Bonding agents for concrete

[phil327]

I have been doing some research on the various methods of bonding new concrete to old concrete. The greater majority of articles suggest a bonding agent that is applied to the existing concrete prior to adding the new concrete. There are lots of different products on the market and they all start with cleaning off the old concrete and then brush, spray or mash the bonding agent in to the old concrete.

Then, there are the bonding agents that you mix in with the concrete mix. Some of these even suggest brushing on the agent to the old concrete as well.

Are the mixed in varieties as good as the brushed on types? any studies as to how long each will last?

Is there a building code that states that a bonding agent must be used? Do the city inspectors look for this?

I would guess that the mixed in variety will cost significantly less money, taking into account the material costs and labor costs.

[Jerry Peck - Codeman]

[phil327]

Thank you for the complete answer. I'll have to read through this again to fully understand.

I think that part of my issue with the mixed in variety is that there is no way to verify what was put in the truck. I have heard of cases where the concrete company will add a gallon, where 5 should have been put in. There are too many places where costs can be eliminated and no one is the wiser.

[Jerry Peck - Codeman]

Just to clarify, you originally stated "bonding new concrete to old concrete", however, I took the other information as you were referring to exterior plaster, not to actual concrete. The ASTM C 926 standard is for exterior plastering (stucco), not concrete.

Typically, if you are placing new concrete next to old concrete (such as making a patio slab larger) there is no need for bonding agents. Instead, dowels are typically used to keep the new slab in place and aligned with the old slab.

If you are 'top coating' an old concrete slab to make it slightly thicker or to align with another surface, that 'top coating' would need to be applied in accordance with it instructions, which may or may not include a bonding agent.

I just want to clarify that you said "new concrete to old concrete" and that I am referring to 'stucco' - if you are not referring to stucco, then I ask for additional information as to what is being done and what the purpose of the "old concrete to new concrete' is. I want to make sure that I am/have properly understanding what you are asking.

[phil327]

I was referring to concrete, not stucco. Let me clarify this a little more.

This is not a patio deck with the concrete on the ground, These are all suspended, up to 7 stories high. The deck is the ceiling for the walkway below.

There are 4 foot wide walkways, the entrance to each unit is through these walkways. The outside edge had a brick wall. The brick wall was not solid and had a cap on the top. Every 4 feet or so, there were 2 vertical rebars that the bricks were attached to. There were a lot of cracks in the bricks, lots of loose bricks and poorly done repairs over the years. The city came in and told the association they had to remove all the bricks. These will be replaced with an aluminum rail system.

The balconies, more specifically, the edges are also poured concrete. The same repair work is being done to them as well.

In almost all places where the vertical rebar was, the underlying concrete slab was damaged. The concrete slab is 4 inches thick and is attached to the vertical walls with rebar. There were also many other areas where the slab was damaged. The company doing the repairs has cut out these bad sections, think notches at least 4 inches deep, or all the way through. I believe that all the cutouts have a piece of rebar exposed. So, there is a mechanical connection as well as a bonded connection.

There are no feathered edges, no thin set used.

While I do not think the repaired sections will fall out, I believe, that over time, the area between the new concrete and the old will crack. The reading I have done, ( as well as the engineer's initial specifications ) call for a bonding agent to be applied to the old concrete.

If you think of concrete as water, portland cement, and sand. Think of the sand the as large stones. If you apply a bonding agent to the old concrete then apply the stone to the bonding agent, it will stick. Now, if you just mix the bonding agent to the new mix, you are assuming that every grain of sand ( or stone ) will be completely covered with a bonding agent. This is not the case, some stones will have cement, some will have only water molecules in contact with the old concrete. The water will not bond the stone to the old concrete.

OK - I am presenting a worst case with the mixed in bonding agent. I guess the proper way to state this is OK, Better, Best. With OK having no bonding agent and only rebars, Better - having a mixed in bonding agent, Best - applying the bonding agent directly to the old concrete.

One of concrete roads in NY had repairs done 20 years ago. The patches that had a bonding agent applied to the old concrete are still there. Other patches have come loose and needed additional repairs. New York has freeze and thaw cycles, so any water getting into the crack will eventually force out the new concrete when it freezes.

I am probably nit picking this to death at this time. However, the restoration project is around a million dollars, a few thousand for bonding agents applied directly to the old concrete seems like a small price to pay for doing it the best way possible.

[Jerry Peck - Codeman]

Phil,

I typed out a long and detailed response yesterday ... but lost it in cyberspace while trying to post it, hopefully I can provide the same information in a condensed form with this post.

I fully understand your project and question now - concrete restoration to repair spalling concrete damage along the edges of balconies, decks, and walkways.

I have inspected many balcony/deck/walkway spalling concrete restoration projects, ranging from a single 7 story condo building to multiple 30+ story condo buildings within the same project - all having balcony edge restoration done at the same time (it was a massive undertaking, to say the least).

Regarding the bonding agent and the two choices: a) surface applied; b) admix which is mixed into the concrete.

