NBSIR 73-101

Penetration Resistance Tests of ^ Reinforced Concrete Barriers

R. T. Moore

asurements Automation Section

tion Processing Technology Division

institute for Computer Sciences and

Technology

Washington, D. C. 20234

December 1972

Interim Report for Period October 31 - November 3, 1972

Prepared for Defense Nuclear Ageney

Washington, D. C. 20305

NBSIR 73-101

PENETRATION RESISTANCE TESTS OF

REINFORCED CONCRETE BARRIERS

R. T. Moore

Measurements Automation Section

Information Processing Technology Division

Institute for Computer Sciences and

Technology

Washington, D. C. 20234

December 1972

Interim Report for Period October 31 - November 3, 1972

Prepared for Defense Nuclear Agency

Washington, D. C. 20305

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

WATIOIMAL BUREAU OF STANDARDS, Richard W. Roberts, Director

CONTENTS

Page

CONTENTS 1

ILLUSTRATIONS 3

SUMMARY 7 ACKNOWLEDGMENTS 7 1. TEST SPECIMENS 9

2. TEST INSTRUMENTATION 13

3. ATTACK DESIGNATION AND TOOLING DESCRIPTION 15

A. Sledgehammer 15

B. Rotohammer 15

C. Diamond Drill 15

D. Burning Bar 15

E. Jet-Ax 15

F. Demolition Saw 16

G. Pneumatic Jackhammer 16

4. ROUND VERSUS RECTANGULAR HOLES 16

5. ATTACK RESULTS 18

SLAB 1 18

ATTACK A 18

ATTACK B 22

ATTACK D 25

ATTACK E 28

ATTACK F 31

SLAB 2 33

ATTACK Al 33

ATTACK B 37

ATTACK D 40

ATTACK F 42

ATTACK E 45

COMPARISON OF RESULTS ON 4-INCH THICK

FIBROUS AND REGULAR CONCRETE SLABS 49

SLAB 3 49

ATTACK A 49

ATTACK D 51

ATTACK E 53

SLAB 4 55

ATTACK B 55

ATTACK E 56

SLAB 5 58

MULTIPLE TOOL ATTACK 58

ATTACK D 67

ATTACK E 67

SLAB 6 71

ATTACK D 71

ATTACK E 71

SLAB 5 72

ATTACK G 72

ATTACK C 72

SLAB 6 76

ATTACK C 76

1
CONTENTS (cont'd)

Page

ATTACK G 76

CONCLUSIONS 78

RECOMMENDATIONS 81

2
ILLUSTRATIONS —Figure No. ^ Page

1 Configuration of Test Slabs 8

2 Slabs Prior to Beginning of Tests 8

3 Vibration Transducer Location 11

4 Acoustic, Ultrasonic and Vibrational

Data Recording Instrumentation 12

5 Target Attack Locations, Slab 1 17

6 Exterior View of l A 18

7 Interior View of lA 19

8 lAl, Sound, Multiple Sweep 19

9 lAl, Vibration, Multiple Sweep..... 20

10 1A3, Sound from Cutting Torch 21

11 1A3, Vibration from Torch 21

12 Background Noise, Vibration 21

13 Drilling with Rotohammer 22

14 IBl, Acoustical Disturbances from the

Rotohammer and Drill 23

15 IBl, Vibrational Disturbances from the

Rotohammer and Drill 23

16 1B2, Acoustical Disturbance from

Rotohammer and Chisel 24

17 1B2, Vibrational Disturbance from

Rotohammer and Chisel 24

18 1D2, Front of Slab 1 after Burning

Bar Attack 26

19 1D2, Interior of Slab 1 after

Burn-through 26

20 ID, Vibrational Disturbances from

Burning Bar 27

3
^^Bure No.

21 Jet Ax Ready for Firing 28

22 Effects of JA-3, Outside of Slab 1 29

23 Effects of JA-3, Inside of Slab 1 29

24 Interior View of Slab 1 after Jet-Ax

and Sledgehammer Attack 30

25 IFlj Vibrational Disturbance from

Demolition Saw and Abrasive Cutoff Wheel.. 31

26 Target Attack Locations, Slab 2 32

27 Exterior, Slab 2, Attack Al, after

Breakthrough 33

28 Interior, Slab 2, Attack Al, after

Breakthrough 34

29 2A, Vibrational Disturbances 34

30 Acoustical Spectra of Cutting Torch 35

31 2A3, 16 lb. Sledgehammer on Slab 2 36

32 2A3, 16 lb. Sledgehammer on Slab 2 36

33 Attack 2B 37

34 Vibrational Spectra from Rotohammer

Drilling, Slab 2 38

35 Acoustical Spectra of Rotohammer

Drilling, Slab 2 38

36 Vibrational Spectra from Rotohammer

and Chisel on Slab 2 39

37 Acoustical Spectra from Rotohammer

and Chisel on Slab 2 39

38 Burning Bar Attack on Slab 2 40

39 Acoustical Spectra from Burning Bar

Attack on Slab 2 41

40 Vibrational Spectra from Burning Bar

Attack on Slab 2 41

4
Figure No. Page

41 Demolition Saw, Slab 2 42

42 Acoustical Spectra from Demolition Saw . 43

43 Vibrational Spectra from Demolition Saw 43

44 Hammering on Plug Left in Slab 2 by

Demolition Saw 44

45 Exterior after JA-5 45

46 Interior after JA-5 45

47 Exterior after JA-3 on Slab 2 46

48 Interior after JA-3 on Slab 2 47

49 Comparison of Measured or Calculated

Penetration Times for Four-inch Thick

Reinforced Concrete Slabs 48

50 16 lb. Sledgehammer Attack on Slab 3 ... 50

51 Spalling Produced by 12.10 Minutes of

Attack by 16 lb. Sledgehammer 50

52 Attack 3D, Burning Bar Hole. 51

53 Background Vibrational Noise 52

54 Vibrational Disturbance from Burning Bar 52

55 Exterior View of JA-4 Effects on Slab 3 53

56 Interior View of Slab 3 after JA-4 Attack 54

57 Vibration from 1" Drill 55

58 Vibration from 1-1/2" Drill 56

59 Spalling Produced by Jet-Ax on Slab 4... 56

60 Slab 5, Target Attack Locations 57

61 Early in Multiple Tool Attack 59

62 Material Chiseled Out of Plug to Make

Room for Saw Hub 61

5
Figure No. page

63 Sawing Reinforcing Rods 62

64 Chiseling Webs Between Holes on the Inner

Half of the Opening 63

65 Final 9 X 14-inch Opening 64

•66 Close-up of Slab 5 Burning Bar Results .. 67

67 Results of JA-4 on Exterior of Slab 5 ... 68

68 Close-up of Exterior Spalling 68

69 Interior Cracking Produced by JA-4 69

70 JA-5 in Place. 69

71 External Spalling from JA-5 70

72 Interior Cracking Produced by JA-5 72

r 73 i

. External View of Slab 6 after Burning

Bar and Jet-Ax Attacks 71

74 Interior View of Slab 6 after JA-4 Attack 72

75 Pneumatic Jackhammer on Slab 5. 73

76 Opening Made by Jackhammer. 74

77 . ,, Diamond Core Drill. 75

. 78 .. ,, Vibration from Diamond Core Drill 76

79 .. . Opening Made in Fibrous Concrete by

18-minute Jackhammer Attack 77

80 Stuck Jackhammer Tool Point 77

81 Drilling Rate in Concrete

versus Drill Size 79

6
PENETRATION RESISTANCE TESTS

OF REINFORCED CONCRETE BARRIERS

by

R. T. Moore

SUMMARY

This report describes the results of a series of penetration tests

on six concrete slabs. The tests were conducted at the U. S Army Corps

of Engineers Construction Engineering Research Laboratory in Champaign,

Illinois, during the period October 31 through November 3 1972

Several alternative methods of attack were employed, a^d the time to

penetrate, and in some instances, to produce a 96-inch opening was measured, and estimates were developed regarding the acoustic, ultrasonic and vibrational disturbances produced by the attacks.

