Protecting Concrete in Marine and Coastal Environments: UK Guide

The Marine Environment: Why It Is So Aggressive to Concrete

Marine and coastal environments subject concrete to a combination of deterioration mechanisms that is unmatched in severity by any other common UK exposure category. Structures located within the splash zone — the range of elevations alternately wetted and dried by wave action — and the tidal zone — the range submerged and exposed by the tide — experience cyclical wetting and drying with chloride-rich seawater, mechanical abrasion from wave-borne particles, biological attack from barnacles and algae, and freeze-thaw cycling in the winter months. Structures above the splash zone, in what is classified as the atmospheric marine zone, are exposed to salt-laden spray and to elevated levels of airborne chloride that deposit on concrete surfaces and are drawn into the concrete pore structure by surface moisture cycling.

The dominant deterioration mechanism in all marine exposure zones is chloride-induced reinforcement corrosion, driven by the chloride concentration of seawater — approximately 35 g/litre — which is several orders of magnitude higher than the chloride concentration in inland groundwater or de-icing salt runoff from roads. At these concentrations, chlorides penetrate even well-specified concrete rapidly: diffusion modelling of concrete with a water-cement ratio of 0.45 suggests that chlorides will reach the critical threshold at a cover depth of 40 mm within 15–30 years of first marine exposure, and on structures with inadequate cover or high water-cement ratio, within 5–10 years. The resulting corrosion, cracking and spalling represents both a structural risk and an ongoing maintenance liability that requires a proactive, systematic management approach.

MPS Concrete Solutions provides condition assessment, structural repair and protective coating services for marine and coastal concrete structures including jetties, piers, sea walls, tidal barriers, coastal car parks and promenade structures across the UK. Our related service on Industrial Concrete Repairs covers the full range of structural concrete repair techniques applicable to marine structures, and our concrete defect diagnosis guide describes the investigation techniques used to characterise marine deterioration before a repair programme is specified.

Deterioration Mechanisms in Different Marine Zones

The classification of marine exposure zones is defined in BS EN 206:2013+A2:2021 (Concrete — Specification, Performance, Production and Conformity) and in BS EN 1992-1-1 (Eurocode 2 — Design of Concrete Structures). For repair and protection work, understanding which zone a structure occupies is essential to selecting the correct repair material system and protection strategy, as the exposure demands differ significantly between zones.

In the permanently submerged zone — below the lowest astronomical tide — concrete is in continuous contact with seawater. Oxygen availability is limited, which slows the corrosion rate once chlorides have reached the reinforcement, but the continuous wetting means chloride ingress is sustained and there is no drying cycle to interrupt the process. Biological colonisation by barnacles, tube worms and marine algae can occur on the concrete surface, and the mechanical attachment of these organisms can physically damage the surface layer over time. Repair access in the submerged zone requires diving operations or dewatering, both of which significantly increase programme duration and cost compared with above-water repairs.

The tidal zone and splash zone represent the most aggressive combination of exposure factors: chloride concentration is high, wetting and drying cycles are frequent, oxygen availability is adequate to sustain active corrosion, and mechanical wave action provides both abrasion loading and hydrostatic pressure cycling. Structures in these zones typically show the most rapid rate of visible deterioration, with spalling appearing first at tidal zone pile sections and at the soffit of bridge beams and jetty deck slabs within 15–30 years of construction on inadequately specified structures. Repair access in the tidal zone requires careful tidal window management — works must be completed within the available dry period at each tide — which makes programme planning and daily logistics significantly more complex than on permanently accessible structures.

In the atmospheric marine zone — above the splash zone but within 500 metres of the shoreline, or further depending on local wind and wave conditions — the primary exposure is to airborne chloride deposition. Visible deterioration in this zone typically begins with map cracking and rust staining, progressing to spalling over a 20–50 year timescale depending on concrete quality and the severity of the salt environment. This zone includes many coastal car parks, promenades, sea wall parapets and coastal building structures, where owners often assume the concrete is simply "weathered" without recognising that active reinforcement corrosion is the underlying mechanism.

Materials for Marine Concrete Repair

Material selection for marine concrete repair is more demanding than for inland applications because the repair system must perform not only in terms of adhesion, strength and shrinkage compatibility but also in terms of chloride resistance, resistance to biological attack and durability under cyclical wetting and drying. Standard polymer-modified cementitious repair mortars (BS EN 1504-3 Class R2–R3) are appropriate for repairs in the atmospheric marine zone; for repairs in the tidal zone and splash zone, higher-specification materials are required.

