Two basic forms of energy are released when high explosives react. The first type of energv will be called shock energy. The second type will be called gas energy. Although both types of energy are released during the detonation process, the blaster can select explosives with different proportions of shock or gas energy to suit a particular application.
If explosives are used in an unconfined manner, such as mud capping boulders (commonly called plaster shooting) or for shearing structural members in demolition, the selection of an explosive with a high shock energy would be advantageous. On the other hand, if explosives are being used in boreholes and are confined with stemming materials, an explosive with a high gas energy output would be beneficial.
To help form a mental picture of the difference between the two types of energy, compare the difference in reaction of a low and high explosives. Low explosives are those which deflagrate or burn very rapidly.
These explosives may have reaction velocities of two to five thousand feet per second and produce no shock energy. They produce work only from gas expansion. A very typical example of a low explosive would be black powder. High explosives detonate and produce not only gas pressure, but also another energy or pressure which is called shock pressure.
During a detonation in high explosives, the shock pressure at the reaction front travels through the explosive before the gas energy is released. This shock energy normally is of higher pressure than the gas pressure.
After the shock energy passes, gas energy is released. The gas energy in detonating explosives is much greater than the gas energy released in low explosives. In a high explosive, there are two distinct and separate pressures.
The shock pressure is a transient pressure that travels at the explosives rate of detonation. This pressure is estimated to account for only 10% to 15% of the total available useful work energy in the explosion.
The gas pressure accounts for 85% to 90% of the useful work energy and follows thereafter. However, the gas energy produces a force that is constantly maintained until the confining vessel, the borehole, ruptures.
Reference
Rock Blasting and Control Overbreak, National Highway Institute, US. Department of Transportation