CES Blast Effects & Damage Assessment 3 — Questions and Answers
Question 1: The Mach stem forms during a surface burst when:
- The incident and reflected waves merge along the ground surface (Correct answer)
- The blast wave is refracted downward by a temperature inversion
- Two separate charges detonate simultaneously
- The shock wave enters a confined underground tunnel
Correct answer: The incident and reflected waves merge along the ground surface
The Mach stem is a near-vertical, reinforced shock front created when the incident wave and the ground-reflected wave coalesce at a specific angle.
Question 2: Cratering from a buried explosive is classified as 'camouflet' when:
- Surface soil is thrown upward forming a visible crater
- No surface rupture occurs and only a subsurface cavity is formed (Correct answer)
- The blast vents through existing rock fractures
- Water table intrusion collapses the cavity immediately
Correct answer: No surface rupture occurs and only a subsurface cavity is formed
A camouflet is an underground explosion that creates a subsurface void without breaking the surface, leaving no visible crater.
Question 3: Which metric is used to compare the brisance of different explosives?
- Heat of detonation
- Plate dent test (copper crusher) value (Correct answer)
- Oxygen balance percentage
- Critical diameter
Correct answer: Plate dent test (copper crusher) value
The plate dent (or copper crusher) test measures the depth of indentation caused by detonation, providing a comparative brisance index.
Question 4: In blast damage assessment of reinforced concrete, spalling on the distal face (far side from blast) indicates:
- Shear failure at the column base
- Reflected tensile stress exceeding the concrete's tensile strength (Correct answer)
- Thermal degradation of the aggregate
- Rebar corrosion pre-existing the blast
Correct answer: Reflected tensile stress exceeding the concrete's tensile strength
Spalling on the far face occurs when the compressive wave reflects as a tensile wave, and concrete — weak in tension — fractures and ejects material.
Question 5: The TNT equivalence factor for a non-TNT explosive is determined by comparing:
- Density ratios at standard temperature
- Heat of explosion values or peak overpressure at a reference distance (Correct answer)
- Critical diameter under confinement
- Sensitivity to electrostatic discharge
Correct answer: Heat of explosion values or peak overpressure at a reference distance
TNT equivalence is derived from either thermochemical heat of explosion ratios or empirical overpressure measurements referenced to TNT at the same standoff.
Question 6: What is the primary purpose of a pressure-time (P-t) recorder placed at a field blast test?
- To measure ground vibration peak particle velocity
- To record overpressure history and calculate impulse (Correct answer)
- To detect toxic gas concentrations post-detonation
- To time the detonation sequence for multi-hole rounds
Correct answer: To record overpressure history and calculate impulse
A P-t recorder captures the pressure waveform over time, from which both peak overpressure and impulse can be derived for blast characterization.
Question 7: Which building component typically fails FIRST under a far-field blast (low overpressure) scenario?
- Reinforced concrete shear walls
- Steel moment frame connections
- Window glazing (Correct answer)
- Roof structural members
Correct answer: Window glazing
Window glazing has low resistance to blast overpressure and fails at pressures as low as 0.5–1 psi, well before structural elements.
CES Blast Effects & Damage Assessment 3
The Mach stem forms during a surface burst when: