So, What is Thioflex 600?

Thioflex 600 is a two-component polysulfide joint sealant manufactured by Fosroc, used to seal expansion and movement joints in building and civil engineering structures. It’s a general-purpose construction sealant rather than a pavement-specific product, and it’s specified across high-rise buildings, basements, subways, retaining walls, brickwork, water-retaining structures, bridges, and floor expansion joints. It’s available in two grades: Gun Grade for vertical joints and general application, and Pouring Grade for horizontal joints and floor stress-relief joints. Movement accommodation is 25% on butt joints and 50% on lap joints under BS 6093.

At Shepherd and Sons, we specify and apply Fosroc sealants across UK pavement, airfield and civil infrastructure work. While our core specialism is pavement joint sealing under BS EN 14188, we’ve worked alongside Thioflex 600 on civil engineering projects where it’s the right product for structural movement joints adjacent to our pavement work, particularly on airfield civil structures and bridge interfaces. This article covers what Thioflex 600 is, where it’s the right specification, how it differs from Thioflex 555, and what proper installation involves.

What Thioflex 600 actually is

Thioflex 600 is a two-component polysulfide-based elastomeric sealant supplied as a base and a separately-packaged curing agent. The two components are mixed on site to initiate the polysulfide cross-linking reaction. Once mixed, the sealant has a defined pot life (around 2 hours at 25°C) during which it must be applied. After application, it cures over hours to weeks depending on temperature, ending as a tough, elastic, chemically resistant seal.

The product is supplied in two grades:

Gun Grade. A thixotropic (non-sag) sealant packed in a 2.5-litre tin containing the base and curing agent in the correct ratios. Suitable for general application, including vertical and overhead joints, brickwork pointing in movement joints, retaining wall joints, and curtain wall interfaces.

Pouring Grade. A self-levelling sealant supplied in a 5-litre tin with the base in the main container and the hardener in a removable sachet or tray. Suitable for horizontal joints, floor expansion joints, stress-relief joints in floors, and any application where the sealant needs to flow into the joint under gravity.

The product carries Fosroc’s general construction sealant designation and is widely specified through NBS specifications across UK building and civil engineering work.

Key technical properties

The technical specification for Thioflex 600 differs significantly from pavement-grade products like Thioflex 555:

• Movement accommodation: 25% on butt joints, 50% on lap joints (BS 6093)
• Operating temperature range: -20°C to +60°C
• Elastic recovery: 86%
• Density: 1.62 kg/litre
• Pot life: 2 hours at 25°C
• Tack-free time: Over 24 hours
• Initial set: 18 hours at 25°C / 36 hours at 15°C / 72 hours at 5°C
• Full cure: 1 week at 25°C / 2 weeks at 15°C / 4 weeks at 5°C
• Rate of cure: Over 85% at 48 hours
• Application temperature range: 5°C to 50°C
• Flash point: Over 65°C

The slower cure compared to pavement sealants like Thioflex 555 (which returns to service in 30 minutes at 20°C) reflects the different application. Thioflex 600 is used in building joints that aren’t typically subject to fast return-to-service traffic requirements. The long cure window allows it to bond properly with a wider range of substrates including brickwork, glass, aluminium, stainless steel, primed concrete and primed timber.

When Thioflex 600 is the right specification

Thioflex 600 is specified across five main application categories.

Building expansion joints. High-rise buildings, residential blocks, commercial superstructures. The 25% butt joint movement accommodation handles thermal cycling and structural movement in framed buildings. The product’s adhesion to glass, aluminium and stainless steel makes it suitable for curtain wall and cladding interface joints.

Basements, subways and below-ground structures. Thioflex 600 is widely specified in below-ground civil engineering work because of its durability and resistance to water and ground chemicals. It’s used on basement floor joints, subway expansion joints, retaining wall joints and underground service structures.

Floor expansion and stress-relief joints. Pouring Grade is the standard for horizontal floor joints in industrial buildings, warehouses, plant rooms and superstructure floors. It accommodates the differential movement between large floor slabs without breaking the seal.

Water-retaining structures. With the appropriate primer (Fosroc Primer 13, a two-component epoxy with hydrolytic stability), Thioflex 600 can be specified in water-retaining structures including reservoirs, water tanks and treatment works.

Brickwork and masonry movement joints. Used in vertical movement joints in brickwork, blockwork and stone facing, particularly where the joint accommodates thermal and structural movement in long facade runs.

In our experience working on airfield civil infrastructure, Thioflex 600 Pouring Grade works better than standard one-part polyurethane sealants on floor expansion joints in plant rooms and ancillary buildings because the polysulfide chemistry handles long-term thermal cycling without losing elasticity, where one-part polyurethanes can become brittle after five to seven years. On structures with limited maintenance access, that longer service life is what justifies the higher product cost.

How Thioflex 600 differs from Thioflex 555

The two products are often confused because they share the Thioflex brand and the polysulfide chemistry. They are different products for different applications.

Application category. Thioflex 555 is a pavement joint sealant for concrete roads, runways and hard standings. Thioflex 600 is a general construction sealant for buildings and civil structures.

Movement accommodation. Thioflex 555 delivers 35% on butt joints. Thioflex 600 delivers 25% butt / 50% lap. The higher 555 figure reflects the larger thermal cycling on pavement joints exposed to direct sun.

