Movement Joints and Expansion Joint Sealing: Why They Fail and How to Fix Them

Movement joints are one of the most carefully engineered features in any concrete structure — and one of the most frequently neglected once the building is in use. They exist for a precise reason: to give the structure a controlled place to move, rather than cracking unpredictably elsewhere. When the sealant in a movement joint fails, that control is lost, and the joint becomes instead a direct pathway for water ingress, debris accumulation, and freeze-thaw damage.

In car parks, industrial floors, external paving, and below-ground structures, failed movement joint seals are among the most common causes of progressive structural deterioration. They are also among the most cost-effective problems to address — when caught early. Left untreated, water entering through a failed joint seal reaches the reinforcement, initiates corrosion, and eventually causes the kind of spalling and structural damage that costs orders of magnitude more to repair than the original joint resealing would have.

Types of Movement Joint

Not all joints in a concrete structure are the same, and the remediation approach differs depending on the joint type and the movement it is designed to accommodate.

Expansion Joints

Expansion joints are full discontinuities through the structure — the concrete on either side of the joint is completely separate, with a physical gap between them. They are designed to accommodate thermal expansion and contraction, preventing the compressive forces generated by a heating structure from cracking the surrounding concrete. The gap is typically 15 to 25 mm wide, filled with a compressible filler board during construction and sealed at the surface with a flexible sealant.

Expansion joints are found at regular intervals in large floor slabs, on bridge decks, in multi-storey car park decks, and wherever a long run of concrete is exposed to significant temperature variation.

Contraction Joints (Induced Crack Joints)

Contraction joints are partial cuts through the slab — typically one-third to one-quarter of its depth — that create a weakened plane where the concrete will crack in a controlled, straight line as it shrinks during curing and thermal contraction. The crack that forms at the base of the cut is invisible at the surface; only the saw-cut is apparent. These joints are sealed to prevent water and debris ingress through the induced crack.

Construction Joints

Construction joints occur wherever two separate concrete pours meet. They are not designed to accommodate ongoing movement — they are simply the boundary between pours — but they represent a plane of potential weakness and a route for water ingress if not properly treated. In floor slabs, construction joints are often armoured with steel edge protection to resist the impact and shear forces from wheeled traffic.

Isolation Joints

Isolation joints separate a floor slab from adjacent fixed elements — columns, walls, drainage channels, and machinery bases — allowing the slab to move without being restrained by or transmitting forces into those elements. Their seals are often overlooked during maintenance because they are not the obvious straight lines across a floor that expansion and contraction joints form.

Why Joint Seals Fail

Understanding the failure mechanism is essential before selecting a remediation approach. Applying a new sealant over a failed one — or into a joint that has not been properly prepared — is one of the most common and costly mistakes in concrete maintenance.

Age and UV Degradation

Even the best polyurethane and polysulphide sealants have a finite service life. UV radiation, ozone, and thermal cycling progressively degrade the polymer chains that give the sealant its flexibility. A sealant that was correctly specified and installed will typically last 10 to 20 years before it becomes brittle, cracks, and begins to debond from the joint faces. On external structures exposed to direct sunlight and extremes of temperature, degradation occurs faster.

Incorrect Original Specification

Many joint seal failures are not the result of age — they are the result of the wrong sealant being specified or installed. A rigid epoxy sealant in a joint designed to accommodate 5 mm of thermal movement will fail cohesively (cracking through the sealant body) within the first few years. An over-soft sealant in a heavily trafficked vehicle joint will be extruded or torn out by tyre shear forces. Matching the sealant's movement accommodation capability and hardness to the joint's actual service conditions is the first principle of correct specification.

Inadequate Joint Preparation

Sealant adhesion depends entirely on the condition of the joint faces. Contamination with dust, oil, laitance, or the residue of a previous sealant will prevent adequate bonding regardless of the sealant's intrinsic quality. The joint faces must be cut clean, shot-blasted or ground, blown clear of debris, and primed where required. This preparation stage is frequently under-resourced in competitive tender situations — with predictable consequences for longevity.

Three-Sided Adhesion

One of the most technically important — and most widely ignored — principles of joint sealing is the prevention of three-sided adhesion. If a sealant bonds to both faces of the joint and to the back of the joint (the bottom), it cannot deform as the joint moves. Instead of stretching, it tears. A correctly installed joint seal bonds only to the two faces; the back is isolated using a bond-breaker tape or a compressible polyethylene backer rod of appropriate diameter. The hourglass profile this creates — wider at the faces, narrower at the centre — is engineered to distribute stress across the sealant body as the joint opens and closes.

Backer rods are specified as either closed-cell (for joints exposed to water, where the rod must not absorb moisture) or open-cell (where outgassing of the sealant during cure requires air movement through the back of the joint). Using the wrong type introduces either moisture or cure failure.

Debris Ingress and Incompressibility

In external expansion joints — particularly on car park decks and bridge surfaces — hard debris (grit, stones, aggregate) accumulates in the open joint over time. When the joint closes under thermal expansion, this debris prevents full closure and forces load transfer into the sealant rather than across the joint faces. The result is sealant tearing and, eventually, spalling of the concrete arris on either side of the joint. Regular joint cleaning is a maintenance task that is frequently omitted.

Remediation: The Correct Approach

Assessment Before Specification

Before any joint remediation is specified, the following should be established for each joint type on the structure:

Joint Preparation

Correct preparation is not negotiable. The sequence for most joint resealing work is:

Sealant Selection

The most commonly specified joint sealants for concrete structures in the UK are:

Maintenance Intervals

Joint seals on trafficked structures should be inspected as part of any annual condition survey. The inspection should check for:

A joint seal showing early signs of adhesion failure or surface degradation can often be remediated with a partial resealing at significantly lower cost than a full joint preparation and resealing programme. Early intervention is almost always more economical than waiting for complete failure.

The Structural and Financial Case for Proactive Joint Maintenance

Water entering a failed expansion joint on a car park deck does not stay at the surface. It migrates through the concrete to the reinforcement, initiates corrosion, and generates the expansive pressure that causes spalling. On a multi-storey car park, the repair cost for advanced reinforcement corrosion — which requires breaking out damaged concrete, treating or replacing corroded steel, and applying new repair mortar — can run to hundreds of pounds per square metre. The cost of resealing the joint that allowed the water in is typically a fraction of that.

The same logic applies to industrial floors, external paving, and below-ground structures. Movement joint maintenance belongs in every planned preventive maintenance programme for concrete structures — not as a reactive response to visible deterioration, but as a scheduled intervention that keeps the primary routes of water ingress closed.

MPS Concrete Solutions carries out movement joint surveys, arris repair, and joint resealing on car parks, industrial facilities, and below-ground structures across the UK. Contact us to arrange a condition survey or to discuss a joint maintenance programme for your site.