The bonding agent does not hold the new concrete in place, that is the purpose of the steel reinforcement. At most, the bonding agent helps hold the concrete at the joint of the new concrete/old concrete together a bit tighter (and thus helping with water intrusion issues at the joint). There is no need to worry about the 'seal tightness' of the bonding agent if using surface applied bonding agent as there will always be voids in the bonding agent itself, between the bonding agent and the old concrete, and around the steel.

The mixed-in bonding agent is similar to mixing in the aggregate and concrete, it all mixes together. If you are concerned that the mixed-in bonding agent is may not be fully mixed in and coating the aggregate, then I would be more concerned that the concrete paste (concrete and water) is not sufficiently mixed to fully coat the aggregate and hold the aggregate together into solid concrete.

The mixed-in bonding agent is the simplest to use and the type susceptible to the least errors in application and use as it is mixed into the concrete mix and becomes part of the concrete. The surface applied bonding agent is more susceptible to application and use errors as it is to be applied to a dusty, dirty, too wet/too dry irregular surface with steel bars projecting out of the surface ... trying to get a proper cleaning of the surface and applying the bonding agent in accordance with its installation instructions is difficult and is seldom done 'exactly' as specified in the instructions (drying/curing/flash exposure time which is sufficient but not overly long such that the bonding agent needs to be re-coated) due to the elements one is working in (from rain to mist to cloudy to hot sun).

It is far more important that the architect/engineer's drawings and specifications be followed and that the architect/engineer be kept advised of site conditions which vary from those shown in the drawing.

For example, the drawings may show a 4" thick slab which may only be 3-3/4" thick, or steel size and placement in the original concrete slab which is not as shown in the drawing, or steel in the original slab which does not allow for the intended steel to be installed as intended.

Contractors take liberties with the architect/engineer's specifications by making undocumented changes to allow them to complete the work given the unexpected conditions encountered during the work - the architect/engineer needs to be advised of all such conditions which do not meet the original drawings so that the architect/engineer can provide a revised method suitable for each such condition. Once the architect/engineer provides the revised method for one condition, the contractor can use that method for all future conditions they encounter (provided the conditions fall within the revised method details, a similar condition covering a larger area may require revisions over what was detailed for the same condition in a smaller area).

The best thing is to hire an independent inspector/consultant/engineer/etc to inspect the work being done by the contractor. The independent inspection party is working for you and is inspecting the installed repair work in relation to what the approved engineering shows.

Another thing which is critical is planning and allowing for the guard rail installation at the end of the restoration project - the mounting of the guard rail posts needs to be addressed as does the guard rail post locations, otherwise all the repair work may be for naught as the guard rail installers core/drill through newly replaced steel, cutting the tension element (the steel) into discontinuous pieces instead of the steel being continuous along the edge. This is probably the second most common problem I find, the first most common problem is making changes to adapt to the site conditions without architect/engineer review/approval of those changes.

[phil327]

[Jerry Peck - Codeman]

[phil327]

The vertical walls are 4 inch poured concrete on the balconies. For example, let's say an area 14 inches front to back, and 28 inches top to bottom is removed, leaving a notch in the vertical wall. Take 2 pieces of plywood, 20 inches by 36 inches and attach to the sides of the wall. Keep the front flush with the front of the existing concrete. This will 'seal' the 2 walls. Now, take a third piece of plywood, 8 inches wide by 34 inches tall. Screw this to the front of the wall, but. the top of the plywood should be 2 inches below the top of the notch. This will leave an opening to pour the new concrete in. Then add a few small pieces of plywood to create a 'funnel' at the opening at the top. I could not see if there was any exposed rebars in any of these fixes.

Get your 5 gallon pail of concrete and start to shovel in the new mix into the opening. While you are doing this, every now and then tap the wood form. You can not see anything inside this box. There could be water at the bottom. ( the boxes have been assembled over the last few weeks and left empty. ) There could even be some critters in there.

After 2 days, remove the wood forms. From what I can see, they are going over each of these repairs with a thin coating of concrete to fill in any voids and to get the edges flush with the original wall.

The consultant was hired on a hourly basis, not to oversee the project. Basically to answer any questions.

I have no idea at this time as to the attachment method of the new railings. I am trying to be selective in my questions to the various parties, picking the ones that would seem to have the greatest impact.

[Jerry Peck - Codeman]

Ah, I see ... the spalling repair area has been chipped out back to, and into, the structures walls around the balconies (I was envisioning just around the outer edges of the balconies/walkways.

Not really many options for pouring those with concrete other than what they are doing, that method is used quite frequently, and any critters in them will simply become cast in concrete ... (bad joke) ... the contractor could take an air hose and spray around the inside of those forms just prior to pouring the concrete to help try to blow out any water (and critters). If the forms are wood, then let the water set a day and it will be gone anyway - it will soak into the wood and the wood will be damp (allowing the concrete to cure better than with dry wood which would suck some of the water out of the concrete).

