PAS 108 Specification Guide: Bale Dimensions, Density, and Quality

Updated on: 26th February 2026 By:     Conor Murphy

PAS 108 is the publicly available specification published by BSI that defines what a tyre bale must be to qualify for use in civil engineering and construction. The full standard is available from BSI. This guide provides a practical interpretation of the key specification requirements for tyre bale producers and civil engineering procurement teams, covering what each requirement means, why it exists, and how to meet it consistently in production.

The guide is aimed at two audiences. First, tyre recycling operations establishing or improving PAS 108 bale production capability: the people who need to understand what they are producing and how to achieve it consistently. Second, civil engineers and contractors procuring tyre bales for projects: the people who need to understand what they are specifying and how to verify that what they receive meets the specification.

PAS 108 Specification: Bale Dimensions Explained

PAS 108 specifies the target dimensions for a compliant tyre bale, with tolerances defining the acceptable range around each target. The standard’s dimensional requirements apply to the bale as produced and ejected from the baler, not to the bale after any period of storage or transport.

The rationale for dimensional specification is straightforward. Engineers designing structures with tyre bales assume consistent bale geometry. The design cross-section of a slope stabilisation berm, the layer depth of an embankment fill zone, and the drainage capacity calculations for a retaining wall backfill all depend on the actual dimensions of the bales being installed. Dimensional variance from the specification introduces uncertainty into these calculations.

Length and width. The length and width of a PAS 108 bale are defined by the internal dimensions of the baling chamber. For a baler producing standard PAS 108 bales, the chamber geometry is fixed. Variance in length and width typically results from bale spring-back after ejection (reduced by proper compression and good tie specification) or from physical wear of the baling chamber over time.

Height. The height of the bale (the dimension in the direction of compression) is the most variable of the three dimensions and the most sensitive to the number of tyres loaded and the compression force applied. Too few tyres per load, and the bale is shorter than specification. Too many and it may be taller, which affects whether the tie wire can be applied at the correct positions. The height dimension is the primary indicator that the loading quantity and compression force are correct.

Dimensional tolerance. PAS 108 allows variance around the target dimensions within specified tolerances. Bales outside tolerance are non-compliant regardless of how close they are to the limit. Production monitoring that checks bale dimensions on every bale, rather than spot-checking a sample, is the only way to catch a drift toward the tolerance boundary before non-compliant bales are ejected.

Bale Mass: Minimum Requirements and Production Practice

PAS 108 specifies a minimum mass for compliant bales. The mass requirement exists because it is a direct indicator of rubber content: a bale that meets the mass specification contains at least the amount of tyre material necessary to achieve the required density when compressed to the specified dimensions.

The mass specification is a minimum, not a target. Producing bales that just meet the minimum is acceptable; producing bales well above the minimum is also acceptable (and typically indicates good loading practice), provided the height dimension remains within tolerance.

Car tyre bales. For car tyre bales, the mass specification is achieved through consistent loading of the appropriate number of tyres per bale. The number of tyres per load varies by tyre size and type; a loading protocol established through initial production trials and confirmed by weighing bales provides the per-load quantity for each tyre category processed.

Truck tyre bales. Truck tyre bales have a higher mass per bale than car tyre bales because truck tyres are substantially heavier. The mass specification for truck tyre bales is correspondingly higher. Sidewall cutting before baling, using the truck tyre sidewall cutter, is essential for achieving the density specification from truck tyre bales. The removed sidewall eliminates spring-back that would otherwise reduce the compressed density below specification.

Weighing in production. Meeting the mass specification requires a means of weighing bales in production. A floor scale or weigh beam positioned at the bale ejection point allows every bale to be weighed as it is produced. Operations that rely on counting tyres per load without weighing bales cannot confirm mass compliance and should not claim PAS 108 certification for their output.

Bale Density: The Engineering Property That Links Mass and Dimensions

Density is the material property that connects mass and dimensions into a single value used in design calculations. PAS 108 specifies a minimum bale density that must be achieved for compliance.

The density of a tyre bale is calculated as:

Density (kg/m³) = Bale Mass (kg) / Bale Volume (m³)

Where bale volume = length × width × height measured on the produced bale.

A bale can be within the specified dimensional tolerances and above the minimum mass specification, but still fail the density requirement if the relationship between them is wrong. This occurs most commonly when a bale is correct in mass but too large in volume (because it was insufficiently compressed). The density requirement catches this case that mass and dimensional checks alone would miss.

The target density for PAS 108 bales represents the minimum compression at which the tyre material achieves the engineering properties (compressive stiffness, resistance to creep deformation) used in civil engineering design. Below this density, the bale material is softer and more deformable than the design assumes.