Depending upon the attack technique, a man-passable opening can be

made in four-inch thick reinforced concrete in times ranging from 4 to

nearly 15 minutes. The corresponding times for eight-inch thick re- inforced regular concrete range from about 7 to 25 minutes and from

about 20 to 30 minutes for fibrous concrete.

With the exception of the diamond core drill, detection should be

relatively easy by the combined acoustic, ultrasonic or vibrational

disturbances produced by the attacks even though not all techniques

produce all three types of disturbances.

ACKNOWLEDGMENTS

These tests were accomplished through the support and active

participation of many individuals in several organizations. In

particular, we wish to thank Col. Reisacher, E. A. Lotz, Bob Gray,

Frank Kearney and Bob Gunkle of CERL for preparation of the test

specimens, provision of the test site, instrumentation support and

general support to the visiting test participants. The penetrations

were made by Frank C. Veirs, Lt. V. Sunderdic and Spc. P. Schmidt of the

Intelligence Materiel Development Office which also provided much of the

attack tooling and key portions of the instrumentation. Photographic

coverage of the tests was provided by IIT Research Institute through the

courtesy of R. J. Bouk and the Federal Reserve Board. Sound level

measurements, test timing and data recording were accomplished by

R. Blackmon, Naval Security Group, and John Cecilio, Naval Facilities

Engineering Command. Vibrational data acquisition was handled by

John Ramboz and R. Koyanagi of the NBS Vibration Section.

The tests were sponsored by Defense Nuclear Agency under the

direction of Marvin C. Beasley who arranged the multi-agency participation and was directly involved in all phases of the test activities.

7
Figure 2. Slabs Prior to Beginning of Tests

8
1. TEST SPECIMENS

The six test specimens were reinforced concrete slabs five feet by seven and one-half feet in size. Three of the slabs were cast using a fibrous concrete mix and the other three were made using a regular

concrete mix in accordance with the mix design shown in table 1. Each

slab was given an identifying number of from one to six with the odd

numbers being assigned to the slabs made from regular concrete and the even numbers fibrous concrete.

Slabs 1 and 2 were four inches thick and were reinforced with

number 5 bars in the center of the slab on five-inch centeis both ways.

Slabs 5 and 6 were eight inches thick and with the same reinforcing

rod configuration.

Slabs 3 and 4 were eight inches thick but with a different reinforcing

pattern. One-quarter inch welded steel mesh with a six-inch mesh size was located two inches inside each 'face of the slab. The two layers of

mesh were staggered from each other three inches in each direction.

At the time the slabs were cast, samples of each mix were cast as columns and beam.s for compression, splitting tensile and flexural

strength testing. These samples were cured with the slabs in a temperature and humidity-controlled curing room and were subjected to

strength tests on the day that slab penetration tests were started. The

test results are shown in table 2.

A few days prior to the scheduled start of penetration testing the

slabs and mix sample test pieces were removed from the curing room. The

slabs were moved outside to a black top parking area at the east end of

the building and placed on edge in a hexagonal configuration and wooden

timbers were attached across their tops so as to provide bracing and

support. (See fig. 1.) On the day prior to the start of testing, attack

locations were laid out on the slabs and identified by spray paint

lettering. The general arrangement is shown in figure 2.

In addition, vibration transducers were mounted on slabs 1, 2 and

3 on the inside at the center at a point about six inches below the

top edge as shown in figure 3.

9
Table 1. Mix Design

CONCRETE PLAIN FIBROUS

Cement, lbs.

94#/sack 564 517

ELy Ash, lbs.

75#/sack) 225

Water, lbs. 250 320

Fine Aggregate, lbs • 1300 1400

Coarse Aggregate, lbs.

(Fibrous 3/8" Max. , Plain 3/4" Max.) 1675 1400

Air Entrancement Agent, fl. oz. (3/4 oz./sack of cement) 4-1/2 3

Water Reducer, fl. oz.

(9 oz./sack of cement ) 50

Fiber, lbs.

(Steel 0.010 :X 0.022 X 1.0 in.) 200

Table 2. Mix Sample Test Results

Plain Concrete

Compression Splitting Tensile Flexural

#1. 5730 psi 385 psi 515 psi

#2 6150 psi

#3 6150 psi

464 psi 595 psi

655 psi

580 psi

Avg. 6010 psi 424 psi

Fibrous Concrete

585 psi

Compression Splitting Tensile Flexural

7090 psi 765 psi 1200 psi

5780 psi 796 psi 1195 psi

1015 psi

1075 psi

Avg. 6440 psi 780 psi 1120 psi

10
gure 3. Vibration Transducer Location

11
MICROPHONE

1/4 INCH

POWER

SUPPLY

PREAMP

i^JjlOO FT.

VOICE

ANNOTATION

MICROPHONE

7 CHANNEL MAG

TAPE RECORDER

SPECTRUM

ANALYZER

CAMERA STORAGE

OSCILLOSCOPE

PIEZOELECTRIC

VIBRATION

TRANSDUCER

CHARGE

AMPLIFIER

C<< 100 FT,

Figure 4. ACOUSTIC, ULTRASONIC AND VIBRATIONAL

DATA RECORDING INSTRUMENTATION

12
2. TEST INSTRUMENTATION

Penetration time was recorded manually using a stopwatch. The times

reported represent only those time intervals when the attack was actualfy

underway. Time intervals used to set up tooling, to change tools, or to

consider new methods of attack were excluded. The probable error in

timing data is believed to be on the order of +5 seconds, except, in the case of short attacks where no change of tooling occurred, measurements

are believed to be within +1 second.

Acoustic, ultrasonic and vibrational data collection was accomplished

using instrumentation as shown in figure 4. A 1/4-inch capacitance

microphone, together with its power supply and preamplifer, was mounted

on a roller-equipped tripod. This was positioned at a nominal distance

of 10 feet from each attack point. A 100-foot length of coaxial cable was led back inside the building to the magnetic tape recorder and

spectrum analyzer. The charge amplifier associated with the vibration

transducer was located within the hexagonal area enclosed by the slabs

and connected to a second length of coaxial cable. Magnetic tape

recordings were made of both data channels but only one channel at a time could be connected to the spectrum analyzer. The output of the

spectrum analyzer was used to drive the storage oscilloscope which

permitted capture and subsequent photography of sample spectra during

the course of the tests.

The microphone preamplifier gain was set to +20 dB for all penetration tests and overall calibration was performed using a calibrator

which provides a reference sound pressure level of +124 dB at 250 Hz.

Vibration transducers were calibrated and full scale g values related to

charge amplifier gain settings and peak voltages prior to the test

series. Transfer of calibrations to the spectrum analyzer was accomplished with a signal generator and digital voltmeter. The

uncertainty associated with data derived from either the acoustic or vibrational channel is estimated to be on the order of 3 dB and is small

compared to the influence of other uncontrolled factors.

In particular, it is important to recognize that the acoustic,

ultrasonic and vibrational data which were observed in this test series

may not be representative of those which might result from a similar

attack on the same materials if they were employed in the construction

of a vault or room. In most of the test series, each slab was resting

on one edge and the other three edges were free. Thus, it is possible

that vibrational modes may have existed which would not occur in a similar slab which was bonded on all edges to form a structure. These

modes, if present, could also influence the observed sound levels which

may be further affected by reverberation and reflection from nearby

objects.

13
The frequency response of both the acoustic-ultrasonic and the

vibration transducer and their respective amplifiers is reasonably flat

to well above 50 kHz and this frequency was selected as the upper limit

in the spectrum analyzer. All spectral samples appearing in this report

were obtained with a 500 Hz analyzer bandwidth, 5 kHz per cm sweep, and

25 kHz center frequency. Amplitude reponse is logarithmic, 10 dB per cm, and the scale was adjustable in 20 dB steps with the input attenuator of

the spectrum analyzer.

Where temperature rise measurements were made, a resistance

thermometer was employed in conjunction with a digital ohmmeter.

Ohmmeter readings were recorded manually and converted to temperature

values using a calibration chart furnished by the manufacturer who

claims accuracy of +1 F in the observed temperature range.

