blast resistance structure
TRANSCRIPT
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CHAPTER 1
INTRODUCTION
1.1 GENERAL
The tragic effect cause by a blast on any structure cannot be eliminated completely,
but can be minimised to a certain level. The main idea of designing a blast resistance
structure is to reduce the tragic effect. In the past few decades considerable emphasis
has been given to problems of blast and earthquake. Conventional structures,
particularly that above grade, normally are not designed to resist blast loads; and
because the magnitudes of design loads are significantly lower than those produced by
most explosions, conventional structures are susceptible to damage from explosions.
With this in mind, developers, architects and engineers increasingly are seeking
solutions for potential blast situations, to protect building occupants and the
structures. In the recent years, there has been a new approach of designing a blast
resistance structure by using elastomeric polymers as a structural retrofitting material
and adding polyethylene fibre in a reinforced concrete.
1.2 BLAST LOAD
1.2.1 Definition
The load that is accounted normally to different structures is its dead load and Live
load. The loads that are impacted on it are not very often encountered, but it has been
seen and encountered in different places around the globe. The loads which are
impulsive in nature are known as BLAST LOAD. It is a high frequency loading, but
not last for a long period mostly.
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The difference between Blast loads and static loads is the impulsive nature of the blast
loads. Since most of the existing buildings are not designed to withstand the dynamic
loads cause by the extreme blast, it will result in the failure of the main load bearing
frames of the structure leading to the failure of the whole structure and collapse after a
very short period of explosion and will further result in the loss of life and loss in
economics.
1.2.2 Effects on structure
The behaviour of structure under blast loadings is more complicated than that of static
loadings. In particular, the fracture modes of reinforced concrete (RC) slabs subjected
to blast loadings are characterised by spalling, due to the tensile stress wave being
reflected from the back side of the slab. To protect human lives inside the structure
and nearby under such conditions, it is necessary to prevent the launch of concrete
fragments that accompany spalling. Therefore reducing spall damage is the most
important problem faced by designers of blast resistance structures.
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CHAPTER 2
PRESENT BLAST RESISTING PRACTICES
2.1 BLAST RESISTING SOLUTIONS
One of the approaches to enhance the resistance of the structural elements (i.e.
columns, beams, walls and slabs) to blast loads is by increasing their mass and
ductility. These may be done by using additional concrete and reinforcement for
concrete structures, and by using larger sections for steel structures, or alternatively,
by using external strengthening techniques such as composite laminates or steel
jacketing. Extensive experimental and numerical investigations have been undertaken
in recent years to evaluate the performance of existing structural strengthening
applications to withstand blast effects. Most of the present practices in strengthening
of structures against blast loads are focussed on the utilisation of composite laminates
such as fibre reinforced polymer (FRP) applications. This can be attributed to the
improved properties of modern FRP composites, which include its high strength to
weight ratios and their corrosion free characteristics, as well as the cost effectiveness
when compared to other strengthening techniques such as using bonded steel plates.
Research and the subsequent application of this technology have largely focussed on
the use of carbon fibre reinforced polymers (CFRP) and glass fibre reinforced
polymers (GFRP), even though other materials such as aramid fibre reinforced
polymers (AFRP), aramid/glass (A/G) hybrid applications and GFRP rods have also
been studied. Malvar et al. (2007) and Buchan & Chen (2007) have undertaken
comprehensive reviews and summarised the findings from researches in recent years,
in the area of strengthening and retrofitting of structures subjected blast effects. While
a lot on focus have been dedicated towards identifying new approaches to enhance the
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efficacy of structural retrofitting against blast effects, and improvising the properties
of existing strengthening materials, there is yet to be any specific and cost effective
technique or material established to be considered as principally suitable in
retrofitting structures facing the risks associated to blast and impact effects. A similar
observation was provided by Buchan & Chen (2007), who also suggested that a more
systematic direction is required to determine the advantages and limitations of the
various strengthening applications.
Steel stud walls can be applied to the interior of existing walls to increase ductility
and energy absorption. To maximise this ductility, the connection to the floor and
ceiling must be well designed so they do not fail but instead the stud yields and failure
can occur due to strain elongation.
The various Reinforced Concrete (RC) structures such as columns, beams, walls etc
can be protected at the site by providing an externally bonded steel plates at the
surface. The mechanism of strengthening RC structures like walls, beams and slabs is
by increasing their flexural strength. While columns can be strengthened by providing
lateral confinement of the concrete which enhances the compressibility and ductility.
