Sawn joints in concrete slabs

Fresh concrete contains more water than the cement needs to set. The surplus water gradually leaves the slab and the slab shrinks. While this happens, the ground underneath holds it back, and walls and columns restrain it at the edges. This creates tension, and concrete is weak in tension, so it tears somewhere. A saw cut in the top of the slab weakens it at that line, so the crack forms right below the cut, straight and out of sight.

A sawn joint is not the same as a movement joint

In everyday speech, people call every joint in concrete an "expansion joint". Professionals distinguish three types, and it helps to know them when you talk to a contractor:

• Contraction (control) joints are sawn into fresh concrete through part of the slab thickness. This article is about these.
• Isolation or movement joints separate the slab through its full depth from walls, columns, stairs or other parts of the building. They are formed before the pour with a polyethylene foam strip or a joint profile, not by sawing.
• Construction joints form where the pour stopped and continued the next day.

This price-list item covers cuts into freshly hardened concrete and is measured in linear metres of cut. Perimeter isolation strips at walls, joint profiles and later sealing of the joints are separate jobs.

Joint layout should come from the design, and for industrial floors and structural slabs always from the structural engineer. For a garage, patio or yard, experienced concreters work from a few proven rules of thumb.

Size and shape of the bays

• Each bay between joints should be as close to square as possible. As a rule, the side ratio should not exceed roughly 1 : 1.5, because long narrow strips crack across.
• The thinner the slab, the smaller the bays. A common guide is a bay side of roughly 25 to 30 times the slab thickness, so a 150 mm slab gives bays of around 4 m.
• External slabs exposed to sun and frost are divided into smaller bays than indoor floors.
• Concrete with steel or synthetic fibres and heavily reinforced slabs allow larger bays, but only according to the design or the concrete supplier's advice.

Places where a joint must not be missing

Why does a floor almost always crack from the corner of a column or a wall return? Stress concentrates at every internal corner. Joints should therefore run out of column corners, out of the corners of L- or T-shaped plans, from the corners of manholes, gullies and drainage channels, and from the edges of door openings. Around columns, the bays are usually turned into a diamond pattern so that joints meet the corners.

Joints should connect with each other. A cut that stops in the middle of a bay and goes no further simply gets extended by a crack. If the floor will be tiled, it pays to align the cuts with the tile size, so the joint in the base can later be carried into a tile joint.

With sawn joints, the hour matters more than the date. Cut too early and the blade rips out pieces of aggregate, so the joint edges ravel and crumble. Cut too late and the concrete has already cracked its own way, so the cut is pointless. In practice, a slab poured on a hot summer day often shows its first hairline cracks before the next morning.

What the timing depends on

• Air and ground temperature: in summer the window opens much earlier, in cold weather it can be many hours later.
• Cement type and mix design, plus any admixtures that speed up or slow down setting.
• Wind and sun, which dry out the surface.
• Slab thickness and how the slab is cured after the pour.

With a standard floor saw, cutting usually happens roughly 6 to 24 hours after the pour, closer to the lower end in warm weather. There are also lightweight early-entry saws that can work on concrete after just a few hours. They make a shallower cut, and not every contractor owns one.

How to recognise the window on site

An experienced operator does not rely on the clock. First, the surface gets a scratch with a nail or a key: if the groove stays shallow and no aggregate comes loose, the concrete is ready. Then comes a short test cut at the edge of the slab. If the edges are clean and the blade does not pull out stones, sawing can continue. This is why the contractor needs the exact pour time and has to be ready to come in the evening, at night or at the weekend.

What must be ready before the saw operator arrives

• A joint layout plan, ideally drawn on the floor plan with dimensions.
• For underfloor heating, a pipe layout drawing, or at least photos of the pipes with measurements taken before the pour.
• The slab thickness and the position of the reinforcement.
• Clear access for the saw and free edges along the walls.
• A water supply for cooling the blade and somewhere the slurry can drain.
• For an electric saw indoors, a power supply with adequate circuit protection.
• Lighting if the work will continue after dark.