Two portable sound level meters were also used and data from these was recorded manually. One was operated in the linear mode providing

an indication of peak sound pressure levels at frequencies up to about

50 kHz. This meter was positioned at a distance of about eight feet

from the attack point except where otherwise noted.

A second Sound Level Meter, USASI Type 3, was operated with an A

weighting network. dBa sound pressure levels were observed at a distance of 30 feet from the attack point unless otherwise noted.

Photographic coverage of the test series included 35 TruTi color still

photographs (slides) and 16 mm color motion picture footage of selected

highlight activities.

14
3. ATTACK DESIGNATION AND TOOLING DESCRIPTION *

Each attack was assigned an Identifying letter. This was associated

with the physical location of the attack point on the slab and was kept

the same from slab to slab to the greatest extent practical. It also

identified the primary attack tooling or technique which was employed as follows

A. Sledgehammer

Sledgehammers were available in weights of 10, 12, 16, and 20

pounds. In practice the 10- and 16-pound weights were used almost

exclusively.

B. Rotohammer

A Skil model 736 rotohammer was used with 3/4", 7/8", 1", and

1-1/2" diameter masonry bits and a 1-1/2" toothed chisel.

C . Diamond Drill

A 3-1/2" diameter water-lubricated core drill was operated at

900 r.p.m. The drill was mounted in a portable press stand which

was equipped with a motor current monitoring meter. Drill pressure

was manually applied so as to develop a drill motor current of 7 to

7-1/2A, 110 VAC.

D. Burning Bar

The bar was one-half inch black iron pipe with nineteen iron

and one aluminum filler rods. The bar had been cut to five-foot

lengths to facilitate shipment. Operation during all tests was at

200 p. s.i. oxygen pressure. This high pressure, together with the

short lengths between couplings, occasionally resulted in ejection

of filler rods.

E. Jet Ax

This is a commercially available linear-shaped charge mounted in

a foamed plastic damper and equipped with a 30- to 45-foot long

detonator cord terminated with a screw-on, hand operated, percussion

initiator. Three different sizes were used in the test series. The

JA-3 has 5.6 ounces of explosive arranged to make a 24" X 36"

rectangular cut (.04 oz. per linear inch). The JA-4 has 5.6 ounces

of explosive arranged to make a 17" diameter circular cut (.104 oz. per linear inch). The JA-5 has 3.4 ounces of explosive arranged to

make a 10" diameter circular cut (.108 oz. per linear inch).

* See footnote at bottom of page 16.

15
F. Demolition Saw

A rotary saw powered by a 5-horsepower gasoline engine and

equipped with a 16" diameter abrasive cutoff wheel.

G. Pneumatic Jackhammer

A pneumatic jackhammer operated at 100 p.s.i. air pressure from a gasoline engine powered air compressor mounted on a small trailer.

Tool size was 1-1/4" (across flats of the hexagonal shaft) and a spike point and 3" spade point tool were used,

4. ROUND VERSUS RECTANGULAR HOLES

It is generally conceded that a small man can squeeze through an opening having an area of 96 square inches provided that one dimension of

the opening is at least 6 inches. The required area can result from a circular opening having a diameter of just over 11 inches or from a rectangular opening having dimensions of 8 by 12 inches, for example. We

believe that the rectangular opening may admit a slightly larger man than

the circular opening of equivalent area as it better accommodates human

shoulders and hips. Because of this, most of the penetration tests were

targeted toward the production of a rectangular opening except in

instances where the physical nature of the attack tooling dictated otherwise. A 96 in. circular opening has a perimeter length of 34.73 inches

while an 8" X 12" rectangular opening has a perimeter length of 40 inches.

As a result, whenever penetration time is a linear function of perimeter

length, it is likely that the rectangular opening may have taken on the

order of 115 percent of the time which might have been required for a circular opening. Obviously this does not hold in the case of tooling

such as the abrasive cutoff saw which is only useful for making straightline cuts or for tooling such as the sledgehammer which is used for volumetric removal of material over the entire area of the opening. In

selected instances, calculations have been made of the time which might

be required to produce openings of both shapes to illustrate the difference.

* Certain commercial equipment and materials are identified in this

paper in order to adequately specify the experimental procedure. In

no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the

material or equipment identified is necessarily the best available for

the purpose.

16
17
5. ATTACK RESULTS

SLAB 1

Slab 1 was 4-inch thick ordinary concrete with 5/8" reinforcing rods

in the middle of the slab on 5-inch centers vertically and horizontally.

The target attack locations are dimensioned in figure 5.

ATTACK A

This attack was conducted with a 10 lb. sledgehammer to break

out the concrete to expose the reinforcing rods for cutting.

After 29 blows in 52 seconds, a depression was well developed

on the front surface and a large circular crack could be observed on

on the rear surface. After 53 blows and 1:30 minutes, an opening of

approximately 22 in. was developed through the slab between the

re-rods. A much larger area spalled away from the re-rod in the

rear. After 2 minutes and 73 blows, the opening had been enlarged

to approximately 75 in. with maximum dimensions on the order of

10" X 8". At 90 blows and 2.43 minutes, the opening had maximum

dimensions of 11" X 14" and a clear area of about 100 in.^ and the

sledgehammer portion of the attack was judged completed. (See figs.

6 and 7.)

Figure 6. Exterior View of lA

18
Figure 7. Interior View of lA

Figure 8 is a multiple sweep exposure of the spectrum analyzer

output showing sound pressure level observed during the hammer

attack. dBa readings ranged from 83 to 86.

Frequency kHz

Figure 8. lAl, Sound, Multiple Sweep

19
Figure 9 is a multiple sweep exposure of the vibrational

spectrum. It should be borne in mind that the spectrum analyzer is

repetitively sweeping the frequency range from a few Hertz to 50 kHz.

When a transient, such as a hammer blow, occurs, only those spectral

components which are within the passband of the analyzer at that

time will be displayed. Thus, the total spectrum might be approximated by a curve connecting the peaks of the multiple sweep traces.

0 25 50

Frequency kHz

Figure 9. lAl, Vibration, Multiple Sweep

To cut the re-rod remaining in the opening developed by the

sledgehammer attack, an oxygen MAPS torch was employed after it was

found that No. 3, 36" long bolt cutters could not handle the 5/8"

rod. The torch cutting required 2,05 minutes to make eight cuts

and release four sections of rod, yielding a total penetration time

of 4.48 minutes.

20
Acoustical and vibrational spectra from the cutting torch are shown in figures 10 and 11, respectively. The dBa readings ranged

from 56 to 68.

126

106

db

25

Frequency kHz

50

Figure 10. 1A3, Sound from Cutting Torch

0.1

.01 I

0 25

Frequency kHz

50

Figure 11. 1A3, Vibration from Torch

.01

,001 mmmi

0 25

Frequency kHz

60

Figure 12. Background Noise, Vibration

21
Figure 12 shows the background noise level with no attack in

progress. It is not at all clear that any vibrational disturbances

introduced by the cutting torch rose above the noise.

ATTACK B

This attack involved the use of a Skil model 736 Rotohaimner

to drill a series of holes around the perimeter of a 6-1/2" X 15"

opening and to chisel out the web between holes to expose the

reinforcing rods enough that they be cut with a demolition saw equipped with a 12" diameter abrasive cutoff wheel.

Drilling eighteen holes one inch in diameter required 7 .125

minutes for an average time of 23.75 seconds per hole (see fig. 13).

Figure 13. Drilling with Rotohammer

22
The acoustical and vibrational disturbances produced by the

rotohammer and drill are shown In figures 14 and 15. The dBa readings were 84 to 92. Peak linear sound pressure level readings of

104 to 110 dB were obtained with the sound level meter Inside the

hexagonal enclosure formed by the six slabs.

