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Strength Matters for Concrete Spacers

Max Frank

Do your concrete spacers hit the bar?

“The best spacer won’t help if not used at all!” – Number of spacers used

Weight of reinforcement as well as foot traffic clearly influences the deflection of the first layer of reinforcement. BS7973 (Part 2, Figure 1) clearly states that spacers for the bottom layer of reinforcement in slabs, in case of welded fabric, need to be at a maximum of 500mm centres. Staggered placement helps to spread the load.

“How can a 42mm spacer give 50mm cover?” – Dimensions of spacers used

Due to the nature of construction, tolerances are higher than in other industries, but are becoming increasingly controlled. Again BS7973 (Part 1, clause 7.2) gives a clear indication. The reinforcement must be positioned as specified to ensure the structural capability of the concrete member. Using the correct cover also prevents corrosion of the reinforcement. Max Frank extruded fibre concrete spacers outperform cast spacers due to the controlled manufacturing process.

“Plastic in concrete” – Materials of spacers used

The success of reinforced concrete is mainly due to similar and complimentary attributes of concrete and steel: Steel has a high ductile strength, whereas concrete has a high compressive strength. If other materials are introduced, such as plastic, this is not the case.

Also, the bond between plastic spacers and the surrounding concrete is significantly inferior. Water penetration to the reinforcement presents a potential problem as a result of the inferior bond.

“Aggregate and blowholes!?” – Concrete quality of spacers used

Blowholes in a cast spacer can occur due to insufficient compaction during the casting process, a problem that won’t occur with Max Frank extruded spacers. If the spacers have a lower compressive strength than the surrounding concrete, the overall specification will not be satisfied. The concrete member is then weakened by the spacer, corrosion and water ingress is not prevented sufficiently, which has a negative impact on the service life of the building. Max Frank Ltd. closely monitors the compressive strength according to a quality plan which is integrated within the ISO 9001:2008 documentation.

Max Frank manufactures spacers for special applications with strengths of 50 N/mm² and also 60 N/mm². Cover options range from 25mm up to 100mm. In addition to the commonly used square bars, Max Frank also manufactures block spacers in a variety of covers and strengths.