Rusts in bars inside beams

[releky]

If there are rusts inside bars inside beams.. how do you remove the rusts when concrete enclosed it. And If you inject epoxy into any hairline opening and the rust is isolated, would the rust further corrode the remaining bar inside the beam? How is this thing usually handled?

This is similar to any flexural hairline crack where the bars have micro exposure to air. Or can't any rust form without spraying the beam with water?

 

[CELinOttawa]

Firstly you can never entirely eliminate the rust.... And there is no need to do so. Even new construction may be undertaken with rusted bars, so long as all lose rust is removed before the pour.

Traditionally all delaminate concrete is removed, all lose rust is removed, and new topping concrete is poured. New technologies are available to further produce rust inhibiting constrete, as well as zinc pucks to create protective galvanic reactions with the finished concrete topping.

Frankly the natural passivating alkalinity of new concrete provides a great level of protection, but the new systems do appear to be further increasing this protection.

FYI: Recall that rust is approx 10 times the thickness of the original base steel, and as such you need to be facing a tremendous thickness of rust to have lost any real steel. I much more frequently find myself replacing stirrup steel, particularly in columns, but this is not at all challenging once you've done it a few times and become familiar with the safe approaches for doing so.

 

[CELinOttawa]

Further to above, I have seen (but frankly do not believe in the use of, aside potentially for in bridges) longitudinal strips of undelaminated concrete cover removed and replaced with new in order to install zinc pucks... That is a potential solution to not removing most concrete cover and still achieving protective action.

In your case, where you are looking to inject, this means you have delamination and visible surface cracking. Don't reinvent the wheel; remove all delaminated concrete, sand blast the bars clean, check the remaining thickness of the stirrups (replacing where necessary) and then install your new cover or system with cover.

If you are not familiar with this type of work, please ensure you get some real world advice. Not all beams can be repaired without propping, and recognising what solutions will be effective takes a measure of experience. Done incorrectly the rust can show through again VERY quickly, which could land you in a whole lot of hot water. Tread carefully, and good luck.

 

[releky]

CELinOttawa, the reason rebars have grooves is so the concrete can bind to it.. the so called development length related to it. If you remove any stirrup or ties from beams or columns and put a new one. It is not connected to the original concrete and can no longer function as required (for example. for shear where it has to be binded to the concrete), how did you handle this scenario?

 

[CELinOttawa]

Hi Releky... You do not remove the old steel, you blast it clean (to a minimum of no-lose rust standard) and then add new stirrups if you have lost too much steel to meet minimum confinement or shear strength requirements.

The new concrete needs to bond to the old. This is possible in a variety of ways, from SSD through slurry washes, all the way through to specialised bonding compounds and epoxies. I have never, however, been a fan of so called bonding agents (the aforementioned compounds and epoxies) as any mistake in application such as poor timing or delay can make for a very effective bond BREAKER.

 

[CELinOttawa]

Two more thoughts:

1) You are nearly always removing cover steel in tension zones, which have no effective strength. Where you remove compressive steel, it would nearly always be where it has already delaminated and no longer has effect. Regardless of the repair, you must undertake a careful review as to the. Red of propping.

2) You seem concerned about the bonding of the bars, and well you should be if you are being overly aggressive as to the extent of repairs. Most repairs have to be staged, and the amount of sound cover removed should be very limited. Bars do not need to be surrounded to have mechanical bond, and do not need to be in contact to have a good lap length, but both effects need to be considered.

The repairs you are looking to make are a fairly specialised field of engineering, best learnt (in my opinion) as a member of s Bridge or Marine/Port Maintenance Engineering group.

 

[releky]

For the stirrup function in beam shear capacity, the stirrups are the only thing holding diagonal cracks from coming apart, so why did you say that "Bars do not need to be surrounded to have mechanical bond, and do not need to be in contact to have a good lap length, but both effects need to be considered." Unless you think the strengths of the stirrups is in enclosing the tension and compression top and bottom bars and this has more strength than development length of the bars in concrete?

 

[CELinOttawa]

Hi releky,

My comment referred to the fact that steel bars do not need to be in contact in order for a splice to work. That means that you can remove concrete cover, add new steel, and have the restored cover concrete cause the new steel go act as part of a new "whole" beam.

The bond between the concrete and steel is important, but perhaps I'm not being detailed enough in my explanation. I'll try to break it down for you.... I also strongly recommend you read non-code reinforced concrete texts before you undertake this type of repair work. Codes are great, but a recipe book solution is *NOT* what is needed here. I strongly suggest ACI408R-03 as well as the old stand-by text "Reinforced Concrete Structures" by New Zealanders Park & Paulay.

- Reinforcing bars still work when only partially surrounded by concrete, particularly if they are still embedded at the ends while you repair along the centre. Longitudinal bars are also quite capable of functioning (sometimes only partially, but this is still useful) when they have lost all covering steel below their midline. Lots of bridges would fall down if this were not the case.

- Stirrups crossing theoretical cracks is not the only thing holding the shear from causing the beam to fail. That's why ∅V=vc+vs, both the concrete and stirrups contribute. The stirrups are critical, however, and I am not suggesting you remove any of them, far from it: Once you've removed the FAILED cover concrete, you need to assess how much stirrup steel you've lost. Frequently when the longitudinal bars (deeper inside the beam) are corroding, the stirrups are more far gone. Once you've supplemented the stirrups, the new concrete cover you have installed will enable the new stirrups to add shear strength to the beam and restore (likely increase) the beam's capacity.