Product Technical Guides : US-EN Post-Installed Rebar Guide - Page 32

Post-Installed Reinforcing Bar Guide
6.0 WHAT ’ S THE BACK STORY ?
The particular assumptions used for the application of anchor theory to bar development ( e . g ., bar yield and bond strength values ) will depend on the specific circumstances of the design . For groups of bars and for bars near edges , this approach obviously becomes more complex , but can be solved by inclusion of the necessary edge distance and spacing adjustments in ACI 318-19 Chapter 17 .
Note : ACI 318-19 Section 17.3.3 states : “ For adhesive anchors with embedment depths 4da ≤ hef ≤ 20da , the bond strength requirements shall be considered satisfied by the design procedure of 17.6.5 .” This requirement recognizes the limits of the uniform bond model adopted by ACI . In some cases it may be justifiable to specify embedment in excess of 20 bar diameters in order to ensure the desired margin of safety . Contact Hilti for further information .
Example : Shear dowels ( compare with example provided in 2.6.1 ) Requirement : Determine the embedment requirement for post-installed reinforcing bars used to connect a new 10- inch thick shotcrete ( pneumatically-placed ) shear wall to an existing concrete wall ( 9 ). Bars are # 5 at 24 in . x 24 in . over face of wall . Existing shear wall is 12 in . thick with 4 ksi normal weight concrete . Based on an assessment in accordance with AC308 , the bond strength , cr
, of the adhesive is 1090 psi and the k c value is 17 . The bond strength of the adhesive in uncracked concrete , uncr
, is 1560 psi .
Assume spacing is sufficient to use simplified expressions for development of bars based on application of anchor theory in accordance with Charney , et al .
l d , breakout
= 1.2
0.310 x 60000 = 8 in .
0.3 x 0.625 x 60000 l d , bond
= = 10.3 in .
1090
Check spacing assumption for concrete breakout : 3 x 8 = 24 in . ≤ 24 in . ∴ ok
Check spacing assumption for bond failure :
2c Na
= 20 x 0.625
1560 1100
2 / 3
= 15 in . ≤24 in . ∴ ok
Use # 5 hooked dowels embedded 10-1 / 2 inches = 16.8 bar diameters < 20 ( limit of uniform bond model per ACI 318-19 Section 17.3.3 ).
6.6.2 USE OF CONFINEMENT TO INCREASE BOND EFFICIENCY
As shown in Figure 45 , the bond stresses associated with typical development lengths are low relative to the bond strengths that can be achieved with post-installed adhesives ( compare , e . g ., with Figure 39 ). The term associated with confinement in Equation 25.4.2.4a of ACI 318-19 is shown in equation 17 :
where
c b
A tr c b
+ d b
40A tr s∙n ≤ 2.5
( Eq . 17 )
the lesser of : ( a ) the distance from center of a bar or wire to nearest concrete surface , and ( b ) one-half the centerto-center spacing of bars or wires being developed ( in .)
area of transverse reinforcement effective to prevent splitting ( in 2 )
s spacing of transverse bars ( in .) n no . of bars being spliced or developed along the line of splitting ( in .
The limit of 2.5 placed by Section 25.4.2.4 on the confinement term reflects the relatively conservative assumption regarding the effectiveness of confinement in suppressing splitting and pullout failures . Research sponsored by Hilti [ 17 ] indicates that , for specific adhesives , the limit on this term can be increased by nearly 100 %, to 4.5 . The particular conditions under which this adjustment can be made are given in the literature . In addition , testing of laterally loaded columns anchored with post-installed bars has demonstrated that the confinement effect provided by the compression toe of the column can effectively be used to reduce the required development length for these cases [ 12 ].
6.6.3 STRUT-AND-TIE MODELS
ACI 318-19 Chapter 23 provides procedures for the development of strut-and-tie models to design reinforced concrete structures or members that contain D-regions ( an area around a force or geometric discontinuity ). This approach is particularly suitable for the design of post-installed reinforcing bars where the bar is installed perpendicular to the primary reinforcement in the existing concrete member . The structure is divided into B- and D-regions . B-regions are parts of a structure in which Bernoulli ' s hypothesis of straight-line strain profiles applies . The internal stress state of B-regions can be easily derived from the sectional forces and the region can be designed on the basis of classical beam theory .
30 2021