processes

Article Experimental and Numerical Study of Polymer-Retrofitted Masonry Walls under Gas Explosions

1, 1, 2 1, 1 Meng Gu †, Xiaodong Ling †, Hanxiang Wang , Anfeng Yu * and Guoxin Chen 1 State Key Laboratory of Safety and Control for Chemicals of SINOPEC Qingdao Research Institute of Safety Engineering, Qingdao 266101, China; [email protected] (M.G.); [email protected] (X.L.); [email protected] (G.C.) 2 College of Mechanical and Electronic Engineering, China University of Petroleum (Huadong), Qingdao 266580, China; [email protected] * Correspondence: [email protected]; Tel.: +86-0532-8378-6327 These authors contributed equally to this work. †


Received: 24 October 2019; Accepted: 14 November 2019; Published: 20 November 2019


Abstract: Unreinforced masonry walls are extensively used in the petrochemical industry and they are one of the most vulnerable components to blast loads. To investigate the failure modes and improve the blast resistances of masonry walls, four full-scale field tests were conducted using unreinforced and spray-on polyurea-reinforced masonry walls subjected to gas explosions. The results suggested that the primary damage of the unreinforced masonry wall was flexural deformation and the wall collapsed at the latter stage of gas explosion. The presence of polyurea coatings could effectively improve the anti-explosion abilities of masonry walls, prevent wall collapses, and retain the flying fragments, which would reduce the casualties and economic losses caused by petrochemical explosion accidents. The bond between the polymer and masonry wall was critical, and premature debonding resulted in a failure of the coating to exert the maximum energy absorption effect. A numerical model for masonry walls was developed in ANSYS/LS-Dyna and validated with the test data. Parametric studies were conducted to explore the influences of the polyurea-coating thickness and spray pattern on the performances of masonry walls. The polyurea-coating thickness and spray pattern affected the resistance capacities of masonry walls significantly.

Keywords: polyurea; gas explosion; masonry wall; field test

1. Introduction Most materials involved in petrochemical plants are flammable and explosive hydrocarbon liquids and gases. Leakage can lead to vapor cloud or gas explosions, which creates a great hazard to existing structures and factory staff. Meanwhile, for historical reasons, unreinforced masonry walls have been extensively used in the petrochemical industry as non-load-bearing components in reinforced-concrete frame buildings, and they are one of the most vulnerable components to blast loads. Most casualties and injuries sustained during external explosions are not caused by the direct effect of the explosion, but rather by the disintegration and fragmentation of a wall, which can be propelled at high velocities by the blast [1]. To reduce the potential hazards to employee safety, it is necessary to study the performances of masonry walls under explosions and find a cost-effective strengthening method. Due to the potential threats of terrorist attacks and accidental explosions, many researchers have focused on the behaviors and dynamic responses of masonry structures under blast loads. Shi et al. [2] conducted a series of field experiments to investigate the failure mechanisms and fragment characteristics of unreinforced masonry walls. The fragment size distribution was also analyzed, and

Processes 2019, 7, 863; doi:10.3390/pr7120863 www.mdpi.com/journal/processes Processes 2019, 7, 863 2 of 18 the size distribution of the fine portion of the fragments followed a Weibull distribution. Keys and Clubley [3] carried out two experimental trials using trinitrotoluene (TNT) explosives, which were used to subject nine small masonry structures to blast loading. They described the structural geometry as an array of simple modular panels and proposed a new method to predict the debris distribution produced by masonry structures. Zapata and Weggel [4] conducted a blast test inside a building with conventional walls. Based on the test data, a single degree of freedom (SDOF) model of a masonry wall was established. The SDOF model was able to predict the deflections of the wall quite well if effective input parameters were carefully considered. Solveig et al. [5] employed a full-scale experiment on a three-story reinforced concrete building exposed to an air blast to evaluate their finite element (FE) simulations. They concluded that the FE simulations with solid and structural elements could provide an adequate representation of the global response of the building at minimum computational cost. The resistance of a masonry wall to blast loads can be enhanced by increasing the equivalent section and ductility of the wall with additional reinforcement materials, including a steel wire mesh, spray-on polyurea, carbon, and glass fiber-composites [6–12]. However, it is important that upgrade costs include not only construction costs but also the potential downtime costs due to the interruption of operations. These costs can increase significantly if the upgrades affect the normal production significantly. The application of stiff materials faces difficulties of high development costs and lack of time-efficient methods for applying the reinforcement to the structure. The masonry wall tests conducted by the Air Force Research Laboratory (AFRL) indicated that the spray-on polymer approach could overcome these issues, and the ability of the material to absorb strain energy was a key factor in the effectiveness toward preventing intrusion of wall fragments into occupant areas [13,14]. A series of impact tests were performed by Wang et al. [15] to investigate the dynamic impact responses of walls strengthened with spray-on polyurea. The test results suggested that the presence of a polyurea layer could transform the collapse of an unreinforced wall into local mortar joint separation and the development of flexure in the walls. The polyurea layer could effectively prevent the collapse and structural failure of the wall and minimize the production of deadly wall fragments. Based on a series of full-scale tests, Moradi [16] developed a single-degree-of-freedom equation to predict the response of membrane-retrofitted concrete masonry walls to impulse pressure loads. The masonry wall subjected to out-of-plane loads (static, high explosive loads, and gas explosions) were studied by conducting field tests, numerical simulations, and theoretical analysis, as summarized in Table1. The above-mentioned studies focused on the performances of polymer-retrofitted masonry walls under the blast loads generated by high explosives. Li et al. [17] only investigated the performances of unreinforced masonry walls subjected to vented gas explosions. Studies of spray-on polyurea reinforced masonry walls subjected to gas explosions are very limited in the literature.

Table 1. Summary of previous studies on the masonry wall strengthening. RC: reinforced concrete; CMU: concrete masonry unit; AAC: autoclaved aerated concrete; SDOP: single degree of freedom; AEM: applied element method; CFRP: carbon fiber reinforced polymer; GFRP: glass fiber reinforced polymer

Strengthening Researchers Year Structures Experimental Numerical Theoretical Material Zapata and Weggel [4] 2008 Brick walls Explosive SDOF − Shi et al. [2] 2016 Clay brick walls Explosive YES − London brick Keys and Clubley [3] 2017 Explosive AEM walls − Li et al. [17] 2017 Clay brick walls Gas explosion LS-DYNA − RC walls, Leca ConWep and Solveig et al. [5] 2018 Explosive block walls − LS-DYNA Brick masonry Valluzzi et al. [7] 2001 GFRP, CFRP Static YES vaults Davidson et al. [13,14] 2005 CMU walls Sprayed-on polymer Explosive DYNA3D Alsayed et al. [8] 2016 CMU walls GFRP Explosive AUTODYN Elsanadedy et al. [9] 2016 CMU walls GFRP Static YES Wang et al. [15] 2016 Clay brick, AAC Sprayed-on polymer Explosive CFRP/sprayed-on chen et al. [10] 2019 AAC panels Explosive polymer Processes 2019, 7, 863 3 of 19

Table 1. Summary of previous studies on the masonry wall strengthening. RC: reinforced concrete; CMU: concrete masonry unit; AAC: autoclaved aerated concrete; SDOP: single degree of freedom; AEM: applied element method; CFRP: carbon fiber reinforced polymer; GFRP: glass fiber reinforced polymer

Strengthening Researchers Year Structures Experimental Numerical Theoretical Material Zapata and 2008 Brick walls − Explosive SDOF Weggel [4] Clay brick Shi et al. [2] 2016 − Explosive YES walls Keys and London 2017 − Explosive AEM Clubley [3] brick walls Clay brick Li et al. [17] 2017 − Gas explosion LS-DYNA walls RC walls, Solveig et al. ConWep and 2018 Leca block − Explosive [5] LS-DYNA walls Brick Valluzzi et al. 2001 masonry GFRP, CFRP Static YES [7] vaults Davidson et al. Sprayed-on 2005 CMU walls Explosive DYNA3D [13,14] polymer Alsayed et al. 2016 CMU walls GFRP Explosive AUTODYN [8] Elsanadedy et 2016 CMU walls GFRP Static YES al. [9] Wang et al. Clay brick, Sprayed-on 2016 Explosive [15] AAC polymer Processes 2019, 7, 863 CFRP/sprayed- 3 of 18 chen et al. [10] 2019 AAC panels Explosive on polymer In this study, a series of full-scale field tests on masonry walls subjected to gas explosions In this study, a series of full-scale field tests on masonry walls subjected to gas explosions were were conducted. A simplified micro-model was also developed to predict the dynamic responses conducted. A simplified micro-model was also developed to predict the dynamic responses of of polymer-retrofitted masonry walls using ANSYS/LS-DYNA. The predictions from the numerical polymer-retrofitted masonry walls using ANSYS/LS-DYNA. The predictions from the numerical simulations were compared with the test data to validate the numerical model. Parametric studies simulations were compared with the test data to validate the numerical model. Parametric studies were carried out to evaluate the effects of the polyurea layer thickness and the spraying pattern on the were carried out to evaluate the effects of the polyurea layer thickness and the spraying pattern on responses of strengthened masonry walls subjected to gas explosions. the responses of strengthened masonry walls subjected to gas explosions. 2. Experiments 2. Experiments A specially designed testing system was prepared for the gas explosion tests. The test system consistedA specially of a pipeline, designed wall specimens,testing system data-recording was prepared instruments, for the gas and explosion gas distribution tests. The instruments, test system asconsisted shown in Figureof a 1pipeline,. wall specimens, data-recording instruments, and gas distribution instruments, as shown in Figure 1.

