Chapter 11: Explosives Properties and Performance Characteristics

Basic Explosive Properties

Property

• Explosives are chemical compounds or mixtures initiated by heat, shock, impact, friction, or a combination of these conditions.
• Explosives can increase very rapidly in detonation and reaction by a stimulus or booster charge.
• Explosives rapidly release heat and large quantities of high-pressure gases upon detonation that expand rapidly with sufficient force to overcome the confining forces (i.e. the confining forces of a surrounding rock formation).

Table 11.1 – Basic explosive properties.

If ignited by a flame, all explosives can burn. The rate of burning increases with pressure so that, in strong enough confinement or when a large enough pile of the explosive burns, the burning may transition into detonation. The flame burning of high explosives that can occur without the help of oxygen from the atmosphere is sometimes called deflagration. In commercial blasting the energy released by the detonation of an explosive produces the four basic effects listed in table 11.2.

Detonation Effects From Energy Release Of Commercial Explosives

Table 11.2 – Detonation effects from energy release of commercial explosives.

Each explosive has specific characteristics or properties that differentiate it from other explosives. Typical properties of explosives are the velocity of detonation (VOD), the detonation pressure, the energy release and the fumes produced. Explosive industry scientists have developed methods of measuring these properties and relating some of them to rock breakage. By knowing what an explosive's properties are, engineers can choose the best explosive type for a specific blasting situation. Meaningful predictions of relative explosive performance can also be made so rock fragmentation can be evaluated and employed in blast design.

EXPLOSIVES AS SOURCES OF ENERGY

Explosives are referred to as energetic materials because they are substances that contain large amounts of chemical energy. This energy is released through the process of detonation. Simply stated, a detonation is a rapid exothermic reaction in which a shock wave is propagated forward and is supported by the chemical reaction behind it.

Upon detonation all explosives react chemically to produce shock, heat (exothermic reaction), and mostly gaseous reaction products. When the energy is released inside rock properly, it produces fragmentation and throw (heave). The subject of explosive energy is a broad one, with methods of determination ranging from the simplest calculations or field tests to extremely complex computations, which have as their underlying base theoretical studies of chemistry and thermodynamics.

The amount of available energy is fixed by the manufacturer of the explosive, and this energy can be calculated for any explosive. Explosive manufacturers perform this calculation with thermodynamic computer code calculations. The result depends on the calculation method of detonation used since some differences may be expected between different manufacturers, even for the same explosive. For most explosives, the energy falls between 700 calories/gram and 1,500 calories/gram. Whether or not this energy is completely released is dependent on how well the explosive is formulated, primed, confined, and protected from influences such as water or other adverse conditions prior to or at the time of detonation.

An explosive's ability to fragment and displace rock into muck piles is superimportant. An engineering model that clarifies how explosives energy causes and influences fragmentation and displacement, also called brisance and heave, is helpful when selecting an explosive for a specific application.

A useful model partitions an explosive's total energy into three components: (1) shock energy, (2) heave energy, and (3) lost energy (Lownds, 1991). This is shown in figure 11.1. As the detonation products apply pressure on the wall of the borehole, the rock deforms until an equilibrium point is reached. The energy consumed up to this point is used in producing strain in the rock mass and propagating the wave. For products to expand, further expansion volume must be created through fracturing and displacement. Energy released until products vent in the atmosphere, is used in heaving the rock and probably enlarging radial fractures. Once products vent in the atmosphere, a large portion of the energy is lost in air overpressure. However, some energy is still used in rock displacement. The goal of blasting is to select an explosive with the proper shock and heave energy for the application and material being blasted. Energy is lost through air overpressure, ground vibrations and the generation of undesirable amounts of fines. Thus, it is important to minimize energy losses. Losses are minimized with sound blast design and proper explosive use.

CURRENT ENERGY REPORTING METHODS

In most commercial explosives, only part of the ingredients react immediately at the detonation front, leaving a considerable remaining part to be consumed by afterburning during the expansion phase. In extreme, unusual cases some fraction of the ingredient materials may even be left completely unreacted. The detonation velocity and detonation front pressure are essentially determined by the amount of energy released by the fraction of the ingredients that react immediately at the detonation front. Many commercial explosives have long reaction zones. In such cases velocity of detonation and pressure are not sufficient to describe explosives performance in blasting. Additional information to enable energy partition is very important in these cases.

Scientists have examined factors, which predict more accurately the ability of an explosive to fragment and move material efficiently. These include (1) calculated properties—theoretical energy (Q) and expansion work (Eaw), and (2) measured properties—underwater shock and bubble energy, shock impulse, and stress wave measurements in the material being blasted.

These tests and calculations, are good indicators of explosives performance but isolated they may not be able to suggest the effectiveness of an explosive in a certain blasting environment. The complexity of the performance prediction problem is increased by the fact that most modern rock blasting explosives have a relatively low reaction rate that results in performance dependent on the borehole diameter, confinement and proximity between oxygen and fuel in the explosive composition. Tests to measure energy are affected by the same parameters; thus their interpretation may not be straightforward. The main tests used today are discussed later in this chapter and listed in table 11.3.

To date, no single test or calculation can predict the blasting action of a commercial explosive, principally because of the complex nature of the materials being blasted and the way energy is partitioned.

As mentioned previously manufacturers provide energy ratings for their products as well as data on energy partitioning. Each manufacturer may use a different formula or code to rate explosives and partition their energy. Therefore, relative strengths of two different products are only meaningful when comparing products from the same manufacturer.

Even for the determination of relative strengths, there is no agreement among manufacturers for the base energy in cal/g for ANFO. This lack of agreement is due to the use of the "available energy" versus the total energy output of the product. In the case of the "available energy" a pressure cut off is established and energy released below this pressure is ignored.

The performance of an explosive is not determined simply by knowing the total energy released by the explosive. It depends also upon the rate of energy release and how effectively the energy is utilized in the fragmenting. In short, both the explosive properties and the properties of the material being blasted influence the effectiveness of an explosive.

Some of the current tests or calculations used to measure and characterize an explosive's energy are the various underwater tests, in-situ measuring techniques in rock, and theoretical calculation techniques, discussed in the following pages.

Theoretical Energy

Most calculated energy values generated by thermodynamic computer codes assume that explosive detonation reactions are ideal. An "ideal" detonation is one in which the reaction zone is extremely small and all the chemical reactions are completed at the detonation front with no afterburning during the expansion of the reaction products.

This is a reasonable assumption for single-molecule, military type explosives such as TNT and pentolite, but it is not correct for many commercial products as which oxidizer and fuel are on different molecules. However, the error of this assumption is not significant as far as total energy output is concerned since the major product gases of detonation are not influenced by this assumption.

The calculated or theoretical energy of an explosive is the difference between the heat of formation of the products of the explosion and the heat of formation of the ingredients of the explosive. This energy, known as the heat of explosion (Q) represents the total thermal energy released by the chemical reaction in detonation and includes the heat obtained by the products of detonation after expansion to atmospheric pressure.

The calculation of heat of explosion requires knowledge of the chemical composition and the density of the explosive and represents the maximum amount of energy an explosive will deliver. Products of detonation can be estimated using hierarchy rules. According to such rules the explosives of the carbon (C), hydrogen (H) nitrogen (N), and oxygen (O) composition (CHNO), nitrogen from fuel forms nitrogen gas (N₂), hydrogen with from water (H₂O), and carbon will form carbon monoxide (CO) and carbon dioxide (CO₂) according to the availability of oxygen in the composition.

