HILTI HIT-CT1 Post Installed Rebar Connection Instruction Manual
- June 14, 2024
- HILTI
Table of Contents
- HIT-CT1 Post Installed Rebar Connection
- INTRODUCTION
- GENERAL
- POST-INSTALLED REINFORCING BARS IN TUNNELS – DEFINITION
- STATIC DESIGN OF POST-INSTALLED REBAR CONNECTIONS
- FATIGUE DESIGN OF STRUCTURAL POSTINSTALLED REBAR CONNECTIONS IN TUNNELS
- SEISMIC DESIGN OF STRUCTURAL POSTINSTALLED REBAR CONNECTIONS IN TUNNELS
- REQUIREMENT FOR 100 YEAR DESIGN LIFE
- FIRE
- CORROSION
- HILTI PRODUCT BASKET FOR POST-INSTALLED REBAR CONNECTIONS IN TUNNEL
- ON SITE TESTING TO SUPPORT IMPROVING INSTALLATION QUALITY OR DESIGN
- SUMMARY
- REFERENCES
- Read User Manual Online (PDF format)
- Download This Manual (PDF format)
HIT-CT1 Post Installed Rebar Connection
TUNNEL STRUCTURES
Concrete to concrete connections with post-installed reinforcing bars Authors: Dr. Jörg Appl, Dr. Philipp Grosser and MSc. Riccardo Figoli
INTRODUCTION
Concrete work in underground structures is often difficult due to space restrictions and/or cross-section geometry. For example, in many tunnels suspended ceilings are installed to create separate chambers for longitudinal ventilation. It is nearly impossible to create this in one construction step with the inner lining. In the end, any concrete element that has to be connected to the lining, resulting in a monolithic connection such as walkways, suspended ceilings, vertical track dividers and corbels, must be done in a subsequent process and may result in a post-installed rebar connection. Figure 1 provides a schematic overview of possible concrete to concrete connections in tunnels by connecting cast-in-place concrete, prefabricated concrete units or UHPFRC structures with the concrete lining or within the components themselves.
Figure 1
Example of possible concrete to concrete connections using post-installed
rebars in tunnels; wall to slab connection, slab extension, wall extension and
slab to wall connections.
To realize concrete-to-concrete connections in tunnels with post-installed
rebar, chemical injection adhesives are preferred over the traditional bagged
cement grout because of their ease of use and quality of application by
providing a complete installation and cleaning system to minimize installation
errors. There are numerous systems readily available in the market with
different or similar product and performance characteristics covered in
European Technical Assessments (ETAs). However, if not dealing with post-
installed rebar systems every day one may find it difficult to understand what
kind of technical boundary conditions are considered in case of these
different ETAs and what kind of product should be used for the design of such
post-installed rebar connections.
It is the intention of this article to provide an overview of the use of post-
installed rebar in concrete-toconcrete connections in tunnels. It should be
noted that this paper does not distinguish between the differenttunnel types
(rail tunnel, road tunnel, utility tunnel, etc.) in detail but focuses on the
technical requirements of post-installed rebar connections in general.
GENERAL
The post-installed rebar systems for concrete-to-concrete connections in
tunnels are in general selected based on structural considerations and are
typically designed and detailed by a structural engineer. A detailed technical
design is needed because post-installed rebar failures can lead to safety
hazards and significant economic loss.
The design establishes whether the requirement of the ultimate limit state
(ULS) and serviceability limit state (SLS) are met. At the ultimate limit
state, it must be verified that the design values of actions do not exceed the
design value of the fastening resistance. The serviceability limit state
includes requirements for limiting deformation or requirements on durability
as corrosion, chemical attack, temperature and other factors that may occur in
tunnels. The following aspects need to be considered in the analysis of
the ultimate limit state and serviceability limit state for post-installed
rebar connections:
- Type of action (static [short-term vs. long-term], fatigue, seismic, shock and fire)
- Corrosion
- Design life
- Applicable design code or guideline
Additional economical or quality aspects may be considered already in the design or the specification by, for example, specifying proof loading or test loads.
POST-INSTALLED REINFORCING BARS IN TUNNELS – DEFINITION
A post-installed rebar connection consists of a reinforcing bar (rebar)
installed with chemical adhesives in holes drilled into the existing concrete.
