars, but the plan calls for 2-#6.  calculations show that 5-#6 would be neccesary.
the 12"x48" beam carries 3000 plf dead load, including the building front wall, which is a brick/block cavity wall.
the basement extends 6 feet beyond the superstructure, and there is a 12 slab that connects the top of the foundation wall to the bottom of the beam.  the base mat is 16 inches thick, but has no bottom reinforcing.
i'm looking for ideas as to how to handle this.  any help would be appreciated.
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it sounds like you have already done some analysis and design checks and this is the first thing one should always do when investigating a damaged concrete beam.  you should thoroughly understand the applied loads and capacity of the original section.
with this knowledge, the next thing needed is an understanding of the current material properties and condition.  the concrete should be tested for compressive strength (via drilled cores if possible) and for the presence of chlorides as this might indicate whether the concrete is "diseased" throughout.  cleaning and removing all loose concrete from around portions of the bottom bars and also along sidewalls to inspect stirrup conditions.
how much bottom bar steel area is left and how does that compare with the required area?
once you have a handle on the above, then you will most likely have a way to choose between the following:
1.  cleaning the reinforcing and remaining concrete surfaces and re-patch.  usually remove concrete behind the bars about 3/4" min. to allow the patch to hold to the bars.
2.  cleaning and patching per item 1 above but also include additional bottom bars spliced and lapped with original bars if the bar damage is local (vs. along the entire length of the bar).
3.  concluding that the bars are too far gone, and deciding on a supplemental beam system (either steel wf, tube or new concrete beam) placed under the damaged beam.
there are multiple repair methods and approaches to take..the key is truly understanding the full condition of the beam as well as how reinforcing and stirrups work within it.  stirrups are much more difficult to repair.
jae,
thanks for the suggestions.  we'll start on the testing and removal of deteriorated concrete.
what's really got me stymied is this:  i'll need to somehow support/shore the (substantial)dead load during the repair process. because of the cavity wall above, the shoring will have to be quite rigid.  i can't support from the bottom of the beam, as that's where the repair will be made. i can't shore from the upper slab, as it's only present in the middle third of the beam.  i can't shore from the lower slab, as it has no top reinforcing.
i'm thinking some sort of "needling", penetrating the beam with steel beams and jacking them.  we did this once to underpin a foundation.  sounds expensive, difficult, and not so rigid.  any other ideas?
jim
you could set up discrete points of support along the length of the beam and repair portions at a time...assuming you don't have to thread a full length bar down the full length.
there is another way:  this looks like a perfect case for the epoxy bonded carbon fiber systems.  no shoring is required.  they come in, clean the bottom of the beam, apply layers of epoxy and carbon fiber sheets and that's it.  if the concrete above is in good enough shape to take the compression, the beam will actually be stronger than what it was before because the reinforcement will have been lowered to the bottom of the beam.
i forgot to mention cost.  yes, the stuff is expensive, but so is any other repair.  if you can get away without shoring, you may actually come out ahead of any other type of repair.
carbon frp may be an economic repair/strengthening method, but frp's should not be applied to distressed concrete that has corroded reinforcing steel unless the distressed concrete and corroded rebar is repaired. the expansive forces from corrosion are huge and will debond the strongest applied laminates, overtime.
also, fire protection/rating is a condideration with frp as external reinforcement - this may not be an issue for a digester building.
i did something like this in a tank containing ferric.  i shored the existing slab on both sides of the beam using posts and wood cribbing.  you'll probably only need one set of shores on each side for 18'-0".  i had the contractor cut the concrete beam flush with the bottom of the floor. i then replaced the concrete beam with a steel member and added concrete support columns at each end.  non shrink grout was added to make up any gap between the steel and the existing concrete.  after that the shores were removed.
i was lucky in that the tank was not being used for ferric storage any more, so i didn't have to worry about my new beam corroding.  you'll have to coat any steel member with some kind of coating resistant to the gasses that caused this problem.