Common misdiagnoses

What gets concluded too early about concrete

Most bad concrete decisions are not caused by bad information. They are caused by a reasonable-sounding conclusion drawn from an observation that did not support it — usually one that is right often enough to feel safe.

This page is not here to tell you what your concrete is doing instead. Replacing one premature conclusion with another is the same mistake pointing the other way. Each entry says what an observation does not establish, and what would settle it.

A corner has broken off

Does not establish: A broken corner does not establish that the slab is failing. Corners are the most vulnerable part of any slab and often the only part affected.

Cause categories worth considering: Loss of support beneath a corner, which is supported on two sides at best; Impact from a vehicle, a plough or a dropped object; A crack that had already isolated the corner.

A crack in a foundation wall

Does not establish: A crack in a foundation wall does not establish that the building is unsafe, and we will not attach a width threshold to it — those numbers are context-specific and inventing one would be worse than saying nothing. What the direction and any displacement do is help somebody qualified decide how urgently to look.

Cause categories worth considering: Shrinkage as the wall cured, which is common and usually vertical; Ground movement outside the wall; Pressure from saturated soil or from ground freezing against the wall.

A crack in the basement floor

Does not establish: A crack in a basement floor does not establish a foundation problem. Basement slabs are commonly not structural elements, and shrinkage cracking in them is very common.

Cause categories worth considering: Ordinary shrinkage of the slab as it cured; Ground beneath the slab settling; Water pressure beneath the slab.

A crack that is getting wider or keeps coming back

Does not establish: A widening crack does not establish that a structure is at risk, and it does not establish which of the possible causes applies. What it does establish is that something is still moving, which is the reason to have it looked at rather than sealed again.

Cause categories worth considering: Ongoing ground movement beneath one side of the crack; Loss of support from water carrying material out of the base; Seasonal moisture change in the ground, which recurs rather than accumulates; A repair that bridged a moving joint with a rigid material.

A joint has broken up along its length

Does not establish: Joint deterioration does not establish a problem with the slabs either side. It is usually the joint's own edges failing, which is a localised repair.

Cause categories worth considering: Water entering an unfilled joint and freezing; Traffic loading the unsupported edges of an open joint; Debris in the joint preventing it from closing, so the edges take the load.

A retaining wall is leaning or bulging

Does not establish: Movement does not establish how close a wall is to failing, and nobody can establish that from a description or a photograph. What it does establish is that the assessment needed is a structural one.

Cause categories worth considering: Water pressure building behind the wall because drainage has stopped working; Loading behind the wall increasing — a vehicle, a structure, stored material; The wall's own support moving.

A step you can catch a foot on

Does not establish: A trip edge does not establish anything about the structure it sits on. It is a use-and-safety observation rather than a condition assessment, and it is one of the few things on this list worth acting on before the cause is known.

Cause categories worth considering: Settlement of one side; Lifting of the other side; A combination, where one side dropped and the other rose.

A walkway section has sunk

Does not establish: A settled panel does not establish who is responsible for repairing it. Where a walk is in a public right of way, that is set locally.

Cause categories worth considering: Ground beneath one panel consolidating or eroding; Water running along or under the walk; A trench crossing beneath the affected panels.

Adjacent sections no longer sit level with each other

Does not establish: A height difference does not establish which side moved. That distinction changes the repair completely, and it usually needs a reference point neither section provides.

Cause categories worth considering: One section settling because support beneath it was lost; The other section being lifted by something below it; Different amounts of fill under each section from original construction.

Concrete has lifted above where it used to be

Does not establish: Lifting does not establish its cause, and the cause matters more here than with settlement: a root will continue after the concrete is put back, whereas seasonal ground movement may reverse on its own.

Cause categories worth considering: Ground beneath expanding, which can happen with moisture change or freezing; A root growing beneath the section; Adjacent slabs pushing against each other where there is nowhere to expand.

Concrete has sunk below where it used to be

Does not establish: Settlement does not establish that the concrete is defective — sound concrete settles when the ground under it moves. Nor does it establish whether movement has finished, which is the thing that decides whether correcting elevation will last.

Cause categories worth considering: Ground beneath consolidating, particularly fill placed during construction; Fine material washed out of the base by water moving under the slab; Seasonal moisture change in the ground beneath.

Fine cracking across the surface

Does not establish: Fine surface cracking does not establish that the slab has moved, that support is lost, or that anything structural is involved. It is one of the most common and least significant things concrete does.

Cause categories worth considering: Ordinary shrinkage as concrete cured, which is extremely common; Surface drying faster than the body of the slab during placement; Early-life temperature change while the concrete was still gaining strength.

Ground beside the concrete has washed away

Does not establish: Visible erosion at an edge does not establish how far under the slab the loss extends. That is the question that decides the repair, and it is not answerable from outside.

Cause categories worth considering: Concentrated water discharge running along or under the edge; Run-off from higher ground crossing the boundary of the slab; Loss of base material carried out by moving water.

It appeared after digging nearby

Does not establish: Timing is suggestive rather than conclusive. It does not establish that the excavation caused the damage, and it does not establish that anybody is liable — but it is exactly the kind of fact worth recording contemporaneously rather than reconstructing later.