The reinforcing bars connect the new and existing concrete by casting the new
elements against the existing structure after the chemical adhesive is
hardened (Fig. 2). A post-installed rebar connection can be used equivalent to
a straight bar cast in concrete if the adhesive is qualified accordingly. An
example is shown in Figure 3 for the connection of a corbel to the unnel
lining. A post-installed rebar application can be characterized as follows:
(a) Post-installed reinforcing bars are straight or can be equipped with hooks
or heads on the cast-in end and are necessarily straight on the post-installed
end (Fig. 2 and Fig. 3).
(b) Post-installed reinforcing bars, in contrast to adhesive anchors, are
often installed with small concrete cover (2φ < c < 3φ, where φ is the
reinforcement bar diameter and c is the concrete cover). This geometrical
boundary condition can be given by the individual geometry of the pre-cast
concrete segments of the tunnel lining in case of a TBM driven tunnel. In such
cases, the strength under tension loading of the post-installed rebar
connection is typically limited by the splitting strength of the concrete (as
characterized by splitting cracks forming along the length of the bar).
(c) Post-installed reinforcing bars are typically not designed to resist
direct shear loading, compared with rebars designed as bonded anchors or
concrete overlay connections (shear dowels). In case of postinstalled rebar,
shear is typically transferred by a roughened surface between existing and new
concrete (Fig. 2).
(d) Post-installed reinforcing bars are in general embedded as required to
“anchor” their design stress σsd using the required anchorage length and
splice length provisions of Eurocode 2: “Design of concrete structures – Part
1-1: General rules and rules for buildings” [1]. In order to achieve ductility
of the structure, the design stress is often close to the design yield
strength.
(e) Also the basic provisions for the anchorage length regulated in the EOTA
Technical Report 069 with improved bond-splitting behavior [2] as compared to
EN 1992-1-1” [1] can be applied. Under such conditions, failure can be
governed by steel failure of the rebar under tension or concrete related
failure modes. EOTA TR 069 [2] is a combination of reinforced concrete design
and anchor design in which several boundary conditions must be considered when
using this design approach. For more details concerning the application and
the design concept of EOTA TR 069 see [3].
Figure 2
Post-installed reinforcing straight ohooked bar (typ.) [4]Figure 3
Post-installed rebar connection (corbel to tunnel lining) 3.1 Application range
Post-installed reinforcing bars are typically used to create a monolithic
connection between new concrete elements and the existing tunnel lining. Post-
installed reinforcing bars are used in both retrofitting of tunnels and in new
construction and are suitable for a wide range of applications in tunnel
construction.
Examples of common applications of post-installed reinforcing bars in tunnel
construction are:
- Opening of tunnel lining and partly closing due to installation of edge reinforcement
- Reinforcement of concrete whaler/ diaphragm wall
- Securing and positioning of reinforcement steel meshes
- Replacement of misplaced cast-in rebar couplers
- Moment resisting connection of corbel and tunnel lining for intermediate slab
- Concrete-to-concrete connection of concrete foundation with tunnel lining (may also be designed as shear-dowel application according to EOTA TR 066 [5])
Above-mentioned applications usually require the placement of a large number of bars with often close spacing. To help avoid drilling through or damaging existing reinforcing bars in the tunnel lining, reinforcing detection equipment, such as the Hilti PS 250 or Hilti PS 1000 scanning systems, can be used.
STATIC DESIGN OF POST-INSTALLED REBAR CONNECTIONS
Figure 4
Schematic loadcarrying mechanism of deformed reinforcing barsAlthough the load-carrying behavior of cast-in rebar in concrete is
not identical with a post-installed rebar, the basic load transfer of an
acting tension force into the concrete is similar. Both cast-in and
postinstalled rebar generate a rotationally symmetric stress pattern around
the bar. Equilibrium is provided by the hoop stresses (tangential) in the
concrete. Same failure modes of cast-in and post-installed rebar can be
observed. The rebars can fail by steel rupture, pullout/bond failure and
splitting failure. The only difference is that for post-installed reinforcing
bars, the tension loads are transferred by mechanical interlock from the
reinforcing bar’s ribs to the mortar and via bond (combination of adhesion and
micro keying) from the mortar into the concrete member whereas for cast-in
reinforcing bars, the tension loads are directly transferred from the rebar to
the base material (Fig. 4).
Until 2018 the EOTA Technical Report 023 “Assessment of Post-installed
Reinforcing Bar Connections” [6] provided guidance for verifying that post-
installed reinforcing bar connections installed with a specific mortar system
exhibit comparable behavior to cast-in-place reinforcing bar connections in
terms of load and displacement behavior under several environmental
conditions. Since 2018 EOTA TR 023 [6] has been replaced by EAD 330087-00-060
“Systems for post-installed rebar connections with mortar” [7]. As a result, a
post-installed reinforcing bar system assessed by [6] results in at least
comparable bond strength and comparable displacement behavior as cast-in-place
reinforcing bars taking into account the influencing factors stated in the EAD
330087. Due to this core philosophy, the design of post-installed reinforcing
bar connections can be done according to the provisions for cast-in-place
reinforcing bars according to EN 1992-1-1 [1]. Usually, the application range
of post-installed rebar is limited to:
(a) Overlap joints of rebar connections for slabs and beams and overlap joints
at a foundation of a column or wall by means of a non-contact splice. In this
case the tension loads are transferred between adjacent bars via compression
struts. The tension forces generated by the hoop stresses are taken up by the
stirrups or transverse reinforcement, respectively, in the splice area.
(b) Simply supported beams and anchoring of reinforcement to cover the line of
acting tensile forces.
To overcome these limitations an EOTA Technical Report (EOTA TR 069 [2]) was
developed and published in 2019.
Figure 5
Design bond strength as a function of the related concrete cover,
schematically
EOTA TR 069 [2] allows to design moment-resisting post-installed rebar
connections without the execution as a lap splice according to EN 1992-1-1
[1]. EOTA TR 69 [2] is utilizing the bond splitting behavior of post-installed
rebar systems taking into account the concrete cover in the design equations.
According to Figure 5 the value of the minimum concrete cover is greater than
2φ (where φ is the diameter of the reinforcing bar). Post-installed rebar
systems (e.g. Hilti HIT HY 200 R V3) exhibit significantly higher bond-
splitting behavior than cast-in-place bars of equivalent bar diameter and
anchorage length. This behavior can be qualified and assessed according to EAD
332402-00-0601 “Post-installed reinforcing bar (rebar) connections with
improved bond splitting behavior under static loading” [8]. It should be noted
that the testing is extensive when compared to post-installed rebar
connections that are limited to the design according to EN 1992-1-1 [1] where
only the comparability of the post-installed rebar with a castin rebar is
verified.
However, both EADs (EAD 330087-00-0601 [7] and EAD 332402-00-0601 [8]) provide
safeguards to restrict post-installed reinforcing bar systems that exhibit
very low stiffness or brittleness compared o a cast in bar.
The allowable concrete-to-concrete connections taking into account connection
type, allowable forces, design method, required EADs and covered load cases as
shown in Figure 6.
Figure 6
Allowable concrete-toconcrete connections taking into account connection type,
allowable forces, design method, required EAD and covered load cases
Connection as…| Splice| End-
Anchorage| End-
Anchorage| End-Anchorage
---|---|---|---|---
Forces and Moments| Yes| Forces only| Predominant compression or Strut & Tie
models| Yes
Examples| All members connected via a splice (extension,
slab to wall, etc.)| Simply supported
beams or slabs| Wall/column to foundation| Column to foundation| Wall to
foundation| Slab to wall| Beam to wall| Beal to
column
Applicable design method| Eurocode 2 / Eurocode 8| EOTA TR 069
Required EAD| EAD 330087-0| EAD 332402
Load cases| Static, sustained loading, fire,
50 years, 100 years, seismic| Static and sustained loading, 50 years, 100
years, seismic
FATIGUE DESIGN OF STRUCTURAL POSTINSTALLED REBAR CONNECTIONS IN TUNNELS
When a high-speed train is entering or passing through a tunnel, a complicated
system of pressure waves develops and propagates through the tunnel and in
addition the concrete foundation may be exposed to repeated loads. The
resulting loads during train-tunnel passage may play an important role in the
structural design of concrete-to-concrete connections. Material fatigue is
relevant not only for high-speed train tunnels but also in road tunnels
designed for an additional operational loading due to the wind pressure and
suction caused by the moving vehicles, especially when entering the tunnel.