A simple heat of explosion calculation for ANFO is given below. In this calculation, the chemical formula for fuel oil has been simplified to CH₂. This simplification makes the chemical reactions particularly easy to follow, and the error is minimal. In reality, fuel oil is a blend of many oils of different molecular weight that are mostly long straight hydrocarbon chains, on the average perhaps similar to Dodecane C₁₂H₂₆ that is sometimes used as a model for fuel oil in more complex calculations. Using heats of reaction of (-87.3, -13, -94.1, and -57.8) kilocalories/mole for ammonium nitrate, fuel oil, carbon dioxide and water (steam) respectively, we can calculate the heat of explosion. The following equations are provided to illustrate the detonation reaction:

Detonation Reaction

$$3NH_4NO_3 + CH_2 \rightarrow CO_2 + 7H_2O + 3N_2$$

Where:

• Reactants =
• Products =

Heat of formation of reactants

$$Q_R = 3 \times (-87.3) + (-12) = -274.9 \text{ kcal}$$

Heat of formation of products

$$Q_P = (-94.1) + 7 \times (-57.8) + 3 \times (0) = -498.7 \text{ kcal}$$

Heat of explosion

$$Q = Q_P - Q_R = -498.7 - (-274.9) = -223.8 \text{ kcal}$$

The above calculation was for one mole of CH₂ and three moles of NH₄NO₃.

Molecular weight of compound

$$3 (AN) + 1 (FO) = 3 \times (80.1) + 1 \times (14) = 254.3$$

Calculate energy output in kilocalories per kilogram—Heat of Explosion

$$\frac{223.8}{254.3} \times 1000 = 880$$

Expansion Work

The actual work done by the reaction products as they expand from the detonation state to atmospheric pressure can be determined by computing the calculated energy of the reaction products at the beginning and end of the expansion. This difference, called the expansion work (Eaw), can be calculated using a computer program that has the ability to calculate the energy of the reaction products at different states of expansion. To do this, a thermodynamic computer program must use an "equation of state" of the reaction products that relates all the combinations of values of the state variables pressure, volume, and internal energy. Then the program can calculate the commonly used isentropic or adiabatic expansion (an expansion so rapid that it occurs without appreciable transfer of heat to the surrounding material) from a given initial state, the detonation state.

The equation of state of the detonation products is the most uncertain of all assumptions in the calculations, because it is based on dynamic measurements or estimations of the volume and internal energy at pressures too high for the material to be contained as even the highest strength container. Most computer programs are equipped for making the calculations with a variety of different equations of state.

Expansion work is one of the most realistic measures of explosive power. It approximates the amount of work the gaseous products of the explosion can do on the rock surrounding the charge as they expand from the initial detonation conditions to atmospheric conditions.

The calculated values for Q and Eaw are similar for many explosives. The Eaw is lower for the explosive compositions that yield significant quantities of solid, high-temperature products at atmospheric pressure. Energy tied up as heat in the expanded detonation products is not useful blast energy.

Explosive Energy Tests

A number of tests have been developed to estimate energy. Most of them utilize small quantities of explosive and very small diameters. Furthermore most of them are indirect, meaning that they are comparing a measured property related to energy to the same property of a standard explosive. The main tests used to estimate explosives energy are listed in table 11.3.

Table 11.3 – Main tests to estimate explosives energy.

Underwater Test Methods

Explosive manufacturers began using underwater tests in the 1960s as a means of comparing the relative effectiveness of various explosives. Tests were based on work carried out during World War II and reported by Cole in his classic book, "Underwater Explosions" (Cole, 1948). This method was based on the hypothesis that "shock energy" from an explosion under water is a measure of the explosive's shattering action (brisance) in other materials, such as rock, and that "bubble energy" from the underwater explosion is a measure of the "heaving action" of the explosive.

The test consists of an underwater detonation, where the shock pressure features are recorded and interpreted. After the detonation of the charge a shock wave is sent in the water, while the detonation products expand creating a bubble. The bubble expands to a maximum and then starts to implode, since the hydrostatic pressure is lower than the pressure of the products in the bubble. The bubble then contracts to a minimum size and then expands again to create another oscillation. The times of the oscillations are significant for calculating the explosive energy and are recorded when the shocks sent out during the implosion. It is important that the test be conducted at sufficient depth to allow oscillations of the bubble and minimize boundary effects. It is recommended that the depth of the charge and the distance between charge and bottom be more than twice the maximum bubble radius. The distance between the charge and the walls of the pond should be more than three times the maximum bubble radius.

Scientists and explosives engineers have found underwater tests to be a useful tool in evaluating the relative strengths of various explosives provided that these tests are carefully interpreted in conjunction with theoretical calculations and field performance. Experience has shown that bubble energy values frequently overrate an explosive's ability to fragment and heave hard rock, but are more closely related to its capability of displacing weak rock.

Two types of energy are measured in underwater tests, shock and bubble energy. The shock energy in the tests is the compressional energy radiated from an underwater detonation and is derived using the pressure-time curve at a known distance from the explosion. The bubble energy is the potential energy of the displaced water or the maximum size of the bubble. Methods of evaluating bubble energy can be found in literature (Bjarnholt, 1980). Essentially the pressure of the shock wave in the surrounding water is measured and from it the impulse density and the energy of the shock wave can be derived. The bubble energy is determined from the bubble period, which is the time interval between the detonation and the first implosion of the bubble (Bjarnholt and Holmberg, 1976).

Initially, the underwater test was set up and used by researchers as a means of comparing the relative outputs of various explosives. This provided useful in-house data. However, because of significant variations in the test setup and methods, it was difficult to compare the results from different laboratories. Through the work of Bjarnholt and Holmberg of the Swedish Detonics Research Foundation in the 1970s, Bjarnholt (1980) has suggested underwater test specifications in an effort to standardize the test method throughout the commercial explosives industry. These specifications cover charge geometry and initiation method, dimensions of the pond required for the test, pressure gauges, use of standard and control explosives and evaluation standards for peak shock and bubble energies.

The capability of the underwater test method to separate the explosive energy into shock and bubble energy components has made it a valuable instrument for commercial explosive development and applications. Because the test involves the actual detonation of a significant-sized charge of an explosive, the energy measures are often more in agreement with the explosive's actual performance than that obtained from theoretical calculations. However there are some problems in relating the underwater test to the rock breaking capacity of explosives. Water does not have the strength of the rock and the impedance of water is typically much lower than those of rock.

The underwater energy test has proven to be a useful tool for evaluating the relative energies of various industrial explosives. Because of the nature of the test method, it is better suited to the measurement of relative or comparative energy values, as opposed to absolute energy values.

Detonation Calorimeter

The detonation calorimeter provides the only direct method of measuring energy output of explosives. It is similar to a bomb calorimeter; however it is substantially larger and measurements need to be more precise. A picture of a detonation calorimeter is shown in figure 11.3.

The instrument consists of the bomb, in which the detonation of 25 grams of charge takes place, the bucket with a known quantity of water inside which accurate measurements of the temperature rise are taken, the jacket that maintains an accurately controlled environment and the supporting mechanism. The unit measures total energy yield directly. Energy can also be calculated from the analysis of the detonation products in the bomb. Very few commercial explosives have been tested in a calorimeter. Table 11.4 shows results obtained by Katsabanis and Liu (1993) for the case of ANFO and an emulsion.

Table 11.4 – Heats and products of detonation from a calorimetric experiment. (Courtesy: P.D. Katsabanis)

Despite the small quantities of the test and the marginal detonations achieved in the small diameter samples, which were tested, the energy yields are close to the theoretical ones. Detonation products indicate complete reaction, in accordance with the full energy values measured. Clearly total energy was not affected by the low, probably marginal, reaction rates; therefore it is considered as a sensitive indicator of blasting performance.