Usually, when those 'funnels' are made such that they can be removed shortly after the concrete is poured which allows the excess concrete to be more easily trimmed off while the concrete is still 'green' (not yet 'hard', but good and 'stiff' to hold its shape).

[phil327]

Thank you for your complete answers to this. You made me aware of a few things that I had not thought of.

In a perfect world, certain methods would be the best. Given the fact that this is not a perfect world, and you have to take into account practical 'construction' methods are the better way to go.

I will see what I can find out about the railing attachment method they are planning on using.

thanks again

[phil327]

[Jerry Peck - Codeman]

[phil327]

We are sending a letter to the engineer with a request that they respond within 3 business days.
The three pictures are a sample of the core drilling they have done showing the cut rebar, Also interesting to note is the cracks in the concrete.

what would be the a proper way to install railings in this environment?

I found a few ways - core drilling ( the method being used here ) OR surface places OR I have seen edge clamps ( probably the most expensive )

This one the rebar is only cut part way through.


This one -the rebar has been cut all the way through.



This one shows the two pieces of rebar that run along the edge being cut. Also cut part way through is the rebar that runs from the wall to the edge, and a few cracks as well....

[Jerry Peck - Codeman]

[phil327]

The foam blocks would work if all the edge was being removed, Unfortunately, not all the edge was damaged and the repairs on the edge are hit and miss. This is old construction and not all the original rebar is located in exactly the same distance from the edges

[Jerry Peck - Codeman]

I recommend using the foam blocks where the repairs are, and locating the rebar at the proposed post locations by using x-ray or some of the new electronic instruments for locating rebar.

Then adjust the exact post locations to miss the rebar, which will also mean that the guard rail lengths will need to be adjusted to match the revised post locations.

[phil327]

I do not know how much thought went into planning where the posts would be for the railings. They have been manufactured and delivered here with a 'standard' length between the posts. The rebar running along the edge varies in distance from the edge by as much a 2 inches or so. I do not know if the rebar that comes out from the wall is any standard distance apart.

I am trying to get my head around the logistics of locating each piece of rebar on the walkways, then creating an order for the railing manufacturer to vary the post distance on each segment of railing, then having all this delivered and then having the installers drill out cores at a variable distance, then getting the correct piece of railing segment put in place. certainly not impossible, but most likely beyond the abilities of the companies involved here. There is around 3000 feet of railing to be installed

[Jerry Peck - Codeman]

This is one way to it with varying spaces between posts:

Let's presume the original intended post spacing was 8 feet, using a 100 foot or longer tape measure, mark the center of one end post then the centers of the other posts along the tape measure.

You have eliminated 96% of the area where you need to locate the rebar.

Check a 4" diameter area around the first end post center and the next post center for rebar, if none is found - good - the posts can be located as marked.

If rebar is found, check a larger area (lengthwise with the railing) for rebar. When you find a suitable area for the post without rebar mark that for center and go to the next post location.

The plan is to not keep reducing the spacing between posts but to vary the spacing around the original spacing.

That means trying for (as an example) 8 feet, 8 feet -2 inches, 8 feet + 4 inches, etc.

The 8 feet or less spacing uses railings built for the 8 foot spacing and cut to length.

The longer spacings will need longer railings to cut to fit. The railing company would need to supply some railing sections which were longer.

[phil327]

The catwalk railings have already been manufactured. We do not have all of them on site because some are still in the 'paint' shop. I did ask about cutting through the rebar for the cores. I was told ' That is not a problem because the material used to fill the cores will prevent any rust from forming. I also asked about the railing being segmented - nope - not a problem.

Is cutting through existing rebar a code violation, or is it just not the best solution ?

This is a picture of one of the balconies. The edge was removed back to the original pre cast slab. The are only 2 pieces of rebar. Some of the balconies are going to get a railing, which will require cores being drilled out. I think that there are 2 posts, so there will be 2 cores drilled out. I am reasonably sure that these will cut through the rebar. I asked if the center section would be at risk, told once again the it would take over 500 PSI to break the concrete. I would think that with no bonding agent holding the new concrete against the old, the mechanical strength of the rebar compromised - that down the road there is the potential for a problem.

[Jerry Peck - Codeman]

[phil327]

OK - they removed the edge of the concrete walkway, about 8 inches in. The steel posts ( stanchions ) that were clamped to edge were loosened to allow the concrete to be removed. The steel posts hold the 2x4s and the orange netting, creating the temporary railing system. I am not an engineer, but it seems to me that bailing wire attached to a jackpost and the other end attached to the top of the steel post to hold the temporary railing is - well - not the best practice. The bottom of the stanchion is 'resting' on the newly exposed rebar.

[Jerry Peck - Codeman]

Phil,

Sounds like you expected them to so something proper? You must be a man of never-ending hope for humankind.

You could put in a call to the local OSHA office to have an OSHA inspector visit the site by sending them the photo and description and expressing a concern for the workers. Not much else you can do, based on past information.