The authors see an increasing demand on fatigue-approved solutions in tunnels,
especially in rail tunnels with high to very high load cycles over the service
life of the connection. Unfortunately, while the research in case of anchors
in concrete loaded under fatigue gained importance in the last decades,
research on post-installed rebars under fatigue loading is rather limited. As
a result, current design [10] and assessment provisions [11] for post-
installed anchors in concrete include provisions for fatigue whereas
qualification and design provisions for post-installed rebars loaded under
fatigue are not existing.
Therefore, the following discussion has to be seen as a possible approach to
tackle post-installed rebar applications under fatigue. A simplified method is
provided in the following. The actions to be used in design may be obtained
from national regulations or in absence of them in the relevant parts of EN
19921-1 [1].
As a simple and conservative approach, fatigue is verified if the following
equation is fulfilled:
Steel failure:
ΔNEd < ΔNRd,E,n
where ΔNEd = design fatigue action
ΔNRd,E,n = design fatigue resistance of the post-installed rebar for pulsating
or alternating load taking into account the required number of load cycles
In cases where actions consist of a combination of a non-negligible lower
cyclic load and a fatigue relevant part, it is necessary to determine the
influence of the lower cyclic load on the fatigue resistance.
This is achieved by using the Goodman diagram which is in general available
for rebars. In absence of such a diagram, ΔNEd shall be replaced by
ΔNEd,simple as follows:
Concrete cone or pullout failure:
ΔNEd,simple < kfat,red ∙ Rd
where ΔNEd,simple = simplified fatigue design action
kfat,red = Reduction factor for fatigue in case of bond and concrete failure
taking into account the number of load cycles
Rd = design static resistance
For a simplified design in case of pullout and concrete cone failure, all
loads are assumed to be fatigue relevant (ΔNEd,simple = NEd + ΔNEd). It is
obvious that in case of low percentage of the fatigue load compared to the
static value, this approach may yield to relatively conservative results. With
this approach a normal “static” PROFIS Rebar calculation may be performed
applying the reduction factor for fatigue in case of bond and concrete failure
taking into account the number of load cycles.
5.1 Proposed method for evaluating the reduction factor in case of concrete
cone failure when applying the logic of EOTA TR 069 [2]
Figure 7
Comparison of S-ncycl curves corresponding to concrete cone failure with S-n
curves for other failure modes; curve 1: concrete cone breakout, curve 2: bond
failure, curve 3: concrete under uniaxial tension, curve 4: concrete under
uniaxial compression, taken from [12]
EOTA TR 069 [2] covers the design of concrete-related failure modes under
static loading. To apply the same design concept for fatigue loading, a
reduction factor for concrete related failure modes needs to be considered.
Typical concrete related failure modes under tension loading are pull-out
failure, concrete cone failure and splitting failure. The relative capacity of
headed studs as a function of load cycles (Sncycl curve) is shown in Figure 7
for concrete cone breakout, bond failure and concrete under both uniaxial
tension and compression. Figure 7 can be also transferred to reinforcing bars
causing concrete splitting failure as expected in the esign philosophy of
EOTA TR 069 [2]. It is assumed that the fatigue strength for concrete related
failure modes is at least equal to the fatigue strength of concrete subjected
to uni-axial tension. This assumption is justified limiting the maximum
fatigue load to 50% of the average static failure load and the number of load
cycles to n = 107 (kfat,red,CC ~ 0,5 for n = 107).
5.2 Proposed method for evaluating the reduction factor in case of pullout
failure when applying the logic of EC2 [1] and EOTA TR 069 [2]
Research on the fatigue behavior of the bond between steel element and mortar
or mortar and concrete is limited [12]. Spot testing is presented in [13].
Test parameters were chosen in such a way that pullout failure occurred. The
results indicate that, in principle, the behavior found with cast-in-place
reinforcing bars also applies to threaded rods anchored with resin. The ratio
of Nmax/Nu is shown in Figure 8 as a logarithmic function of the number of
load cycles at failure for specific epoxy and hybrid systems. For comparison,
the results of tests with cast-in bars are shown as well. The fatigue strength
at two million load cycles (log N = 6.3) is approximately 60% of the short-
term bond strength (kfat,red,bond ~ 0,6 for n = 2 ∙ 106).
In conclusion, the fatigue design bond strength of a post-installed
reinforcing bar system is not provided in an ETA due to missing regulations on
the design and qualification side. For simple cases, it may be reasonable to
apply assumptions in design considering the material behavior under fatigue
loading of concrete, bond and steel.