Crater Test

The crater test (See chapters 9 and 14) can be used for rating explosives detonating in the same geological material. It consists of performing a series of crater tests in which an explosive with a length to diameter ratio of 8:1 is detonated at various depths in rock. The craters are measured and scaled and from the interpretation of the measurements the strain energy factor can be calculated. One should however note that many tests need to be conducted due to the variability of the rock medium, the test is relevant to crater blasting and the result is tied to the geometry of the blast.

Cylinder Test

Explosives are detonated inside an oxygen free high conductivity annealed copper tube of which expansion is monitored using a high-speed camera. The test is suitable for ideal explosives and is used to obtain equation of state parameters to describe explosive performance parameters used in numerical modeling. The result is related to the expansion energy of the explosive; however at the conditions offered by the tube of the test.

Air Overpressure Test

In this test a charge is detonated suspended in air and its pressure-time history is measured using an Air blast measurement equipment at a known location. Comparisons of peak pressures and impulses against TNT can provide energy information. The measurement is not without trouble since the charge and the transducers have to be placed at sufficient height to avoid reflections from the surface of the earth.

Ballistic Mortar Test

In the ballistic mortar test various charges of the test explosive are fired in a heavy steel mortar, which is attached to a pendulum bar. The swing of the pendulum is measured upon detonation and compared to the swing obtained with the detonation of a standard explosive (usually TNT). The test is an indirect measurement of energy. Furthermore the test conditions are far from representative of explosive performance in a blast.

Correlation of Energy Results with Blasting

In most commercial explosives, only part of the ingredients react immediately at the detonation front, leaving a considerable remaining part to be consumed by afterburning during the expansion phase. In extreme, unusual cases some fraction of the ingredient materials may even be left completely unreacted. The amount of energy released by the fraction of the ingredients that react immediately at the detonation front determine the VOD and detonation front pressure.

Many commercial explosives have long reaction zones. In such cases velocity of detonation and pressure are not sufficient to describe explosives performance in blasting, especially by itself. The expansion work includes the effect of afterburning to an extent that is not well understood, so the rock mass and the geometry of the explosive affect explosives performance. Obviously the shock and bubble energies cannot provide complete information for the blasting engineer.

Equilibrium thermodynamic computer programs, currently in practical use, lack the ability to treat long reaction zones or, in other words, to describe non-ideal performance. Their assumption is independent of diameter and confinement suggesting that the calculated explosive values may not be applicable to calculations under non-ideal conditions.

It is worth noting that large deviations from ideal detonation, i.e., where a fraction of the chemical reaction energy is released by afterburning, may be beneficial for the blasting result, especially when blasting in soft materials such as clay, dirt or sand, in which the expansion goes on for a long time without venting to the atmosphere. The late release pressure that may lead to a lost fraction of reaction occurring in the detonation head leads to a too low an items of shock energy by reducing the shock wave amplitude in the material immediately surrounding the borehole.

EXPLOSIVE MALFUNCTION

Due to the interaction between shock waves generated by explosives and explosives that are supposed to detonate at a later stage, a variety of phenomena can occur, which are typically reported as "malfunctions". Such phenomena include: (1) sympathetic detonation, (2) shock desensitization, (either of which can be temporary or permanent), (3) modification of firing times of detonators, and (4) dead pressing.

Malfunctions can be the cause of poor blasting results and increased vibration records. Tracing malfunctions using vibration monitoring cannot provide sufficient information regarding the nature and the reason for the malfunction. Misfires (deflagration or mechanical damage to the connecting cord) may also appear as malfunctions; therefore seldom definitive explanations as to the cause can be offered regarding the cause of malfunctions.

Malfunction is closely related to pressure and its effects. A stress wave, produced by an earlier detonating charge, will result in temporary or permanent desensitization of nearby charges. This may result in temporary or permanent loss of sensitivity and malfunction. If the pressures are sufficiently high they may result in structural damage of explosives and accessories. Some explosives, such as emulsions, suffer permanent structural changes. For example, microballoons may break, or the matrix of the emulsion may break down so that supersaturated oxidizer solution and crystallites form. Detonators can also be affected by impact. Shock initiation of their sensitive components is easily understood; however structural effects resulting in malfunctions are possible, depending on impact pressure and duration.

A special case of malfunction is the external channel effect, which affects decoupled charges. The mechanism of the channel effect is shown in figure 11.4 (Lin, 1987).

According to this mechanism (Lin et al., 1988), as the detonation front advances in the explosive charge placed along the axis of a borehole with a diameter larger than the diameter of the charge, the gases produced by the detonation expand and push the air ahead of it to produce a shock front. A cone shaped oblique shock front is formed which separates from the gaseous products after a certain distance. When the oblique shock front reaches the wall, it is reflected and depending on the shock velocity and the angle of incidence, a strong wave, running parallel to the axis of the charge, may result. This may outrun the detonation and compress the charge laterally increasing the density of the charge. If the density is increased above the dead press density of the explosive or if the explosive has been compressed so that its critical diameter is larger than the diameter of the application, a failure occurs.

Liu et al. (1988), found that the channel effect was possible in all kinds of commercial explosives if the charge diameter was small enough. The limits of the channel effect were expressed as the ratio of the borehole diameter to the diameter of charge as expressed in the following relationship:

For example, the channel effect was observed using a 25 millimeter (1 inch) slurry when:

$$1.51 = \frac{d_b}{d_e} < 4.83$$

where:

• $d_b$ = borehole diameter
• $d_e$ = diameter of explosive charge.

In another instance the channel effect was observed using a 32 millimeter (1.25 inch) slurry when:

$$1.16 = \frac{d_b}{d_e} < 3.16$$

In yet another instance the channel effect was observed using a 75 millimeter (3 inch) emulsion when:

$$1.38 = \frac{d_b}{d_e}$$

STRENGTH REPORTING METHODS

Explosive strength reporting is a method of comparing energies on a relative basis to a standard. When dynamite products prevailed in the marketplace, the percentage of nitroglycerin content of the dynamite grade was considered the standard for compare energy among solids. Today ANFO is considered the standard basis of energy comparison among the AN based products: emulsions, water gels, slurries, and blends. Relative comparisons are commonly made on a mass (relative mass strength) or volume (relative volume strength) basis. Following are detailed descriptions of these reporting methods. Care must be taken making comparisons between relative strength values of products of different manufacturers. Different manufacturers may use different thermodynamic calculations for the absolute weight and bulk strengths of ANFO. Always check to ensure comparisons are made on the same basis.

DYNAMITE STRENGTH REPORTING

The percentage strength markings on dynamite, which have become a part of the brand and grade naming of those products, was developed as a result of efforts to standardize the characterization of dynamite by the Institute of Makers of Explosives (IME) in Safety Library Publication (SLP 12, 2007).

This percentage strength characterization utilized the ballistic mortar, a cannon-like device suspended as a pendulum. With this, 10 gram charges of explosive formulations were fired to determine the relative value of various ingredients. After accumulating a large base of data, relative numbers were assigned to the ingredients usually found in dynamite. Nitroglycerin, for example, was assigned a value of one. Ammonium nitrate was assigned a value of 0.3 nuts. The explosive TNT (the most widely used military explosive and a fairly common dynamite ingredient in the early 1900's) was found to have a value of one unit. Sodium nitrate and the "dopes" (wood pulp, walnut hulls, starch, etc.) were assigned zero unit value. Thus, if a dynamite formula contained by weight 30% nitroglycerin (NG), 40% ammonium nitrate (AN) and 30% "dopes," relative strength is calculated as shown in table 11.5.

