Fallon, Chanel

McShane, Graham J.

English

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        Citation for published version:Fallon, C & McShane, GJ 2018, 'Retrofit of a reinforced concrete slab using a spray-on elastomer coating forblast mitigation'.Publication date:2018Link to publicationUniversity of BathAlternative formatsIf you require this document in an alternative format, please contact:openaccess@bath.ac.ukGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Download date: 19. Feb. 2024 ICILSM 2018          7-11 May 2018, Xi’an, China   Retrofit of a reinforced concrete slab using a spray-on elastomer coating for blast mitigation  Chanel Fallona and Graham J. McShanea*  a Department of Engineering, University of Cambridge, Cambridge, UK * gjm31@cam.ac.uk   Abstract  Recent research has highlighted the merits of a spray application elastomer coating for enhancing the blast resistance of structures. Despite some encouraging results in the literature, there still remains a great deal to be understood about the mechanisms by which the coating achieves its effect. This investigation specifically examines the influence of a spray-on elastomer coating, applied to a reinforced concrete (RC) structural element. A fully coupled numerical approach is developed to predict the influence of fluid-structure interaction (FSI) on the air blast response of coated concrete. The results suggest that for a wide range of cases, there is a negligible FSI effect and thus a purely Lagrangian modelling technique, in which the pressure-time history is applied directly to the structure, is deemed sufficient to accurately capture the behaviour. By examining the response to a wide range of blast intensities, we postulate the existence of three response regimes. At lower blast intensities, Regime 1 is characterised by elastic behaviour and Regime 2 is characterised by elastic-plastic behaviour. Here, we observe the slab to be relatively insensitive to the application of a coating. This is verified by simple analytical models. At higher blast intensities, in Regime 3, the slab undergoes severe damage which introduces difficulties with our modelling strategy. While we remain cautious about interpreting numerical results in this regime, certain simulations indicate a significant reduction in slab deflection with the coating applied. Further, at the highest blast intensities investigated, we qualitatively observe that the failure mechanism becomes dominated by transverse shear failure at the supports. An analytical model predicts that the coating contributes a significant effect in protecting from this particular failure mechanism.  Keywords: Reinforced concrete, Elastomer, Blast  1. Introduction  With growing global security concerns at the forefront of political and industrial agendas, greater attention is being focused on how to protect critical infrastructure. While new structures can be designed with a higher threat level in mind, existing, ageing infrastructure remains vulnerable. Blast and impact are extreme load events – while the likelihood of occurrence is low, the consequences can be catastrophic. Thus, a careful balance must be achieved between various design constraints. Although performance is key, consideration must also be given to cost and ease of installation, life-time maintenance and preserving aesthetics. Structural retrofit for blast mitigation is one solution to this problem.  In this work, we consider a particular retrofit solution – the application of a low-cost, spray-on, elastomer coating. Encouraging results regarding the blast mitigating capabilities of this coating have been reported in the literature for steel and masonry substrates but less attention has been given to reinforced concrete (RC) despite this material representing a significant proportion of infrastructure that could benefit from such a solution. This investigation will employ primarily a numerical technique to interrogate the details of when, where and why this retrofit solution is effective at protecting RC against the detrimental effects of blast. A Coupled Eulerian-Lagrangian (CEL) numerical approach in Abaqus/Explicit was first used to assess the possibility of a fluid-structure interaction (FSI) effect on the air blast response of a coated concrete slab [1]. We observe only a small FSI effect and thus conclude that a simplified, decoupled approach is appropriate to capture the slab response. In this FSI study, we observe that while the total impulse imparted to the ‘combined’ concrete-elastomer structure increases, the impulse imparted to the  ICILSM 2018          7-11 May 2018, Xi’an, China  concrete itself is reduced. Thus, the net effect of the elastomer remains yet to be determined and motivates the present study.  We seek to interrogate the regimes of response of a RC slab subjected to a range of blast intensities in three configurations: uncoated, coated on its blast-receiving face and coated on its non-blast receiving face. Our objective is to determine the regimes in which the coating is effective.  2. Methodology  A purely Lagrangian numerical model in Abaqus/Explicit is used to interrogate the displacement-time history of a RC slab of span, 1000mm and depth, 50mm, reinforced by steel rebar of diameter, 5mm at a pitch of 30mm. For computational efficiency, the slab is modelled to deform in 2D, with plane strain boundary conditions. The concrete constitutive model is chosen as the Concrete Damaged Plasticity (CDP) model with a compressive strength of 39.5MPa and a tensile strength of 4.2MPa. The CDP model treats the concrete as a continuum that exhibits isotropic damaged elasticity and pressure-dependent plasticity. It assumes that the concrete can undergo two failure mechanisms - compressive crushing and tensile cracking. For the coated cases, a 5mm elastomer layer is positioned on either the blast-receiving or non-blast-receiving face of the RC slab and a perfect bond is assumed between the concrete and elastomer. A hyperelastic constitutive model is chosen for the elastomer, based on a fit to experimental measurements. A Prony series is used to represent the viscoelastic behaviour.  A blast pressure loading is directly applied to the top surface of the slab and is approximated as a simple exponential decay function, 𝑝(𝑡) = 𝑝𝑒−𝑡/𝑡𝑖 where 𝑝 is the peak blast pressure and 𝑡𝑖 is the blast decay time. Thus, the impulse is given by, 𝐼 = ∫  𝑝(𝑡)𝑑𝑡 = 𝑝𝑡𝑖 . Various blast intensities are investigated and the displacement and damage time histories are interrogated.    