Meeting of the TG 4

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					     Meeting of the TG 4.5 in Stuttgart, 13 November 2006

Minutes of fib “Bond Models” Task Group 4.5 meeting on Monday 13th November 2006, at
the University of Stuttgart, Germany.

PRESENT: C. Alander (Helsinki), G. Balázs (Budapest), J. Cairns (Edinburgh), R.
Eligehausen (Stuttgart), M. Herzog Stuttgart), Appa Rao (), G. Metelli (Brescia), S.J.
Pantazopoulou (Xanthi), A. Wildermuth (Stuttgart), S. Williamson (Birmingham).


1. Opening and welcome. Apologies for absence. Approval of agenda.
2. Presentation and discussion of drafts for new MC Chapter 5 Section 1:
       1.     Splitting Model
       2.     Application rules for lap splices
       3.     Bundles, Corrosion, ECT, AAR
       4.     Influence of fR
       5.     Crack control - fR
       6.     Post-installed bars
       7.     Bond of GFRP bars and corrosion
       8.     Rib properties

3. Conclusion & Work for the next period

1. Opening and welcome. Apologies for absence. Approval of agenda.
Meeting starts on Monday 13th November at 9.00 am. J. Cairns (convenor) welcomes those
Apologies for absence were received from S. McCabe (Kansas), L. De Lorenzis (Lecce), J.
Jirsa (Austin), Lundgren (Gothenburg), Pilakoutas (Sheffield), G. Plizzari (Bergamo), S.
Tastani (Xanthi), R. Tepfers (Gothenburg), den Uijl (Delft), L. Vandewalle (Leuven).
Approval of Minutes from the Meeting in Budapest Feb 2006.
J. Cairns illustrates the agenda.

2. Activities of the New Model Code
Balázs reported on the actual time schedule, new chapters and general activities for the New
Model Code.

3. Presentation and discussion of drafts for new MC Chapter 5

1. Splitting Model (J. Cairns)

Presentation of the paper “Model of splitting for strength of lapped splices and anchorages”
by J. Cairns.
The actual model was presented and it was shown that the coefficients of variation are 13%
and 16% for test with and without links, respectively. The formulation considers the strength.
For strength up to 30 MPa the expression is relatively close to other expressions. In general
existing rules overestimate the influence of the concrete strength on bond strength.

Noted that same basic design bond strength should be given for anchorages and for laps.

Agreed the model represented a useful limit state simplification of physical behaviour and
should be presented on the left hand page of the new MC without partial safety coefficients
for use in assessment of existing structures.

Short discussion about the consideration of the geometry of the ribs.
Eli we shouldn‟t consider the fR-value for ULS.

Furthermore the second part of the presentation discussed the safety in order to formulate
application rules. Problem: global or partial safety factor, where to put the safety factors…
Also the ductility was mentioned and proposals to avoid brittle failure.

There had been some confusion over the definition of „anchorage length‟. Agreed that an
anchorage consisted of one bar (or bundle), whereas a lap consisted of two bars pulled in
opposite directions. The issue of anchorage was distinct from that of curtailment.

2. Application rules for lap splices (Stuttgart)

The expression of Stuttgart is based on defining the worst conditions for obtaining the bond
strength. A second proposal is based on a probabilistic study: the probability of concrete
failure is assumed to be 10-6.

It should be decided how to present the model: by taking the bond length or the strength?
Headed bars and hooks should be treated through a contribution to anchorage capacity
rather than as a coefficient on bond strength, as the current approach underestimates the
contribution with short bond lengths.

Agreed to provide design bond strengths based on development of design strength of bar
under least favourable conditions of confinement and with 100% bars lapped at section. The
design bond strength to be based on reinforcement attaining full yield, based on probabilistic
approach. Rules to provide for lesser bar stresses by linear interpolation, hence introducing
additional conservatism. Adjusting coefficient could be provided where confinement provides
an alternative to yielding of reinforcement to ensure a ductile failure mode.

3. Bundles, corrosion, ECT, AAR (Cairns)

The phenomena of corrosion (electrochemical restoration), freeze-thaw-damage and AAR
were shortly presented and the different possibilities to define the perimeter of bundled bars
were explained. A student at Brescia is conducting a literature review of bundled bars.

4. Influence of fR(Alander)

The ratio of the measured strength to the calculated increases with increasing fR.

Eli: Top bar effect is more important the fR-value.
 the general definition/influence should perhaps mentioned (LHS)
5. Crack control - fR (Alander)

With increasing fR the crack widths decreases, also influences on the deflection.
 the general definition/influence should perhaps mentioned (LHS)

6. Post-installed rebars (Simons)

It was decided that the part of “connections with post-installed reinforcing bars” in the design
guide for fastenings be reduced to fit within two pages, including references.

7. Bond of GFRP bars and Recovery of corrosion (Pantazopoulou)
Summary of bond behaviour of glass-fiber-reinforced plastic bars in concrete.

Presentation of an analytical model to describe the mechanical implications of corrosion
strength recovery is considered

8. Rib properties (Metelli)

Pull-out tests with ribbed bars were performed in confined concrete, perpendicular to the bar.
It was realised that the bond strength decreases with the diameter of the bar (~10%).
 again discussion with S. Lettow

4. Conclusion/Work for the next period

   The introduction has to be worked out for the final version
   Probabilistic study of Stuttgart: with a factor of 10-4 instead of 10-6
   Stuttgart + Plizzari  section for the local bond slip law
   Chapter 5.1: Basic local bond slip model (for normal materials) and one point dealing
    with alternative materials (self-compacting concrete  plizzari)
   Headed bars + hooked ends: Stuttgart + ???
   Prestressed anchorages  den Uijl with Carlo Pellegrino
   Special circumstances (slip forms, conditions of service) max. one page. Cairns
   Cyclic loading  Balázs
   SAG wishes a draft for January
   Consent: meetings in form of video conferences  next meeting in January or

A Wildermuth
Jan 2007

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