How to Fill Control Joints in Concrete Floors: Semi-Rigid Fillers Done Right

Saw-cut control joints keep a concrete slab from cracking randomly — but on floors that carry forklift, pallet-jack or hard-wheel traffic, an unfilled joint is a row of unsupported edges waiting to spall. Filling those joints with a semi-rigid filler transfers load across the gap and protects the edges. Here is when to fill, what to fill with, and the mistakes that let filled joints fail anyway.

What control joints do — and which ones to fill

Control (contraction) joints are tooled or saw-cut lines that create a weakened plane so the slab cracks in a straight line below the cut as it shrinks. They are not the same as expansion or isolation joints, which are wider and built to absorb movement. On a warehouse or industrial floor, control joints in traffic areas should be filled so hard wheels roll across a supported edge instead of chipping into an open gap. Joints in areas that never see wheeled traffic often don't need filling at all.

When to fill: wait for the slab to shrink

Concrete keeps shrinking as it dries, and control joints keep opening for months. Fill too early and the filler is pulled apart or debonds as the joint continues to widen. The standard guidance is to wait as long as the schedule allows — commonly around 90 days, or until the building is enclosed and heated so the slab has given up most of its drying shrinkage. On fast-track jobs where you can't wait, use a semi-rigid filler that can be topped up or re-cut later, and set expectations that some joints may need a second pass.

The right material: semi-rigid, not soft

Control joint filler is not a flexible sealant. A soft, high-movement sealant will extrude and roll under a forklift wheel, leaving the edge unsupported. Floor filler needs to be hard enough to support the joint edges under load — typically a semi-rigid two-component polyurea in the Shore A 80-plus range. Polyurea fillers such as Sika Loadflex-524 EZ set fast (often shaveable within about an hour) and cure rigid enough to protect the edges while still tolerating the small seasonal movement of a mature joint. For crack repair and narrower joints, ResinTech PPM-SL is a semi self-levelling option, and epoxy pastes like Gel-Loc handle spall repair and rebuilding damaged edges before filling.

How it's done

1. Clean the joint full depth — the joint must be dry and free of dust, debris and old sealant. Vacuum it out; contamination is the number-one cause of debonding.
2. Fill solid, full depth — interior floor control joints are filled full depth to the bottom of the saw cut. Do not use backer rod or foam in a floor control joint: a void under the filler lets it collapse under a wheel load.
3. Overfill slightly — fill just proud of the surface so there is material to shave back flush.
4. Shave flush after cure — once set, razor or scrape the filler level with the floor. Flush is the goal: filler left recessed leaves the edges exposed, and filler left proud gets sheared off by traffic.

Where filled joints still fail

• Filled too early — the joint keeps opening and tears the filler or pulls it off the walls.
• Soft sealant instead of rigid filler — it extrudes under wheels and the edges spall anyway.
• Not filled full depth — the filler bridges a void and drops under load.
• Recessed or proud fill — recessed leaves edges unsupported; proud gets torn out.
• Dirty or damp joint faces — the filler never bonds and lifts out in strips.

One important distinction

This is about interior floor control joints. Exterior expansion joints, and any joint built to accommodate real structural movement, are sealed with a flexible elastomeric sealant over backer rod — not a rigid floor filler. Matching the material to the joint type is the whole game: rigid filler where you need edge support under wheels, flexible sealant where you need movement.

Get the right filler for the floor

Canopus Supply stocks semi-rigid polyurea control joint filler, crack and joint repair products, and the prep equipment to clean and finish the work. Planning a warehouse floor that has to take forklift traffic? See our guide to matching a coating system to the traffic, and getting the surface profile right. Pickup in North Vancouver; ships across Canada and the USA. Call 250-233-3000 or email order@canopussupply.com for product selection and coverage estimates.

General information for professional and trade audiences. Confirm cure times, joint-width limits, Shore hardness, recoat windows and compatibility against each product's current TDS, SDS and your project specification.