How to Dowel Rebar Into Existing Concrete

To dowel rebar into existing concrete, holes are drilled at the size and depth the adhesive maker specifies, cleaned of every bit of drilling dust, filled with a structural adhesive, and the bar is set to its full depth and left alone until the adhesive cures. The bar size, embedment depth, spacing, edge distance and the adhesive product itself are set by an engineer or designer, not by rules of thumb. How well the hole is cleaned and filled matters as much as how deep it is.

This post explains the process so you know what to expect, who decides what, and where dowels go wrong. It is written for owners and contractors planning the work, not as a do-it-yourself guide for structural connections. It is not a substitute for the engineer's details or the adhesive manufacturer's instructions, which govern every step.

When you dowel into existing concrete

Drilled-in dowels (often called epoxy dowels, though not every adhesive is an epoxy) are used wherever new concrete has to be tied to old:

• Additions: new footings and foundation walls tied into the existing foundation.
• Slab extensions: a new garage, patio or porch slab joined to an existing slab or wall.
• New walls or curbs on an existing slab or footing.
• Stairs, landings and ramps anchored to an existing foundation.
• Repairs, where damaged concrete is removed and replaced.

Before anything is drilled, decide what the joint is supposed to do. Deformed rebar dowels set in adhesive lock the two pieces together. A slab-on-grade joint that should be allowed to move needs smooth dowel bars instead, which is a different detail. Our post on dowel bars vs rebar explains the difference.

Connecting to an existing foundation is structural work. For additions and new structural elements, expect a building permit and a designer or engineer to specify the connection.

What the engineer specifies

A drilled-in dowel is designed, the same way a cast-in one is. The engineer will normally give you:

• Bar size and grade. Usually the same deformed bar used in the new work.
• Embedment depth into the existing concrete, and the projection into the new pour.
• Spacing and edge distance. Dowels too close to an edge or to each other can break out a cone of concrete before the bar reaches its strength.
• The adhesive. A specific product, or a performance requirement, from an adhesive system that has been tested and has published design data for post-installed reinforcing bar.
• Hole diameter and drilling method, usually by reference to the adhesive's installation instructions.
• Inspection or testing, where the design calls for it.

The design draws on the adhesive manufacturer's tested data and on CSA A23.3, which covers anchorage to concrete. The cast-in development length tables do not apply directly to drilled-in dowels, because the bond is between bar, adhesive and the existing concrete, not fresh concrete cast around the bar. Our post on how long rebar dowels should be explains how cast-in and drilled-in dowel lengths are worked out.

The condition of the existing concrete also matters. Cracked, deteriorated or low-strength concrete may not support the design values, and the engineer may ask for it to be assessed.

What a proper installation looks like

Whether you hire an installer or are checking a subcontractor's work, this is the sequence you should expect to see. The exact steps, tools and times come from the adhesive manufacturer's printed instructions for the product being used, and those instructions win over anything here:

1. Scan before drilling. Existing rebar, post-tensioning tendons, conduits and pipes can be hidden in the concrete. Ground-penetrating radar or similar scanning is common practice so holes can be shifted to miss them. Cutting through existing bars can weaken the structure, so do not drill through steel without the engineer's approval.
2. Lay out the holes at the specified spacing and edge distance, and mark the depth on the drill bit.
3. Drill to the specified diameter and depth. Hammer drilling is the usual method. Diamond core drilling leaves a smooth, wet hole, and some adhesives are not approved for cored holes or need extra preparation, so check the instructions before coring.
4. Clean the hole. This is the step that decides whether the dowel works. Instructions typically call for repeated cycles of blowing out with compressed air and brushing with a wire brush of the right size, until no dust comes out. Dust left on the walls of the hole acts like a release agent between the adhesive and the concrete.
5. Check conditions. Adhesives have limits on concrete and air temperature and on whether the hole can be damp or wet. Cold weather lengthens cure times considerably.
6. Inject the adhesive from the bottom of the hole up, using a mixing nozzle and extension tube so no air pockets are trapped. Discard the first squeeze from a new nozzle until the colour is uniform, as most instructions require.
7. Insert the bar with a slow twisting motion to its depth mark, so adhesive coats the full length and comes up to the surface. The bar should be clean and free of oil, mud and loose rust.
8. Leave it alone. Do not move, bend or load the bar during the gel time, and do not pour around it or load it until the full cure time has passed.

Hole diameter for each bar size is set by the adhesive maker and is usually a few millimetres larger than the bar. Our guide to rebar dowel hole size and epoxy covers how to read those tables.

Common mistakes that weaken dowels

Most drilled-in dowel failures come from installation, not design:

• Dirty holes. The single biggest problem. A full-depth dowel in a dusty hole can carry far less than its rating.
• Hammering bars into tight holes with no adhesive, or with a non-structural product such as construction adhesive or patching mortar.
• Short embedment, because the hole hit a bar or a stone and the dowel was cut down to fit.
• Holes too close to an edge, which spall the edge or crack the concrete when the bar is loaded.
• Air voids from filling the hole from the top instead of the bottom.
• Loading too early, or bending the bar into position after it was set.
• Bending dowels on site to make them fit. If the projection needs an L or a hook, order it bent so the straight leg goes into the hole and the bend sits in the new concrete.

Bars for drilled-in dowels

Drilled-in dowels are usually straight bars cut to an exact length, or L-shaped bars with the hooked end in the new pour. A few points help on site:

• Order bars cut to length so the embedment and projection both match the details, rather than cutting stock bars on site.
• Mark the embedment depth on each bar, or confirm it against the depth mark on the bit.
• Keep bars clean and dry until they are installed.
• Check the projection against the lap or development length needed in the new concrete, which is set by the drawings.

We supply straight rebar cut to length and fabricated dowels bent and tagged to your bar list, from 10M to 55M, delivered across the GTA and Ontario. For the drilling and adhesive work, your engineer and installer follow the adhesive maker's system. Send us your drawings or bar list for a quote, or see our rebar products.