How is asphalt made?
How is Asphalt Made? The composition of asphalt includes bitumen, sand, types of crushed stone or gravel, as well as mineral additives and fillers. Bitumen remains the only unchanged component, and the rest of the components can be added in different proportions.
Functions of fine components of blacktop mixture
The sand contained in the asphalt composition acts as a filler and a fine base, contributing to the distribution of pressure from the road to the ground. Without sand, the bituminous binder would flow out, and the crushed stone would be squeezed up.
In the case of special asphalts containing cement, the sand takes part in the carburizing process and gives the surface additional hardness.
Mineral filler – is a rock crushed to a dusty state (sandstone, limestone, or chalk), designed to fill the residual voids. Sandstone is the most versatile, as it is inert to almost any chemical attack. Calcium carbonates (limestone and chalk) are commonly used on public roads, while sandstone can be used near chemical plants.
Rubber – added to asphalt in the form of crumb rubber (1-1.5 mm), gives the coating high water resistance and plasticity. Asphalt treated with rubber is much less likely to crack, which increases the period between repairs. The disadvantage of such roads is their high cost, so their use is limited to laying the most critical sections of highways.
One of the main parameters is the size of the crushed stone used, dividing the asphalt into the following groups:
- Dense – used for laying the top layer of the coating and contain fine gravel. In the case of a crushed stone fraction of less than 5 mm, such asphalts are used for light-load pavements (sidewalks and pedestrian crossings) and are called fine-grained. Larger fractions of crushed stone (5-15 mm) are suitable for creating the upper layer of highways;
- Porous – used in the lower part of a multi-layer coating and contains less bitumen than dense asphalt;
- Highly porous – optimal as a base for high-traffic roads. In their production, the largest crushed stone of 15-40 mm fraction is used. These dimensions provide the necessary water permeability, creating drainage in lowlands and swampy areas. The large coarseness reduces road base shear and indentation into the topsoil, reducing the risk of surface washout and subsidence.
The basis of any asphalt production is the preparation of the initial components, mixing at high temperatures, and storage in special heated silos.
When laying roads, it is important that the plant is located close to the construction site since the material must be transported for laying in a heated state. If the mixture cools, it will be very difficult to tamp it, and the resulting coating will not be strong enough. Consider the stages of asphalt production.
Preparation of the components from which asphalt is made
This includes drying and sieving. Sand, crushed stone, and rock usually enter the plant in a wet or air-dry state. The presence of residual moisture is fraught with a decrease in the strength of the coating and spraying of a hot bitumen mixture when water gets into it.
To eliminate possible consequences, the material is dried at a temperature of 150-160? C – this temperature allows you to get rid of the moisture adsorbed in the pores of the material.
Sifting of crushed stone is carried out using a screen. The mineral filler is pre-crushed in a crusher, after which it is also subjected to fractionation. Depending on the production technology, drying can be single or double, repeated after crushing or sieving.
Crushed stone and sand are fed to a belt conveyor that transports them to a common hopper. Mixing with filler and bitumen can occur simultaneously or be carried out after reaching a homogeneous crushed stone-sand mass.
After adding bitumen, the temperature is maintained at 160-170? С. After reaching the required consistency, the mixture (already asphalt) enters the storage bin, where it can remain heated for up to 4 days. During this period, it must be shipped to the consumer in order to avoid loss of strength characteristics.
Modification of the asphalt with additives that impart useful performance properties is carried out simultaneously with the mixing of the asphalt. When creating a rubber-asphalt mixture, rubber crumb is added to a heated, ready-to-use product.