Table 11.5 – IME relative strength value for dynamite. (Source: Reprinted by permission of IME: SLP 12,2007)

Dynamite strength values are rounded to the nearest whole 5%, and thus this dynamite would be marked as having a strength value of 60%. This system worked for its intended purpose, which was to put all manufacturers (and their advertising departments) reporting on an equal basis.

For obvious reasons, this rating system had little to do with the actual rock-breaking value of an explosive. For example, sodium nitrate and the "dopes" actually do add some energy to the reaction but were assigned the value of zero (0). Also, it is easy to show (although in this brief explanation we will not do so) that an explosive marked "60% strength" is not being twice as strong as one marked "30% strength".

AN BASED PRODUCT STRENGTH REPORTING METHODS

An unfortunate modern side-effect of this dynamite system is confusion between the term strength as shown in the example above, and the strength terms now frequently used. Today product strengths are calculated on both weight and bulk bases. For comparison purposes, the absolute energy of ANFO is used as the standard as opposed to simply the ANFO content. These methodologies are discussed below.

Weight Strength

Weight (mass) strength is the energy output (heat of detonation) of an explosive material per unit of weight. Weight strength is reported as an absolute value and a relative value as compared to the absolute weight energy of ANFO.

Absolute Weight Strength

The absolute weight strength (AWS) is the measure of explosive energy per unit weight. It is common in the explosives industry to represent absolute weight strength in terms of joules/kilogram or calories/gram.

Relative Weight Strength

The relative weight strengths (RWS) is the ratio of the absolute weight strength of and explosive to the absolute weight strength of ANFO.

$$RWS_e = \frac{AWS_e}{AWS_{ANFO}}$$

Equation 11.1

Where:

• $RWS_e$ = Relative weight strength of explosive
• $AWS_e$ = Absolute weight strength of explosive (calories/gram)
• $AWS_{ANFO}$ = Absolute weight strength of ANFO (calories/gram)

EXAMPLE 11.1

Calculate the RWS for an emulsion that has an AWS of 800 calories/gram and assume ANFO has an AWS of 880 calories/gram using equation 11.1.

$$RWS_e = \frac{AWS_e}{AWS_{ANFO}}$$

$$RWS_e = \frac{800}{880}$$

$$RWS_e = 0.91$$

The relative weight strength of this emulsion is 0.91 or 91% of ANFO.

Bulk Strength

Bulk (volume) strength is the energy output of an explosive material per unit of volume. Bulk strength is reported as an absolute value and a relative value as compared to the absolute energy of ANFO.

Absolute Bulk Strength

Absolute bulk strength (ABS) is the measure of explosive energy reported per unit of volume or the heat of reaction available in each cubic centimeter of explosive. It is common in the explosives industry to represent absolute bulk strength in the units of calories/centimeter³.

Relative Bulk Strength

Relative bulk strength (RBS) is the ratio of ABS of the explosive to the ABS of ANFO.

$$RBS_e = \frac{ABS_e}{ABS_{ANFO}}$$

Equation 11.2

Where:

• $RBS_e$ = Relative bulk strength of explosive
• $ABS_e$ = Absolute bulk strength of explosive (calories/centimeter³)
• $ABS_{ANFO}$ = Absolute bulk strength of ANFO (calories/centimeter³)

EXAMPLE 11.2

Calculate the RBS of an emulsion with an ABS of 920 calories/centimeter³, and assume an ABS for ANFO of 748 calories/centimeter³ using equation 11.2.

$$RBS_e = \frac{ABS_e}{ABS_{ANFO}}$$

$$RBS_e = \frac{920}{748}$$

$$RBS_e = 1.23$$

The RBS of this emulsion is 1.23 or 23% more bulk energy per unit volume than ANFO.

PROPERTIES AND PERFORMANCE CHARACTERISTICS

Explosives exhibit two important features; physical properties and performance characteristics. These features vary over ranges that offer the blaster choices in product selection for an application. Product choices are made to optimize the fragmentation and throw processes based on these features. When bulk explosives are field manufactured, field quality control and blaster training standards directly influence these explosive features. Following, explosives properties and performance characteristics are defined and discussed.

Borehole Pressure

Borehole pressure (explosion pressure) is the pressure applied to the borehole wall after the reaction products have expanded to occupy the volume of the borehole. Borehole pressure should not be confused with detonation pressure. The borehole pressure is theoretically about 45% to 50% of the detonation pressure, assuming there is complete reaction at the detonation front and the explosive couples the borehole. The effective borehole pressure is blasting depends on how well the explosive fills the borehole. For composite explosives, the borehole pressure is difficult to determine, since the time required for the detonation reaction to go to completion may extend far into the period of large expansion of the borehole.

See Velocity of Detonation on page 211.

Dead Pressing

Dead pressing is a field malfunction phenomenon that occurs when an explosive is subjected to pressures that increase the product's density to a value above a critical density (or Critical Density) where the explosive loses its sensitivity. Thus, dead pressing is a desensitization process. Dead pressing usually occurs to an explosive charge when a stress wave from a previous detonation compacts the explosive to an adjacent or nearby borehole above its critical density prior to initiating by its primer. High hydrostatic pressures can also desensitize an explosive by increasing product density to such a value that the explosive cannot detonate with the given primer or at the diameter of application. For example, water filled, deep drill holes used in seismic work require special formulations and procedures.

Explosives sensitized by chemical gassing must be used with the knowledge that excessive bored pressures created by water or the weight of the explosive column itself may produce dead pressing pressures at the bottom of the borehole.

Explosives that contain glass or plastic bubbles (microballoons) for sensitivity are influenced less by pressure, up to pressures that can cause the bubbles to break. All commercially available explosives products are designed to detonate under the normal pressures encountered in their recommended field applications. If an explosive is used under normally high pressures, special procedures may be needed, and the explosive manufacturer should be consulted.

Density

The term density is used in three different ways when using and applying explosives (1) critical density, (2) explosive density, and (3) stick count. Critical density describes a specific performance limitation, while explosive density and stick count describe specific product weight characteristics.

Critical Density

Typically an explosive's sensitivity decreases as its density increases. Every explosive has a maximum density up to which it retains adequate sensitivity to perform properly; this density is called the critical density. An explosive's sensitivity can be reduced to a level where its ability to detonate can be completely destroyed by a large enough increase in density. At critical density a quality primer will not detonate the explosive.

Explosive Density

Explosive density is one of its most important explosive properties. Density is the property that controls how much of a product can be loaded into a borehole. Density also influences other explosives properties and performance characteristics such as sensitivity, detonation velocity, and critical diameter of the charge. Sometimes density and specific gravity are terms used interchangeably. Technically they are not the same. Density is defined as the mass of an explosive per unit of volume, usually expressed in grams/centimeter³ or pounds/foot³ (IME SLP 12, 2007). Specific gravity is the ratio of the density of an explosive to the density of water. Because the density of water is 1 gram/centimeter³, the specific gravity is numerically equal to its density. However, specific gravity is a ratio and never has units attached. Density is always represented with units such as grams/centimeter³.

Most commercial explosives have densities that fall within the approximate range from a low of 0.4 grams/centimeter³ to a high of 1.6 grams/centimeter³. ANFO made from AN prills typically has a borehole loading density ranging from approximately 0.68 grams/centimeter³ to 0.88 grams/centimeter³, depending on which manufacturer's prills are used, and the method of loading into the borehole. If AN fines are added to ANFO, the product density increases and its sensitivity changes. The fluid density can approach 1.15 grams/centimeter³. Cartridged emulsions and wet or dry dynamite explosives generally range in density 0.90 grams/centimeter³ to 1.60 grams/centimeter³ with the majority of these products in a density range of about 1.10 grams/centimeter³ to 1.35 grams/centimeter³.

