Understanding Concrete Groovers/Jointers and Edgers
During this transition, the slab’s volume shrinks. Depending on the mix’s water/cement ratio and ambient jobsite conditions, driveways, sidewalks, and floors may shrink at a rate of approximately 1/8" inch per 20 square feet in the hours just after placement.
Shrinkage is a subtle yet powerful force in the concrete. These forces create strains that can cause cracking in the middle of the slab and chipping along formwork. To relieve this strain, finishers edge and groove the concrete while it is still workable. These actions are normally done after any bleed water is gone, and just before final set occurs.
Why Edging is Important
The American Concrete Institute (ACI) defines edging as “the operation of tooling the edges of a fresh concrete slab to provide a rounded corner.” But here are three other ways edging improves the concrete slab:
1. LEAVING A LEVEL EDGE. Sometimes the striking process can drag concrete away from the form edge and can leave an unevenness between the slab’s surface and the top of the form. These high spots are often just a few inches away from the slab’s perimeter. Left uncorrected, the hardened surface could create drainage problems or uneven transitions from nearby slabs exposing them to wear.
So just before applying the edging tool, the finisher eliminates any elevation differences by using a hand float in a fanning motion. This cuts any high spots and moves concrete to the form’s edge making the concrete flush to the top of the form. Once level, edging tools can be used to finish and refine the look.
2. IMPROVING EDGE QUALITY. The first place you’ll see the slab shrink is at the upper edge, along the perimeter where the fresh slab meets the form. If there is any adhesion by the concrete to form, uneven edges can occur. To eliminate any bonding or surface tension, finishers use the edging tool to craft a corner with a short radius. This slight gap is enough of a bond-breaker to allow the slab free movement, reducing the chance of any cracking or spalling near the form.
Edging also helps to reduce the occurrence of damage when forms are removed. When concrete adheres to formwork, there can be damage to the edge when that formwork is removed.
3. HARDENING THE CONCRETE’S EDGE. During floating and troweling, coarse aggregates are pushed into the concrete. This leaves a thin layer of mortar called “the cream.”
Edging the mortar next to a form densifies this creamy concrete, making it stronger. Denser edges are more pronounced and less likely to chip or break off when struck. The edging action also pushes any coarse aggregate near the surface down into the slab to reduce the chance of popouts. The denser concrete seals the joint, helping to reduce the chance of any spalling.
An edge’s radius is dependent on the project specifications. On most residential and light commercial applications, such as sidewalks, stairs, driveways, and patios, most finishers opt for a 1/8" radius. Edge radius specifications are different for curb and gutter projects and cast-in-place stairs to create durable shapes.
Before edging industrial and commercial floors, check the specifications. Some floor treatments require special techniques. For example, when the treatment includes a tile floor, it is important not to edge.
Grooving to Control CrackingRandom cracking from shrinkage can occur in the period just after placement and before hardening. Finishers can “control” this random cracking by placing grooves in the hardening concrete to help release the stress from shrinking. When properly located on the slab, the crack occurs within the groove and is said to be controlled.
Laying out the joint pattern is a mixture of science, art, and experience. ACI recommends that joints should be spaced at the ratio of 24-36 times the slab’s thickness. Many finishers use their recommendation as a guide, but often increase the number of joints based upon the geometry, location of embeds, and desired aesthetics.
While joint layout is somewhat discretionary, the required depth of a control joint is not. There’s a direct relationship between the slab’s thickness and the depth of a groove or cut needed. ACI recommends that a control joint’s depth should be created to 1/4 the slab’s thickness.
Contractors install control joints by either tooling or saw cutting. These options are often dictated by the slab’s thickness. For thick slabs (e.g. larger than 6” where the joint depth would need to 1.5”), hand tooling is impractical. But for common slab thicknesses of 4" or less, control joints can be formed with a jointer, also called a groover jointer.
Timing and Proper Tool ChoiceWhen hand tooling control joints, timing is everything. When conditions are right, finishers often take their first jointing pass before the onset of bleeding. Then when the concrete is stiffer, after most of the bleed water has evaporated, finishers take a second pass along the established control joints. If they wait too long, finishers can damage the slab’s surface. If the concrete stiffens too much, grooving can dislodge coarse aggregate at the groove.
Groove Depth, Shape, and WidthProper tool selection is just as important as timing and should match the slab’s final design requirement. Their primary function is to ensure the proper groove depth. The bit depth on the tool’s bottom can range from 3 ⁄16" to 2".
A jointing tool’s secondary function relates to the control joint’s final shape. While cutting the groove, they also “edge” the concrete as they pass. They create a smooth control surface on both sides of the opening, avoiding unevenness between either side of the slab. This edging action also applies to the sides of the groove. Straight clean grooves result in better crack control.
When selecting a groover’s width, it’s important to know if the joint will be left as is, sealed, or filled. Each of these options can dictate the proper width. Depending on the option, a groove’s width can be range from 1 ⁄8" to 3 ⁄4".
Groove RadiusThe final consideration for tool selection is the radius at the top of the groove. For untreated or sealed joints, the radius should be 1/8" for floors and 1/4" to 1/2" for sidewalks, driveways, or patios. For industrial floors, it’s best to check with the design engineer before groove selection.
Edging and grooving have important purposes in the finishing process. When combined with the right tool, finishers will craft a durable and aesthetically pleasing project.