Catcher systems on the inside face of walls can be used to prevent fragments from
entering an occupied space. For this method, a fabric covers the entire surface of the
wall and is securely anchored at the floor and ceiling with just enough tension to
remove slack. Special arrangements must be made for load bearing walls as this does
not provide structural strength and for walls with windows as the fabric must span
continuously without interruption.
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2.2 LIMITATIONS
2.2.1 Since maintaining the appearance of architecturally and historically important
buildings is important while they are the ones that are liable of attacks as they are
usually government’s and corporate’s properties, increasing the dimension and
changing them into a fortress and bunkers will not be a good idea. Also, the building
should still be functional for its purpose and its maintenance should not be an
additional issue.
2.2.2 The blast resistance of a structure can be improved by increasing its mass and
strength with additional concrete and steel reinforcement. Unfortunately, this solution
can be expensive, add considerable gravity loads to the foundations of the structure
and require a significant amount of time to install.
2.2.3 The disadvantages of steel stud walls is the long installation and loss of floor
space.
2.2.4 Even though FRP have indicated to be a potential solution, they do come with
their own set of limitations. For example, in some situations, the excessively thin
sheets of the material require an impractical number of layers or wraps on the
structure to function effectively. Besides, in cases of close-in detonations, the strain
demand of the strengthening material is beyond the strain capacity of FRP (Malvar et
al. 2007). Another drawback of FRP strengthening is that it may lead to a premature
brittle failure, such as through FRP de-bonding and FRP concrete cover delamination
when subjected to such high intensity loading.
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CHAPTER 3
MATERIALS
3.1 POLYETHYLENE
3.1.1 Definition
Polyethylene is a synthetic polymer which is synthesised from a low molecular weight
compound. It is an important themoplasts formed by the polymerization of ethylene
and can be soften on heating and stiffen on cooling. It has a low molecular weights
unlike thermosetting polymers. Like all the other polymers, the strength is estimated
by means of stress-strain test.
3.1.2 Formation
As mentioned earlier, it is formed by the polymerisation of ethylene. Ethylene gas is
first liquefied under a high pressure of about 1500 atmospheres and then pumped into
a heated pressure vessel kept at 150 to 250o
C. Then by the catalytic effect of traces of
oxygen present, ethylene undergoes polymerization to give polyethylene which comes
out as a waxy solid through the perforation at the bottom of the vessel.
Chemical equation is as given below:-
nCH2 = CH2 Polymeruisation - (CH2-CH2) n-
Ethylene Polyethylene
If free radical initiator is used, low-density polyethylene is the product wheareas if
ionic catalysts are used high-density polyethylene is the product.
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3.1.3 Properties
Low density polyethylene is a rigid, waxy, white, translucent, nonpolar materials
having a specific gravity of 0.91 to 0.93. It is chemically inert and shows good
resistance to the action of acids and alkalies, salt solutions etc. It will not dissolve in
any solvent at room temperature but becomes slighty swollen in liquids like benzene
and carbon tetra chloride in which it is soluble at high temperatures. It is a good
insulator of electricity.
High density polyethylene possesses higher melting point, higher density (0.941-
0.965) and higher tensile strength. It is near crystalline polymer. It finds use in the
production of houseware toys, detergent bottles etc.
3.1.4 Review as reinforcing material
Since 1990’s, new synthetic fibers have prosperously been developed, which have
better mechanical characteristics than conventional fibers. Among the synthetic fibers,
polyethylene fiber having high molecular weight or high density polyethylene fiber is
expected to be utilized as a defensive material, because of its good balance between
the tensile strength and the elongation. Furthermore, this fibre has already been
utilized as a reinforcing material in engineered cementitious composites (ECC) with
pseudo strain hardening behaviour. The influences of mix proportion of concrete
matrix, shape of short fiber, and fiber volume fraction on slump and various
mechanical characteristics of polyethylene fiber reinforced concrete (PEFRC) was
studied and it was shown as a result that, PEFRC with higher flexural toughness than
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steel fiber reinforced concrete (SFRC) and sufficient slump for precast concrete could
be derived by applying high-fluidity matrix and binding short fiber to the PEFRC.
3.2 POLYUREA
Polyurea is a type of elastomer that is derived from the reaction product of an
isocyanate component and a syntheticresin blend component through step-growth
polymerization. The isocyanate can be aromatic or aliphatic in nature. It can be
monomer, polymer, or any variant reaction of isocyanates, quasi-prepolymer or a
prepolymer. The prepolymer, or quasi-polymer, can be made of an amine-terminated
polymer resins will not have any intentional hydroxyl moieties. Any hydroxyls are the
result of incomplete conversion to the amine-terminated polymer resins. The resin
blend may also contain additives, or non-primary components. These additives may
contain hydroxyls, such as pre-dispersed pigments in a polyol carrier. Normally, the
resin blend will not contain a catalysts.