FigureFigure 1. 1.Schematic Schematic diagram diagram of of the the testing testing setup. setup. Processes 2019, 7, 863 4 of 19 2.1. Wall Specimens 2.1. Wall Specimens One unstrengthened specimen and one rear-face-strengthened specimen were prepared. According to theOne Chinese unstrengthened standard (GB50003-2011), specimen and MU one 15 clayrear-f bricksace-strengthened and M 5.0 mortar specimen were usedwere forprepared. the clay brickAccording walls. to The the wall Chinese specimens standard with (GB50003-2011), dimensions of 3 MU2 150.12 clay m bricks were lumpedand M 5.0 with mortar clay blocks were used with × × dimensionsfor the clay brick of 240 walls.115 The53 wall mm specimens and 10-mm-thick with dimensions mortar in a of “running” 3 × 2 × 0.12 pattern, m were as lumped shown in with Figure clay2. × × Largeblocks wallwith dimensions dimensions were of 240 selected × 115 to× 53 avoid mm scale and e10-mm-thickffects and to reflectmortar thein actuala “running” performance pattern, of as a reinforcedshown in Figure masonry 2. Large unit wallwall underdimensions blast loadingwere selected [15]. to avoid scale effects and to reflect the actual performance of a reinforced masonry unit wall under blast loading [15].

Figure 2. Wall specimens.

2.2. Gas Distribution Instruments To maximize the explosion overpressure, ethylene with a volume concentration of 5.5% was chosen as the combustible gas in the tests. The oxygen concentration in the mixture was measured by an oxygen content analyzer, and the ratio of fuel gas to air was adjusted over time to ensure uniform mixing of the flue gas and air.

2.3. Device Main Body The device main body included a pipeline and flaring, as shown in Figures 3 and 4, respectively. The pipeline was used to store gas. Its diameter and length were 500 mm and 50 m, respectively. To create a more uniform explosion load on the wall surface, the flaring, with dimensions of 2 × 2 m, was installed at the end of the pipeline.

Figure 3. Pipe.

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2.1. Wall Specimens One unstrengthened specimen and one rear-face-strengthened specimen were prepared. According to the Chinese standard (GB50003-2011), MU 15 clay bricks and M 5.0 mortar were used for the clay brick walls. The wall specimens with dimensions of 3 × 2 × 0.12 m were lumped with clay blocks with dimensions of 240 × 115 × 53 mm and 10-mm-thick mortar in a “running” pattern, as shown in Figure 2. Large wall dimensions were selected to avoid scale effects and to reflect the actual performance of a reinforced masonry unit wall under blast loading [15].

Processes 2019, 7, 863 Figure 2. Wall specimens. 4 of 18

2.2. Gas Distribution Instruments 2.2. Gas Distribution Instruments To maximize the explosion overpressure, ethylene with a volume concentration of 5.5% was To maximize the explosion overpressure, ethylene with a volume concentration of 5.5% was chosen as the combustible gas in the tests. The oxygen concentration in the mixture was measured by chosen as the combustible gas in the tests. The oxygen concentration in the mixture was measured by an oxygen content analyzer, and the ratio of fuel gas to air was adjusted over time to ensure uniform an oxygen content analyzer, and the ratio of fuel gas to air was adjusted over time to ensure uniform mixing of the flue gas and air. mixing of the flue gas and air.

2.3. Device Main Body The device main body included a pipeline pipeline and and flaring, flaring, as as shown shown in in Figures Figures 3 and 44,, respectively.respectively. The pipeline was was used used to to store store gas. gas. Its Its diameter diameter and and length length were were 500 500 mm mm and and 50 m, 50 respectively. m, respectively. To Tocreate create a more a more uniform uniform explosion explosion load load on the on thewall wall surf surface,ace, the flaring, the flaring, with with dimensions dimensions of 2 of× 2 2 m, 2was m, × wasinstalled installed at the at end the endof the of pipeline. the pipeline.

Processes 2019, 7, 863 Figure 3. Pipe. 5 of 19 Figure 3. Pipe.

FigureFigure 4.4. Flaring. 2.4. Data-Recording Instruments 2.4. Data-Recording Instruments The data-measuring system was used to record the explosion overpressure, displacements, and The data-measuring system was used to record the explosion overpressure, displacements, and wall failure modes. It was composed of a synchronous trigger instrument, data acquisition instrument, wall failure modes. It was composed of a synchronous trigger instrument, data acquisition piezoelectric pressure sensors, displacement sensor, and high-speed cameras. The pressure sensors instrument, piezoelectric pressure sensors, displacement sensor, and high-speed cameras. The were used to measure the peak overpressure and positive pressure acting time of the explosion pressure sensors were used to measure the peak overpressure and positive pressure acting time of shockwaves, as shown in Figure4. The measurement range was 0–10 MPa. The displacement sensor the explosion shockwaves, as shown in Figure 4. The measurement range was 0–10 MPa. The was used to measure the displacement of the central points of the test walls, as shown in Figure5, with displacement sensor was used to measure the displacement of the central points of the test walls, as a measurement range of 5 to +5 cm. High-speed cameras were arranged on the left and right sides shown in Figure 5, with a− measurement range of −5 to +5 cm. High-speed cameras were arranged on of the test walls to record the failure process of the wall during the test, and the speed of high-speed the left and right sides of the test walls to record the failure process of the wall during the test, and cameras were 6400 frames. The signals from the pressure sensor and displacement transducers were the speed of high-speed cameras were 6400 frames. The signals from the pressure sensor and logged by the data acquisition instrument. displacement transducers were logged by the data acquisition instrument.

Figure 5. Arrangement of the displacement sensor.

2.5. Test Procedure The supporting structure was designed to fix the test walls on the ground. All the specimens were mounted 1 m away from the flare. The pressure sensors were installed at the end of the pipeline, and the displacement sensor was arranged at the center of the rear surface of the wall. The spark igniter was installed on the pipe inlet flange, which meant that the igniter was at the edge of the gas cloud. The ignition voltage of spark igniter was 1000 V. The ethylene was pumped from the cylinder gas tank to the pipeline and flaring until the target concentration was reached. Finally, the gas mixture was ignited by a high-voltage power source. The data acquisition system and high-speed cameras were triggered to record data simultaneously.

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Figure 4. Flaring.

2.4. Data-Recording Instruments The data-measuring system was used to record the explosion overpressure, displacements, and wall failure modes. It was composed of a synchronous trigger instrument, data acquisition instrument, piezoelectric pressure sensors, displacement sensor, and high-speed cameras. The pressure sensors were used to measure the peak overpressure and positive pressure acting time of the explosion shockwaves, as shown in Figure 4. The measurement range was 0–10 MPa. The displacement sensor was used to measure the displacement of the central points of the test walls, as shown in Figure 5, with a measurement range of −5 to +5 cm. High-speed cameras were arranged on the left and right sides of the test walls to record the failure process of the wall during the test, and the speed of high-speed cameras were 6400 frames. The signals from the pressure sensor and Processes 2019, 7, 863 5 of 18 displacement transducers were logged by the data acquisition instrument.

