Structural Joints Mastic: Sealing Movement Joints in Concrete Structures

Large concrete structures are built with joints that let whole sections move independently. Our team provides structural joints mastic for those joints, sealing them so the frame can move while water and debris stay out.

Photo: sealing precast wall joints at Guildford train station multi-storey car park.

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Definitions

What we mean by structural joints

Structural movement joints are the planned breaks an engineer designs into a concrete frame: between blocks of a building, between a frame and its cores, or between new and existing structures. They are usually wider and see more movement than everyday brickwork joints, so the sealant and the backing have to be chosen with care.

To be clear about scope: this page covers structural movement joint sealing with elastic sealants. It is not structural glazing, structural repair or mechanical expansion joint systems; those are designed and installed by other specialists, and we are happy to work alongside them.

Horizontal joints between concrete wall panels, sealed in grey, Guildford car park — Horizontal joints between concrete wall panels, sealed in grey (Guildford car park).

What we seal

Structural movement joints we seal

Typical joints on our structural work:

Frame and slab isolation joints

Joints separating a concrete frame from slabs, cores and adjoining structures.

Joints between structures

Movement joints in concrete frames where new buildings meet existing ones.

Precast to in-situ joints

Junctions where precast panels, stairs or beams meet cast concrete.

Walls and soffits

Vertical wall joints and overhead soffit joints, sealed from MEWPs or towers.

Car parks

Car park structures and deck edges

Car park structures move a lot because they are exposed to sun, frost and traffic. We seal wall, upstand and soffit movement joints on car park structures, and we discuss car park deck joints case by case, since trafficked deck joints often need a specific engineered system.

Our Guildford train station multi-storey car park package, carried out between September and November 2024, is a good example of this kind of work: roughly 1,250 metres of joints on the walls of a large concrete structure, sealed with a one-part modified polymer sealant over bond breaker tape.

Access

Sealing structural joints at height

Most structural joints sit well above ground. Our director holds IPAF 3a/3b, PASMA, CSCS and SSSTS cards, and our team works from boom lifts, cherry pickers, scaffold towers and crane-suspended baskets. We plan access with your site team, and RAMS are provided on request. We have had no RIDDOR incidents in 3 years.

Method

Preparing a structural joint properly

Wide, high-movement joints leave little margin for error.

Survey
Joint widths, depths and movement checked against drawings and the sealant’s movement capability.
Prepare
Old material removed, faces ground or brushed clean and primed where required.
Back
Backer rod or bond breaker tape to the right depth so the sealant can stretch freely.
Seal
Low-modulus or modified polymer sealant applied and tooled to a consistent profile.

Products

Sealants for structural movement

For high-movement concrete joints we use low-modulus silicone, polyurethane, polysulphide and modified polymer sealants, matched to movement, exposure and colour. We use and apply Arbo, Sika, Tremco and Dow products, and always work to the engineer’s or architect’s specification.

Colour matters too on exposed concrete. Grey and stone shades blend into most frames, while darker tones can be used to pick out joints as a deliberate architectural line. We confirm colour on a sample joint before the full run, so the finished structure looks consistent from every side.