However, there will nearly always be some adjustment to the chemical admixture quantity to maintain the desired workability for the application. This adjustment will depend on the fibre type, the dosage of fibres, the base concrete mix design and the application, to maintain “Fit for Purpose” compliance.
All fibres provide additional physical “solid” material which requires a coating of cement paste, as for other “solid” constituents, when properly and evenly mixed throughout the concrete. The amount of fibre surface area to be coated depends on the diameter and dosage of the fibres. This then dictates the amount of chemical admixture, typically plasticiser or superplasticiser, to be added to compensate for the loss of static slump caused by the addition of the fibres.
Loss of static slump in FRC can be deceptive, as “workability” may often recover when vibration is added to the mix. This is known as a “thixotropic” state and should be understood by the concrete supplier and contractor before any additional water is added to the mix.
For this reason, it is recommended that the water/cement ratio in a good quality concrete is minimised, to reduce the amount of excess water above that required for full hydration of the cement. A ratio of less than 0.5 is recommended but will ultimately be the responsibility of the technical department of the concrete supplier.
The type and dosage of chemical admixture will also be a decision for the technical management of the concrete supplier, based on experience and the constituent materials in the concrete supplied.
Fibre Types, Dimensions and Dosages
Vertex 12 & 6mm – EN14889-2 Class Ia
Vertex is a fine monofilament polypropylene fibre designed for the optimum balance between performance and function, not only for the plastic phase of the concrete’s early life but also for passive fire protection in high strength hardened concrete mixes.
Typically dosed at 0.9kg/m³, Vertex is nominally 32 microns in diameter and 12 or 6mm in length, meaning it has the fineness to significantly reduce plastic shrinkage cracking and settlement cracking in normal concrete and self-compacting concrete (SCC). It also provides explosive spalling resistance in high strength concrete at dosage rates between 0.6 and 2kg/m³, without significant demand for extra paste coating or chemical admixture addition.
In comparison to fibres with an 18 micron diameter, Vertex has a 32% reduction in surface area (fibre count) per kg of fibre added. This means a greater likelihood of full dispersion without clustering, and less chemical admixture required to offset static slump loss. This is particularly important when using 1-2kg/m³ dosages for passive fire protection.
Vertex fibres are added in pre-weighed degradable paper bags into the back of a concrete truck mixer at the beginning of the batching cycle, to allow water and mixer blades to release the fibres from the bag. Final adjustment to mix water, or chemical admixture, for consistence class compliance should be made after the final bag of Vertex has been added to the mixer and 5 minutes at mixing speed has elapsed. If used at a “wet batch” plant, the mixing time required will be the same as for the plain concrete, although it is recommended to re-mix and adjust consistence in the truck before dispatch to site.
Vertex Screed – EN14889-2 Class Ib
Vertex Screed is a fine fibrillated polypropylene fibre, 6mm in length, used for rapid and even dispersion in dry and semi-dry sand and cement screed mixes. No adjustment to the ratio of sand and cement is required. Typical dosages are 0.9kg per tonne or 0.9kg per cubic metre of screed, depending on supplier and contractor preference and experience.
Vertex FF18 – EN14889-2 Class Ib
Vertex FF18 is a fine fibrillated polypropylene fibre, 18mm in length, used for fine aggregate concrete and render mixes where additional mechanical bond from the micro fibres is required, but without the need for structural compliance.
Nexus 85 & 50 – EN14889-2 Class II
Nexus fibres are coarse monofilament macro synthetic fibres, either 60mm or 50mm in length, designed to be added at dosages of more than 2kg/m³ primarily, but not exclusively, to achieve structural compliance with EN14889-2.
Not all macro synthetic fibres are “structural” – a common misconception. They only become “structural” when the supplied dosage in a concrete mix attains the compliance requirements stated on the CE label and Declaration of Performance (DoP). These two documents are required for all additives sold to the public for concrete in the UK construction industry.
Nexus 85 achieves structural compliance with a dosage of 2.25kg/m³, the lowest compliant dosage in the UK. As with Vertex fibres, an adjustment to the chemical admixture quantity may be required. However, despite the higher comparative dosage, the surface area demand for paste coating is a small fraction of that of Vertex – just 0.5% of the surface area.
Nexus fibres are supplied in pucks within a pre-weighed degradable bag and should be added gradually to the truck mixer after all the other mix constituents have been dosed. In this way, sympathetic dosing of the fibres will prevent any clusters or “fibre balls” from occurring in the FRC. Final adjustment with mix water and/or chemical admixture should be made after 5 minutes at mixing speed to meet the consistence specified.
