ThormaJoint - the proven bridge expansion joint

The first choice for bridge engineers seeking effectiveness, economy and reliability

ThormaJoint is a combination of an elastomer modified binder, BJ200 and a carefully selected aggregate, BJ Stone. The joint is constructed in-situ and is a hot process.

BJ200 is a special blend of bitumen, polymers, fillers and a surface active agent, formulated to combine good fluidity at process temperatures with low temperature flexibility and ambient temperature slump control. It is delivered to site in bags in its solid state, where it is heated in a special pre-heater to its normal application temperature in accordance with the manufacturer's recommendations.

BJ200 is available in a range of grades and is selected according to the climate in which the joint is to be installed. This ensures that it remains flexible even in very cold conditions but does not become too soft in very warm conditions (table 1). BJ Stone is a single-size aggregate preferably from the Basalt, Gritstone, Gabbro or Granite groups. For the standard joint the aggregate size is 20mm to BS63. In special cases other sizes may be specified, e.g. a 14mm size in shallow joints. The use of a single-size aggregate enables a high binder content to be reached and ensures a constant ratio of stone to BJ200, important to give the optimum combination of flexibility and load bearing capacity.

The stone is cleaned, sized and bagged for despatch to site. Immediately prior to use it is further cleaned by being rotated in a perforated drum mixed whilst being heated by hot compressed air (HCA) to a working temperature range of 150°C-190°C. All joints extend the full depth of the asphalt road surfacing and waterproof membrane down to the deck concrete. In certain instances the engineer may decide that a rebate may be created in the concrete, to increase the depth of the joint to the required dimension (table 1).

In gaps up to 30mm wide, an aluminium flashing strip spans the gap to prevent stone entering the gap during joint construction or under the punching action of subsequent traffic. For gaps over 30mm wide, a steel plate is used which also serves to distribute wheel loads across the gap (table 2).

The joint develops a very strong bond to the concrete and to the vertical faces of the adjacent asphalt.



Installation Guidelines
Remove the asphalt surfacing to the desired width by saw cutting and jack hammering. The asphalt should be completely removed to expose the deck.
A clean join to the asphalt is essential. Any waterproofing membrane should be removed.
Clean out the recess and the expansion gap and prepare with a Prismo hot compressed air lance, as this warms the surrounding surfaces.
Caulk the expansion gap using Thormafoam.
Tank the recess with hot BJ200 binder to seal the joint and improve adhesion of the joint to the asphalt.
Install the aluminium strip or steel plate over the gap.
Pour stone heated to 150°C - 190°C into the joint to a maximum depth of 40mm, but not less than 20mm. The layer should then be flooded with binder heated to the correct temperature. This process should be repeated until the joint is within 25mm 5mm of the surface.
Apply the ThormaJoint pre-mix layer.
Compact using a compactor or vibrating roller.
Seal the surface using BJ200.


ThormaJoint will perform well under most conditions of temperature and traffic flow. However, certain site conditions may detract from its performance and the following guidelines are intended to assist the engineer:

The normal maximum recommended gradient is 4% and the Prismo Technical Department should be consulted in respect of steeper gradients.
Guidance should also be obtained from the Prismo Technical Department regarding locations where there is likely to be a build up of stationary traffic, such as junctions and traffic lights and where HGVs will be skewing on the joint.
The standard ThormaJoint width is 500mm but may be increased under favourable conditions to a maximum of 750mm. Please consult the Prismo Technical Department for joints in excess of 750mm.
Skew angles must be taken into consideration since they increase the effective width or running length of the joint. (A 30° skew produces an effective 15% increase of width, while a 45° skew produces a 41% increase.) Skew should not exceed 45° and in this case maximum joint widths must be limited to 550mm. Longitudinal joints may only be considered when they are not in line with the main wheel track.
  Movement Table:
Joint Width (mm)
Joint Thickness (mm)
Maximum Horizontal Movement (mm)
Movement capability is reduced as depth decreases (see table).
Traffic loading will often lead to rapid vertical movement at joints. The total maximum vertical movement capability for ThormaJoint is 3mm.
Installation of ThormaJoint at extremes of temperature should be avoided. If this is not possible, due allowance should be made for the gap position in relation to the expected movement of the joint.
The in-joint drainage system may be specified to remove small amounts of water from the joint/surfacing interface. In-joint drainage is not designed to act as drainage for the whole deck. Where a large build-up of water is expected behind the joint, it is recommended that separate deck drainage is used.
9. The various limiting factors are cumulative and care must be taken to avoid situations which lead to an accumulated level of stress. Further guidance may be obtained from the Prismo Technical Department.
10. Due consideration should be given at the design stage to the installation of ThormaJoint around ducts in service bays. Pipeways should be capable of withstanding installation temperatures of 190°C. A number of ducts located close together will restrict the capability of the crew to install the joint in the footway.
11. Cure Time: Minimum one hour per 50mm of surfacing. The time may vary according to ambient temperature. Contact the Prismo Technical Department if further guidance is required.
Details of Plates
1. For gaps of maximum 30mm, caulk with Thormafoam and plug with BJ200, protected by aluminium flashing strip.
2. For gaps over 30mm, caulking and plugging details as above. For plate size refer to table below.
3. Steel plates should be 1 metre in length to aid bedding onto the base of the excavation. In some circumstances it may be necessary to cut the aluminium strip into lengths to reduce the effects of expansion due to heating during installation

ThormaJoint is an asphaltic plug bridge expansion joint approved by the Department of Transport.
It has the following movement capabilities in accordance with BD 33/88 and BD 33/94:

Horizontal:   Standard ThormaJoint 25mm
                      ThormaJoint 9035mm

Vertical: 3mm total
1. Binder
The binder used in the construction of an asphaltic plug joint must have the following characteristics;
  • remain flexible in cold conditions
  • be stable in hot conditions
  • be flow resistant
  The following tests should be carried out by an NAMAS laboratory at the temperatures stated, and a certificate of compliance issued not less than 12 months before the start of the contract:
A. Extension Test
B. Softening Point Test
C. Flow resistance test

Details of methods and test limits are available from the Prosmo Technical Department
2. Stone
The stone will be a single-sized 20mm aggregated to BS63. It shall be a clean, dry stone, pre-bagged to avoid contamination. It shall be a stone with a polished stone value of 60, and a maximum flakiness index of 25.