For the MKII Tyre Baler operating with properly pre-processed tyres and the correct loading quantities, the density specification is achievable consistently across a full production shift. Hydraulic system maintenance to keep the compressive force at specification is the key ongoing operational requirement for consistent density.

Tie Wire Specification: Holding the Bale to Specification

The tie wire system holds the compressed tyre mass in its baled state after the platen retracts. PAS 108 specifies the number of ties, their placement across the bale, and the wire specification in terms of gauge and tensile strength.

The rationale for each element of the tie specification:

Number of ties. PAS 108 requires a minimum of six wire ties per bale. This number is derived from the load that the wire ties must resist when the bale is released from the baler’s compression. Fewer ties mean each wire carries more load, increasing the risk of tie failure and bale expansion after ejection.

Tie placement. The distribution of ties across the bale face is specified to ensure even restraint of the compressed tyre mass. Ties clustered at the centre of the bale resist the highest compression forces but leave the bale ends inadequately restrained. Even spacing across the bale face, as PAS 108 requires, distributes the restraint load optimally.

Wire gauge and tensile strength. The wire must be of sufficient gauge and tensile strength to hold the bale dimensions after ejection. Under-specified wire stretches and allows bale expansion. The MKII Tyre Baler’s automatic tying system applies six ties at the correct positions using wire of the correct specification when set up and maintained according to Gradeall’s recommendations.

Post-ejection inspection. Bales should be inspected for tie integrity after ejection and again before loading for transport. A tie that has failed, loosened, or been cut accidentally allows bale expansion that changes the dimensions and density. Non-intact bales should be identified and separated from compliant stock.

Quality Management: Turning Specification into Consistent Production

Meeting the PAS 108 specification once is straightforward for a well-equipped operation. Meeting it consistently, across every bale in every production run, requires a quality management system.

The elements of a quality management system for PAS 108 bale production:

Production records. For each bale or production batch, record: date and time, tyre input type, number of tyres per load, bale dimensions, bale mass, and any observations from the operator. These records are the primary evidence of compliance and the first thing a procuring engineer or contractor will ask for.

Loading protocol. A documented loading quantity for each tyre type and size processed, established through initial production trials and confirmed by weighing bales. The protocol should specify the number of tyres per load by tyre category and the target loading orientation, where this is relevant to bale density.

Weighing system. A floor scale or weigh beam at the bale ejection point, calibrated at regular intervals and records of calibrationare kept. Every bale should be weighed, not every tenth or every batch sample.

Dimensional checking. A measuring procedure for checking bale dimensions at regular intervals during production, with the tolerance limits marked for easy reference. Checking every bale is ideal; checking every fifth to tenth bale is the practical minimum.

Non-conformance procedure. A documented procedure for what happens when a bale is found to be non-compliant: how it is identified, segregated from compliant stock, investigated for root cause, and disposed of appropriately. Non-conformances are also production intelligence: a cluster of under-mass bales may indicate a hydraulic pressure drop; a cluster of oversized bales may indicate a loading quantity error.

Equipment maintenance schedule. A documented maintenance schedule for the baler, tying system, and all pre-processing equipment, with records of maintenance carried out. Maintenance records demonstrate that the equipment is kept in a condition to produce to specification.

Tyre Pre-Processing Requirements for Specification Compliance

Consistent PAS 108 compliance requires consistent tyre input quality. The pre-processing steps that ensure input quality are:

Rim removal. All rimmed tyres must have their rims removed before baling using the tyre rim separator for car tyres or the truck tyre rim separator for HGV tyres. A rimmed tyre in the baling chamber produces a non-compliant bale and risks equipment damage.

Tyre sorting. Car and truck tyres must be sorted and baled separately. Mixed-type bales have unpredictable properties and are non-compliant under PAS 108.

Sidewall cutting for truck tyres. For truck tyre bales, sidewall cutting before baling is an essential pre-processing step for consistent density compliance. The truck tyre sidewall cutter removes the sidewall, eliminating the spring-back resistance that prevents the tyre body from reaching the required compressed density.

Car tyre sidewall cutting. For car tyre bales where maximum density consistency is required, the car tyre sidewall cutter provides an additional pre-processing step that reduces variance in bale density across a production run.

The full pre-processing and baling line from Gradeall is detailed in the tyre recycling equipment range. Contact Gradeall International for guidance on establishing or upgrading a production line to consistent PAS 108 output.

Frequently Asked Questions

Tyre bale producers and civil engineers ask the same questions about PAS 108 compliance. Here are the ones that matter most.

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