Cause categories worth considering: Removal of lateral support beside or beneath the concrete; Backfill placed loosely and consolidating afterwards; Vibration during the work.

It appeared after plumbing or utility work

Does not establish: A linear pattern matching a trench is strong evidence about location but does not establish whether the cause is backfill settling or water still escaping. Those need different trades, in a particular order.

Cause categories worth considering: Trench backfill consolidating beneath the concrete; Support removed during the work and not fully restored; A line still leaking after the work.

It appeared or worsened after freezing weather

Does not establish: Seasonal change does not establish permanent damage. Lifting that reverses when the ground thaws is a different situation from settlement that does not, and telling them apart needs observation across a year rather than at one visit.

Cause categories worth considering: Water held in the surface freezing and expanding; Ground beneath expanding as it froze, lifting concrete; Chemical exposure increasing how often the surface froze and thawed.

It appeared or worsened after heavy rain

Does not establish: A change after rain does not establish that the rain caused the underlying condition. It frequently reveals something that was already developing, which is a useful distinction when deciding what to fix.

Cause categories worth considering: Water volume exceeding what the site's drainage could handle; Erosion beneath or beside concrete during a concentrated event; Ground swelling with a rapid increase in moisture.

Part of the slab sounds hollow

Does not establish: Sound alone cannot distinguish a void beneath the slab from delamination within it. Those two have completely different repairs, and separating them needs somebody on site.

Cause categories worth considering: Loss of contact between slab and base over that area; Delamination within the concrete itself, near the surface; A change in slab thickness rather than a loss of support.

Patches of different colour

Does not establish: Colour variation says almost nothing about condition. The exception worth noting is a patch that stays damp when everything around it has dried, which is a moisture observation rather than a colour one.

Cause categories worth considering: Different moisture content across the slab; Deposits left by water moving through the concrete; Substances spilled or dripped onto the surface; Variation between separate pours or batches.

Small cone-shaped holes with a stone at the bottom

Does not establish: Pop-outs do not establish a problem with the slab as a whole. They are localised and generally cosmetic, though they can collect water.

Cause categories worth considering: A particle near the surface expanding and displacing the concrete above it; Freezing acting on a moisture-susceptible particle close to the surface.

Small holes across the surface

Does not establish: Pitting does not establish anything about the slab below the surface. It is usually the top few millimetres reporting how they were finished and what they have been exposed to.

Cause categories worth considering: Loss of the finished surface layer in localised spots; Freezing and chemical exposure acting on a weaker surface; Conditions during finishing that left the surface less durable.

Something has changed and I do not know what

Does not establish: Uncertainty is a legitimate starting point and it establishes nothing either way. Not knowing what is wrong is not a reason to delay asking somebody who can look, and it is not a reason to accept the first explanation offered.

Cause categories worth considering: Any of the mechanisms described elsewhere in this registry; A combination, which is more common than a single cause; Something outside concrete entirely.

The driveway has sunk

Does not establish: Driveway settlement does not establish that the concrete is at the end of its life. Where the slab is sound and has moved as a unit, correcting its height is frequently a better answer than replacing it.

Cause categories worth considering: Consolidation of fill around the foundation, which is why the garage end is most common; Water washing base material out from beneath; A service trench crossing beneath the drive.

The edge is crumbling away

Does not establish: Edge crumbling does not establish that the body of the slab is affected. It is frequently confined to the outer band and reports what is happening beside the slab rather than under it.

Cause categories worth considering: Loss of support along the edge as adjacent ground eroded; Freezing acting on an edge that stays wet; Vehicles running over an unsupported edge.

The flexible filler in a joint has gone

Does not establish: Failed sealant does not establish that anything else is wrong. It is a consumable component with a shorter life than the concrete around it.

Cause categories worth considering: Ordinary end of service life for the material; Movement exceeding what the material could accommodate; Application over a damp or dirty surface so it never fully adhered.

The garage floor has dropped

Does not establish: A settled garage floor does not establish that the building's foundation has moved. In most construction the floor is independent of the walls, which is why a gap opens between them rather than the wall coming down with it.

Cause categories worth considering: Fill placed around the foundation consolidating beneath the slab; Water entering beneath the slab and removing material; A backfilled service trench crossing under the floor.

The patio has sunk or tilted

Does not establish: Patio settlement does not establish that the house has moved. The usual situation is the reverse — the patio moved and the building did not, which is why a gap appears at the wall.

Cause categories worth considering: Fill beside the foundation consolidating beneath the outer part of the slab; Water discharging beside the patio and removing support; Seasonal moisture change in the ground beneath.

The porch or front step has moved away from the house

Does not establish: A porch pulling away does not establish that the building has moved. It commonly establishes the opposite, which is usually the cheaper of the two situations.

Cause categories worth considering: Fill beneath the porch consolidating, since porches are often built on backfill; Water entering beside the porch and removing support; The porch being supported independently of the building and settling on its own.

The steps have shifted or the risers are uneven

Does not establish: Movement in a flight does not establish that it is unsafe to use, and steps that look fine do not establish that risers are still consistent. Only measurement settles the second one.

