Concrete Joint Filler Guide

How deep do I fill a control joint?

Saw-cut control joints: full depth of the cut. Semi-rigid filler has to bear on concrete at the bottom of the joint to support the edges under wheel loads. Foam backer rod compresses, so it is never used in a saw-cut joint. Up to 1/4" of clean, dry silica sand at the bottom is acceptable to seal the cut.

Joints deeper than 2", including formed construction joints: fill 2" minimum. Below that, pack dry silica sand to within 2" of the surface, or set backer rod in compression 2" down; it is a dam, not a support.

Saw cuts on new slabs are typically one-quarter of the slab thickness, so if you don't know the depth, the slab tells you:

When can joints be filled, and how are they prepped?

Timing

• ACI guidance: defer filling 60 to 90 days after the pour, or as long as the schedule allows. Concrete keeps shrinking for a year or more, and every week of waiting is a joint that opens less after it is filled.
• 30 days is the absolute minimum.
• Freezer and cooler floors: bring the room to operating temperature and hold it 7 days before filling.

Preparation

• Clean both joint walls to bare concrete, full depth or at least 2". Laitance, curing compound, and old filler all cause bond failure.
• A dustless diamond-blade joint saw or crack chaser with a HEPA extractor is the standard method. Wire brushes alone don't get the walls clean.
• The joint must be dry. Polyurea foams on damp concrete; if the joint can't be dried, use epoxy.
• Overfill slightly, then shave flush. Polyurea shaves in about an hour; epoxy is usually razored the next day.

Control joint, construction joint, or expansion joint?

The lines in a concrete floor are not all the same joint, and semi-rigid joint filler is for only two of them.

• Control (contraction) joints are the saw cuts made into a fresh slab so it cracks where you want it to. They are filled full depth with semi-rigid filler once the slab has shrunk. This page is about these.
• Construction joints are where one pour stopped and the next started, formed through the full slab. Filled 2" deep with semi-rigid filler over a sand or backer-rod dam.
• Expansion (isolation) joints separate the slab from walls, columns, and other slabs so it can move freely. They are sealed with a flexible sealant over a compressible filler, never with semi-rigid filler, which would lock the slab to what it's supposed to move against. Driveway and sidewalk "expansion joint filler" products are this category.

What is backer rod, and when do you use it?

Backer rod is a round foam cord pushed into a joint to set the depth of sealant above it and keep the sealant from bonding to the bottom. In expansion joints and caulked joints it is the right tool. In saw-cut control joints on a floor that carries wheel loads it is the wrong one: foam compresses, the filler loses its support, and the joint edges spall. Use dry silica sand instead, and keep it to 1/4" or less. In formed joints deeper than 2", backer rod set in compression at least 2" down is acceptable because the filler above it is deep enough to bear on the walls.

Why did the filler pull away from the joint?

Usually it didn't fail. Concrete shrinks for a year or more after placement, so joints keep opening after they are filled. Semi-rigid fillers are designed to release from one wall rather than tear, which keeps the filler in the joint and the edges protected. Fillers at A-80 and above handle no more than about 15% lateral movement, and a 1/8" joint that opens to 5/32" has already moved 25%.

Refill when the gap is wider than a credit card, or when the edge is no longer supported. Filling later in the slab's life is the only real prevention, which is why the 60–90 day wait matters.