General chemical reaction
O O
R R’ R R’
OCN NCO + H2N NH2 N N N N
H H H H n
Di-isocyanate Polyamine Polyurea
3.3 POLYURETHANE
Polyurethane is an elastic polymer which is obtained by the reaction of di-isocyanate
with a diol. It composed of a chain of organic units joined by carbamate (urethane)
links. Polyurethane polymers are formed by combining two bi- or higher
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functional monomers. One contains two or more isocyanate functional groups and the
other contains two or more hydroxyl groups. More complicated monomers are also
used.
The alcohol and the isocyanate groups combine to form a urethane linkage:
ROH + R'NCO → ROC(O)N(H)R' (R and R' are alkyl or aryl groups)
This combining process, sometimes called condensation, typically requires the
presence of a catalyst. Polyurethanes are used in the manufacture of flexible, high-
resilience foam seating; rigid foam insulation panels; microcellular foamseals and
gaskets; durable elastomeric wheels and tires; automotive suspension bushings;
electrical potting compounds; high performance adhesives; surface coatings and
surface sealants; synthetic fibers (e.g. spandex); carpet underlay; and hard-plastic
parts (i.e. for electronic instruments). Polyurethane is also used for the manufacture of
hose as it combines the best properties of both rubber and plastic.
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CHAPTER 4
EXPERIMENTAL INVESTIGATIONS
In designing a blast resistant structure, the technique of employing fiber reinforced
concrete as a slab material has been the most common approach along with the
technique of employing a fiber reinforced polymer composites or steel plates on the
area to be protected. A few decades has it been, since then that new synthetic fibers
have been developed prosperously. These synthetic fibers have better mechanical
characteristics than conventional fibers. Because of the good balance between the
tensile strength and the elongation, high molecular weight polyethylene fiber is
expected to be utilised as a defensive material.
An experimental investigations have been conducted for investigating the
applicability of polyethylene fiber reinforced concrete for use in blast resistant
structure. This test was conducted regarding the evaluation of the damage to PEFRC
slabs subjected to contact detonation.
4.1 EXPERIMENTAL INVESTIGATION
4.1.1 Materials and mix proportions
Binding polyethylene short fiber is used to make the PEFRC. A granulated blast
furnace slag and super plasticizer is used to compensate for the decrease in slump due
to the surface area effects of mixed fibers. High-early strength Portland cement is also
used, in view of the intended application of PEFRC to precast concrete walls. For
comparision a normal ready mixed concrete with a nominal strength of 30 MPa and a
specified slump of 18cm was employed.
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A table below, Table 1 shows the materials used for making PEFRC.
Table 1 Materials used for PEFRC
Series 1 Series 2 Series 3
CementHigh early strength
Portland cement
High early strength
Portland cement
High early
strength Portland
cement
Fine aggregate River sand River sand Crushed sand
Coarse aggregate Crushed sand Crushed sand Crushed sand
AdmixtureBlast furnace slag
Superplasticizer
Blast furnace slag
Superplasticizer
Blast furnace slag
Superplasticizer
Short fiber
High molecular
weight
polyethylene fiber
High molecular
weight
polyethylene fiber
High molecular
weight
polyethylene fiber
The mix proportions of the PEFRC are as shown in Table 2 below. Because of the
spall damage which is caused by the tensile stress wave being reflected from the back
side of the slab, the mechanical characteristic namely, flexural toughness is
considered important.
Table 2 Mix proportions of PEFRC
Series Vf
(%)
W/B
(%)
Sg/B
(%)
s/a
(%)
Unit weight(kg/m ) Sp/B
(%)
Pa/C
(%)C Sg W S G
1 4.0 33 50 65 488 488 325 550 339 0.40 0
2 2.0 33 50 65 488 488 325 565 341 0.25 0
4.0 33 50 65 488 488 325 565 341 0.50 0
3 4.0 33 5 65 49 490 323 562 310 0.80 0.01
Note: Vf – fiber volume fraction, W/B – water-binder ratio, Sg/B – blast furnace slag-
bnder ratio, s/a – sand percentage, C – cement, Sg – blast furnace slag, B(=C+ Sg) _
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binder, W – water, S – fine aggregate, G – coarse aggregate, Sp – superplasticizer, Pa
– powdered antifoamer.
For mixing the PEFRC, a forced double mixer is used: first, the cement, the blast
furnace slag, and aggregates were dry mixed for 15 seconds; secondly, the water,
superplasticizer andpowdered antifoamer (used in series 3) were added and mixed for
90 seconds; finally, the polyethylene short fibers were added and mixed for 3 minutes.
