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