Cold joints are a necessary evil in road construction. They cannot be avoided but they can be planned. Following the correct procedures to work next to them will ensure that the risk of failure of the cold joint is decreased.
Q: What is a cold joint?
The term ‘cold joint’ applies to the interface between an existing asphalt layer and a freshly laid asphalt layer. The temperature of the existing asphalt layer is considerably less than that of the freshly laid asphalt.
Q: Where do cold joints form?
Cold joints thus form at the interface between the cold already laid asphalt and the hot freshly laid asphalt. This may happen for the following reasons:
- Road projects have a start and an end. The freshly laid hot asphalt has to match with existing asphalt of other roads.
- With today’s stringent traffic management measures it is not always possible to lay full width asphalt pavements on major roads. Roads cannot be closed completely for traffic and usually the asphalt is laid in phases.
- When the paver cannot cover the whole width of the area to be asphalted, asphalt is laid on the first lane and then the paver has to reverse on the other lane to lay the asphalt next to the freshly laid asphalt. If the temperature of the joint drops considerably and this is usually the case for long stretches, then the interface between the first lane and the second has to be treated as a cold joint.
Q: What happens at cold joints?
Over time, the interface between the cold and hot asphalt starts to ravel and a crack is formed. This allows the seepage of water into the lower layers of the road structure, which may oxidise the bitumen in the asphalt.
Q: Why does this failure occur?
Failure of the cold joints result from the different densities of the two asphalt layers.
Q: Why do the two layers have different densities?
I will use the following example to illustrate this:
A wide carriageway is being asphalted with a single paver. The first lane has been laid. The asphalt layer in the first laid lane is confined by a kerb or gutter on one side and unconfined on the other side.
A hot asphalt layer has plastic properties and as temperature drops, the plastic properties decrease. During compaction the asphalt layer is still hot and thus it tends to spread, since one of the sides is unconfined. This results in a lower density. Moreover, the unconfined edge will be slightly lower than the required level since some of the asphalt spread sideways. The unconfined edge is usually more difficult to compact and thus will generally have more air voids.
On the other hand, when a freshly laid hot asphalt layer is laid adjacent to the first laid lane there exist a temperature gradient between the two mats and thus they behave differently. The asphalt of the first laid lane being more rigid than the hot asphalt confines the hot asphalt and prevents it from spreading. Hence the second lane is confined at both sides of the asphalt mat and thus cannot spread. This will result in a higher density of the asphalt mat. The low density of the unconfined edge will lead to joint failure.
Q: What do the TM Specifications for Road Works say about joints?
The TM Specifications for Road Works, Series 900, clauses 21 and 22 state that:
21 Except where otherwise specified in this Series, where joints are made in any courses, the material shall be fully compacted and the joint made flush in one of the following ways;
(i) By using two or more pavers operating in echelon, where this is practicable, and in sufficient proximity for adjacent widths to be fully compacted by continuous rolling.
(ii) By cutting back (e.g. with edge roller, blade, joint cutter) the exposed joint for a distance equal to the specified layer thickness to a vertical face, discarding all loosened material and coating the vertical face completely with a suitable hot bitumen (200g/m per 4 cm thickness), before the adjacent width is laid. A polymer modified adhesive bitumen strip with a minimum thickness of 2 mm may also be used. Bitumen emulsion is not suitable.
22 All joints shall be offset at least 300 mm from parallel joints in the layer beneath. Joints in the wearing course shall coincide with either the lane edge or the lane marking, whichever is appropriate. No joints shall be formed between a hardstrip and the edge of the carriageway, nor within a hard strip. Longitudinal joints in materials subject to Percentage Impact Compactor Density (PCD) testing procedures shall not be situated in wheeltrack zones.
Q: How to prevent cold joints?
TM Standards and Specifications prefers the use of two or more asphalt pavers working in echelon, whenever possible. It is very important that the two pavers are not too far from each other. This method has been used in recent major projects such as the construction of the Council of Europe Avenue and Giuseppe Garibaldi Road, in Luqa.
Q: When cold joints cannot be prevented, how can it be constructed?
Clause 21 (ii) of the Standards & Specifications requires that:
- the edge of the unconfined space be cut back to a distance equivalent to the thickness of the asphalt layer forming a vertical face. If the asphalt layer is 80mm thick, then the cut back shall be 80mm from the unconfined edge.
- remove the excess material.
- coat the vertical face completely with hot bitumen or a bitumen strip such as the TOK BAND. DO NOT use bitumen emulsions.
- Lay the adjacent lane.
Q: How to compact the areas next to a cold joint?
- The first roller pass shall be approximately 75mm away from the cold joint.
- The second pass shall be approximately 75mm on the cold asphalt mat.
Q: If cold joints are unavoidable, where shall they be planned?
- Cold joints in wearing courses shall be planned to coincide with the line markings.
- They shall be offset by at least 300mm from cold joints in the lower asphalt layers.