Keystone Setting, Striking Joints, and Centering Removal

Keystone Setting, Striking Joints, and Centering Removal

The final three operations on a masonry arch — setting the keystone, finishing the joints, and removing the centering — decide whether the arch looks professional and whether it carries load safely. Each step must be done in the right order, at the right time, and with care.

Setting the Keystone

The keystone is the last voussoir placed and the one that locks the arch together. Because it is the final piece, the opening it must fill is fixed. If your keystone was cut accurately during layout, it will slide into position without forcing and without excessive gap. Before you set it, double-check that the two adjacent voussoirs (called the skewbacks of the keystone pocket) are clean and free of hardened mortar blobs that would prevent full bearing.

Apply a full, even bed of mortar to both angled faces of the adjacent voussoirs, not to the keystone itself. This gives you better control. Lower the keystone straight down into the pocket — do not rock or twist it, as that breaks mortar bond on the voussoirs already set. Use a rubber mallet and light taps to seat it; heavy blows can crack a brick or displace neighboring units.

Once seated, hold a 4-foot level across the arch face at the keystone position to verify the unit is flush with the adjacent voussoir faces. Being even a small amount proud (sticking out) or recessed will be obvious once the centering is out and the arch is in full view. Check plumb on the keystone's exposed face as well. The tolerance for this course is 1/8 inch. If the keystone is proud, do not hammer it down — remove it, trim excess mortar, reset, and re-check. If it is recessed, remove it, add a thin skim of mortar, and re-set.

Safety note: Wear safety glasses and leather gloves during setting. A dropped keystone, especially a cut stone, can cause serious foot or hand injuries. Keep your feet clear of the drop zone and ensure no one is below you if you are working at elevation.

Finishing and Striking Joints

Joints must be struck (tooled) while the mortar is thumbprint-hard — firm enough that a thumb pressed to the surface leaves a clean print without mortar sticking to the thumb, but not so hard that the jointing tool tears the mortar rather than compresses it. On a warm, dry day this window may arrive in 30–60 minutes after laying; in cool or humid weather it may take longer. Striking too soon produces sagging joints; striking too late produces torn, rough surfaces and poor weather resistance.

For an arch, work all joints on both faces of the wall. Begin with the bed joints (the angled mortar joints between voussoirs) and then do the head joints if any are present. Match the jointing tool to the specified profile:

• Concave (rodded): most common; the tool compresses mortar into a slightly curved recess, creating excellent water runoff and a tight bond at the edges.
• Flush: mortar is cut off level with the masonry face using a trowel or margin trowel; used for interior or painted work.
• Grapevine: a jointing tool with a raised bead that leaves a decorative ridge in the center of the joint; common in historic or residential work.

After tooling, sponge the face with a damp (not wet) sponge to remove mortar smears. Work in light passes — scrubbing too hard with too much water will pull mortar out of the fresh joint and smear it back onto the face. Inspect each face under raking light (hold a flashlight or work light at a low, nearly parallel angle to the wall surface). Raking light reveals voids (holidays), uneven tool pressure, and torn edges that straight-on lighting hides. Mark any void or inconsistency and re-tool before the mortar hardens fully.

Safety note: Dispose of mortar-laden rinse water properly. Do not pour large quantities down a floor drain; cement slurry can harden in pipes.

Removing the Centering (Striking the Form)

Centering is the temporary wooden or steel form that supports the arch voussoirs while the mortar cures. Removing it too soon is one of the most common causes of arch failure or cracking. Your instructor will confirm when adequate cure has been achieved before this step begins — do not remove centering on your own schedule.

Centering is typically held in place by wedges, adjustable shores, or falsework. Remove wedges or lower shores incrementally and evenly — a little from each support point in rotation, not all the way down on one side first. This allows the arch to begin carrying its own load gradually and symmetrically. Sudden, uneven release can create an asymmetric load that cracks the arch or displaces voussoirs before the mortar has developed full strength.

While one student operates the wedges or shores, a second student watches the intrados (the inner curved surface of the arch) closely for any sign of movement, cracking, or joint opening. The moment you observe any of those, stop the removal process and call the instructor.

After the centering is fully removed, both students examine the arch: intrados, extrados (outer curve), and both faces. Record your observations in your lab log — the time of removal, the condition of every joint and unit you can see, and the presence or absence of any cracking or displacement. This written record is a professional habit that carries into real jobsite practice.

Safety notes during centering removal: Wear a hard hat — if any component of the arch or formwork moves unexpectedly, falling material is the primary hazard. Keep bystanders out of the area below and around the arch. Never reach under the arch to pull centering while the arch is still partially supported.

Summary

A successful arch finish depends on accurate keystone placement within 1/8 inch of flush and plumb, consistently struck joints with no voids, and a slow, even centering removal after adequate cure. Document everything. These three operations are what the owner and the inspector will see and judge.