Can you use backer rod with concrete?

What is Backer Rod?

Backer rod is a flexible, cylindrical foam cord that is inserted into joints and cracks to control sealant depth, provide a backstop for the sealant, and improve the sealant’s performance. It is typically made from closed-cell polyethylene foam and comes in various diameters to fit different size joints. Backer rod is used in concrete, masonry, and roofing applications before applying caulks, sealants, or fill material.

Some key things to know about backer rod:

It controls the depth and shape of sealants like caulk, preventing three-sided adhesion which can lead to early failure.

It provides a firm backup to apply sealant against.
It allows proper sealant tooling and bead shape.
It supports elastomeric sealants during joint movement.

It isolates and separates dissimilar materials.

Backer rod is commonly used with building and paving sealants like silicone, polyurethane, polysulfide, and acrylic. It comes in different diameters from 1/4 inch up to over 2 inches to accommodate many joint sizes.

Using Backer Rod with Concrete

Backer rod can be highly useful when working with concrete in several applications:

Control Joints

Concrete control joints are placed during the pouring process to control inevitable cracking as the concrete cures and shrinks. Backer rod can be inserted into these joints before applying a sealant to prevent moisture and debris intrusion into the joint. The backer rod supports the sealant through expansion and contraction cycles.

Crack Repair

For repairing non-structural cracks in concrete up to around 1 inch wide, backer rod can help shape the crack to accept a sealant. The rod is pressed into the crack, then sealant applied over top to create a flexible, water-tight seal.

Cold Joints

Cold joints are construction joints between two pours of concrete that can be prone to leakage. Placing backer rod into the seam before sealing creates a stronger, more durable seal.

Expansion Joints

Expansion joints allow concrete slabs to expand and contract with temperature changes. Backer rod placed prior to a sealant accommodates this movement while maintaining a tight seal. This helps prevent cracks and spalling around the joint.

Sawing Joints

Backer rod can be inserted into decorative saw cut joints in concrete before filling and sealing them. The foam cord shapes the void to accept sealant and prevents three-sided adhesion.

Benefits of Using Backer Rod

There are several important benefits to using backer rod with concrete:

Correct sealant shape and depth – Backer rod ensures the sealant bead has an ideal hourglass shape, improving adhesion and preventing thin spots.
Supports sealants – The foam backer rod provides a firm, resilient backstop for sealants to adhere and cure against.
Accommodates movement – It allows joint expansion and contraction without compromising the seal.

Prevents three-sided adhesion – Three-sided adhesion can lead to premature sealant failure. Backer rod forces two-sided adhesion.
Reduces sealant usage – By controlling depth, less sealant is needed to fill joints.
Isolates materials – The foam prevents sealants like polysulfides from directly contacting reactive materials.

Improves durability – Joints and repairs can last longer when reinforced with proper backer rod and sealant use.

Using the appropriate size backer rod for the joint width is important for achieving optimal performance.

Considerations

There are a few important considerations when using backer rod with concrete:

Make sure the diameter is 25-50% wider than the joint width so it fits snugly when compressed into the crack or joint.
Avoid over-compressing the rod which can lead to sealant failure.
Use roughly 1/8″ of sealant thickness over the rod for best results.

Cut the rod with a sharp utility knife for clean, even ends when installing.
Make sure the foam is compatible with the sealant being used.
Install the rod dry. Avoid water exposure which can cause swelling.

Installation

Here is a quick summary of how to install backer rod in concrete joints:

Clean the joint thoroughly to remove all dust, dirt, debris, old sealant, oils, etc.
Make sure the joint is wide enough to accept the diameter of rod being used.

Select a backer rod at least 25% wider than the joint width.
Cut the rod to the appropriate length for the joint.
Compress and insert the rod into the joint at the proper depth.

Use a tool to press the rod evenly into place, avoiding over-compression.
Apply sealant over the rod, filling the joint.
Tool the sealant with an appropriate tool to create an hourglass shape.

The rod should fit snugly about 1/4 to 1/2 inch below the surface to allow for proper sealant thickness. Take care not to puncture or over-compress the rod during installation.

Types of Backer Rod

There are a few different types of backer rod available:

Cylindrical Foam Rod

The most common type, cylindrical foam backer rod is extruded from closed-cell polyethylene, polyolefin, or polyurethane foam. It is available in diameters from 1/4″ to over 2″ and is the best choice for most concrete joint applications.

Rectangular Foam Rod

Rectangular cross-section backer rod provides a flat side to tool sealants against in some applications. It may be used for control joints or large expansion joints.

