Concrete Dowel Action

Last updated March 17, 2023
By Ian Story

This post contains some working thoughts on dowel action for concrete-concrete interfaces

ACI 318 allows the use of shear friction for concrete-concrete interfaces where rebar can be fully developed on both sides of the joint. By a strict reading of the code, For straight bar, this is

Reduced Development Length

ACI 318 section 25.4.10 allows a reduction in development lengths where excess capacity is provided (with some limitations). The reduction is a straight prorating of the excess forces: providing double the required reinforcement cuts the required length in half.

Another way to formulate this discussion would be to specify the strength developed per unit length.

For straight #4 grade 60 bar in 2,500 psi concrete, the required development length (per 25.4.2.3) is

    \[\frac{3f_y\psi_t\psi_e\psi_s}{40\lambda\sqrt{f'_c}(2.5)}d_b = \frac{3(60,000)(1.0)(1.0)(0.8)}{40(1.0)\sqrt{2,500}(2.5)}(0.5) = 14.4 inches\]

The nominal load the bar can carry in tension is:

    \[\frac{N_T}{A} = f_y => N_T = f_yA = (60,000 psi)\pi(0.5 in)^2/4 = 11,781 lb (LRFD)\]

Which translates to a nominal load developed per unit length of 818 lb/inch.

For hooked #4 grade 60 bar in 2,500 psi concrete, the required development length (per 25.4.2.3) is

    \[\frac{f_y\psi_e\psi_c\psi_r}{50\lambda\sqrt{f'_c}}d_b = \frac{(60,000)(1.0)(0.7)(1.0)}{50(1.0)\sqrt{2,500}}(0.5) = 8.4 inches\]

Which translates to a nominal load developed per unit length of 1,402 lb/inch.

This is equivalent to ACI’s provision for a reduction in development length when excess capacity is provided. Note that the minimum development lengths still apply (12 inches for straight bar, 6 inches for a standard hook).

To summarize, the required development lengths for straight and hooked bar are:

Straight bar: 1 inch per 818 lbs nominal tension force (LRFD), min. 12 inches

Hooked bar: 1 inch per 1,402 lbs nominal tension force (LRFD), min. 6 inches

Shear Friction

ACI 318 22.9.5.1 requires that reinforcement be developed for the full nominal capacity of the bar (i.e. prorating is not allowed). We, along with many engineers, don’t really agree with this code provision, but it appears to be the code requirement. This means that, practically speaking, sections to be developed need to be at least 11.4 inches deep (8.4 inches for a hooked bar, plus 3 inches cover…assuming earth contact)

Post-Installed Epoxy Anchors

To develop the full strength of #4 grade 60 bar in tension, an epoxy embedment would need to be longer than 10 inches. ESR reports for Simpson epoxies cap out at 10 inches depth, so post-installed epoxy anchors cannot satisfy shear friction requirements, by code.

Dowel Action

For situations where shear friction can’t be used in a code-compliant manner, straight dowel action using the provisions of ACI 318 chapter 17 may be used to calculate allowable shear capacities assuming the bars resist the load in straight shear.

For purposes of this analysis, the tensile capacity of reinforcing bar will be assumed zero, which implies that the joint between the surfaces must take zero moment. Given the zero moment condition, pryout will also not be considered.

Given the above, it becomes apparent that no bond is required between the concrete and the dowel in order to resist shear (provided there is zero moment at the joint). It may be advisable to epoxy dowels to strengthen the connection, but the epoxy provides no usable strength under the code.

The following calculations assume the dowels are sufficiently far from the end of the concrete that this mode does not influence concrete breakout (i.e. shear parallel to an edge). Assume shear cones don’t intersect (spacing > 3 times embedment). The values below are LRFD forces and include the reduction factor (0.70, for Condition B)

For #4 grade 60 bar, per dowel (in lbs LFRD per dowel)

Embed\Edge1.5″2″2.5″3″3.5″4″
2.5″8751,3501,8902,4803,1303,820
3″9101,4001,9602,5803,2503,960
4″9651,4802,0802,7303,4404,200
6″1,0501,6102,2502,9603,7304,560
8″1,1001,7002,3903,1403,9504,830

Note: values in the table above are increased by a factor of 1.2 if there is a #4 bar between the anchors and the edge of the concrete

Note on calculations: these values are the smaller of pry-out (based on concrete breakout strength in tension – still applies even if there is no tensile bond between the anchor and the concrete) and concrete side breakout. In all cases examined, concrete side breakout controls