Step by step

1. The operator checks the concrete with a scratch test and a trial cut.
2. Joint lines are marked on the surface with a chalk line. With underfloor heating, the pipe runs are marked too.
3. The cutting depth is set on the saw according to the slab thickness.
4. Each line is cut in one smooth pass along its full length, without stopping halfway, so the cut has no steps and the edges stay intact.
5. Near walls, where the large saw cannot reach, the cut is finished with a hand-held saw or an angle grinder with a diamond blade.
6. Slurry from wet cutting is rinsed off or vacuumed up straight away. If it dries on the surface, it leaves stains and clogs the joint.
7. After sawing, curing continues by wetting or covering the slab with plastic sheeting. The cut does not change the fact that concrete needs moisture.

Wet or dry cutting

Most floor saws use water cooling. Water protects the blade and, above all, binds the dust. Dry cutting with an industrial extractor is chosen indoors where water and slurry would cause problems, for example when cutting an existing floor in an occupied house. Concrete dust contains fine silica, which damages the lungs, so cutting without water or extraction is not acceptable. Petrol saws have no place in enclosed spaces, because the exhaust fumes are dangerous.

How deep to cut

The cut has to weaken the slab enough for it to crack exactly there. For concrete slabs, the usual depth is roughly one third of the thickness. With early-entry saws the cut can be shallower, if the saw manufacturer and the design allow it. For cement and calcium sulphate (anhydrite) screeds, roughly one third of the thickness is also typical. A cut that is too shallow is a very common reason why the crack appears a few centimetres beside the joint.

With a reinforced slab, you need to know where the mesh sits. The steel mesh should lie in the lower or middle part of the slab, and the cut normally does not touch it. If the blade hits the mesh, it sits too high, which is a sign that the reinforcement was not placed correctly. Whether reinforcement should be interrupted at joints is for the structural engineer to decide, not the saw operator.

Width of the cut

A standard blade cut is usually about 3 to 5 mm wide. If the joint will later be filled with a permanently elastic sealant, it is often widened with a second cut after a few weeks, so the sealant has enough room to move. The sealant manufacturer specifies the width-to-depth ratio of the filling.

What to fill joints with

• Polyurethane or hybrid sealant is the usual choice for garages, patios and yards. It follows the movement elastically and keeps water and dirt out of the joint.
• Semi-rigid epoxy or polyurea filler is used in warehouses and industrial units where forklifts with hard wheels run. A soft sealant would not support the edges under those wheels, and the edges would break away.
• No filling is normal under screeds and base layers that will be covered. The joint must still be carried through into the finished floor surface.

Before sealing a deeper joint, a foam backer rod goes in, so the sealant does not stick to the bottom and can stretch freely. The edges are primed according to the manufacturer. Sealing waits until the concrete has gone through most of its shrinkage, usually at least several weeks. A joint sealed too early keeps opening and the sealant tears away from one edge.

Factors that decide how demanding the job is

• The total length of cuts and the slab thickness: a deeper cut means slower progress and more blade wear.
• Aggregate hardness: concrete with hard river gravel or steel fibres cuts more slowly than concrete with softer limestone aggregate.
• Timing: cutting at night, at the weekend or at short notice is harder for the contractor to arrange.
• The shape of the area: many short cuts, corners and finishing cuts at walls take longer than a few long straight lines.
• Access and conditions: an interior with no drainage for water, the need for dry cutting with extraction, carrying the saw up stairs.
• Cutting existing old concrete, which is slower and is often only done after cracks have already appeared.
• Underfloor heating in the slab, which requires accurate setting out and great care.

Coordination with the concreting crew

The saw operator must know when the pour starts and finishes and when power floating is done. It makes sense to book sawing together with the concreting contractor, or to check whether the concreters do it themselves. If the slab is poured in stages, each stage has its own cutting window.

Underfloor heating

With heated screeds, joints are planned together with the heating circuits. Circuit boundaries, doorways and large areas are separated with joint profiles during the pour. Sawing is only used for additional contraction joints, and only where no pipe runs. A cut heating pipe means breaking out screed and a difficult repair, so nobody should saw over underfloor heating without reliable pipe setting out.