Cure speed. Thioflex 555 Machine Grade cures in 30 minutes at 20°C. Thioflex 600 cures fully over 1 to 4 weeks depending on temperature. The 555 product is engineered for fast return-to-traffic; the 600 product is engineered for durability in building joints where cure time isn’t operationally critical.

Standards. Thioflex 555 is certified to BS EN 14188-2 (pavement sealants) Classes B, C and D. Thioflex 600 conforms to BS 4254 and BS 6093 (building sealants and joint design in buildings).

Service temperature range. Thioflex 555 operates across -40°C to +70°C. Thioflex 600 operates across -20°C to +60°C. The 555 product’s wider range reflects pavement exposure to extreme outdoor conditions.

Application method. Thioflex 555 Machine Grade is extruded through a metered 1:1 mixing machine. Thioflex 600 is mixed on-site by hand and applied with a hand gun or poured directly.

Fuel resistance. Thioflex 555 is tested under BS EN 14188-2 against aviation fuels, kerosene, Skydrol, glycols and de-icers. Thioflex 600 doesn’t carry equivalent fuel-resistance certification and is not specified for fuel-exposure applications.

The simple summary: if you’re sealing a joint in a building, a basement, a subway, a bridge deck or a floor, Thioflex 600 is the relevant product. If you’re sealing a pavement joint exposed to traffic and fuel, Thioflex 555 is the relevant product.

What proper Thioflex 600 application involves

Thioflex 600 is more forgiving than pavement-grade sealants on cure speed and pot life, but the preparation and priming discipline still matters.

Joint preparation. Joint surfaces must be sound, dry, clean and free from contamination. Loose material, dust, oil and laitance are removed. Concrete is wire-brushed or lightly grit-blasted as required. Metal substrates are degreased.

Backer rod. A compatible closed-cell backer rod is inserted to control sealant depth, prevent three-sided adhesion and create a proper joint geometry. For joints where Thioflex 600 will be used, the backer rod doesn’t need to be heat-resistant (unlike hot applied work).

Priming. Fosroc supplies a range of primers matched to substrate type:

• Primer 4 for porous substrates (concrete, brickwork, stone)
• Primer 7 for general concrete, brickwork, stone and timber applications
• Primer 13 (two-component epoxy) for joints subject to water immersion
• Glass, aluminium and stainless steel typically require no primer

Primer selection is critical and must follow the manufacturer’s matched primer guidance. Wrong primer or no primer is the most common cause of premature adhesive failure on Thioflex 600 joints.

Mixing. The base and curing agent are combined and mixed thoroughly with a slow-speed drill and paddle. Mix until a uniform colour and consistency is achieved. Air must not be incorporated into the mix.

Application. Gun Grade is applied with a manual or pneumatic gun, working the sealant into the joint to ensure full contact with the primed surfaces. Pouring Grade is poured directly into horizontal joints, or loaded into a ‘G’ Gun for joints under 15mm wide.

Tooling. Thioflex 600 is tooled to a smooth finish using a tool wetted with dilute detergent solution. Masking tape can be used to protect adjacent surfaces, removed before the sealant skins over.

Cure protection. Joints are protected from contamination and from mechanical disturbance during cure. The 18-hour initial set at 25°C means most joints can be lightly handled the next day, but full mechanical performance isn’t achieved until 1 to 4 weeks later depending on temperature.

We’ve found that contractors who treat Thioflex 600 cure times the same as pavement-grade sealants are the ones who get adhesion failures. The long cure window is not optional. Loading a Thioflex 600 joint before full cure compromises the bond permanently.

Standards and compliance

Thioflex 600 conforms to:

• BS 4254 (two-part polysulfide-based sealants for the building industry)
• BS 6093 (design of joints and jointing in building construction)
• Type FC classification under BS 5212 for cold applied joint sealants (where applicable)

For specifiers writing NBS specifications, Thioflex 600 is referenced in standard NBS clauses for two-part polysulfide building sealants and is widely accepted across UK building and civil engineering work.

Storage, shelf life and pack sizes

Thioflex 600 has a 12-month shelf life in unopened original containers, stored at 5°C to 27°C in dry conditions. Storage outside that range shortens shelf life and can cause base component separation.

Pack sizes are:

• Gun Grade: 2.5-litre tin (base and curing agent pre-portioned)
• Pouring Grade: 5-litre tin (base in main container, curing agent in removable sachet or tray)

Primers are supplied in matched sizes: Primer 4 in 250ml tin, Primer 7 in 1-litre can, Primer 13 in 250ml two-component pack.

Specifying Thioflex 600 properly

A meaningful Thioflex 600 specification will identify the grade (Gun or Pouring), the joint dimensions, the substrate type, the matched primer, programme constraints, and cure-window protection. A specification that just lists “Thioflex 600” without grade or primer detail isn’t a complete specification.

If you’re specifying or commissioning building or civil engineering joint work involving Thioflex 600, or you’re working on infrastructure that combines pavement joint sealing with building or structural sealing, get in touch. We’ll discuss the specification, the substrates involved, and where Thioflex 600 fits alongside pavement-grade joint sealing products. You can see recent project work on our LinkedIn and Instagram.