Bi-Cellular Foam Rod

Bi-cellular backer rod has a closed-cell outer layer and open-cell inner layer. The open-cell layer allows for more compression. It’s beneficial for very wide joints.

Elastomeric Foam Rod

Elastomeric foam backer rods are ultra-resilient. They provide maximum support for sealants in expansion joints subject to frequent cyclic movements.

Backer Rod Type	Description	Applications
Cylindrical Foam	Standard closed-cell polyethylene foam, various diameters	Most concrete control joints and repairs
Rectangular Foam	Flat side for sealant tooling	Large expansion joints
Bi-Cellular Foam	Closed-cell outer layer, open-cell inner layer	Wide expansion joints
Elastomeric Foam	Ultra-resilient, high recovery	High movement joints

Choosing Diameter Size

Choosing the right size backer rod is key for proper installation. The diameter should be around 25% larger than the joint width before being compressed into place.

Here is a quick guide to selecting backer rod sizes for concrete joints:

Joint Width	Backer Rod Diameter
1/4″	3/8″
1/2″	5/8″
3/4″	7/8″
1″	1 1/4″
1 1/4″	1 1/2″
1 1/2″	1 3/4″

This ensures a snug, compressed fit without over-squishing the rod. Measure joint widths carefully before purchase.

Advantages vs. Alternatives

Backer rod provides some clear benefits over alternative backstop materials:

vs. Foam Strip

Pre-formed foam strips lack the cylindrical shape that supports controlled sealant geometry. Foam strips are less compressible and may apply stress to the sealant as joints widen.

vs. Rope

Rope fibers can wick moisture into the joint and contaminate or compromise the sealant. Rope also degrades over time.

vs. Extruded Rubber

Continuous rubber tubing doesn’t conform as well to irregular joint surfaces. Rubber also conducts more heat/cold which can impact sealant curing and performance.

vs. Mineral Wool

Fibers from mineral wool can contaminate sealants. It also readily absorbs water which is problematic.

vs. Oakum

Oakum (hemp fiber) has similar issues as mineral wool with contamination, moisture absorption, and degradation over time.

The compressible, moisture-resistant, resilient properties of quality backer rod make it the optimal choice for most concrete joint backstop applications.

Tips for Successful Use

Here are some helpful tips for getting great results using backer rod with concrete:

Make sure the joint is clean before installing rod.

Select the appropriate rod diameter around 25% larger than the joint width.
Cut rods with a sharp utility knife for clean, even ends.
Avoid getting the foam wet before installation.

Insert the rod uniformly to proper depth without excess compression.
Apply 1/8″ to 1/4″ sealant thickness over the rod.
Use concave joint sealant tools to create ideal shape.

Inspect for proper adhesion between sealant, rod, and joint surfaces.

Proper selection, preparation, installation, and tooling are key to maximize performance.

Cost Effectiveness

Backer rod is an extremely cost-effective solution for improving concrete joint repairs and sealing:

It typically costs between 2-7 cents per linear foot, very affordable.
By controlling sealant depth, it can reduce materials cost.
It helps prevent early sealant failures, eliminating costly redos.

Longer lasting repairs mean greater value for the initial investment.
Labor costs are minimal with simple insertion.
No specialized tools required beyond a utility knife.

Backer rod is a simple, inexpensive product that can lead to major long-term cost savings from superior sealant performance. Paying a little more upfront is worthwhile.

Availability

Backer rod for concrete applications is widely available at most hardware stores, home centers, construction supply outlets, and industrial suppliers.

Some sources to purchase backer rod include:

Home Depot
Lowes
Ace Hardware

Sherwin-Williams
Fastenal
Grainger

McMaster-Carr

It can be found near concrete repair and caulk/sealant products. Many locations stock common 1/4″ to 1″ diameter rods suitable for most residential needs. Larger sizes may need to be special ordered. Prices range from $2-$7 per 10-30 foot roll.

Buying the right size foam backer rod for the intended project is quick and convenient.

Conclusion

Using backer rod is an excellent solution for sealing concrete control joints, cold joints, cracks, and more. The compressible foam properly shapes joints, provides strong sealant support, and improves adhesion. It accommodates joint movement and prevents early sealant failures.

With the right size rod, proper installation depth, and quality sealant application, backer rod delivers more durable, long-lasting repairs in concrete. This inexpensive product can save major headaches and costs down the road. For best concrete sealing, make sure to include backer rod as part of the joint preparation process.