0 2 5 50

Frequency kHz

Figure 14. IBl, Acoustical Disturbances from

the Rotohammer and Drill

Frequency kHz

Figure 15. IBl, Vibrational Disturbances

from the Rotohammer and Drill

23
With a 1-1/2" chisel in the rotohammer, it took 3.66 minutes to

chisel out the 18 webs between holes for an average time of 12.36

seconds per web. With a 43" perimeter and 18 one-inch holes, the

length of material chiseled was 25" or 8.8 seconds per inch.

It required 10.79 minutes to drill and chisel an aperture

perimeter 43 inches long. The average rate of perimeter extension

was slightly over 15 seconds per inch.

The acoustical and vibration spectra are shown in figures 16

and 17.

Frequency kHz

Figure 16. 1B2, Acoustical Disturbance

from Rotohammer and Chisel

g

Frequency kHz

Figure 17. 1B2, Vibrational Disturbance

from Rotohammer and Drill

24
During the chiseling, sound level meter readings of 87 to

90 dBa were observed. Peak linear values of 102 - 108 dB were

observed from within the hexagonal enclosure.

Cutting the reinforcing rods with the demolition saw took

1.23 minutes giving a total penetration time of 12.025 minutes.

ATTACK D

This attack employed a burning bar. Two holes were melted

through the slab; one required 29 seconds and used 20-1/2 inches

of bar, the other took 20 seconds and used 21 inches of bar.

A resistance thermometer element was mounted on the back face

of the slab. Burn-through of the first hole occurred 2-1/2 inches

away from the element and produced the following time versus

temperature profile:

Time (Sec.) Temp

. °F

0 (breakthrough) 53

25 55

35 58

60 56

The second hole burned through only 1/16-inch below the

element and produced the following profile:

Time (Sec.) Temp

. °F

0 63

35 108

57 157

Using an optical pyrometer, the apparent temperature of the

molten ejecta coming from the slab during the burn was 2000°C.

Since the emissivity of this material is not known, the true

temperature is undetermined.

25
Figure 18 shows the exterior of the slab. This photograph was

taken only a few moments after completion of the second burn.

Figure 19 is an interior view of the slab.

Figure 18. 1D2, Front of Slab 1 after

Burning Bar Attack

Figure 19. 1D2, Interior of Slab 1

after Burn-through

26
Sound levels of 71-92 dBa were observed and a peak linear

level of 102 dB was observed at a distance of 12 feet. Vibrational

disturbances are shown in figure 20.

Frequency kHz

Figure 20. ID, Vibrational Disturbances

from Burning Bar

Although a complete penetration was not made using the burning

bar, the time which would be required to do this is calculated as follows

Assume 12 holes equally spaced along the circumference of an

11" diameter circle. This would provide a nominal center-tocenter hole spacing of 2.9 inches. With average burn time of 24.5

seconds per hole, the total burn time is estimated to be 294 seconds.

An estimated 246 inches of bar would be required based on the

observed average consumption of 20.5 inches per hole.

Assuming the holes are one inch in diameter, then if the 22.5

linear inches of webs between burn holes are chiseled out using the

rotohammer and 1-1/2 inch chisel at 8.8 seconds per inch, the

estimated time to chisel the 12 webs is 199 seconds.

We now assume that the reinforcing rod was not aligned with

the holes and cut by the burning bar in more than about half of the

necessary instances, so an estimated 60 seconds is allowed to make

four cuts using the torch employed to ignite the burning bar.

27
Finally, we assume that 15 seconds might be required with a sledgehammer to knock out the remaining plug and thus develop an estimated penetration time of 567 seconds or 9.45 minutes.

To provide a more direct com.parison with the results in

Attack IB, let us assume a rectangular opening of 8" X 12" is to be

produced by burning 20 holes around this perimeter. This would

require an estimated 510 seconds. Chiseling out the remaining 20

inches of web would require 176 seconds. Add to this 60 seconds of

torch time and 15 seconds of sledgehammer time under the assumptions

stated above and the total penetration time is 761 seconds or

12 . 68 minutes

ATTACK E

This attack involved the use of a Jet-Ax, size JA-3, with a 5.6 ounce linear-shaped charge arranged to cut a 24" X 36"

rectangular opening.

Actual setup times were not measured. It is estimated that a hangar nail could be placed in one minute and that another 1.50

minutes would permit placing the charge connecting the detonator

and actuating the device.

Figure 21 shows the Jet-Ax in place ready for firing and

figures 22 and 23 show the results which it produced.

Figure 21. Jet-Ax Ready for Firing

28
Figure 22. Effects of JA-3, Outside of Slab 1
Sound level meter readings were off scale on this shot but data was obtained on later Jet-Ax tests.

The JA-3 caused partial penetration of the slab such that 22

blows of a 10-pound sledgehammer in 40 seconds time were adequate

to completely expose the reinforcing rod in a 24" X 31" area. The

Jet-Ax detonation also shifted end of the slab back approximately

1-1/2 inches and the other end was shifted, together with slab 3

which was against it, back 2-1/4 inches.

Figure 24 shows the interior view after the sledgehammer

attack cleared most of the shattered concrete from the reinforcing

bars

Figure 24. Interior View of Slab 1 after

Jet-Ax and Sledgehammer Attack

30
If we assume two minutes with the cutting torch to cut the

reinforcing rods, the estimated total time to penetrate is 5.17

minutes; however, this results in an opening considerably larger

than the 96 in.^ minimum used in other calculations. If the

opening size is restricted to the specified minimum, torch time

could probably reduced by one minute and sledgehammer time reduced

by 10 seconds to give an estimated penetration time of only 4*0

minutes

ATTACK F

It was planned to cut an 8" X 12" perimeter with the demolition saw to a depth of about 3 - 3-'-/2 inches and then knock the resulting plug out with a sledgehammer.

The attack was aborted after a single cut when it was reported that the saw was not operating properly. The single cut was 14"

long on the surface and approximately 2" deep and took 55 seconds

to make. Allowing for the 6" radius of the abrasive wheel, the

full depth of the cut would only extend for 5.4 inches.

Sound levels were 104 - 108 dBa and 105 - 106 dB linear.

Figure 25 shows the vibrational disturbances produced by this

attack. The spectrum is noteworthy in that amplitude does not

fall but increases with increasing frequencies.

Frequency kH

Figure 25. IFl, Vibrational Disturbance from

Demolition Saw and Abrasive Cutoff

Wheel

31
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rO

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32
To calculate the penetration time, we assume that a 12-inch

diameter abrasive wheel cutoff saw, operating normally, could cut

to a depth of three inches and extend the cut at this depth at a rate of 10 seconds per inch, thus the 40-inch perimeter would

require 400 seconds. The remaining inch of concrete beyond the

depth of the perimeter cut would probably yield to 30 seconds of

sledgehammer attack giving an estimated total penetration time of

430 seconds or 7 . 16 minutes.

SLAB 2

Slab 2 was a 4-inch thick slab of fibrous reinforced concrete which

was otherwise identical to Slab 1. Target areas for Attacks B and F

were the same as on Slab 1 (see fig. 5). Figure 26 shows the locations

of Attacks Al, A3, D, El, and E2.

ATTACK Al

Attack Al on this slab was similar to that conducted on Slab 1,

i.e., a 10-pound sledgehammer attack to expose the reinforcing

followed by use of a cutting torch on the rods.

The first interior crack appeared after 110 blows and 2.93

minutes. Initial breakthrough occurred at 129 blows and 3.42

minutes. The reinforcing rod was exposed in an opening 9-1/4" X

10-1/4" at 288 blows and 8.0 minutes. The resulting opening is

shown in figures 27 and 28. Note that the broken areas all

occurred within the spaces between reinforcing bars. In the regular

concrete of Slab 1, the extent of spalling and breakthrough was not

constrained by the reinforcing bars in this way (see figs. 6 and 7).

Figure 27. Exterior, Slab 2, Attack Al,

after Breakthrough

33
Figure 28. Interior, Slab 2, Attack Al

after Breakthroueh

Vibration spectra are shown in figure 29 which is a multiple

trace exposure in which seven hammer blows appear. Visual monitoring of the output of the spectrum analyzer showed disturbances in

the range of 10 to 30 g at frequencies below 5 kHz.

