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Cutting and Sealing a Driveway Loop
Cutting a vehicle loop into a driveway looks like a simple weekend job: a saw, some wire and a tube of sealant. In practice, small decisions about depth, corners, wire turns and sealing decide whether the loop works for fifteen years or fails after the first hot summer. This guide describes the process in plain terms, so an owner can understand what a good installer does and ask informed questions before the saw touches the pavement.
Planning the layout
Start with a chalk outline. Decide where vehicles will actually travel and stop, and choose a loop size that covers the center of a typical car. A common shape is a rectangle several feet wide and several feet long. For a safety loop, place it where a vehicle would stand in the gate's path. For an exit loop, place it where a car rolls slowly before reaching the gate.
Look at what is beneath and around the pavement. Rebar, wire mesh and steel plates interfere with a loop's field. Expansion joints in concrete are a problem if the wire crosses them, since the slab moves. Try to keep the loop within a single slab and away from joints. Check for utilities too, since a saw blade can damage lines buried only a few inches deep.
Cutting the slot
A concrete saw with a narrow blade cuts a groove just wide enough for the wire, often around a quarter inch to a half inch. The depth is typically a couple of inches, enough to cover the wire and leave room for sealant above it. Corners should be cut at an angle, called a chamfer, rather than a sharp right angle. A sharp corner can abrade or cut the wire insulation as the pavement flexes. Many installers cut each corner diagonally across for this reason.
Slot cleanliness matters. After cutting, flush out slurry and dust with water and dry the groove thoroughly with compressed air. Any grit left in the slot can chew on the wire insulation over time. Make sure the slot is free of sharp edges that could cut the wire.
Choosing and laying the wire
Loop wire is insulated with a tough, moisture resistant jacket designed for burial in pavement. Ordinary building wire may not tolerate heat, moisture and compression. The number of turns depends on loop size and the detector's recommendations. Smaller loops need more turns to achieve the same sensitivity, while larger loops need fewer. The detector's manual provides a table.
Lay the wire into the slot with a blunt tool, such as a wooden stick or a rounded plastic wedge, rather than a screwdriver that can nick the jacket. Keep the turns neat and parallel, and avoid crossing wires within the slot. Hold the wire gently in place with short pieces of foam backer rod, which keeps it below the surface and allows sealant to flow around it.
Lead in and twisting
From the loop corner, the two ends travel to the detector as a lead in cable. This section should be tightly twisted, with a number of twists per foot suggested by the manufacturer. Twisting cancels pickup from nearby electrical noise, so an untwisted lead behaves like an antenna. Cut a slot from the loop to the edge of the pavement and continue the lead in conduit to the detector.
Keep the lead in as short as practical. Long runs add resistance and noise pickup. If a long run is unavoidable, use shielded cable and ground the shield at one end as directed.
Testing before sealing
Before filling the slot, test the loop. Connect it to the detector, power it up and watch the indicator while a vehicle drives over. A meter that measures resistance and insulation to ground tells whether the wire is intact and dry. A reading that indicates leakage to ground at this stage means a nicked jacket or a damp slot. Fix it now, because after sealing the problem is hard to reach.
Check that the loop does not respond to metal that should be ignored, such as the gate itself or nearby fence panels. If it does, adjust sensitivity or reposition.
Sealing the slot
Sealant protects the wire and keeps water out. Use a product designed for pavement loops, which stays flexible and resists heat, oil and sunlight. Epoxy that cures hard can crack as the slab moves, so it is generally less suitable than flexible sealants. Fill the slot completely, slightly above the surface, and tool it smooth so water does not pool.
Weather matters. Apply sealant when the slot is dry and the temperature falls within the product's range. Water trapped beneath a sealed slot will cause problems for years. In Central Texas summers, cure times shorten, and a quick surface skin can trap moisture, so follow the manufacturer's directions closely.
Why loops fail anyway
Even good loops can fail when pavement cracks across the slot, when heavy vehicles flex the slab or when the ground beneath shifts. Expansive clay soils common around Round Rock move slabs by small amounts every year, and a crack that crosses a loop will eventually stretch the wire. Water entering through poor sealant can cause intermittent signals, especially after rain. Utility work near the driveway can cut the wire.
Documenting the work
Record the loop size, number of turns, wire type and detector settings. Take photographs of the layout before sealing. A sketch showing where the loop sits relative to the gate and the edge of the pavement helps future repairs. Label the detector with the date of installation. This small amount of paperwork saves a great deal of time when a technician needs to diagnose a problem years later.
Maintenance
Inspect the sealant each year for cracks or gaps and refill them promptly. Keep heavy equipment off the loop. Watch for pavement cracks near the slot and repair them early. If detection becomes unreliable, test the wire's resistance and insulation before replacing the detector. A well cut, well sealed loop is a quiet part of a driveway that does its work without attention.