Figure 5. ArrangementArrangement of the displacement sensor. 2.5. Test Procedure 2.5. Test Procedure The supporting structure was designed to fix the test walls on the ground. All the specimens were The supporting structure was designed to fix the test walls on the ground. All the specimens mounted 1 m away from the flare. The pressure sensors were installed at the end of the pipeline, and were mounted 1 m away from the flare. The pressure sensors were installed at the end of the pipeline, the displacement sensor was arranged at the center of the rear surface of the wall. The spark igniter and the displacement sensor was arranged at the center of the rear surface of the wall. The spark was installed on the pipe inlet flange, which meant that the igniter was at the edge of the gas cloud. igniter was installed on the pipe inlet flange, which meant that the igniter was at the edge of the gas The ignition voltage of spark igniter was 1000 V. The ethylene was pumped from the cylinder gas tank cloud. The ignition voltage of spark igniter was 1000 V. The ethylene was pumped from the cylinder to the pipeline and flaring until the target concentration was reached. Finally, the gas mixture was gas tank to the pipeline and flaring until the target concentration was reached. Finally, the gas ignited by a high-voltage power source. The data acquisition system and high-speed cameras were mixture was ignited by a high-voltage power source. The data acquisition system and high-speed triggered to record data simultaneously. cameras were triggered to record data simultaneously. 3. Testing Results and Discussion

3.1. Unreinforced Wall Explosion tests of the unreinforced masonry wall were conducted to analyze the failure mode and anti-explosion capacity of the wall. The test results of the masonry wall without an explosion-resistant coating under the first explosion (test 1) are shown in Figure6. As shown in Figure6a, large-area concave deformations appeared on the explosion face of the unreinforced wall, and the deformation was almost symmetric. The compression failure of the unstrengthened masonry wall was caused by the high reflected peak pressure. A -shaped penetrating crack appeared in the upper half of the wall, as ⊥ shown in Figure6b. The fracture lengths in the horizontal and vertical directions were approximately 1.8 and 1.4 m, respectively, and the widths of cracks were about 25 mm. Bricks almost fell from the central area of the back blasting surface, and cracks developed radially from the center of the wall to the surrounding area. The wall was severely damaged, which indicated that the explosive load was close to the ultimate load-bearing capacity of the unreinforced wall. The blast load time histories were also recorded during the test, and Figure7 shows that the peak overpressure of the explosive shockwave was about 5.5 MPa. However, no displacement data were obtained due to the mechanical failure. ProcessesProcesses 2019 2019, ,7 7, ,863 863 66 of of 19 19

3.3. Testing Testing Results Results and and Discussion Discussion

3.1.3.1. Unreinforced Unreinforced Wall Wall ExplosionExplosion tests tests of of the the unreinforced unreinforced masonry masonry wall wall were were conducted conducted to to analyze analyze the the failure failure mode mode andand anti-explosion anti-explosion capacity capacity of of the the wall. wall. The The test test results results of of the the masonry masonry wall wall without without an an explosion- explosion- resistantresistant coating coating under under the the first first explosion explosion (test (test 1) 1) are are shown shown in in Figure Figure 6. 6. As As shown shown in in Figure Figure 6a, 6a, large- large- areaarea concaveconcave deformationsdeformations appearedappeared onon thethe explosionexplosion faceface ofof thethe unreinforcedunreinforced wall,wall, andand thethe deformationdeformation was was almost almost symmetric. symmetric. The The compress compressionion failure failure of of the the unstrengthened unstrengthened masonry masonry wall wall waswas caused caused by by the the high high reflected reflected peak peak pressure. pressure. A A ⊥ ⊥-shaped-shaped penetrating penetrating crack crack appeared appeared in in the the upper upper halfhalf of of the the wall, wall, as as shown shown in in Figure Figure 6b. 6b. The The fracture fracture lengths lengths in in the the horizontal horizontal and and vertical vertical directions directions werewere approximately approximately 1.8 1.8 and and 1.4 1.4 m, m, respectively, respectively, and and the the widths widths of of cracks cracks were were about about 25 25 mm. mm. Bricks Bricks almostalmost fell fell from from the the central central area area of of the the back back blasting blasting surface, surface, and and cracks cracks developed developed radially radially from from the the centercenter of of the the wall wall to to the the surrounding surrounding area. area. The The wa wallll was was severely severely damaged, damaged, which which indicated indicated that that the the explosiveexplosive load load was was close close to to the the ultimate ultimate load-bearing load-bearing capacity capacity of of the the unreinforced unreinforced wall. wall. The The blast blast load load timetime histories histories were were also also recorded recorded during during the the test, test, and and Figure Figure 7 7 shows shows that that the the peak peak overpressure overpressure of of theProcessesthe explosive explosive2019, 7 ,shockwave 863shockwave was was about about 5.5 5.5 MPa. MPa. However, However, no no displacement displacement data data were were obtained obtained due due6 of to to 18 thethe mechanical mechanical failure. failure.

FigureFigure 6. 6. Damage Damage of of unstrengthened unstrengthened specimen specimen in in test test 1. 1. ( a( a) )front front face face and and ( b( b) )rear rear face. face.

FigureFigure 7. 7. Pressure-time Pressure-time histories histories in in test test 1. 1.

ToTo study studystudy the thethe distribution distributiondistribution of ofof wall wallwall debris debrisdebris under underunder ex explosiveexplosiveplosive loading, loading, subsequent subsequent tests tests were were carried carried outout on on the the wall. wall. The The peak peak overpressure overpressure of of the the ex explosiveexplosiveplosive shockwave shockwave was was about about 5.2 5.2 MPa, MPa, as as shown shown in in FigureFigure 8.8 8.. TheThe failure failure process process ofof of thethe the unstrengthenedunstrengthened unstrengthened wall wall wall specimen specimen specimen during during during the the the second second second explosion explosion explosion (test (test (test 2) 2)is2) is shownis shown shown in in Figurein Figure Figure9. Compared9. 9. Compared Compared with with with the the teststhe tests tests of high of of high high explosives explosives explosives in Table in in Table Table1, the 1, peak1, the the pressurespeakpeak pressures pressures from fromgasfrom explosions gasgas explosionsexplosions were were lowerwere lower butlower the butbut duration thethe durationduration of pressure ofof pressurepressure waves werewaveswaves longer, werewere whichlonger,longer,lead whichwhich to di leadleadfferent toto differentstructuraldifferent structural structural responses. responses. responses. Local damages Local Local damages damages such as spallingsuch such as as spalling orspalling punching or or punching punching damage da weredamagemage more were were likely more more to likely likely occur towhento occur occur the when when masonry the the masonry masonry walls were walls walls under were were close-in under under clos TNTclose-ine-in explosions TNT TNT explosions explosions as compared as as compared compared to the overall to to the the flexuraloveralloverall Processes 2019, 7, 863 7 of 19 fldeformationsflexuralexural deformations deformations in gas explosions. in in gas gas explosions. explosions. The test The resultsThe test test were results results similar were were to similar similar those carriedto to those those out carried carried by Li out etout al.by by [ 17Li Li ],et et wall al. al. specimens experienced damage modes of flexural deformation and a through crack occurred at the [17], wall specimens experienced damage modes of flexural deformation and a through crack middleoccurred of at the the walls. middle The crackof the developed walls. The with crack the developed rise of pressure with and the therise wall of pressure specimens and collapsed the wall at thespecimens latter stage collapsed of gas at explosion. the latter stage of gas explosion.

Figure 8. Pressure-time histories in test 2.

Figure 9. Failure process of unstrengthened wall specimen in test 2.

The fragment distributions of the wall specimen after the gas explosions are shown in Figure 10. The wall collapsed and broke along the bottom, top, and left joints of the frame, with only a 0.4-m- wide wall remaining on the right side. The debris was dispersed in an area of approximately 7.6 m in length and 5 m in width. Masonry walls are often used as filling walls of frame structures. The debris produced by a masonry wall under a blast wave can cause severe injuries to indoor personnel.

Processes 2019, 7, 863 7 of 19

[17], wall specimens experienced damage modes of flexural deformation and a through crack occurred at the middle of the walls. The crack developed with the rise of pressure and the wall specimens collapsed at the latter stage of gas explosion.

Processes 2019, 7, 863 7 of 18 Figure 8. Pressure-time histories in test 2.