Frequency kHz

Figure 29. 2A, Vibrational Disturbances

34
Sound pressure levels observed ranged from 86 to 96 dBa

(30 ft.) and peak linear values of 122 dB were observed at 8 feet

from the attack point on the outside and 135 dB inside the hexagonal

enclosure

After the completion of the sledgehammer attack, the

reinforcing was removed with an Oxy-MAPS cutting torch. This

involved seven cuts and took 1.43 minutes, giving a total time to

penetrate of 9.43 minutes. The acoustical spectra showed 85 to

90 dB below 5 kHz falling to 65 - 75 dB at 20 - 25 kHz as shown in

figure 30.

Sound levels of 57 to 70 dBa were observed at 30 feet.

db

106

86

i

i.. . mm

Ik mm

25

Frequency kHz

30

Figure 30. Acoustical Spectra of Cutting Torch

Then, in order to assess the influence of sledgehammer weight,

an attack was mounted with a 16 pound sledgehammer at an adjacent

but undamaged area of the slab. The first visible internal crack was developed after 55 blows and L62 minutes; breakthrough

occurred at 105 blows and 3. 07 minutes.

35
Vibration showed 80 g below 5 kHz dropping to about 10 g at

15 - 17 kHz and 3 g at 40 kHz (see figs. 31 and 32).

Sound level was 120 - 122.5 dB on the linear meter.

1000

too

10

am

I

50

Frequency kHz

Figure 31. 2A3, 16 lb. Sledgehammer on Slab 2

25 50

Frequency kHz

Figure 32. 2A3, 16 lb. Sledgehammer on Slab 2

36
ATTACK B

This attack paralleled Attack B on Slab 1 in that the rotohannner was employed to drill and chisel, but only an 8" length of

perimeter was cut on the assumption that the time required for this

could be extrapolated (see fig. 33)

Drilling five 1" diameter holes took 2,07 minutes for an average of 24.8 seconds per hole.

The vibrational spectra were relatively flat and in the range

of 5 to 20 g at all frequencies up to 45 kHz. Figure 34 is a multiple trace exposure of the vibrational spectra.

Figure 33. Attack 2B

37
0 25

Frequency kHz

0

Fibure 34. Vibrational Spectra from Rotohainmer

Drilling, Slab 2

As shown in figure 35, the acoustical spectra showed levels

above 100 dB out to 20 kHz then dropping to about 75 dB at 45 kHz,

108 - 112 dB was observed on the linear meter at five feet and

88 - 91 dBa was observed.

Frequency kHz

Figure 35. Acoustical Spectra of Rotohammer

Drilling, Slab 2

Chiseling out the four webs between the holes took 30 seconds

to remove 3 linear inches of material; thus, the total time to cut

8 linear inches of perimeter was 2.57, corresponding to an average

rate of 19.25 seconds per inch. This time is a little over 4

seconds per inch greater than the average on Slab 1.

At 19.25 seconds per inch, it would require an estimated 770

seconds to complete a 40-inch perimeter. Allowing another 120

seconds to cut the reinforcing rod, the estimated total time to

penetrate is 890 seconds or 14.83 minutes.

38
As shovm in figure 36, the vibrational spectra were relatively

flat ranging from 10 - 30 g at all frequencies up to 42 kHz. Sound

levels were 105 dB up to about 15 kHz falling to 70 - 80 dB between

17 and 45 kHz (see fig. 37).

Ill - 112 dB linear aid 88 - 94 dBa levels were observed at the

usual 8-foot and 30-foot, respective, distances.

Figure 36. Vibrational Spectra from Rotohammer

and Chisel on Slab 2

Frequency kH

Figure 37. Acoustical Spectra from Rotohammer

and Chisel on Slab 2

39
ATTACK D

The burning bar was employed to make two holes; each required

24 seconds; one hole used 18-1/2" of bar and the other used 25"

(see fig. 38).

The average burning bar penetration time of 24.5 seconds for

the fibrous concrete of this slab was the same as the average for

the regular concrete in Slab 1. Chiseling the web out between holes

required 10 seconds per linear inch, however, as contrasted to only

8.8 seconds per linear inch in the regular concrete. Thus, to

estimate the time required to complete a penetration with an 11-inch

diameter circular opening, we add 1.2 seconds per inch times 22.5

linear inches or 27 seconds to the time calculated for Attack ID to

get a total time of 9.90 minutes. In the case of an 8" X 12"

rectangular opening, there are 20 linear inches of web assumed and

the added time factor over the value calculated for Slab 1 is

24 seconds giving a total penetration time of 13.08 minutes..

Figure 38. Burning Bar Attack on Slab 2

40
During this attack with the burning bar on Slab 2, sound

pressure levels of 110 to 113 dB linear and 74 to 80 dBa were

observed. Acoustical and vibrational spectra are shown in figures

39 and 40.

0 2 5 50

Frequency kHz

Figure 39. Acoustical Spectra from Burning Bar

Attack on Slab 2

25 50

Frequency kHz

Figure 40. Vibrational Spectra from Burning Bar

Attack on Slab 2

41
ATTACK F

The demolition saw was used to cut an 8" X 12" rectangular

perimeter to a depth of 2-1/8 to 2-1/2 inches (insufficient to cut

through the reinforcing rod). This required a total of 8.78 minutes

and corresponds to a rate of slightly over 13 seconds per linear

inch. (See fig. 41.)

Figure 41. Demolition Saw, Slab 2

The nominal 2-1/4 inch depth of penetration achieved by the

demolition saw could probably have been extended to 3 inches with

the installation of an unworn abrasive cutoff wheel. This would

involve cutting to a depth one-third greater so we assume that the

13 seconds per linear inch cutting rate would be increased

proportionately to a value of 17.3 seconds per inch. The 40-inch

perimeter cut would then have required 693 seconds. It is assumed

now that with the reinforcing rod cut, the 30-second sledgehammer

attack would have been successful in knocking out the remaining

plug. This would yield a total penetration time of 723 seconds

or 12.08 minutes.

42
The acoustical and vibrational spectra produced by the

demolition saw are shown in figures 42 and 43. Sound levels

104.5 dB linear and 90-98 dBa were observed.

0 2 5 50

Frequency kHz

Figure 42. Acoustical Spectra from Demolition Saw

25 50

Frequency kHz

Figure 43. Vibrational Spectra from Demolition Saw

43
As noted above, the perimeter cut which was made was not deep

enough to completely cut through the reinforcing bars, so the effort

to knock out the remaining plug was rather superficial. When 30

seconds and 16 blows of the 16-pound sledgehammer, figure 44, were

ineffective, the attack was abandoned, since the elapsed time was now

nearly as great as that required for the total penetration in Attack

A.

Figure 44. Hammering on Plug Left in Slab 2

by Demolition Saw

44
ATTACK E

A size JA-5 Jet-Ax was tested against Slab 2 and produced a 10-1/2 inch diameter opening which was clear of all material except

the reinforcing rods. These were slightly bowed but showed no evidence of any cutting action from the shaped charge. Exterior and

interior views of the opening are shown in figures 45 and 46.

Figure 45. Exterior after JA-5

Figure 46. Interior after JA-5

45
As shown in figure 47, a Jet-Ax, size JA-3, produced

external spalling to a depth of 3/4" to 1" in a rectangular area

having external dimensions of 27" X 39" and internal dimensions of

21" X 33", and produced a hairline interior crack, figure 48, along this

perimeter. The JA-3 was judged to be relatively ineffective in

marked contrast to its performance on Slab 1.

Figure 47. Exterior after JA-3 on Slab 2

46
Figure 48. Interior after JA-3 on Slab

47
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Note in Figure 47 how the force of the Jet-Ax has driven the

top of the slab back so that it is only restrained from falling by

the bracing timbers which fastened all six slabs together.

COMPARISON OF RESULTS ON 4-INCH THICK

FIBROUS AND REGULAR CONCRETE SLABS

In figure 49, the measured or calculated penetration time for

the various attack methods tested on the 4-inch thick slabs are summarized in the form of a graph. Inspection of this shows that the

fibrous concrete tested was significantly more penetration resistant

to every method of attack used except the large Jet-Ax. The

penetration resistance of the fibrous concrete exceeds 9 minutes

for any of the other attack techniques used. Its superiority is

most pronounced against the sledgehammer and is least noticeable

with the burning bar.