Figure 8
Ratio Nmax/Nu as a function of number of load cycles at failure [13]
SEISMIC DESIGN OF STRUCTURAL POSTINSTALLED REBAR CONNECTIONS IN TUNNELS
Historically, underground utilities have experienced a low rate of damage
during earthquakes than surface structures for a given intensity of ground
shaking because the imposed ground strains are lower at higher depths.
However, tunnels may suffer from damage due to earthquake loading by showing
lining cracks, shear failure of lining, tunnel collapses caused by slope
failure, portal cracking, leaking and deformation of sidewall/invert damage
[14], [15], [16], [17]. It was found that for peak ground accelerations (PGAs)
equal to or less than about 0.2g, ground shaking caused minor damage. For PGAs
in the range of about 0.2–0.5g, some instances of slight to heavy damages were
observed, whereas for PGAs larger than 0.5g there were many instances of
slight to heavy damages. This may lead to the need – based on the project
specification – that the concrete-to-concrete connection may also be designed
considering seismic conditions. It should be noted that upfront of such a
seismic design the effects of ground motion on the underground structure by
determining the additional loading imposed by ground shaking and deformation
must be assessed and special requirements for reinforcement detailing may be
followed.
With EAD 330087 [18] a qualification process for post-installed rebar is
existing that allows a design according to EN 1998-1 “Design of structures for
earthquake resistance” [19]. The assessment of postinstalled reinforcing bars
under cyclic (seismic) loading is conducted following the same logic adopted
in the case of static loading. The performance of the system in the case of
pullout (bond) and splitting failure is compared and related to the
performance of cast-in bars by means of comparing and assessing the bond
strength degradation of a post-installed bar system with the number of cycles.
In conclusion, the seismic design bond strength of a post-installed
reinforcing bar system fbd,seis that can be used in combination with the
requirements of EN 1998-1 [19] is provided in the related ETA. Additional bond
efficiency factors kb,seis (reduction factor) may be applied to the design
bond strength taking into account the drilling systems and borehole
conditions.
REQUIREMENT FOR 100 YEAR DESIGN LIFE
Nowadays there are more and more requests from owners or operators of tunnels
for an extended service life from 50 years to 80, 100 or even 200 years. The
authors believe that this is a rapidly growing international demand also on
post-installed rebar applications. However, it should be noted that the design
life should not be confused with the service life. The service life relates to
the period that the tunnel is expected to be in operation. In contrast, the
design life represents the period on which the statistical derivation of
transient loads is based on. The requirement for a service life and/or design
life of 100 years is based on the goal of minimizing maintenance requirements
and to help that the investment is spent in a rational way.
The variant of the EAD 332402-00-0601-v01 [9] provides the answer to an
extended working life for postinstalled reinforcing bar connections of 100
years. This EAD is also the basis for Hilti to provide engineering judgments
for a working life of 120 years. The biggest difference in the assessment for
an extended working life in comparison to a working life of 50 years is that
the long-term test is modified from a 50-years bond-strength estimation to a
100-years projection (120-years projection as an engineering judgment outside
of the EAD).
However, it is important to note that the design life assessment in [9] is
limited to the bond between mortar and concrete (bond strength) by providing
bond strength values for 50 years and 100 years. The durability of the steel
element (rebar) and the surrounding concrete is not considered within the
scope of the European Assessment Document. Consequently, the EAD assumes that
the material specific parameters of the concrete and the steel are not
negatively influenced by the design life. Important is the definition of the
correct exposure class in the tunnel projects, maximum water cement ratio,
minimum cement content and consequently the required nominal concrete cover of
the reinforcing bars for an extended working life.
In conclusion, the 100-year design bond strength fbd,PIR,100y of a post-
installed reinforcing bar is provided in the related ETA. The design process
is equal to the design for 50 years by replacing the bond strength of 50 years
fbd,PIR, 50y with the 100-year design bond strength fbd,PIR,100y. Additional
bond efficiency factors kbr (reduction factor) may be applied to the design
bond strength taking into account the drilling systems and borehole
conditions.
FIRE
When post-installed reinforcing bar connections are part of a fire-rated
assembly (floor, roof, etc.), it is important that the fire resistance of the
connection is evaluated using test data for the time-dependent reduction in
bond strength associated with typical geometries and time-temperature loading
protocols. These elements are designed and constructed to provide a specific
period of fire resistance (R), typically rated for 30, 60, 90, 120, 180 or 240
minutes.