4.1.2 Test methods
The three specimens were prepared for a separate test with different measurement as
shown in Table 2. The specimens were cured for 14 days for PEFRC and 28 days for
normal concrete, and then cured in air until testing The tensile toughness of the
PEFRC was evaluated indirectly using the flexural toughness coefficient σb, which is
given by
σb=
Where Tb= an area under load-displacement curve until the displacement
reached 2.0mm in N.mm.
= displacement of 2.0mm
l = span length in mm
b = width of prism specimen
d = depth of prism specimen
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Table 3 Material test methods
Specimen configuration Number Measurement item
Compressive test 100mm dia x 200mm high
cylindrical specimen
3Compressive
Stress-Strain curve
Splitting tensile 100mm dia x 200mm high 3 Maximum load
Flexural test 100x100x400mm prism 3 Load-displacement
The material test results for the contact detonation tests are given in the table below,
i.e., Table 4. The values of the flexural toughness coefficient of the PEFRC were
distributed within the range of 5.96 to 10.3 Mpa.
Table 4 Thirteen types of specimen for contact detonation tests and material test
results.
Specimen Types of concrete T(mm)
W(g)
Slump
(cm) f c
(MPa) E
(GPa) f t
(MPa) f b
(MPa)
(MPa)
A Normal concrete 50 200 13.5 41.5 32.2 3.33 -
B Normal concrete 100 100 20.0 38.7 29.1 3.04 -
C Normal concrete 100 100 17.5 41.6 31.9 3.48 -
D Normal concrete 100 200 17.5 41.6 31.9 3.48 -
E PEFRC (Series
1,V f = 4.0 % )
100 100 6.5 57.8 21.5 8.85 11.2 10.3
F PEFRC (Series
2,V f = 2.0 % )
100 100 20.0 59.9 26.3 6.36 6.34 5.96
G PEFRC (Series
2,V f = 2.0 % )
100 200 20.0 59.9 26.3 6.36 6.34 5.96
H PEFRC (Series
2,V f = 4.0 % )
200 11.5 70.6 23.3 7.28 10.2 9.54
I PEFRC (Series
2,V f = 4.0 % )
200 18.5 59.4 24.3 7.94 9.37 8.69
J PEFRC (Series
2,V f = 4.0 % )
100 13.0 54.6 22.5 7.65 8.79 8.12
K PEFRC (Series
2,V f = 4.0 % )
200 13.0 54.6 22.5 7.65 8.79 8.12
L PEFRC (Series
3,V f = 4.0 % )
100 3.0 76.0 25.5 5.60 9.20 8.46
M PEFRC (Series
3,V f = 4.0 % )
200 3.0 76.0 25.5 5.60 9.20 8.46
Note: T - slab thickness , W – amount of explosives , f c – compressive strength , E –
Young’s modulus , f t – splitting tensile strength , f b – flexural strength , - flexural
toughness coefficient.
4.1.3 Specimen configuration
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For the test, a specimens of thirteen types were made. All the specimens have the
same dimension, i.e., 600mmx600mm. This dimensions is considered large enough
not to disturb the creation of the crater and spall. The slab thickness was changed to
50mm and 100mm . As for the reinforcement, a deformed steel bars SD295A D10 and
a polished steel bars 5mm diameter were used for 100mm thick and 50mm thick
respectively. The specimen were cured in wet conditions for 14 days and 28 days for
PEFRC and normal concrete respectively. Then it was cured in air until testing.
The configuration and bar arrangement of the slab specimen is as shown below
PEFRC or normal concrete
Reinforcement steel bars
Fig 1 Specimen configuration and bar arrangement
600mm
600mm 50 or 100mm
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REFERENCE:
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1. Makoto Yamaguchi, Kiyoshi Murakami, Koji Takeda and Yoshiyuki Mitsui,
“Blast resistance of Polyethylene Fibre Reinforced Concrete to contact
detonation”, Journal of Advanced Concrete Technology, Vol. 9, No. 1, 63-71,
February 2011.
2. Makoto Yamaguchi, Kiyoshi Murakami, Koji Takeda and Yoshiyuki Mitsui ,
“Blast Resistance of Double Layered Reinforced concrete slabs composed of
precast thin plates”, Journal of Advanced Concrete Technology, Vol. 9, No. 2,
177-191, June 2011.
3. S.N. Raman, T. Ngo and P. Mendis, “A Review on the use of Polymeric
coatings for Retrofitting of structural elements against blast effects”,
Electronic Journal of Structural Engineering, 11, 2011.
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