SLAB 3

Slab 3 was constructed of ordinary concrete 8" thick. It was re- inforced with two layers of 1/4" diameter welded steel mesh with 6"

center-to-center spacing vertically and horizontally. One layer of

mesh was placed 2" from each face of the slab and the two layers were

staggered 3" from each other vertically and horizontally.

Only attacks A, sledgehammer, D, burning bar, and E, Jet-Ax, were

conducted on this slab. The target location for these attacks is as shown in figure 5.

ATTACK A

A 16-pound sledgehammer was used in an attempt to break

through. The attack was conducted in a series of 30-second duration

efforts each of which yielded 15 - 19 blows (see fig. 50).

After 5.00 minutes and 162 blows, spalling had occurred

sufficiently to yield a penetration depth of approximately 2". After

8.75 minutes and 298 blows, the penetration depth had been increased

to 3" and a hairline crack was observed on the rear of the slab.

Since a Jet-Ax attack had already been conducted on this slab, it is

uncertain whether the hairline crack was initiated by that or by the

sledgehammer attack.

After 12.10 minutes and 443 blows, the penetration depth had

reached 3-5/8" (see fig. 51) and it became apparent that it would be

necessary to enlarge the attack area considerably to avoid striking

the edges of the hole with the hammer handle. The attack was aborted at this point in concession to this and to the advanced

fatigue of the hammer swingers.

49
Figure 50. 16 lb. Sledgehammer Attack on Slab
ATTACK D

A single penetration was made with the burning bar. It took

52 seconds and burned 46" of bar. Oxygen pressure was again 200

p.s.i. Figure 52 shows the hole which was melted through.

Figure 52. Attack 3D, Burning Bar Hole

Vibrational disturbances were not visible above the background

noise level at frequencies below about 10 kHz, but climbed to about

5 dB above the noise at 45 kHz. Typical amplitude in the middle

frequency range (25 - 35 kHz) was about .01 g.

Figures 53 and 54 show the vibrational background ncdse and the

slight increase in the disturbance level produced when the burning

bar was in operation.

51
0 25 50

Frequency kHz

Figure 53. Background Vibrational Noise

0 25 50

Frequency kHz

Figure 54. Vibrational Disturbance from Burning Bar

Sound levels of 76 - 88 dBa and 100 dB linear were observed.

The apparent ejecta temperature was 1850°C.

52
ATTACK E

A size JA-4 Jet-Ax with a 5.6-ounce charge arranged to cut a 17" diameter circle produced front surface spalling to a depth of

1-1/4" - 1-1/2" in an annular ring with an outside diameter of

approximately 22" and an inner diameter of about 10". The center

area inside the ring was deflected inward approximately 1/16"

and a 3/8" bulge was produced on the back side of the slab. A rather

extensive network of cracks was also produced and the free end of

the slab was driven back about 3/8".

Figures 55 and 56 show exterior and interior views following

the Jet-Ax attack.

Figure 55. Exterior View of JA-4 Effects on Slab 3

53
Figure 56. Interior View of Slab 3 after JA-4 Attack

The sound levels produced by this and the other JA-4 Jet-Axes

ranged from 141 to 147 dB on the linear meter at distances of 75 to

126 feet from the detonation.

54
SLAB 4

Slab 4 was a replica of Slab 3 (8" thick, double mesh reinforcing)

except constructed with fibrous reinforced concrete.

ATTACK B

The rotohammer was used to drill several holes of different

diameters with the following results:

Diam. (in.) Time (sec.)

3/4 33.5

7/8 45

1 55

1 59

1-1/2 246

The times required for hole sizes below 1" diameter show that

drilling rate is inversely proportional to the volume of material

removed. The 1-1/2" diameter size, however, departs drastically from

this behavior and takes nearly twice as long as would be expected on

the basis of volumetric material removal.

The vibrational effects were similar for all drill sizes except

the 1-1/2". Figure 57 shows vibrational spectra from the 1" drill,

which was very similar to that recorded from the smaller drill sizes,

while figure 58 is from the 1-1/2" size. There are less high frequency components in the latter trace. In both cases the separation

between transducer and attack point was about 3-1/2 feet.
g

Frequency kHz

Figure 58. Vibration from 1-1/2" Drill

ATTACK E

A size JA-4 Jet-Ax produced a 21" OD X 12" ID annulus of

spalling to an approximate depth of 1-1/4". The central area inside

the ring of spalling appeared to have been depressed about 1/16"

inward. Cracks on the inside of the slab were less pronounced than

on Slab 3. The detonation moved one end of the slab inward 3/8" and

the other end 1-1/4".

The spalling is shown in figure 59.

Figure 59. Spalling Produced by

Jet-Ax on Slab 4

56
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57
SLAB 5

Slab 5 was 8" thick, ordinary concrete with 5/8" reinforcing rod on

5" centers, vertically and horizontally centered in the slab.

The target locations of the attacks made on this slab are shown in

figure 60.

imTIPLE TOOL ATTACK

In order to get information on the total time that might be

required to make a man-passable opening in this type of slab, a "minimum time" attack was mounted. In this attack, the objective

was to attempt to use whatever tooling appeared most appropriate at

any stage of the process. A 9" X 14" aperture was produced in

30.257 minutes of attack time which involved 1 hour and 42 minutes

total elapsed time.

The sequence of activities began with rotohammering 23 holes,

3/4" diameter, through the slab, along the periphery of the intended

opening. This required 10.106 minutes for an average time of 26.36

seconds per hole. (Compare with 23.75 seconds for drilling 1"

diameter holes through the 4" thick ordinary concrete.)

A couple of holes were then drilled half way through the slab

near the central area of the central area of the plug and a superficial attempt was made to spall off the concrete on the interior

(see fig. 61) . A suitable bull point punch was not available

however, and this approach was quickly abandoned.

58
Figure 61. Early in Multiple Tool Attack

59
Then with the 1-1/2" chisel in the rotohammer, the webs

between holes on the left side (14", vertical) of the hole were

chiseled out about half way through the slab. The time required

was 1.48 minutes or 6.35 seconds per inch of perimeter.

Then the demolition saw was used to cut the webs on the re- maining three sides to a depth of about 3" as limited by the

diameter of the slightly worn abrasive wheel. The time was

3.358 minutes or 6.3 seconds per inch of perimeter. A new

abrasive cutoff wheel was then installed in the saw and cutting

was resumed for 1.65 minutes in an attempt to get through the

reinforcing rods in the center of the slab. Some of this time was wasted as the hub of the demolition saw was limiting wheel

penetration for a while before the operator became aware of the

problem.

Then the rotohammer and 1-1/2" chisel was used to remove

material so as to make room for the saw hub in an effort to provide for greater penetration of the abrasive wheel. The chiseling

time was 1.35 minutes. This was followed by 51 seconds with the

demolition saw, another 2.23 minutes of chiseling and another 1*32

minutes with the demolition saw.

See figures 62 and 63.

60
Figure 62. Material Chiseled Out of Plug

to Make Room for Saw Hub

61
Figure 63. Sawing Reinforcing Rods

At this point, the plug was struck 5 blows with the 16-pound

sledgehammer in 10 seconds with no significant effect. Close

examination showed that the reinforcing rods had not yet been

fully cut through and the demolition saw was used for another 1.17

minutes with the guard removed to try to get additional penetration. This was successful and the reinforcing bars were finally

cut.

The plug was then ineffectively attacked with the 16-pound

sledgehammer for 21 blows and 37.5 seconds.

Chiseling was then resumed with the rotohammer and material

was removed from most of the webs through the full thickness of

the slab (see fig. 64). This required 5.52 minutes.

62
Figure 64. Chiseling Webs between Holes

on the Inner Half of the Opening

63
Finally, 15 more blows from the 16-pound sledgehammer knocked

the plug out in 26 seconds (see fig. 65).