Floor layers and tilers

A joint in the base has to appear in the finished floor too. With bonded tiles, a flexible joint filled with silicone or a movement profile is left above the cut. If tiles are laid continuously across the joint, the crack from the base will sooner or later come through the tiles. For floating floors, joints in the base are handled according to the flooring manufacturer's instructions. Under bonded timber floors, joints are sometimes stabilised with resin before bonding, but only once it is clear they have stopped moving.

Flats and apartment buildings

Cutting existing concrete in a flat or in common areas is noisy and dusty. Agree the timing with the building management or owners' association and warn the neighbours in advance. Sawing joints into a new floor slab or screed is not an intervention in the load-bearing structure. If cutting into a floor slab or another structural element is planned, however, a structural engineer has to decide, and it is worth checking with the local building authority beforehand whether the work needs approval under local building regulations.

• Cutting too late: random cracks formed before the saw arrived, so the slab has both cuts and cracks next to them.
• Cutting too early: ravelled edges with pulled-out aggregate that keep breaking away under load.
• Shallow cut: the crack bypasses the joint and forms a few centimetres beside it.
• Bays too large or too long: cracks across the middle of the bay, typical of long driveways.
• Missing joint from an internal corner: a diagonal crack from the corner of a column, manhole or wall return.
• Cut ending in the middle of the slab: a crack continues in the direction of the cut to the edge.
• Slurry not rinsed off: grey stains on the surface and joints clogged with dried paste.
• Curing stopped after sawing: the surface dries out, giving a network of hairline cracks and a dusty surface.
• Sealing too early: after a while the sealant comes away from one edge and water gets into the joint.
• Tiles laid across the joint: cracked tiles in a straight line exactly above the cut, often after the first heating season.

Most of these mistakes cannot be fully put right. Cracks can be filled with resin or chased out and sealed, but the slab never looks as clean as one with well-timed cuts.

If you are pouring a small patio, path or garden shed floor, you can saw the joints yourself with a hired floor saw with water cooling. Bear in mind that the concrete sets the schedule, not the calendar, and that you need to book the saw in advance. Leave structural slabs, floors with underfloor heating and large areas to a contractor. Wear ear defenders, safety glasses, gloves, sturdy boots and a dust mask rated for silica dust.

1. Draw the joint layout before the pour, with bays as close to square as possible and a joint from every internal corner.
2. Book a floor saw with a diamond blade for green concrete for the day of the pour and the following day.
3. Mark the position of each joint on the formwork edges, so you can set out the lines quickly after the pour.
4. After pouring and finishing, watch the surface and start scratch-testing it a few hours after it has set.
5. Snap the lines with a chalk line between the marks on the formwork.
6. Set the cutting depth to roughly one third of the slab thickness.
7. Make a short trial cut at the edge and check that the edges do not crumble.
8. Turn on the water to the blade and cut each line in one smooth pass from edge to edge, without stopping in the middle.
9. Guide the saw slowly and straight, without forcing it forward.
10. Finish the cuts near the edges, where the saw cannot reach, with an angle grinder, a diamond blade and a straight batten as a guide.
11. Rinse the slurry off the surface and out of the joints straight away.
12. Cover the slab with plastic sheeting again or keep it damp for several days, depending on the weather.
13. After several weeks, clean the joints, insert backer rod and fill them with an elastic sealant according to the manufacturer's instructions.

You can check the cuts straight after sawing, but the real result shows over the following days. A thin hairline crack appearing under a cut means the joint is doing exactly its job. The problem is cracks outside the joints. At handover, walk the whole slab and check:

• The joint layout matches the agreed plan or the design
• Joints are straight and continuous and run from edge to edge or into another joint
• A joint runs from every internal corner, column, manhole and door opening
• The cutting depth, measured at several points, matches the agreement
• The cut edges are clean, with no pulled-out aggregate or broken pieces
• With underfloor heating, the pipes are undamaged, confirmed by a pressure test or a check of the circuits
• The surface and joints are free of slurry and dried paste
• There are no random cracks outside the joints
• Curing of the slab continues after sawing
• It is agreed who will fill the joints, when and with what material
• For a floor that will be tiled, you have a drawing of the joint positions for the tiler