The density of pure clean water is 1.0 grams/centimeter³, but the density of muddy water or water containing suspended solids can be substantially higher than 1.0 gram/centimeter³. Due to this increased water density condition, products loaded into boreholes containing water must have a sufficient density to sink and not float. Packaged products of density near that of the water they are loaded into may tend to float or separate in the column. Care must be taken to ensure this does not happen. Separations cause a break in column continuity and may cause misfires or poor performance.

The ideal detonation velocity is strongly dependent on the explosive density (i.e. loaded packed density in a borehole). A specific explosive's velocity might increase linearly with density from a value of about 2,000 meters/second (6,500 feet/second), at a density of 0.2 grams/centimeter³ to perhaps 6,000 meters/second or 8,000 meters/second (19,685 feet/second or 26,250 feet/second) as the density approaches 1.5 grams/centimeter³.

Cartridge (Stick) Count

When dynamite products prevailed in the marketplace, high explosive density was traditionally expressed by the number of 32 millimeter × 203 millimeter (1¼ inch × 8 inch) cartridges contained in a 22.5 kilogram (50 pound) case. Cartridge counts of the various grades ranged from 83 sticks to 300 sticks per case. Now standard cases of explosives weigh 25 kilograms (55 pounds). Thus cartridge count is represented as the number of sticks of the product at its diameter and length in a 25 kilogram (55 pound) case. Explosive manufacturers represent cartridge counts as a range. This is due to the slight density variations that occur in the manufacturing process. Cases are filled by weight to meet the 25 kilogram (55 pound) nominal weight amount.

When using emulsions, water gels, and ANFO products in bulk form, it is more useful to describe the products in the borehole in terms of their loading density. However, when these materials are used as cartridges from, the stick count is still of value.

Detonation Pressure

The detonation pressure is considered to be the pressure created at the beginning of the reaction zone and is usually expressed in units of kilobars or in gigapascals, (1 kilobar = 1,000 atmospheres = 0.1 gigapascals). To help you compare, 1 atmosphere of pressure is approximately 14.7 pounds/inch². The detonation pressure is mathematically the product of the density, the detonation velocity, and the particle velocity of the reacting explosive at the detonation front. For condensed explosives at an initial density above about 1 gram/centimeter³, the particle velocity is about ¼ the detonation velocity. For lower initial densities, the ratio between the particle velocity and the front of the reaction zone and the detonation velocity becomes greater, for example, for ANFO at the initial density of 0.8 grams/centimeter³, the ratio is 1:3 to 1:6. It should be noted that there are many formulas used by different researchers to approximate detonation pressure. The one given below is a good approximation when the ratio does not deviate much from the ratio 1-4.

Caution To calculate detonation pressure using these equations, VOD given in feet/second units must be converted to meters/second. 1 foot = 0.3048 meters.

$$P_D = 0.25 \times \rho_e \times v_d^2 \times 10^{-6}$$

Equation 11.3

Where:

• $P_D$ = Detonation pressure (gigapascals)
• $\rho_e$ = Explosive density (grams/centimeter³)
• $v_d$ = Detonation velocity (meters/second)

Example 11.3

Calculate detonation pressure in gigapascals of ANFO with a density of 0.82 grams/centimeter³ and detonation velocity of 4,084 meters/second using equation 11.3.

$$P_D = 0.25 \times \rho_e \times v_d^2 \times 10^{-6}$$

$$P_D = 0.25 \times 0.82 \times 4,084^2 \times 10^{-6}$$

$$P_D = 3.419$$

The detonation pressure is 3.419 gigapascals.

An equation commonly used by some manufacturers is equation 11.4.

$$P_D = 2.5 \times \rho_e \times v_d^2 \times 10^{-6}$$

Equation 11.4

Where:

• $P_D$ = Detonation pressure (kilobars)
• $\rho_e$ = Explosive density (grams/centimeter³)
• $v_d$ = Detonation velocity (meters/second)

Example 11.4

Calculate the detonation pressure in kilobars of ANFO with a density of 0.82 grams/centimeter³ and detonation velocity of 4,084 meters/second using equation 11.4.

$$P_D = 2.5 \times \rho_e \times v_d^2 \times 10^{-6}$$

$$P_D = 2.5 \times 0.82 \times 4,084^2 \times 10^{-6}$$

$$P_D = 34.19$$

The detonation pressure is 34.79 kilobars.

Detonation pressure is an important property because it is related to the stress level created as the material to be blasted and is believed to be an important factor in fragmentation. When blasting soft or porous materials, it is generally better to use explosives with a low detonation pressure, to avoid a high amplitude shock wave in the surrounding material near the borehole wall, which leads to a great loss of energy by shock and deformation resulting in excessive amounts of fines.

Very non-ideal explosives with low detonation pressures are preferable for cast blasting and blasting sand and silt. The detonation pressure is also important for the selection of a primer charge for effective and reliable initiation. The mass of the minimum primer to initiate an explosive is inversely related to the detonation pressure of the primer.

Diameter

Diameter, like density, is a term used to describe the two explosive features (1) critical diameter and (2) explosive diameter.

Critical Diameter

Each explosive also has a minimum critical diameter, which is the smallest charge diameter at which the detonation process, once it is initiated, will support itself. This depends on confinement, density and particle size of the explosive. The confined critical diameter is usually smaller than the unconfined critical diameter (i.e., the critical diameter of an explosive confined as a borehole is usually smaller than the explosive's unconfined critical diameter). The density of the explosive affects its critical diameter. In most situations where confinement and density remain constant, Furthermore, the particle size of the explosive affects critical diameter. If other parameters are constant, the critical diameter decreases as the particle size decreases.

Explosive Diameter

The explosive diameter is the cross-sectional width of an explosive cartridge or charge (IME SLP 12, 2010). The diameter of a specific explosive influences its detonation velocity up to a certain diameter. This charge diameter depends on the specific explosive characteristics and characteristics of the confining material. In general, as the explosive diameter increases, its detonation velocity increases up to a limit. At a certain diameter the explosive's ideal detonation velocity is achieved.

Explosive Classification

Explosives are classified for the hazard they present during transportation and storage. The United Nations (UN) has adopted a system of hazard classification and methods to communicate the hazards. Check current UN, Federal or local regulations for the most current classification and hazard communication requirements.

Flammability

Flammability refers to the ease with which an explosive or blasting agent can be ignited by heat. Most dynamites ignite readily and burn violently. This burning can transform into a detonation if the burning takes place in a confined space. Water gels and emulsions are more difficult to ignite than dynamite and in many cases an outside source of flame must be applied continuously to support burning in the open.

Water gels and emulsions can often support combustion without confinement after most of their water is evaporated by an outside flame source. At elevated pressures these explosives burn more readily even before the water has evaporated. Ammonium nitrate (AN) products, emulsions and water gels have a lower tendency to convert the burning into a detonation than does dynamite.

Burn tests are carried to simulate conditions encountered in the field. Currently burn tests are a part of an explosive's testing for hazard classification. The test is defined in the UN Manual of Tests and Criteria (2007). These tests have ranged from burning a single case of explosives to burning a 10,000 pound load of emulsion or water gel confined in a truck. To date, water gels and emulsions have failed to detonate under these conditions. These tests have demonstrated that water-based products have a significantly lower potential to detonate when subjected to flame than does dynamite, but it should be stressed that this is only a greater margin of safety and does not imply that unsafe practices are allowable, or that it is ever safe to fight an explosives fire.