3. Results and Discussion  3.1. Identifying response regimes  Figure 1 presents a 𝑝 − 𝐼 diagram, illustrating the response regimes identified for a RC slab in its uncoated configuration.                    Figure 2 presents the characteristic displacement-time profiles observed in each of the three response regimes. With reference to Fig. 2, we define each regime as follows;   Regime 1: At the lowest impulses, in Fig. 2a, we observe completely elastic behaviour whereby the slab oscillates, but with a zero permanent displacement. We note that the numerical model predicts that the slab remains completely undamaged in this regime.  Regime 2: Increasing the blast intensity, we observe an elastic-plastic response in Fig. 2b, characterised by oscillations about a permanent level of displacement. Our model predicts a small amount of flexural cracking at the supports and on the tensile face of the slab. Further, Fig. 1. 𝑝 − 𝐼 diagram for an uncoated RC slab 0  ∞   ICILSM 2018          7-11 May 2018, Xi’an, China  b) Regime 2: 𝑝 = 1000kPa, 𝐼 = 100Pas d) Regime 3: 𝑝 = 1000kPa, 𝐼 = 500Paswe note that the slab attains a stable plateau in energy dissipated by damage, and this coincides with the slab attaining its maximum displacement.  Regime 3: At higher blast intensities, the slab undergoes extensive damage and typically, does not reach a stable plateau in energy dissipated by damage. We take the first ‘knee’ point in energy dissipated by damage as an arbitrary failure time, marked with a cross in Figs. 2c and 2d. Beyond this point, the concrete is severely damaged, which introduces unreasonable mesh sensitivity and thus, uncertainty in interpreting the results. At the highest blast intensities investigated, we consistently observe a tendency for the slab to undergo severe damage concentrated near the support region which is indicative of a transverse shear failure mechanism.   3.2. Investigating the elastomer coating’s influence  The numerical model is then used to interrogate the influence of an elastomer coating placed on the blast-receiving and non-blast-receiving face of the RC slab. Figure 2 also plots the displacement-time history of RC slabs in their coated configurations, in each of the response regimes. In Figs. 2a and 2b, we notice that the RC slab is relatively insensitive to the application of a coating in both Regimes 1 and 2. It has little effect on the maximum (or permanent) deflection experienced by the slab and the damage patterns observed show no significant coating effect. As described previously, the slab behaviour in       Fig. 2. Typical mid-span displacement-time histories in each response regime observed for a RC slab in three configurations: uncoated, coated on its blast-receiving face and coated on its non-blast-receiving face a) Regime 1: 𝑝 = 100kPa, 𝐼 = 15Pas c) Regime 2 - 3 transition: 𝑝 = 10,000kPa, 𝐼 = 350Pas ICILSM 2018          7-11 May 2018, Xi’an, China  Regime 3 is somewhat obscured at longer times by the development of unreasonable mesh sensitivity that becomes apparent when the concrete becomes severely damaged. Thus, we must define an arbitrary failure time, described in Section 3.1 and focus on the coating effect up to this point in the calculation. By probing the 𝑝 − 𝐼 space, we identify that a blast intensity which lies on the Regime 2 - 3 boundary shows a particularly strong coating effect. Figure 2c shows one such case for a blast intensity, 𝑝 = 10,000kPa and 𝐼 = 350Pas. We observe that while the uncoated slab and the slab coated on its blast-receiving face undergoes Regime 3 behaviour, the slab coated on its non-blast receiving face exhibits a Regime 2 response. Thus, the latter case experiences a significantly reduced permanent deflection and less extensive damage patterns. An analytical study has also been performed to support the numerical predictions for the regime boundaries and coating influence. We find that we achieve very good agreement between analytical theory and numerical predictions for the slab deflections in Regimes 1 and 2, and further obtain a close match for the boundary between Regimes 1 and 2 (at approximately, 𝐼 = 20Pas). The severe concrete damage that occurs in Regime 3 makes analytical modelling challenging, however, we are able to show that the coating is particularly effective at protecting against the transverse shear failure mechanism, qualitatively observed at the highest blast intensities in the numerical model.     4. Conclusions  A numerical study is performed to assess the blast mitigating capabilities of a spray-on elastomer coating applied to a RC slab. The slab is subjected to range of blast intensities and the deflection and damage behaviour are interrogated to establish the regimes of response and the influence of the elastomer coating in each regime. The following conclusions are made:   A fully coupled (CEL) numerical study suggests that the FSI effect contributed by the elastomer coating is negligible, and thus a simplified, Lagrangian model is appropriate to capture the slab behaviour.  Three response regimes are identified - Regime 1, characterised by elastic behaviour, Regime 2, characterised by elastic-plastic behaviour and Regime 3, characterised by severe concrete damage.  The application of an elastomer coating has a negligible effect on the RC slab response in Regimes 1 and 2.  By interrogating the Regime 2 - 3 boundary, we notice that the coating is capable of significantly reducing slab deflections at these blast intensities. This suggests that the coating only serves an effective mitigating role when the concrete is severely damaged.  Analytical models for the Regime 1 and 2 response provide a close match to numerical predictions. While analytical modelling of Regime 3 proves complex due to severe RC damage, a model of the transverse shear failure of a coated beam supports the theory that the coating is particularly effective at protecting severely damaged RC slabs.  Acknowledgements   The authors are grateful for the financial support of the George and Lillian Schiff Foundation and Peterhouse at the University of Cambridge.  References  [1] Fallon C., McShane G.J., 2018, Blast response of elastomer-coated concrete, submitted for publication    

Retrofit of a reinforced concrete slab using a spray-on elastomer coating for blast mitigation

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Retrofit of a reinforced concrete slab using a spray-on elastomer coating for blast mitigation

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