In general, the capacity of post-installed reinforcing bars is reduced when
exposed to fire. The bondstrength degradation is highly product dependent.
Main parameter is the composition of the used adhesive material (inorganic or
organic such as vinyl ester or epoxy). Consequently, if post-installed
reinforcing bars are part of a fire-rated assembly it is important to know the
time-temperature dependent reduction in bond strength to properly design the
connection.
The bond strength of post-installed rebar subjected to fire is assessed on
European level based on tests according to EAD 330087-00-0601 [7]. The
European Technical Assessment (ETA) provides an equation to calculate the
design value of the bond resistance under fire fbd,fi.
The bond resistance in the cold condition is multiplied with a reduction
factor under fire exposure. The bond resistance in the cold condition depends
on concrete class, rebar diameter, drilling method and bond conditions
according to EN 1992-1-1 [1]. In case the temperature along the post-installed
rebar is known, the anchorage length can be calculated according to EN
1992-1-1 [1] using the temperature-dependent bond strength fbd,fi.
The determination of the temperature in the mortar layer is easier in case of
equal distance along the length of the post-installed rebar to the flamed
surface (Fig. 9a). A constant temperature distribution can be assumed that
depends on the exposure time and concrete cover. In case of varying distance,
the determination of the temperature along the length of the post-installed
rebar is only possible with the help of numerical analyses (Fig. 9b). The bond
strength is not affected along the entire anchorage length. The load is
transferred in regions with lower temperature where no decrease of bond
strength takes place.
Figure 9
Simplified temperature distribution in the mortar layer depending on the
location of the rebar relative to the flamed concrete surface a) Post-
installed rebar located parallel to the flamed concrete surface
b) Post-installed rebar with uneven temperature distribution along
the length of the bar
The requirements for the fire load case in a tunnel can be different depending
on the application and type of tunnel. To minimize the damage in case of a
fire event, the temperature on the concrete surface and the temperature in the
reinforcement should be limited. Note, both the concrete and steel temperature
depend on several parameters (e.g. exposure time, concrete cover, protection
of concrete member).
Based on the experience of the authors and as a simplification, the following
temperature limitations should be used: concrete surface (200°C to 380°C) and
reinforcement (250°C to 300°C). In case of higher temperatures on the concrete
surface, fibers of polypropylene or steel should be incorporated into the
concrete. However, it is noted that at such high temperatures organic adhesive
material is showing a very low bond strength fbd,fi which is only 10-20% of
the bond strength in cold condition fbd. Especially for applications in which
the rebar is parallel to the flamed concrete surface the impact is more
pronounced which often leads to challenges in design.
To overcome this challenge, Hilti developed an injectable inorganic calcium-
aluminate-based cement for post-installed rebar connections, named Hilti HIT-
FP 700-R. Compared to organic mortar systems, which show no residual capacity
at 500°C, Hilti HIT-FP 700-R has been tested up to 500°C (932°F) and
experiences a very low reduction of its bond capacity compared to concrete for
which a reduction of 40% is assumed at 500°C, see Fig. 10.
Figure 10
Reduction factor under fire exposure kb,fi ( θ ) for Hilti HIT-FP 700-R
compared to several organic mortar systems in the market and concrete
(example: concrete strength class C20/25))
CORROSION
Concrete is an alkaline material and under normal conditions corrosion of
cast-in reinforcing bars is prevented by passivation of the bar surface.
However, when concrete undergoes carbonation, its decreased pH value can break
the passivation film and allow corrosion. Furthermore, accelerated corrosion
rates (pitting corrosion) are observed if the concrete is contaminated with
chlorides.
Consequently, the qualification of systems for post-installed rebar
connections with mortar includes a specific test for the susceptibility of the
system to long-term bar corrosion.
Figure 11
Test setup to assess the long-term rebar corrosion [6]
After curing of the mortar, a concrete member with an embedded post-installed
rebar is immersed into a container filled with artificial tap water (sodium
sulphate and sodium bicarbonate) while each rebar is connected to a cathode,
see Fig. 11. The current between the rebar and the cathode is determined by
measuring the potential drop while additionally, the corrosion potential of
each rebar is measured by a voltmeter. The measured current flow and the
potential are plotted as a function of the time (duration of the test for at
least 3 months). The measured current flow and the potential must be below a
certain limiting value. In addition, a visual inspection of the rebar after
the test takes place to identify signs of corrosion products. If the
requirements are fulfilled, the post-installed rebar connection is assessed as
comparable with the corrosion resistance of cast-in-place rebars.
