Frequently Asked Questions

What joint width and depth profile is required for Expoflex and how should a saw-cut floor joint be prepared for filling?

For Fosroc Expoflex to perform reliably in a saw-cut floor control joint, the joint must be prepared to the correct geometry and cleanliness before filling. The minimum joint width for gun application of the two-component Expoflex is typically 6 to 10 mm — narrower joints are very difficult to fill without air voids and the joint filling is too thin to resist the shear loading across the joint edge under forklift traffic. For joints narrower than 6 mm, the saw cut should be widened by a second pass with a wider saw blade to achieve the minimum 6 to 8 mm filling width. The joint depth should be controlled by inserting a backer rod (Nitofoam 45 at the appropriate width) to the required depth so that the Expoflex fill depth is not more than 1.5 times the joint width (to avoid excessive epoxy mass in deep joints that generates heat during cure and may cause cracking). For saw-cut control joints in industrial floors, the typical profile is 6 to 8 mm wide by 8 to 12 mm deep, with a backer rod maintaining the depth. Before filling, the joint faces must be: clean and dry (free of cutting slurry, water, and concrete dust from the saw cutting — typically allow 5 to 7 days after saw cutting for the joint to dry completely); free of oil or contamination; and properly primed if the Expoflex technical data sheet requires priming for the specific concrete quality. The Expoflex is gun-applied into the prepared joint, filling to slightly above the floor surface, and levelled flush with the floor by a straight edge or putty knife.

At what age after concrete casting should saw-cut joints in an industrial floor be filled with Expoflex?

The timing of saw-cut joint filling with Expoflex in an industrial floor is critical — filling the joints too early risks joint refilling or cracking as the concrete continues to shrink and the joints continue to open after the Expoflex has been applied. Concrete undergoes the majority of its drying shrinkage movement in the first 28 to 60 days after casting, during which the saw-cut joints open progressively as the concrete shrinkage pulls the two sides of the joint apart. If the joints are filled during this active shrinkage phase, the cured Expoflex (even with its limited flexibility) is subjected to ongoing strain as the joint continues to open, and the joint fill may crack or debond from the joint faces before the concrete reaches dimensional stability. The recommended minimum age for joint filling with Expoflex (and similar semi-rigid epoxy joint fillers) is 28 days after concrete casting for joints in slabs with steel fibre reinforcement (which have lower shrinkage) and 60 days after casting for joints in plain concrete slabs (which have higher long-term shrinkage). The floor programme should allow for this minimum age before the joint filling operation is scheduled. In fast-track construction programmes where floors must be handed over to operations within 28 days of casting, the joint filling should be deferred and the joints left open until 60 days is reached — the concrete can be put into light service with open saw-cut joints (clearly marked and protected from debris and damage), with joint filling scheduled as a follow-up operation after the minimum age.

What is the difference between Expoflex and Nitoseal MS300 for filling control joints in an industrial floor and when should each be used?

Expoflex and Nitoseal MS300 are both used for filling saw-cut control joints and cracks in concrete floors, but they differ fundamentally in chemistry, flexibility, and joint movement accommodation capability. Expoflex is a flexible epoxy compound — it has high adhesion and chemical resistance typical of epoxy, and limited flexibility (designed for joints with very small residual movement after filling). Nitoseal MS300 is a one-component moisture-curing polyurethane/silyl-terminated polymer sealant — it has much higher elasticity and flexibility than Expoflex, capable of accommodating significantly larger cyclic movement at the joint (elongation at break above 100 per cent versus the limited flexibility of epoxy). The selection between the two depends on the type of joint, the expected movement, and the traffic loading. Expoflex is the correct specification for: saw-cut control joints in industrial floors subject to fork-lift and heavy vehicle traffic (the semi-rigid epoxy provides load transfer across the joint edge and prevents edge chipping and spalling that would occur if a soft, elastic sealant is used under traffic loading); joints with small or negligible movement (the concrete shrinkage cycle is essentially complete before filling); and joints in environments requiring chemical resistance to oils or fuels. Nitoseal MS300 is the correct specification for: expansion joints with designed movement allowance (where the joint must accommodate cyclic thermal opening and closing); joints in building facades and external walls; and any joint where significant ongoing movement is expected after filling and where load transfer across the joint is not required.