Fume Classification

There are two different fume classifications for explosives in the United States. The type of classification depends upon whether or not the explosive is a U.S. Mine Safety and Health Administration (MSHA) approved explosive named "permissible." Nonpermissible explosives are rated by the IME fume class classification (table 11.6). Permissible explosives are those approved by the MSHA in the United States, and by authorized governmental agencies in other countries. The actual testing in the U.S. is done at the Bruceton laboratories, formerly operated by the U.S. Bureau of Mines, but is now carried out by its present operator, National Institute for Occupational Safety and Health (NIOSH).

Table 11.6 – IME fume classification. (Source: Reprinted by permission of IME: SLP 12, 2010)

Permissible explosives are used in underground coal mines and other mines or underground construction projects classified as gassy. The use of regular, nonpermissible explosives could cause an explosion by igniting a concentration of gas or dust in the air adjacent to the blast. MSHA prescribes limits to the amount of explosives that can be fired per round in underground coal mines. In addition, ventilation in underground coal mines is generally better than in other mines, in order to minimize methane and dust hazards.

There are several ways to determine an explosive's output of fumes. These include measurement in the Bichel Gauge, the Crawshaw-Jones Apparatus, the Ardeer Tank method, field tests, and theoretical calculations. The IME fume classification is based on fumes generated in the Bichel Gauge, where an approximate 200 gram explosive charge with the proper proportions of its wrapper is detonated inside a heavy closed chamber ("bomb") with a volume capacity of approximately 15 liters.

After firing the charge, the pressure and the temperature are recorded and the gas samples are collected. Solid residues are collected after the gas samples have been obtained and the bomb has been opened. There are several problems associated with extrapolating laboratory fume data to actual field use. Some emulsions, water gels and ANFOs will not usually detonate completely in such small quantities as are prescribed for the tests. In the test, the explosive is also not confined and the explosive gases quickly expand into a relatively large volume, unlike the borehole confinement typical of field use.

The most efficient method to measure fumes is to take on-site measurements after the blast. Only trained personnel that have adequate protection against noxious gases should conduct this test. Some explosive manufacturers use new computer technology to make theoretical estimates of fume production. In general, these theoretical results correlate quite well with measured results, but are not designed to replace laboratory or on-site measurements and should never be considered an alternative to actual testing.

Post Blast Fumes

The reaction product gases resulting from the detonation of commercial explosives and blasting agents consist principally of water vapor (steam), nitrogen, and carbon dioxide, which are nontoxic. However, some toxic gases, including carbon monoxide and nitrogen oxides are also present in some small concentration. In the explosives industry these toxic gases are called fumes. The fume components, carbon monoxide and nitrogen oxides are sometimes referred to jointly as nitrous air. Fumes should not be confused with smoke, which is composed mainly of steam and the solid products of combustion or detonation. Although smoke is nontoxic, excessive exposure to smoke, especially that produced by dynamite, can cause severe headaches and should be avoided. The headache may be the result of small particles of unreacted or partially reacted nitroglycerin/nitroglycerol in the smoke.

Both the nature and of the total quantity of fumes and smoke vary among types of explosives. For example, the detonation of emulsion explosives or water gels may produce significantly less smoke than dynamite. Fumes may also vary according to conditions of use (See chapter 28). For example, water attack on ammonium nitrate products can produce large amounts of nitrogen oxides. In open blasting, fumes usually cause little concern if they can be quickly dispersed by air movement; however, due to local conditions (depth of the excavation) quick dispersal is impossible. Then evacuation of the work place is the only recommended procedure. In underground blasting environments fume considerations have a bearing on the type and amount of explosive used, blast confinement, ventilation, and reentry time.

When fumes are a potential problem, properly formulated and manufactured explosives and blasting agents create minimum fume quantities. However, it must be recognized that some oxides of carbon and nitrogen result from every explosive or blasting agent detonation. In addition conditions of use can drastically shift the types of gases produced. Some factors that increase fumes are poor product formulation, inadequate priming, insufficient water resistance, lack of confinement, reactivity of the product with the rock or other material being blasted, and incomplete product reaction.

Adequate waiting periods are mandatory before returning to the blast area. This is important because some fumes are odorless and colorless. Absence of post blast smoke is no guarantee that hazardous levels of fumes are not still present. Never return to an area before ventilation has cleared the fumes from the area.

Oxygen Balance

The detonation process is simply a very rapid combustion reaction. Oxygen balance refers to the ratio of oxygen to fuel in the explosive. Using ANFO as an example, the oxygen is contained in the ammonium nitrate (NH₄NO₃) and the hydrocarbon fuel (diesel fuel oil) can be represented as C₁H₂. The combustion reaction can be written: C₁H₂ + NH₄NO₃ → CO₂ + N₂ + H₂O. This reaction is said to be oxygen balanced if there is just enough oxygen (O) to convert all the carbon (C) to carbon dioxide (CO₂) and all the hydrogen (H) to water (H₂O).

Oxygen balance is a property of the formulation's chemistry. Variations from ideal oxygen balance can produce deviations in energy output from an explosive's published energy content as well as fumes.

If there is too much oxygen, too little fuel, (positive oxygen balance) there will be more oxygen than is required to convert all the carbon and hydrogens to carbon dioxide and water. Excess oxygen appears in the form of nitrogen oxides (NO₂). If there is not enough oxygen, too much fuel, (negative oxygen balance), the reaction by-products include carbon monoxide (CO) as there is not enough oxygen to completely convert all the carbon into carbon dioxide.

Given that it is extremely difficult to formulate or manufacture to the perfect oxygen balance point every time, the industry traditionally has selected the over-fueled (negative oxygen balance) option, particularly for ammonium nitrate based products, because there is little likelihood of generating large amounts of NO₂ and the energy loss for over-fueled ANFO is less than for slightly under-fueled formulations.

Rheology

Rheology is a description of the flow characteristics, physical consistency or texture of an explosive product. Rheology is most often used to describe water-based explosives (emulsions, suspensions, and gels). Various descriptions of rheology include viscosity, the cohesiveness of the material and how easily the material pumps. The required product rheology is determined by the required use of the product.

Viscosity is a measure of the fluid frictions or thickness of a product and is most commonly used with (emulsions, suspensions, and gels) that have not been cross-linked. Viscosity is measured with a viscometer. In the explosives industry, the unit of measure of viscosity is centipoise (cps). The water-based borehole products have viscosity values that range from 10,000 cps for very thin products to over 100,000 cps for very thick products. It is important to note that viscosity measurements are dependent on the method used, so a direct comparison between manufacturers reported values might not be accurate.

The conditions of the emulsifier (surfactant) and fuels that are used along with the amount of these used in the manufacturing process are the primary factors in determining the rheology of an emulsion. The type of gelling agent and the amount of cross-linking determine the rheology of suspensions and gels. Each type of product (emulsion, suspension or gel) can have a wide rheology range, from a thin easy flowing material like hot honey to solids like candies. Products are manufactured to meet the requirements of a specific application and so are formulated to exhibit a specific rheology type.

There are many different applications for packaged products that require a different rheology. For instance, products that are used in a cartridge loader need to maintain their shape while being loaded, but still be flexible enough to pack tightly when they hit the back of the hole. Other packaged products need to be more fluid so they will slump, completely filling vertical holes and seams. Products need to be very rigid so they can be pushed to the back of a horizontal hole without getting hang up. It is important to select a product with rheology that will meet the application needs.

Bulk products can be manufactured at a plant or on a truck at the blast site. For bulk products that are used in a high ANFO content blend need to be fluid enough to easily coat the ANFO, yet be thick enough to provide the required water resistance. In some cases the product (crude or water conditioned) may favor a thicker product. Again, it is important to select a product with rheology that will meet the application needs.

