Blast Resistance structure

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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.

Blast Resistance structure

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