Dowel rebar is frequently employed in new construction projects and rehabilitation ones like slab replacement. Certain applications require the addition of a dowel rebar into existing concrete. You must understand the basics of dowel rebar before progressing to the working of dowel rebar into existing concrete.
Thus, in this article, you’ll learn the basics of dowel rebar, the purpose of placing dowel rebar into existing concrete, planning the event, and the process.
What are Dowel Rebars?
Dowel bars are short steel rods, which constructors place at the adjacent joints of the concrete slab in construction projects. It usually serves to stabilize the concrete pavement when the civil design requires a connection between two slabs. The formed joint helps load distribution.
Dowel rebar is subject to ACI 3180-19 Reinforced Concrete Design. Specifics of the rebar requirements depend on the function it performs and the stresses it undergoes. For instance, when installed in deep embedments and in seismic applications, the reinforcing bar theory applies.
Rebar usually consists of a pattern of ribs that prevents slippage within the concrete structure. At times, engineers prefer smooth rebar. Smooth rebar fits scenarios where expansion and contraction of concrete occur due to frequent changes in weather and moisture content.
After engineers specify whether the rebar is smooth or ribbed, they need to select the appropriate rebar material.
Carbon Steel Rebar
This general-purpose rebar provides a great value/tensile strength ratio. It is considered the standard type of rebar and fits almost every type and scale of project with few exceptions. Its tendency to corrode equates to its biggest weakness. When rebar corrodes after installation, its expansion may lead to cracking and breaking of the concrete it supports.
Some fabricators may have a concern about the effectiveness of rusty rebar. Generally, this does not cause issues as both the ASTM standard for deformed steel reinforcement (A706) and the Concrete Reinforcement Steel Institute (CRSI) Manual of Standard Practice allow for rust. This is provided that the minimum dimensions and weight of the rebar stay within manufacturing tolerances.
This type of rebar uses manganese steel. This makes for an extremely strong product in overall yield strength but does not perform as well as standard carbon steel rebar in bending. European rebar typically provides an overall lower cost, but generally is perceived unfavorably for standard use.
Epoxy-coated rebar provides a carbon steel rebar with an epoxy coat. It has the same textile strength but is over a hundred to a thousand times more resistant to corrosion. This holds true if fabricators do not damage the rebar during handling or installation.
Galvanized rebar provides around forty times the amount of corrosion resistance than standard carbon steel rebar. Its coating is much more damage-resistant than its epoxy brethren. As such, galvanized rebar fits applications where fabricators cannot ensure delicate handling.
This composite option cannot tolerate field bending. However, it also does not corrode whatsoever. This highly expensive option also offers extremely lightweight. Thus, its use is limited to specialized applications.
Stainless Steel Rebar
Stainless steel rebar is the most expensive reinforcing bar available and functionally provides an outstanding set of physical characteristics. It has high resistivity to corrosion, no coating that may undergo damage, and also can tolerate field bending. Stainless steel rebar’s high cost typically disqualifies it for most applications.
Why Add Dowel Rebar Into Existing Concrete?
Rebar allows for an appropriate distribution of heavy loads. For example, pavement that serves heavy vehicles such as a truck parking lot may experience additional forces on the concrete slab, damaging it. Dowels rebar allows the concrete to distribute the load to an adjacent concrete slab. These dowel bars help specifically when the traffic moves from one end of the slab to the start of another.
Planning the Dowel Bar Location
Proper spacing and alignment of dowel rebars ensure proper load transfer. The dowel rebar reinforcement schema only works properly with balanced spacing. One should always place the dowel bar in the mid-center of the concrete surface vertically.
Specific guidance may vary on application, but generally, installers place rebar 12 inches apart from one another. Placing the dowel rebars any closer could overstress the concrete slabs, damaging them. Most retrofitting projects require adding the dowel rebars in wheel paths to reduce the number of slots that the installations need to cut into the existing concrete.
Properly spaced dowel rebars allow the concrete to flex as per requirements. Otherwise, the concrete pavements may feel a little rigid. This also helps with maintaining the essential connection with the pavement and allows multiple concrete slabs to work as a single unit for maximum strength.
Process of Working a Dowel Rebar Into Existing Concrete
Drilling dowel rebar into existing concrete requires deliberate steps. While exact steps differ per application, the general procedure holds true:
- Ensure planned drilled holes are in accordance with expoxy manufacturer’s recommendations and the required diameter of the dowel rebar.
- Drill holes from the top or side of the concrete depending on the location of the planned addition.
- Verify hole sizes are appropriate.
- Inspect drilled holes to ensure it is free of debris.
- Epoxy the rebar into the hole.
- Pour the new concrete as per specfication.
For a large installation, construction workers may use specialized machines that drill multiple holes at the same time. For rigid pavement applications, contractors may skip the epoxy and apply a lubricant to the dowel rebar. Additionally, contractors may provide small expansion spaces for movement.
New slab shrink and dowel rebars restrain this shrinkage. Thus, dowels placed at the corners or near the end of the pavement may lead to cracks.
As a final step installers may perform diamond grinding to ensure an even surface.
In most scenarios, the addition of a dowel rebar into existing concrete occurs during an expansion project. However, it also may be required as part of a required repair. No matter the reason, appropriate planning, specification, and execution ensure a long-lasting and safe construction project.