Figure 65. Final 9 X 14-inch Opening

64
Not counting the minute or two expended on the abandoned

spalling attempt, the attack times for each type of tooling are summarized below:

Tool Time (min.)

Rotohammer with 3/4" drill

Rotohammer with 1-1/2" chisel

Demolition saw Hammer

10.106

10.583

8.341

1.225

Total 30.255

This attack was clearly suboptimal in a number of respects.

First, the opening was larger than necessary. A 9 X 14 inch

opening has an area of 126 in,^ and a perimeter 46 inches. This is a perimeter length nearly one-seventh greater than that of the

nominal 8 X 12 inch opening.

Second, the abrasive cutoff wheel was rather ineffective. A

12-inch diameter wheel does not have enough radius outside its hub

to reach the reinforcing rods in the center of an 8-inch slab. A

minimum cutting depth of 4-5/8 inches is needed if the reinforcing

rods are accurately centered, and even more depth of cut is re- quired if they are displaced toward the inside of the slab.

A large volume of the material in the plug had to be chiseled

away in an effort to provide clearance for the hub of the abrasive

cutoff wheel to obtain greater penetration. This clearance was

also needed in order to provide chisel access to the webs between

the holes on the inner half of the slab (inside the reinforcing

rods) . It seems likely that most of the time used with the

demolition saw to cut along the perimeter of the opening did little

to reduce the volume of material which had to be chiseled out. Also,

it probably did not have a large influence on the time that the

chiseling required. It is probable that its only real contribution to

the penetration effort was in cutting the reinforcing rods.

Additionally, all but the last 26 seconds of time used with

the sledgehammer was ineffective and therefore wasted.

Finally, it is believed that the lack of a suitable bullpoint punch and the consequent abandonment of the internal

spalling attempts was a major factor in the inordinately long time

used in this penetration. In Attack 1-A it was demonstrated that

a 10-pound sledgehammer could break through 4 inches of reinforced

concrete in about 2-1/2 minutes. It should be easier to break out

4 inches of concrete by impacting it from a point behind the

reinforcing bars than 4 inches of concrete which has the reinforciig

in the center.

65
A hypothetical penetration will be outlined using time

information developed in the course of these tests as follows:

1. Drill 20 holes, 3/4" diameter

on 2" centers (same spacing used

before) along an 8 X 12" perimeter.

Drill alternate holes 6" and 4"

deep. Total drilling depth 100".

Average rate 3.295 seconds per inch

(same as before). Time: 329.5 sec.

2. Using bull-point punch in 6" holes,

spall off 2" from interior of slab.

Then repeat in 4" holes to produce

interior spalling to a depth of 4"

around the perimeter (time allowance

the same as for the sledge attack

on Slab 1) . 146.0 sec,

3. On outside, chisel 25 linear inches

of web between the 20 holes at 8.8

seconds per inch, to a depth of

4 inches (same chiseling rate as developed in IB). 220.0 sec,

4. Cut reinforcing rod with torch. 120.0 sec,

5. Knock out plug with sledgehammer. 30.0 sec,

Total Time: 845. 5 sec. or 14.091 minutes.

Even without the use of the bull-point punch as described in

the hypothetical penetration but employing the same tooling used

in the actual penetration, it seems likely that the opening could

have been made in less than 25 minutes by elimination of the

ineffective portion of the abrasive cutoff wheel time.

66
ATTACK D

The burning bar made a hole through the 8" slab in 66 seconds

consuming 58-1/4" bar. Apparent ejecta temperature was 1905 C and

sound levels were 80 - 89 dBa and 102 dB on the linear meter.

Figure 66 is a close-up view of the hole which was produced.

Figure 66. Close-up of Slab 5 Burning Bar Results

ATTACK E

Two Jet-Axes were used against Slab 5. The JA-4 size produced a 22-1/2" CD by 11" ID annulus that was spalled to a depth of

about 1-1/2". The center area within the annular ring was depressed approximately 1/32". One end of the slab was shifted back

about 3" and the other end, resting against another slab, bounced

forward about an inch. The cracking produced was significant but

less extensive than Slab 3.

Figures 67 and 68 show the exterior and figure 69 the interior of Slab 5 after the attack by the first Jet-Ax.

67

Figure 69. Interior Cracking Produced by JA-4

The size JA-5 produced a 15" OD X 4" ID annulus that was

spalled to a depth of about 1-1/2", and a 1" bulge was produced

on the interior of the slab.

Figure 70 shows the Jet-Ax in place, figure 71 shows the

exterior spalling it produced, and figure 72 shows the interior

bulge and cracking.

Figure 70. JA-5 in Place

69
Figure 71. External Spelling from JA-5

Figure 72. Interior Cracking Produced by JA-5

70
SLAB 6

Slab 6 was 8" thick fibrous concrete with 5/8" reinforcing rods on

5" centers vertically and horizontally in the center of the slab.

ATTACK D

The burning bar made a penetration in 40 seconds and con- sumed 39" of bar. The apparent temperature of the ejecta was

1860 C and a sound level of 77 to 86 dBa was observed.

ATTACK E

A size JA-4 Jet-Ax produced a 21" CD, 11" ID annulus with a penetration depth of about 1-1/4". The center area within the

ring showed an inward deflection of about 1/16".

Figures 73 and 74 show external and internal views of

Slab 6 after these attacks.

Figure 73. External View of Slab 6 after

Burning Bar and Jet-Ax Attacks

71
Figure 74. Interior View of Slab 6 after JA-4 Attack

This concluded the tests which were conducted with the

slabs in a vertical position. The hexagonal configuration was

torn down and Slabs 5 and 6 were supported in a horizontal

position with heavy timbers under two edges for penumatic jackhammer and diamond core drill tests.

SLAB 5

ATTACK G

With the slab horizontal as described above, an attack was made with a pneumatic jackhammer (see fig. 75). The jackhammer

was operated at 100 p.s.i.air pressure, and both spike point and

3" spade point tools were used. Initial breakthrough occurred in

2 .55 minutes and by 5 •55 minutes concrete had been cleared from an area 11" X 11". The reinforcing rods were not cut, see figure 76.

72
Figure 75. Pneumatic Jackhainmer on Slab 5

73
Figure 76. Opening Made by Jackhammer

74
ATTACK C

A 3-1/2" diameter water-cooled diamond core drill was run

for 3.92 minutes and penetrated 3-5/8". The core drill was mounted in a semiportable drill press arrangement and was powered by a drill running at 900 RPM and operating at 7 - 7.5 amps. 110 Vac.

(see figure 77)

Figure 77. Diamond Core Drill

The vibrational disturbances produced by the diamond core drill covered a broad range of frequencies but were quite low in

amplitude as shown in figure 78.

75
25 50

Frequency kHz

Figure 78. Vibration from Diamond Core Drill

SLAB 6

ATTACK C

With the slab in a horizontal position supported on timbers,

the 3-1/2" diameter diamond core drill was operated for 3.92

minutes. The depth of penetration was 2-11/16". This rate of

penetration in the fibrous concrete was 87.4 seconds per inch

which can be compared with the rate of 64.8 seconds per inch in

the regular concrete of Slab 5.

ATTACK G

The pneumatic jackhammer required 2.78 minutes to make an initial breakthrough and 18.07 minutes to clear material away and

expose the reinforcing rods in an open area having dimensions of

approximately 7" X 12-1/2" (see fig. 79). This is more than three

times as long as was required on Slab 5.

