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can i increase footing tie beam size by bonding agent only? 4

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delagina

delagina

Structural
Sep 18, 2010
1,008
can i increase footing tie beam size by bonding agent only?

or do i need dowels to develop new concrete to existing concrete?

if i need dowels, how do i calculate number of dowels required.

thanks,
 
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PS the picture is NOT PLAN view but section view of a footing tie beam
 
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I think you can, but might not comply with some relevant structural checks (computed crack width?); s0, such big new cover would ask for new rebar in the addition; and all else will depende on the quality and magnitude of the forces/stresses/strains for the application. RC codes are not thinking of adhesives, so from their viewpoint if some mechanical attachment is feasible would look preferable from RC codes viewpoint, but it maybe unreasonable for some cases not to use adhesive means. There's an expensive -yet practical- industry of specialties making their living on reinforced carbon fiber and adhesives.
 
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I would not rely on any form of "adhesion" of new to the old.

Kinda depends on why the increase in size is necessary. Consider that the existing concrete has probably undergone most of its shrinkage. The new concrete will shrink with respect to the existing, and will compromise the bond that you're looking for.

If you need the additional cross-sectional size for compressive forces, then your encasement method may work. I would roughen the existing surface, use a good bonding agent, and provide shrinkage/temperature reinforcement using more frequent small bars rather than large.

If you need the additional cross-sectional size for tensile forces, I would consider casting an additional beam (or 2) next to the eisting with no attempt to bond to the existing, then concentrate on developing the end anchorage into the footing or cap at each end. Any shrinkage cracks that develop will be distributed over the length of the new beam(s) and should not be of a size for concern.


Ralph
Structures Consulting
Northeast USA
 
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I prefer dowels, but that is because I don't know much about the newest bonding agents. As ishvaag states, they may be adequate without dowels.

The spacing of dowels would be dependent on the shear force to be transferred between new and existing concrete. The capacity of one dowel in shear is determined by shear friction principles if the dowel is adequately anchored each side of the joint. Dowels can be anchored into the existing concrete with epoxy so that they develop their full yield strength.

BA
 
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adhesives are very very very very dependant on application parameters such as surface preparation, porosity, workmanship e.t.c.

This is why chemical anchors have a factor of safety of 4 and this is just for simply filling a hole with the stuff and shoving in an anchor bolt (much simpler than what you propose).

Glulams undergo a lot of testing to ascertain the effectiveness of their adhesives.

So in summary, I would not recommend the use of adhesives for one off applications that are not backed up by extensive testing.

Is there any reason why the existing beam cannot simply be demolished and replaced with the new larger beam?
 
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thanks,

i'm just checking another person's work, and he increased the size because development length is short on the new tie beam perpendicular to existing.

i guess in this case it may be ok loads are minimal.
 
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Adhesives/bonding agents are not appropriate for this application, particularly since the existing is already failing in shear. Use dowels. As csd72 pointed out,adhesives are dependent on many factors. Bonding agents are even less reliable than adhesives.
 
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If the beam is also failing in shear, I would suggest also extending the concrete under the existing beam so that addidional stirrups can be added. Otherwise, you are just adding concrete technically unreinforced for shear.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
 
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This is a footing tie beam - I would expect axial load to control more than anything else.

As far as expense - one does not need to be fussy with demolition, except at each end of the existing. Proper surface prep (top & sides) and bonding agent application will be time consuming. I have to believe it would be less expensive to replace rather than enhance, and you get a little more peace of mind that the fix will be reliable.


Ralph
Structures Consulting
Northeast USA
 
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footing tie beam superstructure has been removed. currently no load except the new loads.

i do feel demolish and replace is better than roughening the top and side then applying bonding agent.

i'll discuss with him.
 
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Bonding agents are time dependent, so often work as bond breakers when the time parameters are not adhered to. It would be nice to have a "magic" glue, but none exists yet.
 
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You might be able to demo back the existing reinforcement, and in accord with ICRI guidance, under the existing by more than the nominal aggregate size of the new concrete. Since the concrete can be placed into an open top form, consolidation shouldn't bee too much trouble. Use a very low W/C ratio mix with high range water reducer and viscosity modifier to help reduce shrinkage and segregation. Consider shrinkage-reducing admix.

No need to remove the concrete core inside the existing cage. The new stirrups should be fine.

Pre-wet the existing to saturate surface dry. Cure the new material properly to reduce shrinkage.
 
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Whenever bonding new concrete to an existing concrete substrate I have always provided a bonding agent like the product which I uploaded to ensure a good bond of the new concrete to the existing. But I also call up drilled and epoxied reinforcement to cross the joint if I want the sections to act compositely. If I was relying solely on the bonding agent then I would want to make sure I am providing 100% supervision during installation.

To solve your shear problem I would dowel into the existing footing and rely on dowel action to transfer the shear. The calculations on the section will be complicated by the fact that there is pre-existing stresses on the cross-section which need to be considered.

Best of luck.
 
 http://files.engineering.com/getfile.aspx?folder=72c134b9-bcdd-4909-81b7-c058b973bb40&file=Parchem_Nitobond_EP_(Bonding_Agent).pdf
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From a very pragmatic viewpoint - all of the suggestions offered are worth considering, BUT the best solution is the one that offers the highest assurance that design parameters are met for the least cost.

Add up the total cost of surface prep, form & rebar installation, bonding application, concrete placement & curing, form removal, and close supervision for all of this - I submit that the best solution is demo & replace.


Ralph
Structures Consulting
Northeast USA
 
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The product data sheets for the Parchem bonding agent which kikflip attached are worth a read. Note particularly the time requirement (still tacky and not more than 90 minutes) before concrete placement. This is tricky to achieve in practice, thus I don't depend on these materials.
 
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