Product Technical Guides : US-EN Post-Installed Rebar Guide - Page 28

Post-Installed Reinforcing Bar Guide
6.0 WHAT ’ S THE BACK STORY ?
Note : Because the " top bar " factor , k 1
, accounts for bar position effects in freshly poured concrete , it may be neglected for the drilled-in portion of post-installed bars .
k 1 must be applied where applicable for the freshly cast-in portion of new bars and for the spliced portions of existing cast-in-place reinforcement .
Tension lap splices : CSA A23.3-19 12.15.1 A Class B splice taken as the greater of 1.3l d and 300 mm is required in all cases unless 1 ) the area of reinforcement provided is at least twice that determined by analysis over the entire length of the splice and 2 ) one-half or less of the total reinforcement is spliced within the lap length . Where 1 ) and 2 ) are satisfied , a Class A splice taken as the greater of 1.0l d and
300 mm may be used .
Table 8.5 provides a summary of development and splice lengths for a range of concrete strengths for HIT-HY 200 V3 and HIT-RE 500 V3 according to CSA provisions for the specific case where the confinement term ( d cs
+ K tr ) has been taken as the maximum value of 2.5d b
.
6.5 DESIGN OF POST-INSTALLED REINFORCING BARS BASED ON DEVELOPMENT LENGTH CONCEPTS
Figure 43 provides a comparison of the performance of post-installed and cast-in reinforcing bars from tension tests conducted at the University of Stuttgart . For small concrete cover ( approx . 2 in .) the failure loads of post-installed and cast-in reinforcing bars are shown to be nearly identical , verifying that , for splitting failure , qualified post-installed reinforcing bars behave the same as cast-in-place bars . At larger concrete cover , splitting no longer controls the behavior and the bars fail by pullout . In such cases , the bond strength of a post-installed reinforcing bar may be significantly higher than that of a cast-in reinforcing bar , depending on the adhesive used .
Mean bond stress [ N / mm 2 ]
14
12
10
8
6
Displacement [ mm ]
Large concrete cover
d b = 20 mm (# 6 )
4 h ef = 200 mm ( 7.8 in .) f ' c ~ 3000 psi
Concrete cover c = 50 mm 2
Cast-in-place bar Post-installed bar
0 0 2 4 6 8 1 0
In addition , the load-slip performance of post-installed reinforcing bars installed with a qualified system is similar to that of cast-in-place reinforcing bars . Thus , the design provisions for cast-in reinforcing bars in tension can be extended to qualified post-installed reinforcing bars as well .
ACI 318-19 Equation ( 25.4.2.4a ) for deformed bars in tension can be recast in terms of an equivalent bond stress as shown in equation 10 :
A s f y
= bond πd b l d where
bond
( Eq . 10 )
is the equivalent bond stress Substituting A s
= πd b 2 / 4 , the following expression for bond can be derived from the development length equation given in Section 25.4.2.4 , as shown in equation 11 :
bond
= 3.33λ c b
+ K tr
d b ψ t ψ e ψ s
For post-installed reinforcing bars , the “ top bar ” modification factor , ψt , which accounts for defects associated with concrete placement , can be taken as one .
Note that the ACI code limits the extent to which bond stresses in the concrete may be utilized via a 2.5 cap on the value of the quotient ( c b
+ K tr )/ d b in the development length equation in Section 25.4.2.4 . Figure 44 provides a schematic representation of this limit , whereby for bars located well away from edges , it is assumed that splitting no longer controls the behavior at ultimate load . Hilti anchoring adhesives can generate bond stresses that far exceed this limit ; however , strain compatibility and serviceability considerations often dictate the use of the more conservative embedments associated with the code .
Available bond strength τ bond
[ psi N / mm ] 2 ]
3.5 3.0 2.5
0 0
Extension for postinstalled bars with large cover ( productdependent )
1.0 2.5
Figure 44 — Effective limit on bond stress .
1
( psi ) ( Eq . 11 )
Edge distance as a function of bar diameter [ c d / d b ]
Fully-utilized bond strength of adhesive ( productdependent )
Effective limit on bond per ACI 318-19 318-11 § 25.4 ¤ 12.2
Figure 43 — Comparison of bond stresses as a function of edge distance 16
16 ) Spieth , H ., “ Structural Behavior and Design of Bonded Reinforcing Bars ,” doctoral thesis , University of Stuttgart [ 20 ].
26 2021