Comparative Explosive Properties

Explosives and blasting agents are characterized by various properties that indicate how they will perform under field conditions. These properties include fume class, density, water resistance, temperature effects, detonation velocity, detonation pressure, borehole pressure, sensitivity, and strength. Each of these properties will be covered.

2.3.2.1 Fume Class

Class-Fumes are noxious gases that are produced from the detonation of explosives. The production of these gases is most critical in underground and other confined workings. Many factors affect the volume of poisonous gas produced including oxygen balance and adverse loading of explosives. The fume class is a measure of the toxic gases in cubic feet per 0.44 lb (200 g) of un-reacted explosive. Institute of Makers (IME) has developed a fume class classification scheme as seen in Table 2-4. The now-disbanded US Bureau of Mines (USBM) limits the volume of poisonous gases produced by permissible explosives (those used in underground coal and other gaseous mines) to 2.5 lb (1.14 kg).

Table 2-4: Standards for Fume Class

Class Volume of poisonous gas per 200g of explosive, in ft³

1 0.16

2 0.16-0.33 3 0.33-0.67

2.3.2.2 Density

The density of an explosive is defined as the weight per unit volume or the specific gravity. Commercial explosives range in density from 0.5 to 1.7.

Explosives with a density less than 1 will float in water. Therefore, in water-filled holes, an explosive with a density greater than 1 is required. For certain granular explosives such as dynamite, density correlates to the energy released in a given borehole volume. However, for water-based explosives, this is not the case, and often the reverse is true. Density is most useful in determining the loading density or the weight of explosives one can load per unit length of borehole (in pound per foot or kilogram per meter). Note that knowledge of loading density is required for blast-design calculations, and is calculated in English units as:

LD = 0.3405 D²

Where: is density

D is explosive column diameter in inches.

2.3.2.3 Water Resistance

The ability of an explosive to withstand exposure to water for long periods of time without loss of strength or ability to detonate defines the water resistance. A numerical rating is used based on the results of tests performed on the explosive. However, explosive manufacturers individually rate products based on a relative basis as good, fair, or poor rating. The presence of moisture in amounts greater than 5% dissolves chemical components in dry blasting agents and alters the composition of gases produced, contributing to the formation of noxious fumes and lower energy output.

Gelled granular products have good water resistance, and certain water-based mixtures have an excellent rating.

2.3.2.4 Temperature Effects

Extreme low temperatures affect the stability as well as the performance of explosives. The sensitivity and detonation velocity are hampered for certain water-based explosives at low temperatures while dynamites can become dangerously unstable below freezing temperatures. Explosives manufacturers recommend the appropriate range of temperature for storage and use.

2.3.2.5 Detonation Velocity

The detonation velocity is the speed at which the detonation front moves through a column of explosives. For high explosives such as dynamite, the strength of an explosive increases with detonation rate. For dry blasting agents and water-based explosives, field loading conditions greatly affect detonation velocity. Such conditions include (not exclusive list):

borehole diameter density

confinement within the borehole the presence of water

The speed of detonation is important when blasting in hard, competent rock where a brisance effect is desired for good fragmentation. For most explosives, there is a minimum diameter Dmin below which detonation velocity increases nonlinearly with increasing borehole diameter as can be seen in Figure 2-33. Above D_min the explosive has reached its steady-state velocity. At this point, all thermodynamic properties are at a maximum as the reaction front approaches a plane shock front. At diameters less than D_min, complete reactions do not take place, and less than ideal energy and pressure evolve from the slower detonation rates. This represents a loss in terms of dollars spent on explosive energy.

Figure 2-33: Generalized relationship between VOD and Diameter

2.3.2.6 Detonation Pressure

The detonation pressure is the maximum theoretical pressure achieved within the reaction zone and measured at the C-J plane in a column of explosives.

The actual pressure achieved is somewhat less than this maximum due to non-ideal loading conditions always present in practice and due to certain explosive formulation. Most commercial explosives achieve pressures in the range of 0.29 to 3.48 x 10⁶ psi (2 to 24 GPa). Although detonation pressure is related to the temperature of the reaction, a number of simplifying formulas are available for estimating detonation pressure for granular explosives based on detonation velocity and density, for example (in English units):

P = 0.00337 V²

where P is detonation pressure in psi, is density XXX

V is detonation velocity in fps.