Figure 9. Failure process of unstrengthened wall specimen in test 2. Figure 9. Failure process of unstrengthened wall specimen in test 2. The fragment distributions of the wall specimen after the gas explosions are shown in Figure 10. The wallThe fragment collapsed distributions and broke along of the the wall bottom, specimen top, and after left the joints gas of explosions the frame, are with shown only ain 0.4-m-wide Figure 10. Thewall wall remaining collapsed on and the rightbroke side. along The the debrisbottom, was top, dispersed and left joints in an of area the offrame, approximately with only a 7.6 0.4-m- m in widelength wall and remaining 5 m in width. on the Masonry right side. walls The are debris often was used dispersed as filling wallsin an area of frame of approximately structures. The 7.6 debris m in lengthproducedProcesses and 2019 by 5, 7 m,a 863 masonryin width. wallMasonry under walls a blast are wave often canused cause as filling severe walls injuries of fram to indoore structures. personnel. The debris8 of 19 produced by a masonry wall under a blast wave can cause severe injuries to indoor personnel.

Figure 10. Distribution of debris after the failurefailure of the unstrengthened wall.

3.2. Polymer-Retro Polymer-Retrofittedfitted Wall Test 33 waswas carriedcarried outout to investigate the eeffectivffectivenesseness ofof the polyurea layer and the performance of a polymer-retrofittedpolymer-retrofitted wallwall subjectedsubjected toto gasgas explosions.explosions. The thickness of the polyurea layer on the rear face waswas 66 mm.mm. The dynamic response of the rear-face-strengthened wall specimen is shown in Figure 1111..

Figure 11. Structural response of rear-face-strengthened wall specimen in test 3.

Figure 12 shows photos of the damage of the rear-face-strengthened wall after the explosion. The wall was concave and deformed slightly, with X-shaped cracks appearing in the central area of the front face. The maximum plastic deformation at the bottom was about 0.06 m. The polyurea coating on the back blasting surface was intact, and no cracks formed. This suggested that the presence of the polyurea layer could effectively improve the blast resistance of the unreinforced wall and reduce the wall failure to local bending deformation.

Processes 2019, 7, 863 8 of 19

Figure 10. Distribution of debris after the failure of the unstrengthened wall.

3.2. Polymer-Retrofitted Wall Test 3 was carried out to investigate the effectiveness of the polyurea layer and the performance of a polymer-retrofitted wall subjected to gas explosions. The thickness of the polyurea layer on the Processesrear face2019 was, 7, 8636 mm. The dynamic response of the rear-face-strengthened wall specimen is shown8 of in 18 Figure 11.

Figure 11. Structural response of rear-face-strengthened wall specimen in test 3. Figure 11. Structural response of rear-face-strengthened wall specimen in test 3. Figure 12 shows photos of the damage of the rear-face-strengthened wall after the explosion. The wallFigure was 12 concave shows andphotos deformed of the slightly,damage withof the X-shaped rear-face-strengthened cracks appearing wall in the after central the explosion. area of the Thefront wall face. was The concave maximum and plastic deformed deformation slightly, atwith the X-shaped bottom was cracks about appearing 0.06 m. Thein the polyurea central coatingarea of theon thefront back face. blasting The maximum surface was plastic intact, deformation and no cracks at formed. the bottom This was suggested about that0.06 the m. presence The polyurea of the polyureacoating on layer the couldback eblastingffectively surface improve was the intact, blast resistanceand no cracks of the formed. unreinforced This wallsuggested and reduce that the wall failure to local bending deformation. presenceProcesses 2019 of, the7, 863 polyurea layer could effectively improve the blast resistance of the unreinforced9 wall of 19 and reduce the wall failure to local bending deformation.

Figure 12. Local damage of rear-face-strengthened specimen in testtest 3.3. (a) front face and ((b)b) rear face.

As shown in Figures 13 and 1414,, the peak overpr overpressureessure in tests 3 and 4 was 6.0 and 5.5 MPa, respectively. The The peak peak pressures pressures in in all all the the tests tests we werere basically basically the the same, same, which which indicated indicated that that the gas the gasmixture mixture was wasrelatively relatively uniform uniform during during each eachtest. Fi test.gure Figure 15 shows 15 shows photograph photographss of the ofdamage the damage to the tomasonry the masonry wall after wall aftertest test4. The 4. Thereinforced reinforced bric brickk wall wall collapsed. collapsed. However, However, compared compared to to the unreinforced wall, wall, the the traveling traveling distance distance of of the the debris debris was was significantly significantly reduced, reduced, and andthe debris the debris was wasalmost almost completely completely distributed distributed in the in framework the framework of the ofsupporting the supporting body. The body. polyurea The polyurea coating coating peeled peeledoff along off thealong left, theright, left, and right, lower and edges lower of edgesthe fram ofe. the The frame. coatings The sprayed coatings on sprayed the upper on part the upperof the partframe of were the framepartially were torn, partially whereas torn, the whereasremaining the coatings remaining were coatings basically were intact. basically The polyurea intact. layer The polyureacould absorb layer the could explosive absorb energy, the explosive thereby energy,reducing thereby the kinetic reducing energy the of kinetic the debris energy and of preventing the debris andhigh-speed preventing wall high-speed fragments wallfrom fragments harming fromindoor harming personnel. indoor However, personnel. in However,this test, inpremature this test, debonding of the coating occurred, and the wall failed to produce the maximum effect of the coating. The temperature on the day of coating spraying was 9–12 °C, the wind force level was 4–5, and the humidity was about 52%. This may have been due to the low temperature of the outdoor spraying operation, causing the coating to not adhere to the frame well. The spraying operation should be carried out under suitable temperature and humidity conditions to improve the adhesion of the coating. Furthermore, steel angle can also be used to reinforce the coating around the frame to maximize the energy absorption effect of the coating. In order to avoid the loss of displacement data during violent explosion, non-contact digital image correlation (DIC) technology can be used to carry out the real-time monitoring of the surface displacement of the wall in future experiments.

Figure 13. Pressure-time histories in test 3.

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Figure 12. Local damage of rear-face-strengthened specimen in test 3. (a) front face and (b) rear face.

As shown in Figures 13 and 14, the peak overpressure in tests 3 and 4 was 6.0 and 5.5 MPa, respectively. The peak pressures in all the tests were basically the same, which indicated that the gas mixture was relatively uniform during each test. Figure 15 shows photographs of the damage to the masonry wall after test 4. The reinforced brick wall collapsed. However, compared to the unreinforced wall, the traveling distance of the debris was significantly reduced, and the debris was almost completely distributed in the framework of the supporting body. The polyurea coating peeled off along the left, right, and lower edges of the frame. The coatings sprayed on the upper part of the

Processesframe were2019, 7partially, 863 torn, whereas the remaining coatings were basically intact. The polyurea 9layer of 18 could absorb the explosive energy, thereby reducing the kinetic energy of the debris and preventing high-speed wall fragments from harming indoor personnel. However, in this test, premature prematuredebonding debondingof the coating of theoccurred, coating and occurred, the wall and fail theed to wall produce failed the to produce maximum the effect maximum of the ecoating.ffect of theThe coating.temperature The temperatureon the day of on coating the day spraying of coating was spraying 9–12 °C, was the 9–12wind◦ forceC, the level wind was force 4–5, level and was the 4–5,humidity and the was humidity about 52%. was This about may 52%. have This been may due have to been the duelow totemperature the low temperature of the outdoor of the spraying outdoor sprayingoperation, operation, causing the causing coating the to coating not adhere to not to adherethe frame to thewell. frame The well.spraying The operation spraying should operation be shouldcarried beout carried under out suitable under suitabletemperature temperature and humidi and humidityty conditions conditions to improve to improve the adhesion the adhesion of the of thecoating. coating. Furthermore, Furthermore, steel steel angle angle can can also also be beus useded to toreinforce reinforce the the coating coating around around the the frame to maximize the energy absorption eeffeffectct ofof thethe coating.coating. In order to avoid the loss of displacement data during violent explosion, non-contact digital image correlation (DIC) technology can be used to carry out the real-time monitoringmonitoring ofof thethe surfacesurface displacementdisplacement ofof thethe wallwall inin futurefuture experiments.experiments.

Processes 2019,, 77,, 863863 Figure 13. Pressure-time histories in test 3. 10 of 19

Figure 14. Pressure-time histories in test 4.

Figure 15. Failure of rear-face-strengthened specimen in test 4. (a) front face and (b) rear face. Figure 15. Failure of rear-face-strengthened specimen in test 4. (a) front face and (b) rear face.

