1.1.2 Historical Context
Prior to the 1930s, bituminous surfacing of roads was completed using either mixtures of aggregate and binder or crude sealing techniques involving spraying liquid binder (road oil) and scattering graded aggregate to soak up the oil and create a marginally bound layer. The concept of a thin layer of hot bitumen and a clean single‑sized aggregate (that was not submerged in the binder) was developed around the 1930s.
One of the first to articulate a design procedure for this technique was F. M. Hanson of New Zealand (Hanson 1935). His concept involved calculating voids between the aggregates as a function of aggregate size (average least dimension (ALD)) and filling a proportion of those voids with an amount of binder related to traffic volume. This remains the basis of sprayed seals design to this day, although with significant adjustment for changes in traffic loads, and improved understanding of other factors influencing sprayed seal behaviour.
Sprayed sealing techniques were quickly implemented by Australian road authorities. Some equipment was imported from the USA and other countries, but there was also substantial local innovation to meet the requirements developed by the authorities. This included the design and building of equipment such as simple robust bitumen sprayers, heating and storage units, aggregate spreaders and the unique Australian aggregate loaders that screened and precoated aggregates while loading trucks.
Understanding the importance of aggregate characteristics, the use of clean, dust‑free, good quality, single‑sized aggregates, the drive for innovative low-cost techniques and attention to detail were key factors in the development of sprayed seals that performed well. Apart from improvements in mechanical equipment, the basic techniques developed in the 1930s remain the same today.
Prior to World War II, the only sealed roads in Australia were in urban areas and on some of the more important highways near major centres of population. In the 1950s and 1960s sprayed seals played an important part in creating a network of safe, all-weather, low-maintenance, surfaced roads connecting all significant population centres and a substantial portion of the rural arterial road network (Figure 1.4).
Prior to the 1950s, most bitumen was imported in drums. Drums were stacked at site dumps ready for the arrival of the spraying gang. When required for use, the drums were split and heated in open kettles. The sprayer gangs generally camped in tents and would complete work within approximately a 40 or 50 km radius, before moving on to the next dump site.
The construction of oil refineries in Australia in the 1950s saw the introduction of hot bulk bitumen being transported directly from refineries in rail cars and road tankers. With increased mobility of sprayer crews, most sprayed work is now performed by crews operating from fixed depots or modern accommodation in remote areas.
Initially, most sprayed work, particularly state highway and main road networks, was done by direct labour units operated by the state road authorities. Most of this work is now done by private sector contractors. The road agencies, and state and local government remain the major customers so that most work is performed to public authority specifications.