Consequently, it can be said that postinstalled rebars installed with a
qualified system should exhibit similar corrosion rates to cast-in-place bars
installed in the same concrete.
The Swiss Association for Protection against Corrosion (SGK) was given the
assignment to evaluate the corrosion behavior of fastenings post-installed in
concrete using the Hilti HIT-HY 200-R V3 and Hilti HITRE 500 (V4) injection
systems to provide further information about the corrosion behavior in
addition to the “pass/fail” criteria ccording to the related uropean
assessment document.
The results can be summarized as follows:
Hilti HIT-HY 200-R V3
- The Hilti HIT-HY 200-R V3 system in combination with reinforcing bars can be considered resistant to corrosion when they are used in sound, alkaline concrete. The alkalinity of the chemical mortar helps to ensure the initial passivation of the steel.
- If rebar is installed in chloride-free concrete using Hilti HIT-HY 200-R V3, in the event of later chloride exposure, the rates of corrosion are about half of those of rebar casted-in concrete.
- In concrete containing chlorides, the corrosion behavior of Hilti HIT-HY 200-R V3 corresponds to that of cast-in rebar. Consequently, the use of unprotected steel in concrete exposed to chlorides is not recommended because corrosion can be expected after short exposure times.
Hilti HIT-RE 500 V4
- If the Hilti HIT-RE 500 V4 system is used in corrosive surroundings, a sufficiently thick coat of adhesive significantly increases the time before corrosion starts to attack the steel.
- The Hilti HIT-RE 500 V4 system may be used in carbonated concrete containing chlorides if a coat thickness of at least 1 mm can be ensured. In this case, only the unprotected steel in the new part of the concrete joint is critical.
- In none of the cases investigated previously rusted steel (without chlorides) showed signs of an attack by corrosion, even in concrete containing chlorides.
Neither during this study an acceleration of corrosion was found at defective points in the adhesive nor there is any reference to this effect available in literature.
HILTI PRODUCT BASKET FOR POST-INSTALLED REBAR CONNECTIONS IN TUNNEL
CONSTRUCTION
Depending on the requirements – e.g., type of action (static [short-term vs.
long term], seismic, fatigue and fire), corrosion, design life, design
concept, installation – different products are offered by Hilti. Every product
has its strengths but also its limitations. Figure 12 shows the product
portfolio Hilti is offering for anchoring post-installed rebar. The overview
provides guidance on the selection of the product. Hilti is also providing a
software to design post-installed rebar (Hilti PROFIS Engineering) that allows
for a
faster and safer design of post-installed reinforcement connections.
Figure 12
Overview of Hilti products used for postinstalled rebar connections in tunnels
| | | |
---|---|---|---|---
Product name| HIT-FP 700| HIT-RE 500 V4| HIT- HY 200-R V3| HIT-CT 1
ETA-Rebar (EC2, static and quasi -static, 50 years design life)| e 8-40| 1
8-40| c 8-80| e 8-25
ETA-Rebar (EC2, static and quasi -static, 100 years design life)| (I) 12-40| 1
10-40| (v 10-40| –
ETA-Rebar (TR069, static and quasi-static, 50 years design life)| –| c1 8-40|
(i) 8-32| –
Seismic assessment for EC2| No| Yes| Yes| No
Max. fire temperature [it]| > 500| 152| 275| 376
Reduction at max. fire temperature| 0%| –,.. 80%| = 95%| = 90%
Working time at 21°C| 20 minutes| 30 minutes| 9 minutes| 4 minutes
Curing time at 21°C| 10 days| 7 hours| 60 minutes| 75 minutes
Pre-load time at 21°C| 5 days| –| –| –
Installation temperature [°C]| +5 to +40| -5 to +40| -10 to +40| -5 to +40
Simplified overview, details can be found in the relevant ETA. Pre-load time: 75% of final performance indicating the time when scaffold can be removed (available as Hilti technical data)
ON SITE TESTING TO SUPPORT IMPROVING INSTALLATION QUALITY OR DESIGN
ASSUMPTIONS
If a post-installed rebar system carries an ETA and is installed according to the manufacturer’s instruction for use (IFU) in a base material within the scope of the assessment, there is no need to verify the performance with on- site testing. There are only two reasons why on-site testing in tunnel construction is performed:
- In cases where the base material is not covered in the ETA non-destructive (proof loading) or destructive tests can be performed to determine the design resistance. One example is the use of concrete with a mix composition that is outside of the scope of the qualification according to the related European Assessment Document (EAD).