4. Numerical Model

4.1. Material Parameters The anisotropic material model *Mat _96 (MAT BRITTLE DAM-AGE) in LS_DYNA has been used and validated in many studies to model a variety of brittle materials under explosions. Therefore, it was adopted for the clay bricks and mortar in this study. The parameters of the blocks and mortar used in the numerical simulations are listed in Table 2 [15,17,18–20]. The MAT PIECEWISE LINEAR PLASTICITY material model was used for the polyurea coating. As provided by the supplier, the composite material had a tensile strength of 15.4 MPa,MPa, breakingbreaking elongationelongation ratioratio of 200%, and tear resistance of 105 N/m. The mechanical properties of the composite material are summarized in Table 3 [21].

Table 2. Material parameters for brick and mortar.

Young Tensile Shear Compressive Fracture Shear Density Poisson’s Modulus Strength Strength Yield Stress Toughness Retention (kg/m33) Ratio (MPa) (MPa) (MPa) (MPa) (N/m) Factor Brick 1800 15,400 0.15 1 0.5 19.4 120 0.03 Morta 2100 4450 0.25 0.297 0.9 8.9 140 0.03 r

Table 3. Material parameters of blast-resistant polyurea coating.

Tensile Tear Density Elasticity Poisson’s Ratio Strength Resistance Elongation (%) (kg/m³) Modulus (MPa) (MPa) (N/m) 1000 100 0.4 15.4 105 200

Processes 2019, 7, 863 10 of 18

4. Numerical Model

4.1. Material Parameters The anisotropic material model *Mat _96 (MAT BRITTLE DAM-AGE) in LS_DYNA has been used and validated in many studies to model a variety of brittle materials under explosions. Therefore, it was adopted for the clay bricks and mortar in this study. The parameters of the blocks and mortar used in the numerical simulations are listed in Table2[ 15,17–20]. The MAT PIECEWISE LINEAR PLASTICITY material model was used for the polyurea coating. As provided by the supplier, the composite material had a tensile strength of 15.4 MPa, breaking elongation ratio of 200%, and tear resistance of 105 N/m. The mechanical properties of the composite material are summarized in Table3[21].

Table 2. Material parameters for brick and mortar.

Young Tensile Shear Compressive Fracture Shear Density Poisson’s Modulus Strength Strength Yield Stress Toughness Retention (kg/m3) Ratio (MPa) (MPa) (MPa) (MPa) (N/m) Factor Brick 1800 15,400 0.15 1 0.5 19.4 120 0.03 Mortar 2100 4450 0.25 0.297 0.9 8.9 140 0.03

Table 3. Material parameters of blast-resistant polyurea coating.

Density Elasticity Modulus Poisson’s Tensile Strength Tear Resistance Elongation (kg/m3) (MPa) Ratio (MPa) (N/m) (%) 1000 100 0.4 15.4 105 200

4.2. Geometric Model The simplified micro-models for the masonry walls were developed in LS-DYNA, and the mortar, blocks, and polyurea layer were modeled separately. The nodes on their interfaces were merged. The wall model dimensions were a thickness of 120 mm, height of 2 m, and width of 3 m to match the tests described above, as shown in Figure2. The sizes of blocks were 240 115 53 mm, and the thicknesses × × of the mortar and polyurea layer were 10 and 6 mm, respectively, identical to the thicknesses in the test specimens. The three-dimensional solid element SOLID164 was used to discretize the geometrical model. Considering the computational accuracy and efficiency, the combination of a 20-mm mesh size for the blocks and one layer for the mortar joint and polyurea coating was adopted for the numerical model.

4.3. Numerical Model Calibration In the tests, the pressure sensor was located at the end of the pipeline. To obtain the explosion overpressure value at the wall, after the tests, a pressure sensor with a range of 0–1 MPa was also arranged at a distance of 1 m from the flaring. The sensor was destroyed in the first measurement and reached the maximum range in the second measurement, but it was not destroyed. According to the information provided by the supplier, the damage pressure of the sensor was 2 MPa, and the positive-pressure-action time measured by the sensor at the end of the pipeline was about 2 ms. These were used as the explosion loads in the numerical simulations. Figure 16 shows the comparison between the numerical simulation and the experimental results of the unreinforced masonry wall in test 1. A large depression area and deformation appeared on the face of the explosion. The numerical simulation predicted depressions caused by the blast wave, as shown in Figure 16a. The destruction area of the front surface of the wall was basically symmetric, and the edge of the area was arc shaped. A -shaped crack penetrated the wall in the test. The calculation ⊥ results showed that the same main crack penetrated to the back of the masonry wall, as illustrated in Figure 16b, and the brick in the center of the wall almost fell out. The fracture lengths in the horizontal Processes 2019, 7, 863 11 of 19

4.2. Geometric Model The simplified micro-models for the masonry walls were developed in LS-DYNA, and the mortar, blocks, and polyurea layer were modeled separately. The nodes on their interfaces were merged. The wall model dimensions were a thickness of 120 mm, height of 2 m, and width of 3 m to match the tests described above, as shown in Figure 2. The sizes of blocks were 240 × 115 × 53 mm, and the thicknesses of the mortar and polyurea layer were 10 and 6 mm, respectively, identical to the thicknesses in the test specimens. The three-dimensional solid element SOLID164 was used to discretize the geometrical model. Considering the computational accuracy and efficiency, the combination of a 20-mm mesh size for the blocks and one layer for the mortar joint and polyurea coating was adopted for the numerical model.

4.3. Numerical Model Calibration In the tests, the pressure sensor was located at the end of the pipeline. To obtain the explosion overpressure value at the wall, after the tests, a pressure sensor with a range of 0–1 MPa was also arranged at a distance of 1 m from the flaring. The sensor was destroyed in the first measurement and reached the maximum range in the second measurement, but it was not destroyed. According to the information provided by the supplier, the damage pressure of the sensor was 2 MPa, and the positive-pressure-action time measured by the sensor at the end of the pipeline was about 2 ms. These were used as the explosion loads in the numerical simulations. Figure 16 shows the comparison between the numerical simulation and the experimental results of the unreinforced masonry wall in test 1. A large depression area and deformation appeared on the face of the explosion. The numerical simulation predicted depressions caused by the blast wave, as shown in Figure 16a. The destruction area of the front surface of the wall was basically symmetric, and the edge of the area was arc shaped. A ⊥-shaped crack penetrated the wall in the test. The calculationProcesses 2019 results, 7, 863 showed that the same main crack penetrated to the back of the masonry wall,11 of as 18 illustrated in Figure 16b, and the brick in the center of the wall almost fell out. The fracture lengths in the horizontal and vertical directions were 1.78 and 1.5 m, respectively, and the widths of cracks and vertical directions were 1.78 and 1.5 m, respectively, and the widths of cracks were about 23 mm. were about 23 mm. The values of the three parameters in the test were 1.8 m, 1.4 m, and 25 mm The values of the three parameters in the test were 1.8 m, 1.4 m, and 25 mm respectively. The results of respectively. The results of the simulation are basically consistent with the experimental data. the simulation are basically consistent with the experimental data.

Figure 16. Failure pattern predictions of unreinforced specimen under the first explosion. (a) front face Figure 16. Failure pattern predictions of unreinforced specimen under the first explosion. (a) front and (b) rear face. face and (b) rear face. Figure 17 shows the numerical simulation results of the polymer-retrofitted masonry wall in test 3. Figure 17 shows the numerical simulation results of the polymer-retrofitted masonry wall in test In Figure 17a, the deformation area of the central area of the wall was approximately 0.12 m2, and the 3. In Figure 17a, the deformation area of the central area of the wall was approximately 0.12 m2, and numerical simulation predicted a value of approximately 0.14 m2. In Figure 17b, the coating on the the numerical simulation predicted a value of approximately 0.14 m2. In Figure 17b, the coating on back burst surface remained intact, and the calculated results showed that the coating was also intact, the back burst surface remained intact, and the calculated results showed that the coating was also with slight arching in the center of the coating. Therefore, the numerical model developed in this study intact, with slight arching in the center of the coating. Therefore, the numerical model developed in yielded acceptable predictions and could be used to predict the performances of polymer-retrofitted thisProcesses study 2019 yielded, 7, 863 acceptable predictions and could be used to predict the performances of polymer-12 of 19 masonry walls against gas explosions. retrofitted masonry walls against gas explosions.