Calcium aluminate cement (CAC)-based repair mortars offer significantly better resistance to chloride penetration and sulfate attack than Portland cement systems, and they are specified for repairs in the tidal and splash zones on structures where the long-term chloride resistance of the repair is a primary design requirement. Their higher material cost — typically 50–100% above equivalent Portland cement systems — is justified by their superior marine durability and the potential service life extension they offer relative to a Portland cement repair that may require repeat intervention within 10–15 years. CAC systems have specific application requirements — temperature limits, curing procedures and substrate preparation standards that differ from Portland cement systems — and should be applied by contractors with specific experience of the material.

Epoxy and epoxy-modified cementitious systems are used for thin-section repairs in marine applications where a very high chloride resistance and good tensile strength are required, particularly at the pile-deck interface of jetties and piers where the repair is subjected to bending and shear as well as compression. Epoxy systems are sensitive to high substrate moisture content and require a surface-dry substrate for optimum bond, which can be difficult to achieve in the tidal zone. Substrate drying by hot-air blower or the use of moisture-tolerant epoxy primer systems is required in these situations. Sika products including Sika MonoTop and SikaTop repair mortar systems are regularly specified for marine repair applications and are available through our Sika product range.

Protective Coatings for Marine Concrete Structures

A protective coating applied to sound concrete in the atmospheric marine zone — and to repaired surfaces in all marine zones above the tidal zone — significantly slows the rate of future chloride ingress and extends the time before further repair is required. The appropriate coating type depends on the marine exposure intensity, the substrate condition and the level of pedestrian or vehicle trafficking the coated surface must withstand.

Silane or siloxane impregnating treatments penetrate the concrete pore structure and render it hydrophobic without significantly altering the surface texture or appearance. They do not form a surface film — which means they cannot be damaged by abrasion or impact — but they dramatically reduce the rate at which liquid water and dissolved chlorides enter the concrete. Silane impregnation is the standard protective treatment for atmospheric marine zone concrete on structures where appearance must be preserved and where a film coating would be visually obtrusive, such as heritage sea walls, promenade balustrades and listed coastal structures. Application requires the concrete surface to be dry and the ambient temperature to be above 5°C; silane treatments should not be applied in rain or fog conditions. Retreatment is typically required after 10–15 years as the hydrophobic effect gradually diminishes.

Elastomeric polyurethane or PMMA coatings are specified where the concrete surface requires a higher level of protection than an impregnant can provide — typically on heavily chloride-contaminated structures in the lower atmospheric zone, on structures with existing fine cracking, or on trafficked areas such as coastal car park decks. These coatings form a continuous film that bridges existing fine cracks and resists both liquid water and chloride penetration. Their limitation in the marine environment is resistance to UV degradation (polyurethane coatings gradually yellow and lose flexibility with prolonged UV exposure) and the potential for blistering if the substrate moisture content is too high at the time of application. For marine structures, coating systems should be selected from a manufacturer's marine-approved range and should have documented performance data from marine exposure testing or field reference projects in similar environments.

Inspection, Maintenance and the Long-Term Management of Marine Concrete

Marine concrete structures require a structured inspection and maintenance programme to manage deterioration proactively and to avoid the large-scale, high-cost remediation that results from deferred maintenance. The Concrete Society Technical Report 65 (Guide to the Design, Construction and Maintenance of Marine Structures) and the Highways England (now National Highways) Design Manual for Roads and Bridges (DMRB) both provide guidance on inspection regimes for marine structures, recommending principal inspections at intervals not exceeding 6 years supplemented by routine visual inspections annually.

A principal inspection for a marine concrete structure should include: visual survey of all accessible surfaces with photographic record; half-cell potential mapping of exposed reinforced concrete surfaces to identify active corrosion zones; chloride sampling at defined depths to assess the current penetration front; carbonation depth testing; concrete cover measurement at defined survey points; and structural assessment against the original design load case if section loss is suspected. The outputs of the principal inspection drive the maintenance strategy for the following 6-year inspection cycle, including the scope of physical repairs, the protective treatment to be applied to sound areas, and any cathodic protection measures required to control ongoing corrosion.

MPS Concrete Solutions provides marine concrete inspection, structural repair and protective coating services for coastal structures across the UK. If you own or manage a marine or coastal concrete structure showing signs of deterioration — rust staining, cracking, spalling or delamination — contact our team to arrange a condition assessment. We can advise on the most appropriate repair and protection strategy for your specific exposure conditions, and our guide to what to expect during a commercial concrete repair project describes how we plan and deliver complex repair programmes in challenging access environments.