Cause categories worth considering: Ground beneath the flight settling unevenly; Water entering behind or beneath the steps; The flight rotating away from the structure it serves.

The surface deteriorated over a winter

Does not establish: Winter surface loss does not establish that the concrete was poorly made or that it will keep deteriorating at the same rate. It also does not establish a service life, which nobody can give you.

Cause categories worth considering: Repeated freezing and thawing of water held in the surface; De-icing chemicals increasing how often the surface freezes and thaws; A slab exposed to freezing conditions in its first season.

The surface is breaking away in flakes or patches

Does not establish: Surface loss does not establish that the slab is structurally compromised. In most residential flatwork it is a surface problem on concrete that is otherwise doing its job.

Cause categories worth considering: Freezing and thawing acting on a wet surface; De-icing chemicals concentrating where meltwater collects; Finishing or curing conditions when the slab was placed; Corrosion of steel close to the surface pushing concrete off.

The surface leaves powder on your hand

Does not establish: Dusting does not establish a structural problem. It is a surface-layer condition, though it does tell you a coating applied over it would probably not bond well.

Cause categories worth considering: A weak surface layer left by conditions during finishing or curing; Moisture moving up through the slab and depositing material at the surface; Abrasion of a soft surface layer.

There may be a gap under the slab

Does not establish: A hollow sound does not establish the size of a void or that one exists at all — surface thickness and reinforcement change how concrete sounds. Nor does an absence of hollow sound establish full support.

Cause categories worth considering: Base material carried away by water; Ground consolidating away from the underside of the slab; Fill placed loosely and compressing over time.

Water is coming in at a crack or joint

Does not establish: Water at a crack does not establish that the crack is structural, and sealing the crack does not establish that the water has stopped — it may simply arrive somewhere else.

Cause categories worth considering: Surface water reaching the outside of the wall or slab and finding a path; Groundwater rising seasonally; Exterior drainage that has stopped functioning.

Water runs toward the house rather than away

Does not establish: Water reaching a wall does not establish that it is getting inside, and it does not establish that any damage has occurred. It does establish a condition worth correcting before it does.

Cause categories worth considering: Settlement of the outer edge of a slab, reversing its fall; Original construction with inadequate fall; Ground beside the slab being built up over time by landscaping.

Water stands on the surface after rain

Does not establish: Standing water does not establish that the slab has moved. Plenty of flatwork was placed with too little fall and has never shifted since.

Cause categories worth considering: Insufficient fall in the original construction; Settlement creating a low point that was not there before; A surrounding surface being raised so water can no longer leave.

Settlement

Shrinkage. Concrete shrinks as it cures, which produces cracking without anything sinking. Settlement usually leaves one side of a crack lower than the other; shrinkage usually does not.

Differential settlement

Uniform settlement, which is usually harmless and frequently unnoticed. The damage comes from the difference, not from the depth.

Void

A hollow sound, which suggests a void but also has several other explanations — including a thin section sitting on perfectly sound base.

Control joint

A crack. A crack running neatly along a straight groove is usually the joint doing its job. A crack that ignores the grooves is the more interesting one.

Expansion joint

A control joint. A control joint controls cracking within one slab; an expansion joint separates one thing from another. Filling the second with rigid material defeats the point of it.

Spalling

Scaling. Scaling is the loss of a thin surface layer over an area; spalling is pieces coming out, leaving depth. They can share a cause and they do not share a remedy.

Scaling

Spalling, which has depth. Also normal wear: a surface that has simply been walked and driven on for twenty years is not necessarily scaling.

Curling

Heave or root lifting. Curling is the concrete deforming; heave is the ground moving. A curled edge is usually at a joint or a corner and is often accompanied by a gap underneath.

Heave

Root lifting and curling, both of which also raise concrete. Heave tends to be seasonal and reversible; the other two are not.

Mudjacking

Polyurethane foam lifting. They achieve the same thing by different means, with genuinely different trade-offs in weight, hole size and behaviour in wet ground.

Polyurethane foam lifting

Spray foam insulation, which is a different product entirely. Also mudjacking — see that entry for the actual comparison.

Grinding

Levelling. Grinding removes the hazard; levelling moves the slab. Both produce a flush surface on the day.

Resurfacing

Replacement. A resurfaced slab is the old slab with a new coat. Anything wrong below the bond line is still there.

Full-depth repair

Partial-depth repair, which removes only the deteriorated top portion and leaves sound concrete beneath. The word 'partial' appears in both and means different things.

Reinforcement

Crack prevention. Reinforcement mostly controls what a crack does after it forms rather than stopping it forming.

Mix strength

Thickness. A stronger mix in a thinner slab is not automatically better than the reverse; they are separate specifications doing different jobs.

Curing

Drying. Concrete does not harden by drying out — it hardens by reacting with water, which is why keeping it damp early on helps rather than hurts.

Sealer

A coating, which builds a film with its own thickness and appearance. Also joint sealant, which is a different product for a different job.

Efflorescence

Scaling and mould. Efflorescence brushes off or dissolves; a deteriorating surface does not.