2.3.2.7 Borehole Pressure

Borehole pressure is the maximum pressure exerted within the borehole upon completion of the explosive reaction measured behind the C-J plane.

Such measurements cannot be made directly and are done during underwater tests performed for energy and strength determinations. With the use of hydrodynamic computer models, theoretical calculations of borehole pressures are made. There is little agreement in the literature regarding specific estimates of actual borehole pressures. In general, pressures after detonation within the borehole are estimated to be less than 30% of the theoretical detonation pressure.

2.3.2.8 Sensitivity

The definition of explosive sensitivity is two-fold. It includes sensitivity against accidental detonations in addition to the ease by which explosives can be intentionally detonated. From the standpoint of safety and accidental detonations, the sensitivity of an explosive to shock, impact, friction, and heat determine its storage and handling characteristics. Standardized tests for high explosives have been adapted for commercial explosives that include the friction (pendulum), impact (fallhammer), and projectile tests, among others.

The term properly used to define the propagating ability of an explosive is sensitiveness. In this respect, tests such as the No.8 strength blasting cap test, air-gap test, and the minimum critical diameter test are used. The cap sensitivity test measures the minimum energy required for initiation and is used to classify explosives (e.g., cap sensitive vs. noncap sensitive products) or the ability to initiate an explosive directly with a standard cap.

The No.8 cap is an industry standard cap of specific dimensions and charge characteristics. The air-gap test measures the distance between the ends of adjacent cartridge explosives for which reliable initiation can be propagated from one cartridge to another. The critical diameter of an explosive is the smallest diameter at which an explosive will maintain a steady-state detonation. Below this critical diameter, explosives may deflagrate or "dead press." Dead pressing occurs when an explosive is densified to a point that no free oxygen is available to ensure the start or progression of detonation.

2.3.2.9 Strength

The strength of an explosive is a measure of its ability to break rock. The terms "weight strength" and "bulk strength" were useful many years ago when explosives were primarily comprised of nitroglycerin cartridges, packaged in 50 lb (23-kg) boxes. In recent years, with the development of bulk blasting agents and less sensitive ingredients, new testing methods have been established to determine relative energies for all commercial products regardless of ingredients or packaging. The performance potential of an explosive is a function of the detonation velocity and density, as well as the volume of liberated gases and the heat of the reaction. A number of methods are used to establish this energy including the use of theoretical computer models and tests such as crater, ballistic mortar, and underwater tests. Of these methods, underwater tests give the best correlation to rock-breakage performance. Underwater tests were developed to measure both the shock energy and the gas (bubble) energy released during the detonation of standard test samples. These energy values have been useful in predicting the rock-breaking capabilities of explosives for comparative

Other terminology widely used by manufacturers is based on the theoretical heat of reaction determined by explosive formulation. Absolute bulk strength (ABS) in calories per cubic centimeter and absolute weight strength (AWS) in calories per gram are computed from the heat liberated during the detonation and formation of gaseous end products. Note ABS and AWS can be computed from one another if density is known, and it is the volumetric basis of reaction heat which correlates with energy. Most manufacturers of explosives will include either value with technical product literature.

A mixture of ammonium nitrate and fuel oil (ANFO) is by far the most widely used commercial blasting product. Depending on the proportions of the mix, the heat of reaction is approximately 850 cal/g. As a dry, free-running blasting agent, ANFO is capable of being loaded or packaged at varying densities. For a typical density of 0.85 and an AWS of 850 cal/g, the ABS = (850 cal/g) (0.85) = 723 cal/cm3. Other common strength terms are the relative weight strength (RWS) and relative bulk strength (RBS) in which the relative measure of energy available per unit weight or volume of an explosive is compared to an equal weight or volume of the standard commercial explosive ANFO. The RWS and RBS are computed as a percentage of that available from ANFO.

Example 1 explosive B is more powerful.