In using the jackhammer on the fibrous concrete, the point

of the tool became stuck on several occasions. It was then

necessary to release the tool point from the hammer, install

another tool point and cut out additional material around the

stuck point to free it. (See fig. 80)

76
Figure 80. Stuck Jackhammer Tool Point

77
6. CONCLUSIONS

At the time the test plan was formulated for this series of tests it was anticipated that it would be possible to complete the same series of

tests on the eight-inch thick slabs that were done on the four-inch

thick pair. Due to unusually adverse weather conditions this was not

possible even though the test duration was extended a day. As a result,

several of the planned penetrations of the eight-inch sections were

either cut short or eliminated. Despite this, enough test data was obtained to support a number of conclusions regarding the penetration

resistance of reinforced concrete and the characteristics of detection

equipment which might be useful in alarming as a result of attempted

penetrations

There is usually a factor of more than three to one between the

penetration times for the slowest and the fastest methods of attack that

were tested in regular concrete, but this difference is significantly

narrowed in fibrous concrete. Most of the narrowing in range is due to

the increased resistance of fibrous concrete to impactive attack, in

particular by sledgehammer and pneumatic jackhammer. Here differences

of more than two to one were observed in tests lA versus IB and 5G veraas

6G. The fibrous concrete suffered less damage from the Jet-Ax than did

the regular concrete, but in the four-inch thickness both are readily

penetrated by one or more of the sizes which are available. None of the

Jet-Axes cleanly penetrated the eight-inch slabs of either type of

concrete but the rather extensive cracking produced by the size JA-5

in Slab 5 would probably have yielded to a fairly brief sledgehammer

attack.

The drilling, chiseling and cutting resistance of fibrous concrete

is also greater than that of regular concrete. Drilling rates are shown

in figure 81 for both materials. This figure shows inches per second

versus drill diameter squared. A rate that is inversely proportional to

volume of material removed plots as a straight line in this form of

presentation. The dashed line indicates the expected rate for the 1-1/2

inch drill if it followed that pattern in the same way as the smaller

drill sizes. For equivalent drill sizes of one inch or less the rates

in fibrous concrete are about 25 percent slower than in regular concrete.

Drilling rate with the diamond core drill was 34 percent slower in

fibrous concrete. Chiseling rate using the rotohammer and 1-1/2 inch

chisel was 13 percent slower in fibrous concrete based on the times re- quired to chisel out webs between holes. Cutting rates with the abrasive

cutoff wheel appear to be somewhat slower in fibrous concrete but the

variations in operating circumstances with this tool did not provide

completely reproducible data to allow for a quantitative determination

of the relative cutting rates.

78
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79
There was no significant difference in the melting rates of regular

and fibrous concrete using the burning bar, but this is not considered

significant as the burning bar represents a rather poor choice of attack

tooling for concrete barriers in the four- to eight-inch thickness range.

The test data indicates than an eight-inch thick regular reinforced

concrete barrier can be penetrated in about seven minutes with a pneumatic jackhammer and cutting torch. Although not demonstrated, it

is likely that use of a second Jet-Ax at the attack point might make a penetration possible in about the same amount of time, particularly in

the case of a double mesh reinforced barrier such as Slab 3 which

sustained rather extensive external and internal cracking from a single

Jet-Ax. The concept of double reinforcing as a penetration deterrent

is good, but the size of the reinforcing used in Slab 3 was probably

too small to fully exploit this concept.

If the use of the pneumatic jackhammer and Jet-Ax is excluded, the

time required to penetrate an eight-inch thick regular concrete barrier

increases to values in the range of 14 to 25 minutes. The penetration

resistance time of the eight-inch fibrous reinforced concrete appears

likely to start with values on the order of 18 to 20 minutes using the

pneumatic jackhammer and extend upwards to perhaps 25 to 30 minutes

against other types of attack.

A review of the acoustic, ultrasonic and vibrational disturbance

data collected during the test series provides an indication of the

detection sensitivities which might be required in intrusion alarm

equipments. For example, assuming a passive ultrasonic detector with

a one-third octave bandwidth centered at 25 kHz and a detection

threshold of 60 dB located 10 feet from the attack point, all of the

impactive tooling, the abrasive cutoff wheel, the torch and the burning

bar would have been readily detected. A vibrational detector at the same frequency and bandwidth and a detection threshold of 0.1 g located

four feet from the attack point would have alarmed on all but the

torch, burning bar, the 1-1/2 inch drill with rotohammer and the

diamond core drill. The diamond drill was probably the quietest of the

attack tools tested. Vibrational disturbances were quite low, generality

below .05 g,and while acoustical measurements were not obtained, the

noise in the audible range at least appeared to be lower than any of

the other tools tested. If the drilling rate in regular concrete is

extrapolated and one minute of added time allowed for cutting through

the reinforcing, it would require about 9-1/2 minutes to make a single 3-1/2 inch diameter hole. Such a hole might admit an intruder's

arm but it would probably require seven or eight more such holes to

develop a pattern which would admit his body. This would involve at

least an hour of working time. Since the drill must be water-cooled

and lubricated during this period, the use of a moisture sensitive

detector might be a useful technique to consider if the protected area

is in an interior location and not exposed to weather.

80
In summary, all of the attack techniques tested except the diamond

drill produced noise or vibration levels, or both, which should be

easily detected by sensor equipment which is operated with its alarm

threshold set high enough to provide an acceptable nuisance alarm rate.

Detection of the higher frequency components of the disturbances is

particularly attractive from the standpoint of minimizing the nuisance

alarm rate. Many of the potential sources of nuisance alarms are either

low in high frequency energy or have it heavily attenuated during

propagation to a sensor.

7. RECOMMENDATIONS

This series of tests leaves unanswered a number of interesting

questions, such as:

1. On figure 80, is there a break in the curve between the

drilling rate that is inversely proportional to volumetric

material removal and the rate for the 1-1/2 inch size drill

that is not, or is there a gradual transition? Does regular

concrete exhibit similar characteristics.

2. Would the hypothetical attack on Slab 5 be as rapid as the calculated time?

3. What would be the performance of a larger, say 16-inch

diameter, abrasive cutoff wheel?

4. How long would it take to complete penetrations in the areas damaged by the Jet-Ax on the eight-inch thick concrete

slabs?

5. Would a 200- to 250-pound battering ram be an effective

penetration tool?

6. Would an eight-inch thick fibrous reinforced concrete

barrier with two layers of No, 5 bars on five-inch centers

both ways located two inches in from each face and staggered

with respect to each other offer 25 minutes minimum resistance

to any of the attack methods tested plus use of multiple Jet-Axes

at the same location?

Further tests are recommended to try to develop answers to these and

other related questions.

81
FORM N!>S-n4A (1-71) U.S. DEPT. OF COMM. 1 1. PUbLlCATlON OU RISPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA j i^gSIR 7.3-101

3. "Recipient's Accession No.

4. TITLE AND .SUBTITLE Penetration Resistance Tests

of Reinforced Concrete Barriers

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

R. T. Moore

8, Performing Organization

NBSIR 73-101

9. PERFORMIN'G ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. Project 6509419

11. Contract/Grant No. lACRO DNA EO 72-201

and 73-406

12, Sponsoring Organization Name and Address Defense Nuclear Agency (DNA)

Washington, D. C. 20305

13. Type of Report & Period Covered Interim

10/31 - lA.mil

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) SUMMARY. This report describes the results of a series of penetration tests on six

concrete slabs. The tests were conducted at the U. S. Army Corps of Eiigineers

Construction Engineering Research Laboratory in Champaign, Illinois, during the

period October 31 through November 3, 1972. Several alternative methods of attack were employed, and the time to penetrace, and in some instances, to produce a 96-inch^ opening, was measured, and estimates were developed regarding the acoustic,

ultrasonic and vibrational disturbances produced by the attacks.

Depending upon the attack technique, a man-passable opening can be made in

four-inch thick reinforced concrete in times ranging from 4 to nearly 15 minutes. The

corresponding times for eight-inch thick reinforced regular concrete range from about

7 to 25 minutes and from about 20 to 30 minutes for fibrous concrete.

With the exception of the diamond core drill, detection should be relatively

easy by the combined acoustic, ultrasonic or vibrational disturbances produced by the

attacks even though not all attack techniques produce all three types of disturbances.

17. KEY WORDS (Alphabetic.il order, separated by semicoloris) barrier penetration; intrusion detection; physical security; reinforced concrete

18. AVAILABILITY STA'lEMENT

[xi UNLIMITED. U I'OR OFFICIAL DISTRIBUTION. DO NOT RELEASE

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