Figure 17. Failure pattern predictions of polymer-retrofitted wall under the first explosion. (a) front Figure 17. Failure pattern predictions of polymer-retrofitted wall under the first explosion. (a) front face and (b) rear face. face and (b) rear face. 5. Numerical Simulations 5. Numerical Simulations The masonry walls of petrochemical buildings are larger than the specimens used in this study, and mostThe are 240masonry mm thick. walls Therefore, of petrochemical based on buildings the calibrated are larger models, than numerical the specimens models used with indimensions this study, andof 4 mmost3 mare 240 mmmm werethick. established Therefore, according based on to the patterncalibrated shown models, in Figure numerical 18. In the models simulation with × × dimensionscalculations, of the 4 m peak × 3 overpressurem × 240 mm were was 69 established kPa, and theaccording positive-pressure to the pattern time shown was 20 in ms Figure [22,23 18.]. In the simulation calculations, the peak overpressure was 69 kPa, and the positive-pressure time was 20 ms [22,23].

Figure 18. Wall with 240-mm thickness.

According to the Chinese standard (GB50779-2012) [22], the peak reflected pressure can be calculated as follows: = ()+ Pr 2 0.0073Pso Pso , (1)

where Pr is the peak reflected pressure (kPa). The stagnation pressure can be expressed as = + Ps Pso Cdqo , (2)

where Ps is the stagnation pressure (kPa), and Cd is the drag coefficient. The drag coefficient depends on the shape and orientation of the obstructing surface. For a rectangular building, the drag coefficient may be taken as +1.0 for the front wall and −0.4 for the side and rear walls and the roof. The duration of the equivalent triangle was calculated as follows: = < tc 3S U td , (3) = =()− + te 2Iw Pr td tc Ps Pr tc, (4)

Processes 2019, 7, 863 12 of 19

Figure 17. Failure pattern predictions of polymer-retrofitted wall under the first explosion. (a) front face and (b) rear face.

5. Numerical Simulations The masonry walls of petrochemical buildings are larger than the specimens used in this study, and most are 240 mm thick. Therefore, based on the calibrated models, numerical models with dimensions of 4 m × 3 m × 240 mm were established according to the pattern shown in Figure 18. In the simulation calculations, the peak overpressure was 69 kPa, and the positive-pressure time was 20 ms [22,23]. Processes 2019, 7, 863 12 of 18

Figure 18. Wall with 240-mm thickness. Figure 18. Wall with 240-mm thickness. According to the Chinese standard (GB50779-2012) [22], the peak reflected pressure can be According to the Chinese standard (GB50779-2012) [22], the peak reflected pressure can be calculated as follows: calculated as follows: Pr = (2 + 0.0073Pso)Pso, (1) = ()+ (1) where Pr is the peak reflected pressurePr (kPa).2 0 The.0073 stagnationPso Pso pressure, can be expressed as where Pr is the peak reflected pressure (kPa). The stagnation pressure can be expressed as Ps = Pso + Cdqo, (2) = + Ps Pso Cdqo , (2) where Ps is the stagnation pressure (kPa), and Cd is the drag coefficient. The drag coefficient depends whereon the P shapes is the and stagnation orientation pressure of the (kPa), obstructing and C surface.d is the drag Fora coefficient. rectangular The building, drag coefficient the drag coe dependsfficient may be taken as +1.0 for the front wall and 0.4 for the side and rear walls and the roof. on the shape and orientation of the obstructing− surface. For a rectangular building, the drag coefficientThe duration may be taken of the as equivalent +1.0 for the triangle front waswall calculatedand −0.4 for as the follows: side and rear walls and the roof. The duration of the equivalent triangle was calculated as follows: t = 3S/U < t , (3) Processes 2019, 7, 863 = c < d 13 of 19 tc 3S U td , (3) te = 2Iw/Pr = (t tc)Ps/Pr + tc, (4) d − I =0.5()P −P t +0.5Pt , (5) w= Iw = 0.5=r ()(P−rs cPs)tc ++0.5s Pd std,(4) (5) te 2Iw Pr td −tc Ps Pr tc, wherewhereI wIwis is the the positive positive pressure pressure impulse, impulse,S is S the is clearingthe clearing distance, distance, which which is the smalleris the smaller of H or of B/ 2(m),H or tB/2(m),e is the durationte is the of duration the equivalent of the triangle equivalent (s), and triangletc is the (s), duration and tc ofis thethe reflected duration overpressure of the reflected effect (s).overpressure The calculated effect front (s). The wall calculated loading is front shown wall in loading Figure 19 is. shown in Figure 19.

210 180 150 120

P/kpa 90 60 30 0 0 5 10 15 20 25 t/ms

Figure 19. Wall loading. Figure 19. Wall loading. 5.1. Unreinforced Wall 5.1. UnreinforcedFigure 20 shows Wall the structural response of an unsprayed wall. According to the figure, the brick wall collapsed.Figure 20 shows Selecting the thestructural center pointresponse of the of wallan unsprayed as the observation wall. According point, the to displacementthe figure, the curve brick iswall shown collapsed. in Figure Selecting 21. In thethe rangecenter of point 0–0.2 of s, the the wa displacementll as the observation tended to poin increaset, the significantly, displacement and curve the maximumis shown in displacement Figure 21. In wasthe range 0.7 m, of which 0–0.2 was s, th largere displacement than the thicknesstended to of increase the clay significantly, brick wall. Theand velocitythe maximum curve isdisplacement shown in Figure was 0.722. m, As which shown was in the larger figure, than the the wall thickness velocity of increased the clay brick first, afterwall. whichThe velocity it remained curve basically is shown constant, in Figure reaching 22. As ashown maximum in the velocity figure, ofthe 3.84 wall m /velocitys at 0.028 increased s. first, after which it remained basically constant, reaching a maximum velocity of 3.84 m/s at 0.028 s.

Figure 20. Structural response of the clay brick wall without sprayed blast-resistance coating. (a) front view and (b) side view.

Processes 2019, 7, 863 13 of 19

= ()− + Iw 0.5 Pr Ps tc 0.5Pstd , (5) where Iw is the positive pressure impulse, S is the clearing distance, which is the smaller of H or B/2(m), te is the duration of the equivalent triangle (s), and tc is the duration of the reflected overpressure effect (s). The calculated front wall loading is shown in Figure 19.

210 180 150 120

P/kpa 90 60 30 0 0 5 10 15 20 25 t/ms

Figure 19. Wall loading.

5.1. Unreinforced Wall Figure 20 shows the structural response of an unsprayed wall. According to the figure, the brick wall collapsed. Selecting the center point of the wall as the observation point, the displacement curve is shown in Figure 21. In the range of 0–0.2 s, the displacement tended to increase significantly, and the maximum displacement was 0.7 m, which was larger than the thickness of the clay brick wall. The velocity curve is shown in Figure 22. As shown in the figure, the wall velocity increased first, Processes 2019, 7, 863 13 of 18 after which it remained basically constant, reaching a maximum velocity of 3.84 m/s at 0.028 s.

ProcessesFigure 2019, 20. 7, 863Structural response of the clay brick wall without sprayed blast-resistance coating. (a) front14 of 19 Processes 2019, 7, 863 14 of 19 viewFigure and 20. ( bStructural) side view. response of the clay brick wall without sprayed blast-resistance coating. (a) front view and (b) side view. 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3

Displacement/m 0.3 0.2 Displacement/m 0.2 0.1 0.1 0 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 Time/s Time/s

Figure 21. Displacement-time history curve of the unreinforced wall. Figure 21. Displacement-time history curve of the unreinforced wall.