Apex 50, 60 & 65 – EN14889-1 Group 1
Apex steel fibres are available with different aspect ratios to suit the application and performance requirements. Dosage rates range from 7.5 to 50kg/m³, with 7.5kg/m³ of Apex 60 and Apex 65 being the lowest structurally compliant dosage in the UK. As already explained, the extra “aggregate” provided by the steel fibres may require some adjustment, particularly for the higher dosage applications.
However, the specific gravity and diameter of Apex 60 steel fibres, in comparison to Nexus 85 synthetic macro fibres, means that the surface area (fibre count) demanding cement paste coating equates to about 27.5kg/m³ of Apex 60, compared to 2.25kg/m³ for Nexus 85 synthetic macro fibre.
Apex steel fibres are supplied in cartons of 7.5kg (Apex 60) or 25kg (Apex 65), and the dosage to be added will be decided by the application and performance requirements. Apex 60 dosed at 7.5kg/m³ is the standard addition rate for Beam & Block floors where NHBC compliance is required. Apex 65 dosed at 45kg/m³ is commonly used for pile supported slabs. The dosage rate required for the application will dictate the adjustment of the mix, primarily the quantity of plasticiser or superplasticiser used when dosage rates are higher than 7.5kg/m³.
As with Nexus fibres, sympathetic addition of Apex steel fibres to the truck or wet batch plant is essential to ensure no clustering or fibre balls occur. Most concrete plants (or sites, if applicable) will employ conveyors or blowers to introduce the fibres into the truck.
Addition of the Apex fibres after all other mix constituents have been dosed, while the truck is spinning at mixing speed, is recommended. Final adjustment with mix water and/or chemical admixture should be made after 5 minutes at mixing speed to meet the consistence specified.
Concrete Mix Design Requirements and Applications
As previously mentioned, the base concrete mix will depend on the general “Fit for Purpose” requirements and the application.
Beam & Block Floor Toppings
For Beam & Block floor toppings the minimum strength requirement is C20/25, but SCC mixes may achieve higher strengths than this. Nexus 85 dosed at 2.25kg/m³ or Apex 60 dosed at 7.5kg/m³ will require minimal adjustment to the mix, which will ultimately be determined by the consistence class (or flow, with SCC) requested by the contractor. Holding back some of the mix water until the end of the batching process is recommended.
Although Beam & Block floor toppings are generally only 75mm deep and will most likely be covered by a floor finish, when using SCC and Apex 60 fibres it is recommended to incorporate Vertex micro fibres into the mix to resist the sedimentation of the heavy steel fibres. This is essential in deeper floor slabs and is considered good practice by most concrete suppliers.
Industrial Slabs
For industrial slabs that are commonly power-floated, there is a general rule to have a minimum cement content of 330kg/m³, and good practice is to keep the water/cement ratio below 0.5. These values may sometimes be specified for a specific project and will also be determined by the specified strength and aggregates used locally.
Large Panel Ground Supported and Pile Supported Slabs
For large panel ground supported slabs and pile supported slabs using minimum dosages of Apex 60 at 30kg/m³ and Apex 65 at 45kg/m³ respectively, there will be a requirement to adjust the plasticiser or superplasticiser quantity to maintain consistence.
Pumped Concrete
Pumped concrete will require the normal adjustments to the base concrete mix for pumping purposes, and additional adjustment depending on the type and dosage of fibres. Long pumping distances may require high pump pressure, which in turn may cause segregation of coarse aggregates and steel fibres. If segregation leads to pump line blockages, it may be wise to consider adding Vertex fibres as a pumping aid. As a rule, it is recommended that S4 rather than S3 FRC consistence is supplied.
External Slabs
For external slabs, FCS recommends RC40/50XF as the non-air entrained alternative to PAV 2 for the highest level of freeze-thaw protection. There are set minimums and maximums for cement content and water/cement ratio in this mix, and other adjustments will only be required for consistence specification compliance, depending on the type and dosage of fibres to be used.
Composite Metal Deck (CMD)
For Composite Metal Deck (CMD) applications, the pumped concrete height (pressure) will often dictate the base concrete mix design, and it may be wise to consider Vertex fibres in conjunction with the specified dosage of Apex 60 as a pumping aid to resist segregation of the steel fibres and coarse aggregates in the mix.
The extra cement demanded for the tallest buildings may deliver much higher strength concrete than required in the design. Vertex fibres in this instance will not only provide resistance to segregation but also reduce the likelihood of explosive spalling in a high temperature fire.
Conclusion
For further information and guidance regarding our tailored fibre reinforcement solutions, contact Fibre Concrete Solutions by phone on 03332 408970 or by email at info@fibreconcretesolutions.com.