A third type of rheology consistency refers to water gels for products designed to be pumped in smaller volume bulk operations or poured from jugs into water filled boreholes similar to bulk loading. Water gels of this type are thickened with a very long chain linear polymer. They are lightly cross-linked so that the final consistency is a third yet cohesive gel that will flow from a bag, fill and fully couple with the borehole wall and yet have sufficient water resistance to pass through any borehole water with no serious degradation of properties.

Sensitiveness

Sensitiveness is a measure of an explosive's ability to propagate from cartridge-to-cartridge under certain test conditions. It is expressed as the distance through air that a primed half-cartridge (donor) will detonate an unprimed half-cartridge (receptor) (IME, SLP 12). Sensitiveness is sometimes referred to as "Gap Sensitivity." In the past, the measure of sensitiveness was the distance through air that one-half of a 32 millimeter × 206 millimeter (1¼ inch × 8 inch) cartridge would propagate to another one-half of a 32 millimeter × 206 millimeter (1¼ inch × 8 inch) cartridge. Both halves with flat-faced facing were enclosed in a paper tube made from about five wraps of manila paper, and shot unconfined.

The testing of popular commercial explosives, such as ANFO, some emulsion explosives, and water gels, is not practical in 32 millimeter (1¼ inch) diameter, unconfined conditions. This is because many of these products are not available or tested at these diameters, or they require confinement to shoot in these diameters. These products are routinely tested by manufacturers in larger diameters and under various conditions of confinement.

See Sympathetic Detonation.

Sensitivity

A physical characteristic of an explosive material that classifies its ability to be initiated upon receiving an external impulse such as impact, shock, flame, friction or other influence which can cause explosive decomposition (IME SLP 12, 2007). The shock initiation sensitivity of an explosive is the ease with which it can be induced to detonate. It depends on the amplitude and the duration of the pressure pulse it receives. In the case of priming, the shock initiation sensitivity of a given explosive depends on the intensity or size of the initiation charge. Some explosives can be initiated by a single detonator, while others require a large booster charge. Initiation of detonation by transition from burning to detonation often called deflagration-to-detonation transition (DDT) is another important mechanism for initiation of detonation after detonator failure in a drill hole when the borehole diameter is near to the critical one. It is also a likely mechanism for initiation of detonation in large accidental fires in manufacturing facilities or during transportation.

Impact Test

A variety of tests are conducted to measure an explosive's susceptibility to initiation by accident. Accidental initiation often involves situations where the explosive is exposed to impact or becomes squeezed between impacting objects. The standard drop test for dynamite and military explosives is designed to model such situations. This is called the Bruceton up-and-down method. This method consists of dropping a specified weight on a thin section of explosive on emery paper on a metal anvil from varying heights, until a drop distance is reached at which the tested explosive produces a reaction 50 percent of the time.

The Bruceton method is explained in the UN Manual of Tests and Criteria. Restriction is indicated by the noise of an explosion or by analyzing the reaction product gases. With the widespread use of ANFO and water-based explosives such as when testing these products, no explosions are detected unless extremely large heights of drop are used. In fact, generally no reaction is detected for ANFOs, emulsions, or water gels even when they are subjected to the impact of 40 pound (18 kilogram) weight dropped from a height of 4-5 meters (14-15 feet).

However one has to bear in mind that the drop weight test is mainly intended for the case of military explosives, which are also very sensitive under the conditions of the test (small mass of explosive, powder form). Comparing the sensitivity of commercial explosives against the sensitivity of known insensitive compositions (i.e. cast TNT), using results of the drop weight test, is not recommended.

Friction Test

Friction sensitivity testing was formerly done using the friction pendulum. However, at the UN, the following tests and apparatuses are used: the BAM Apparatus, the rotary friction test, and the friction sensitivity test. These are explained in the UN Manual of Tests and Criteria (1999).

Detonator Sensitivity Test

One of the most frequently used sensitivity tests is the detonator (cap) sensitivity test. This test, often referred to as the cap sensitivity test (formerly called "blasting caps"), not only characterizes an explosive's ease of initiation by a standard detonator, but also is used to classify products for safety in storage, transportation, and use. The No. 8 detonator sensitivity test is the standard used by the explosives industry. The test consists of placing the material to be tested in a 3 inch (86 millimeter) inside diameter by 102 millimeters (6½ inch) deep, open-top paper container. The sample is packed into the container at the same density "as packaged for shipment" to the top of the paper container. The container is placed on a 50 millimeter (2 inch) diameter by a 200 millimeter (4 inch) long steel cylinder. The No. 8 test detonator is fully embedded in the center of the product, which should be at ambient temperature.

A No. 8 test detonator is specifically defined as one containing two grams of a mixture of 80% mercury fulminate and 20% potassium chlorate, or a detonator of equivalent strength. An equivalent strength detonator contained 0.40 gms to 0.45 grams of PETN base charge pressed in an aluminum shell with bottom thickness not to exceed 0.76 millimeter (0.03 inch), loaded to a density of 1.4 grams/centimeter³ and primed with standard weights of primer material. This is known as the Institute of Makers of Explosives (IME) No. 8 test detonator. A compression of the cylinder of more than 3.2 millimeters (1/8 inches) indicates a shot, and shows failure of the explosive to pass the UN test. The test is fully described in the UN Manual of Tests and Criteria (1999).

To obtain different numeric values for the sensitivity of different products, detonator sensitivity tests can be carried out with detonators having different weight base charges. Some explosives are sensitive to detonators with very small base charges, while others are only sensitive to a booster initiated by a detonator. The detonator sensitivity of a given explosive can then be indicated by the smallest detonator charge weight that initiates that explosive. Knowing how the sensitivities of different explosives compare can guide formulators of new explosives and provide a basis for recommending priming levels for these explosives.

Shelf Life

Shelf life is the maximum storage period during which an explosive material retains adequate performance or physical characteristics (IME SLP 12, 2007). Explosives are often purchased and stored in magazines for future use. If explosive materials are stored properly, they will retain their performance characteristics for their published shelf life time period. However, all materials can degrade over time. Thus, it is important when explosives are stored, that their inventory is managed to ensure oldest products are used within recommended time limits.

All manufacturers specify optimum storage environments and the shelf life for their products. Manufacturers recommend that inventories be rotated, using the oldest products first. If products begin to show signs of degradation, immediately consult with the manufacturer or manufacturer's representative for advice on use. If products begin to show signs of degradation, immediately consult with the manufacturer for proper disposal advice or action.

In packaged form, water gel and emulsion explosives have shelf lives in excess of one year under most normal storage conditions encountered in North America. Please consult your manufacturer for product specific use.

Manufacturer's Recommendation Please consult your manufacturer for product specific shelf life information.

Sympathetic Propagation

Some types of explosives are sufficiently shock sensitive so that the explosive in one charge will initiate as a result of the detonation of a charge in a nearby hole. Such charge-to-charge initiations may occur with some explosives over considerable distances, depending on the type of material being blasted, the explosive, the size of the charge, the distance between charges, and other factors, such as the presence of water. The term "sympathetic propagation" or "sympathetic detonation" is sometimes used to describe such initiation. Under most conditions it is important that individual charges do not sympathetically propagate, but rather detonate adequately at a predetermined delay interval. ANFO, emulsions and water gels have a reduced tendency to propagate between charges when compared with dynamite.

Temperature

Depending on the type of explosive, changes in initial temperature have an effect on the detonation velocity and sensitivity. As a product's temperature decreases its sensitivity is always reduced. This is because the energy required to increase the temperature to that where rapid reaction takes place is higher. Typically, explosives that are porous solids at normal temperatures and contain little or no liquid are relatively unaffected at the normal low temperatures experienced in commercial blasting. Examples are ANFO and cast boosters. This is because these explosives contain many small voids that act as intense hot spots for setting off the reaction when hit by the detonation wave.

