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RE: Values of dowels in existing concrete

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Thanks for raising this subject, Mr. Tang.  I'd be interested to hear what
others have to say.  A couple of comments from Hilti's perspective:

1.  The design of reinforcing dowels to connect old and new concrete
elements often has little to do with traditional anchor design theory.  We
conducted tests with Prof. Marti at the ETH in Zurich and with Prof.
Eligehausen at the University of Stuttgart to investigate the tension
performance of rebar set with our hybrid bonding mortar. (You can get a
design guide from your local Hilti field engineer covering the Marti tests.)
These tests were conducted with both large and small edge distances and
spacings, and involved long and short dowels.  The critical factor is
usually splitting, as opposed to the concrete breakout cone that anchor
theory is predominantly concerned with.  

2. We also ran tests on old to new concrete interfaces at our labs in
conjunction with the University of Innsbruck in order to determine design
shear transfer values for various surface roughening techniques and dowel
densities.  In this case, the dowels were relatively short, as our research
centered around the problem of bonding new topping slabs to existing
concrete bridge decks.  A thesis was published, and we have put together a
design aid based on this research.

3. With respect to special inspection, we would recommend that this be a
part of the specifications regardless of what bonding material you choose.
Workmanship is critical to the performance.

4.  Along with certain epoxies, Caltrans specifies the use of magnesium
phosphate grout (Master Builders markets one under the name SET 45) based on
tests conducted by John Dusel in the 70s.  This material can develop
relatively high bond stresses, but is difficult to get into a drilled hole
(my experience), and is generally only appropriate for down-hole
applications.  Again, special inspection is critical.

5.  The ICBO Evaluation Service issues evaluation reports based on
acceptance criteria (I strongly recommend that design professionals who rely
on these reports become familiar with the acceptance criteria on which they
are based.  You will find them on ICBO's web site.)  AC58, Acceptance
Criteria for Adhesive Anchors in Concrete and Masonry Elements, is really
only appropriate for the cases involving the introduction of discrete loads
into concrete or masonry via relatively shallow embedments, i.e., problems
that center around traditional anchor design theory.  It is probably not
appropriate for the problem you are facing.  On this note, Hilti recently
obtained an approval in Europe for our hybrid bonding mortar to bond
reinforcing bars using the rules of reinforced concrete design.  This
approval was based on the tests mentioned in 1.) above.  It is the first
approval of its kind, and may serve as a model for future developments in
this area.

Hope this helps.

J. Silva, SE
Hilti, Inc.

> ----------
> From: 	LtTang(--nospam--at)[SMTP:LtTang(--nospam--at)]
> Reply To: 	seaint(--nospam--at)
> Sent: 	Wednesday, February 17, 1999 9:28 PM
> To: 	seaint(--nospam--at)
> Subject: 	Values of dowels in existing concrete
> hello all,
> I'm curious about the use of dowels to connect existing concrete with new
> as
> in the case existing to new foundation.  I am wondering about what type of
> strength can the engineer assume or rather rely on.  Is there any theory
> behind this?  
> In most condition, one can use epoxy and rely on the testing data of
> manufacturers for design, but suffer that wrath of complaints regarding
> special inspection.  The alternative is to use other grouting material
> like
> "rapid set."  
> I have yet review the specs for "rapid set," but have seen engineers
> specify
> them on plans. 
> What type of values can engineers use?  
> Has "rapid set" been reviewed by ICBO or other testing agencies?
> Is there any special concern regarding this grout?  
> If design relies on dowels to carry load from existing to new, how can one
> design or justify through calculations for the use?  
> Your comments are always greatly appreciated.  Thanks
> Long T. Tang