4.5 4.5 4 4 3.5

） 3.5 3 ）

m/s 3 2.5 m/s （ 2.5 （ 2 2 1.5

Velocity/ 1.5 1 Velocity/ 1 0.5 0.5 0 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 Time/s Time/s

Figure 22. Velocity-time history curve of the unreinforced wall.wall. Figure 22. Velocity-time history curve of the unreinforced wall. 5.2. Coating Thickness 5.2. Coating Thickness 5.2. Coating Thickness Masonry walls strengthened at the rear face with layer thicknesses of 3, 6, and 8 mm were Masonry walls strengthened at the rear face with layer thicknesses of 3, 6, and 8 mm were considered.Masonry Figure walls 23 strengthened shows the response at the rear of the face strengthened with layer wall thicknesses with a 6-mm of 3, coating.6, and 8 The mm mortar were considered. Figure 23 shows the response of the strengthened wall with a 6-mm coating. The mortar andconsidered. bricks were Figure damaged 23 shows by thethe blastresponse loading, of the but st therengthened coating remained wall with intact a 6-mm and coating. effectively The reduced mortar and bricks were damaged by the blast loading, but the coating remained intact and effectively theand formation bricks were of wall damaged debris. by the blast loading, but the coating remained intact and effectively reduced the formation of wall debris. reducedThe the displacement formation historiesof wall debris. at the centers of masonry walls are shown in Figure 24. The notation 0–3, 0–6, and 0–8 represent the polyurea coatings with corresponding thickness (mm) sprayed on the rear surface of clay brick wall and no coating on the front surface. The displacement of all the reinforced walls increased first and subsequently decreased gradually. The maximum displacements

Figure 23. Structural response of the 0–6 clay brick wall. (a) front view and (b) side view. Figure 23. Structural response of the 0–6 clay brick wall. (a) front view and (b) side view. The displacement histories at the centers of masonry walls are shown in Figure 24. The notation The displacement histories at the centers of masonry walls are shown in Figure 24. The notation 0–3, 0–6, and 0–8 represent the polyurea coatings with corresponding thickness (mm) sprayed on the 0–3, 0–6, and 0–8 represent the polyurea coatings with corresponding thickness (mm) sprayed on the rear surface of clay brick wall and no coating on the front surface. The displacement of all the rear surface of clay brick wall and no coating on the front surface. The displacement of all the reinforced walls increased first and subsequently decreased gradually. The maximum displacements reinforced walls increased first and subsequently decreased gradually. The maximum displacements

Processes 2019, 7, 863 14 of 19

0.8 0.7 0.6 0.5 0.4 0.3

Displacement/m 0.2 0.1 0 0 0.05 0.1 0.15 0.2 Time/s

Figure 21. Displacement-time history curve of the unreinforced wall.

4.5 4 3.5

） 3 m/s 2.5 （ 2 1.5 Velocity/ 1 0.5 0 0 0.05 0.1 0.15 0.2 Time/s Processes 2019, 7, 863 14 of 18

Figure 22. Velocity-time history curve of the unreinforced wall. of the 0–3, 0–6, and 0–8 walls were 0.29, 0.21, and 0.19 m, respectively. With the increase in the coating 5.2.thickness, Coating theThickness maximum displacement of wall decreased, which indicated that the anti-explosion ability was significantly improved. The kinetic and internal energies of the coatings are shown in Figures 25 Masonry walls strengthened at the rear face with layer thicknesses of 3, 6, and 8 mm were and 26, respectively. The internal energy absorbed by the coating was much greater than the kinetic considered. Figure 23 shows the response of the strengthened wall with a 6-mm coating. The mortar energy. With the increase in the coating thickness, the amount of wall deformation decreased, the and bricks were damaged by the blast loading, but the coating remained intact and effectively internal energy of the coating decreased, and the wall absorbed more explosive energy through the reduced the formation of wall debris. destruction of the mortar or brick.

Processes 2019, 7, 863 15 of 19 Processes 2019, 7, 863 15 of 19 of the 0–3, 0–6, and 0–8 walls were 0.29, 0.21, and 0.19 m, respectively. With the increase in the coating of the 0–3, 0–6, and 0–8 walls were 0.29, 0.21, and 0.19 m, respectively. With the increase in the coating thickness, the maximum displacement of wall decreased, which indicated that the anti-explosion thickness, the maximum displacement of wall decreased, which indicated that the anti-explosion ability was significantly improved. The kinetic and internal energies of the coatings are shown in ability was significantly improved. The kinetic and internal energies of the coatings are shown in Figures 25 and 26, respectively. The internal energy absorbed by the coating was much greater than Figures 25 and 26, respectively. The internal energy absorbed by the coating was much greater than the kinetic energy. With the increase in the coating thickness, the amount of wall deformation the kinetic energy. With the increase in the coating thickness, the amount of wall deformation decreased, the internal energy of the coating decreased, and the wall absorbed more explosive energy decreased, the internal energy of the coating decrea sed, and the wall absorbed more explosive energy through the destruction of the mortar or brick. through theFigure destruction 23. Structural of the response mortar ofor thebrick. 0–6 clay brick wall. (a) front view and (b) side view.

Figure 23. Structural0.35 response of the 0–6 clay brick wall. (a) front view and (b) side view. 0.35 0-3 0.3 0-3 The displacement histories0.3 at the centers0-6 of masonry walls are shown in Figure 24. The notation 0-6 0–3, 0–6, and 0–8 represent the0.25 polyurea0-8 coatings with corresponding thickness (mm) sprayed on the 0.25 0-8 rear surface of clay brick wall and no coating on the front surface. The displacement of all the 0.2 reinforced walls increased first0.2 and subsequently decreased gradually. The maximum displacements 0.15 0.15 Displacement\m

Displacement\m 0.1 0.1 0.05 0.05 0 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 Time/s Time/s

Figure 24. Displacements of walls strengthened with different coating thicknesses. Figure 24. DisplacementsDisplacements of of walls walls strengthened strengthened with different different coating thicknesses.

250 250 0-3 0-3 200 0-6 200 0-6 0-8 0-8 150 150

100 100 Kinetic Energy/J Kinetic Energy/J 50 50

0 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 Time/s Time/s

Figure 25. Kinetic energies of coatings with didifferentfferent thicknesses. Figure 25. Kinetic energies of coatings with different thicknesses.

3500 3500 0-3 3000 0-3 3000 0-6 0-6 2500 0-8 2500 0-8 2000 2000 1500 1500 1000 Internal energy/J 1000 Internal energy/J 500 500 0 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 Time/s Time/s

Figure 26. Internal energies of coatings with different thicknesses. Figure 26. Internal energies of coatings with different thicknesses.

Processes 2019, 7, 863 15 of 19 of the 0–3, 0–6, and 0–8 walls were 0.29, 0.21, and 0.19 m, respectively. With the increase in the coating thickness, the maximum displacement of wall decreased, which indicated that the anti-explosion ability was significantly improved. The kinetic and internal energies of the coatings are shown in Figures 25 and 26, respectively. The internal energy absorbed by the coating was much greater than the kinetic energy. With the increase in the coating thickness, the amount of wall deformation decreased, the internal energy of the coating decreased, and the wall absorbed more explosive energy through the destruction of the mortar or brick.

0.35 0-3 0.3 0-6 0.25 0-8

0.2

0.15

Displacement\m 0.1

0.05

0 0 0.05 0.1 0.15 0.2 Time/s

Figure 24. Displacements of walls strengthened with different coating thicknesses.

250 0-3 200 0-6 0-8 150

100 Kinetic Energy/J

50

0 0 0.05 0.1 0.15 0.2 Time/s

Processes 2019, 7, 863 15 of 18 Figure 25. Kinetic energies of coatings with different thicknesses.

3500 0-3 3000 0-6 2500 0-8 2000 1500 1000 Internal energy/J 500 0 0 0.05 0.1 0.15 0.2 Time/s

Figure 26. InternalInternal energies energies of coatings with different different thicknesses.

5.3. Spray Pattern To study the effect of the spraying method on the explosion resistance of the reinforced walls, 3-mm-thick explosion resistant coatings were sprayed on the front (3–0), back (0–3), and both sides of the clay brick walls (3–3). ProcessesThe 2019 coatings, 7, 863 of the walls strengthened using three different spray patterns were not damaged16 of 19 under explosive loading. The side views of the reinforced walls are shown in Figure 27. When the 5.3. Spray Pattern polyurea was sprayed only on the front face of the masonry wall, debris was launched from the back of theTo wall. study the effect of the spraying method on the explosion resistance of the reinforced walls, 3- mm-thickThe displacement explosion resistant curves coatings of the diff wereerent sprayed spraying on methods the front are (3–0), shown back in Figure (0–3), 28and. The both maximum sides of thedisplacements clay brick walls of the (3–3). 3–0, 0–3, and 3–3 walls were 0.55, 0.29, and 0.11 m, respectively. This suggested thatThe the masonrycoatings wallof the with walls the strengthened double-side reinforcement using three different exhibited spray the bestpatterns explosion were resistance.not damaged For underthe 3–3 explosive wall, the loading. thickness The of theside coating views onof the frontreinforced side was walls the are same shown as that in onFigure the back27. When side. Thethe polyureakinetic and was internal sprayed energies only on of the the front coating face on of the the front masonry sidewere wall, larger debris than was the launched corresponding from the values back ofof the the wall. back side, as illustrated in Figures 29 and 30, which showed that the energy absorption effect of the front face coating was better than that of the back face coating.