The detonation velocity of less sensitive explosives that contain liquids (e.g. oxidizer solution in some quantity, such as water gels and emulsions, are more affected by temperature, although formulations can be designed to minimize this effect in practical applications.

As a general rule, phase transitions within the explosive will result in changes of sensitivity and performance. Temperature, being the parameter affecting such phase transitions, is therefore a controlling parameter.

Velocity of Detonation

Velocity of detonation (VOD) is the speed a detonation wave travels though a column of an explosive. VOD of commercial explosives reaches its optimum at large diameters and significant confinement. Figure 11.8 shows how the velocity of detonation of ANFO varies with diameter.

Detonation velocities of commercial explosives range from as low as 1,600 meters/second (5,250 feet/second) for certain permissible explosives used in coal mining to as high as 7,600 meters/second (25,000 feet/second) for cast boosters and detonating cords. Most commercial explosives used today have detonation velocities that fall in the range of 3,900 meters to 6,000 meters/second (10,000 feet to 20,000 feet/second). High-speed photographs (See figures 11.9 and 11.10) contrast the difference between a high-velocity and a lower velocity explosive detonated as water. The most noticeable difference is the angle of the shock wave generated by the explosive. This angle is dependent upon the difference between the detonation velocity and the shock velocity in a medium, similar to a sonic boom.

Ideality of detonation is controlled by size of the reaction zone behind the shock wave, during detonation. If the reaction zone is small, the all of the energy released by an explosive particle can support the detonation wave front, since the relief wave coming behind the detonation front down the mass doves so that communication between the mass, which is involved in the shock, which is propagating, is lost (Leiper and du Plessis, 1987). So energy released behind the relief wave cannot support the detonation wave. Furthermore the detonation front is not one dimensional; it has curvature, which results in energy losses laterally. Since total energy is constant, as the stall losses increase, axial velocity decrease. Thus, parameters that have lateral losses have a bearing on VOD. Increased confinement decreases lateral losses; a larger diameter decreases the proportion of total energy lost laterally.

A non-ideal detonation is shown in figure 11.5 and 11.6.

Thus, in non-ideal detonation, as a commercial explosive's confinement or diameter increase its velocity of detonation increases. Particle size, controlling the reaction zone length. VODs with the non-ideal zone VOD increases as particle size decreases. This is the main reason why crushed ANFO (that has higher VOD than prilled ANFO). Also, emulsions, having a small particle size, have ideal performance at smaller diameters.

Figure 11.7 shows the effect of increasing borehole diameter on the VOD of ANFO. Clearly the type of confinement affects the VOD. This is the result of the effect of confinement on the curvature of the wave and the reaction cross behind it. A stronger confinement will result in less curvature and higher reaction rates; thus a larger fraction of the material will react in the zone between the front and the sonic locus, which is responsible for the energy release supporting the detonation front.

When explosives detonate, they don't perform at their optimum velocity of detonation instantaneously. Then from the time of initiation, time is required for the VOD to "run-up" to its optimum or steady state velocity. Until an explosive achieves its steady state velocity, optimum energy delivery is not realized.

Water Resistance

The water resistance of an explosive is defined to be the ability of an explosive to withstand the desensitizing effect of water penetration. (IME: SLP 12, 2007). Water resistance is generally expressed as the number of hours a product may be submerged in static water and still be reliably detonated. Many explosive products penetrated by water have their efficiency only slightly affected. However, virtually all explosive products can be adversely affected if they have been in contact with water for too long.

A product's water resistance depends not only on its packaging and the explosive's inherent ability to withstand water, but also on the water condition. Static water at low pressures will not affect water resistance as much as flowing water at high pressures. Borehole conditions and applications should be carefully monitored by the blaster. Product water resistance values must be considered in relation to the particular conditions of each blasting operation. Water resistance values are conservative guidelines that must be properly interpreted by trained and experienced personnel.

Commercial explosives differ in their ability to resist the effect of water penetration. Ammonium nitrate-fuel mixtures have no inherent water resistance, since ammonium nitrate is very soluble in water. The thin film of fuel oil offers little protection. ANFO poured into a borehole filled with water will quickly dissolve. Packaged ANFO products, if used in wet boreholes, depend entirely on their package integrity to resist water penetration. The packaging may be polyethylene-lined spiral tubes, or woven polypropylene plastic bags. Either of these may be used to package high-density ANFO products but great care must be taken to ensure their integrity when loaded into water-filled boreholes. Even if the package is intact many methods of sealing them are not adequate for water conditions. Also, these products are not solid so they need to be of higher density that the water they are loaded into, or the cartridges will float. Cartridge floating and separation in the borehole is a technique (See chapter 19) often used in large diameter boreholes where once the standing water is pumped out the borehole is not wet.

Certain grades of dynamite such as ammonia gelatin and straight gelatins do offer good water resistance. Other grades of dynamites may depend primarily on their packaging for water resistance, the same as high density packaged ANFO. Emulsions and water gels have an inherent ability to resist the effects of water. These materials can be loaded into water-filled boreholes and stored successfully if the exposure time "sleep-time" in the borehole is not too long.

Always follow manufacturers' recommendations for maximum exposure time. Emulsion/ANFO blend explosives have a range of water resistances, ranging from no resistance for low percentage blends (30% emulsion/70% ANFO) to very good resistance with high percentage blends (60% emulsion/40% ANFO). Despite this variable water resistance, some severe field applications still require a specific type of water-resistant product to meet the application needs.

In general it is recommended that products should not be removed from their packaging and that they should not be cut into small sections. Packaging does improve a product's water resistance. Cutting explosives into small pieces reduces their effective water resistance by increasing the surface area exposed to water. Similarly, the smaller the diameter of the drill hole, the greater the surface to volume ratio in relation to the volume of explosive. As a result, in small diameter drill holes the water resistance of a given product is lower than that of the same product in its larger drill holes. When loading a bulk emulsion, water gel, or an emulsion/ANFO mix, pumping the product from a hose extending to the bottom of the borehole is usually recommended in water filled boreholes.

"Dewatering" and in some cases even inserting a plastic "borehole liner" into which the explosive is loaded to prevent water from reentering the borehole, is a technique (See chapter 19) often used in large diameter boreholes. The technique is mainly used when loading low emulsion percentage blends, and straight ANFO. It is particularly successful in areas where once the standing water is removed, there is limited water inflow into the borehole.

Various standard water-resistance tests have been developed which simulate field conditions. These tests quantify water resistance providing numbers that can serve as a guideline for field use and as a useful measure to compare different explosives. These tests are conducted at pressure typical of most water-filled drill holes. Water resistance numbers, if provided by a manufacturer, should be used only as guidelines since field conditions vary so widely. In severe water conditions a water-resistant product such as an emulsion explosive, watergel, or an emulsion/ANFO blend should be loaded as packaged and shot as soon as possible. Under some severe conditions published water resistance numbers may not be applicable. Blasters should know their water conditions and use products they know will perform under those conditions.

References

Cole, R.H. 1948: Underwater Explosions. Princeton University Press, New Jersey.

Additional Resources

Bjarnholt, G. and Holmberg, R. 1976. Explosive expansion works in underwater detonation. 6th Symposium (International) on Detonation., pp. 540-550, August 24 – 27, Naval Surface Weapons Center, White Oak.

Bjarnholt, G. 1980. Suggestions on standards for measurement and data evaluation in the underwater explosion test. Propellants and Explosives 5, pp. 67-74.

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