- To enable customer to control and potentially validate the quality of installation of the post-installed rebars, non-destructive tests can be performed on the job site (proof tests).
In case of non-destructive loading (proof loading), a tension load is applied
to the rebar. The customer must select the appropriate load level depending on
requirements. But in any case, loading on the system should not be so high as
to result in damage (e.g. in the form of yielding or permanent slip). Proof
loads should be defined by the responsible engineer and maintained long enough
– for non-destructive testing it’s normally minimum 60 seconds holding the
proof load – to guarantee no rebar movement. Proof loads are set as a
percentage of the tension capacity of the post-installed rebar, not as the
design tension load. Note that, depending on the embedment depth to diameter
ratio and the steel grade, the proof load might lead to yielding of the
reinforcing bar. In any case it should be verified that the proof load does
not exceed 80% of the nominal yield stress of the rebar.
Proof loading equipment is arranged with sufficient spacing to the post-
installed rebar to observe movement due to incorrect installation. If proof
load is used to determine the design resistance, loading equipment is arranged
with close spacing to the post-installed rebar to avoid failure of the base
material. Hilti provides a complete on-site testing engineering service with
appropriate testing equipment and a service for the customer for evaluation of
the result/full documentation.
In case of destructive loading a tension load is applied to the rebar up to
failure. The failure is typically characterized by yielding of the steel or
pullout failure of the rebar.
However, it is noted that on site testing (neither non-destructive or
destructive loading) cannot replace approval testing to assess the suitability
of the product. Also onsite testing does not serve to conclude on product
performance between different products (product A is better than product B).
SUMMARY
Post-installed rebar connections are important in tunnel construction to connect new concrete elements (e.g. ceiling- or floor connections) with the existing concrete structure. Knowledge about the different technical application conditions but also selecting the right post-installed rebar system is crucial. It is the intention of this paper to provide relevant background information about concrete-to-concrete connections in tunnels realized with post-installed rebar and give guidance for the selection and design of the post-installed rebar system.
REFERENCES
-
Eurocode 2 (EN 1992-1-1): Design of Concrete Structures – Part 1-1: General rules and rules for buildings, Brussel (2004)
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EOTA Technical Report 069 (2019): Design method for anchorage of post-installed reinforcing bars (rebars) with improved bond-splitting behavior as compared to EN 1992-1-1. June 2021
-
P. Woerle, J. Appl, G. Genesio: Bewehrungsanschlüsse für momententragfähige Verbindungen nach EOTA TR 069, Beton- und Stahlbetonbau 2020
-
Seismic Assessment of Post-Installed Reinforcing Bars with Mortar Based on the European
Organization of Technical Assessment. Available from (PDF):
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EOTA Technical Report TR 066 (2019): Design and requirements for construction works of post-installed shear connection for two concrete layers. April 2019, Amended October 2019
-
EOTA Technical Report TR 023 (2006): Assessment of post-installed rebar connections, November 2006
-
EOTA European Assessment Document EAD 330087-01-0601 (2020):
Systems for post-installed rebar connections with mortar. December 2020 -
EOTA European Assessment Document EAD 332402-00-0601 (Pending for citation in OJEU): Post-installed reinforcing bar (rebar) connections with improved bond-splitting behavior under static loading
-
EOTA European Assessment Document EAD 332402-00-0601-v01 (Pending for citation in OJEU):
Variant: Post-installed reinforcing bar (rebar) connections with improved bond-splitting behavior under static loading: 100 years working life -
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K. Uenishi, S. Sakurai: Characteristics of the vertical seismic waves associated with the 1995 Hyogo-Ken Nanbu (Kobe), Japan earthquake estimated from the failure of the Daikai underground station, J Earthquake Eng. Struct. Dyn., 29 (6) (2000), pp. 813-821
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