Figure 27. Structural responses of clay brick walls (0.07 s). (a) 3–0, (b) 0–3 and (c) 3–3.

Figure 27. Structural responses of clay brick walls (0.07 s). (a) 3–0, (b) 0–3 and (c) 3–3.

The displacement curves of the different spraying methods are shown in Figure 28. The maximum displacements of the 3–0, 0–3, and 3–3 walls were 0.55, 0.29, and 0.11 m, respectively. This suggested that the masonry wall with the double-side reinforcement exhibited the best explosion resistance. For the 3–3 wall, the thickness of the coating on the front side was the same as that on the back side. The kinetic and internal energies of the coating on the front side were larger than the corresponding values of the back side, as illustrated in Figures 29 and 30, which showed that the energy absorption effect of the front face coating was better than that of the back face coating. The displacements of the walls strengthened with the same thicknesses are shown in Figure 31. The maximum displacements of the 0–6, 3–3, and 2–4 walls were 0.21, 0.11, and 0.09 m, respectively. The explosion resistances of the masonry walls could be further improved by optimizing the combination of the coating spraying method and thickness.

0.6 3-0 0.5 0-3 3-3 0.4

0.3

0.2 Displacement/m 0.1

0 0 0.05 0.1 0.15 0.2 Time/s

Figure 28. Displacements of walls strengthened with different spraying patterns.

Processes 2019, 7, 863 16 of 19

5.3. Spray Pattern To study the effect of the spraying method on the explosion resistance of the reinforced walls, 3- mm-thick explosion resistant coatings were sprayed on the front (3–0), back (0–3), and both sides of the clay brick walls (3–3). The coatings of the walls strengthened using three different spray patterns were not damaged under explosive loading. The side views of the reinforced walls are shown in Figure 27. When the polyurea was sprayed only on the front face of the masonry wall, debris was launched from the back of the wall.

Figure 27. Structural responses of clay brick walls (0.07 s). (a) 3–0, (b) 0–3 and (c) 3–3.

The displacement curves of the different spraying methods are shown in Figure 28. The maximum displacements of the 3–0, 0–3, and 3–3 walls were 0.55, 0.29, and 0.11 m, respectively. This suggested that the masonry wall with the double-side reinforcement exhibited the best explosion resistance. For the 3–3 wall, the thickness of the coating on the front side was the same as that on the back side. The kinetic and internal energies of the coating on the front side were larger than the corresponding values of the back side, as illustrated in Figures 29 and 30, which showed that the energy absorption effect of the front face coating was better than that of the back face coating. The displacements of the walls strengthened with the same thicknesses are shown in Figure 31. The maximum displacements of the 0–6, 3–3, and 2–4 walls were 0.21, 0.11, and 0.09 m, respectively. The explosion resistances of the masonry walls could be further improved by optimizing the Processes 2019, 7, 863 16 of 18 combination of the coating spraying method and thickness.

0.6 3-0 0.5 0-3 3-3 0.4

0.3

0.2 Displacement/m 0.1

0 0 0.05 0.1 0.15 0.2 Time/s

Processes 2019, 7, 863 17 of 19 Processes 2019, 7, Figure863 28. 17 of 19 Figure 28. DisplacementsDisplacements of walls strengthened with different different spraying patterns. 160 160 front face 140 front face 140 120 rear face 120 rear face 100 100 80 80 60 60 kinetic energy\J kinetic

kinetic energy\J kinetic 40 40 20 20 0 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 Time/s Time/s Figure 29. Kinetic energy of the 3–3 wall coating. FigureFigure 29.29. Kinetic energy of the 3–33–3 wallwall coating.coating.

1800 1800 front face 1600 front face 1600 rear face 1400 rear face 1400 1200 1200 1000 1000 800 800 600

Internal energy/J 600 Internal energy/J 400 400 200 200 0 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 TIme/s TIme/s Figure 30. InternalInternal energy energy of of the the 3–3 3–3 wall wall coating. Figure 30. Internal energy of the 3–3 wall coating. The displacements of the walls strengthened with the same thicknesses are shown in Figure 31. 0.25 The maximum displacements0.25 of the 0–6, 3–3, and 2–4 walls were 0.21, 0.11, and 0.09 m, respectively. The 0-6 explosion resistances of the masonry walls0-6 could be further improved by optimizing the combination 0.2 3-3 of the coating spraying method0.2 and thickness.3-3 2-4 2-4 0.15 0.15

0.1 0.1 Displacement/m Displacement/m 0.05 0.05

0 0 0 0.05 0.1 0.15 0.2 0 0.05 0.1 0.15 0.2 Time/s Time/s

Figure 31. Displacements of walls strengthened with the same thickness. Figure 31. Displacements of walls strengthened with the same thickness.

6. Conclusions 6. Conclusions In this study, the dynamic impact responses of walls strengthened with polyuria layers were In this study, the dynamic impact responses of walls strengthened with polyuria layers were investigated through a series of impact tests under the blast loads generated by gas explosions instead investigated through a series of impact tests under the blast loads generated by gas explosions instead of high explosives. The calibrated numerical model was conducted on parametric studies. The of high explosives. The calibrated numerical model was conducted on parametric studies. The conclusions are summarized as follows: conclusions are summarized as follows:

Processes 2019, 7, 863 17 of 19

160 front face 140 120 rear face 100 80 60 kinetic energy\J kinetic 40 20 0 0 0.05 0.1 0.15 0.2 Time/s

Figure 29. Kinetic energy of the 3–3 wall coating.

1800 front face 1600 rear face 1400 1200 1000 800 600 Internal energy/J 400 200 0 0 0.05 0.1 0.15 0.2 TIme/s Processes 2019, 7, 863 17 of 18 Figure 30. Internal energy of the 3–3 wall coating.

0.25 0-6 0.2 3-3 2-4 0.15

0.1 Displacement/m 0.05

0 0 0.05 0.1 0.15 0.2 Time/s

Figure 31. Displacements of walls strengthened with the same thickness. 6. Conclusions

In this study, the dynamic impact responses of walls strengthened with polyuria layers were investigated6. Conclusions through a series of impact tests under the blast loads generated by gas explosions instead of high explosives. The calibrated numerical model was conducted on parametric studies. The In this study, the dynamic impact responses of walls strengthened with polyuria layers were conclusions are summarized as follows: investigated through a series of impact tests under the blast loads generated by gas explosions instead (1)of highThe explosives. wall damage The modes calibrated under numerical gas explosions model were was analyzed, conducted the on masonry parametric walls studies. experienced The conclusionsbending are damage summarized and then as follows: collapsed. The presence of the polyurea coatings could effectively improve the anti-explosion abilities of masonry walls, prevent wall collapses, and retain the flying fragments. Excellent reinforcement effect and no influence on normal production make polyurea coating an alternative method to strengthen existing petrochemical buildings. (2) The bond between the polymer and masonry wall is critical for the reinforcement effectiveness. Premature debonding results in a failure of the coating to exert the maximum energy absorption effect. Spraying operations should be carried out under suitable temperature and humidity conditions to improve the adhesion of the coating. Furthermore, steel angle can also be used to reinforce the coating around the frame. (3) When polyurea was sprayed only on the front face of a masonry wall, debris was still launched from the back of the wall. The blast-resistant ability of the front polyurea-coated wall was not significantly enhanced, and it was weaker than the rear and double-side polyurea-coated walls. (4) Compared with the rear polyurea-coated wall, the blast-resistant ability of the double-side polyurea-coated wall improved significantly. Double-sided reinforcement should be used in building reinforcement to improve the wall explosion resistance. Rear-face reinforcement can be selected when construction conditions are limited. In addition, the explosion resistance of masonry walls can be further improved by optimizing the combination of the spray method and coating thickness Subsequent tests can be carried out to investigate characteristics of wall fragments, proposing an analytical model to accurately predict the debris distribution at low computational cost. Several other parameters can be included in future tests such as elongation, tensile strength, tear strength, and hardness of polyurea coating. The performance criteria for coatings could be optimized to better blast reinforcement.

Author Contributions: Conceptualization, A.Y. and M.G.; methodology, M.G. and X.L.; software, M.G.; formal analysis, G.C.; data curation, X.L.; writing—original draft preparation, M.G.; writing—review and editing, A.Y. and H.W. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. Processes 2019, 7, 863 18 of 18

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