PSI - Issue 17

Maria Apostolopoulou et al. / Procedia Structural Integrity 17 (2019) 914–923 Maria Apostolopoulou et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

Natural hydraulic lime mortars have been applied for joining building elements of masonry structures since antiquity (Lanas et al 2004, Maravelaki-Kalaitzaki et al 2003, and Zhang et al 2018). Natural hydraulic lime occurs from the calcination and subsequent slaking of marly limestones (limestones with clay impurities, which after calcination become reactive silicates and aluminates), as illustrated by Figueiredo et al (2016a) and discussed by Pozo-Antonio (2015). It thus sets through both hydration and carbonation processes, leading to the formation of hydraulic compounds and the formation of calcite, respectively. Due to the relatively low calcination temperatures required, it is considered as an eco-friendly material in relation to modern binders, such as cement, as also pointed out by Cho et al (2017) and Velosa and Cachim (2009); furthermore, during hardening, part of the CO 2 emitted through the limestone calcinations is consumed during the carbonation process, thus further lowering the total environmental impact associated to the greenhouse effect gases. Natural hydraulic lime mortars have also been used extensively in recent years for the restoration of historical structures and much research has been conducted in recent years regarding natural hydraulic lime mortars. This is mainly on account of their enhanced compatibility with the historical materials comprising the masonries, as discussed by Faria and Silva 2019, Vyšvařil et al 2017, Isebaert et al 2016, Amenta et al 2017, and Silva et al 2015, while the fact is that these mortars harden in damp conditions and underwater is an advantage and they present over more commonly used aerial lime mortars, as stated by Figueiredo et al (2016a), Arizzi et al 2015 and Kalagri et al 2014. However, during the restoration of a historical building or monument, performance of the restoration materials is also of paramount importance, as discussed by Apostolopoulou et al 2017 and 2018. This indicates that it is crucial, not only to ensure compatibility in terms of chemical and physical characteristics, but also to ensure a compatible, yet adequate level of mechanical strength. Furthermore, an adequate level of mechanical strength can also allow for the use of this eco-friendly material, natural hydraulic lime, in new structures as well. In 2010, the regulations regarding the classification of natural hydraulic limes into different categories (NHL5, NHL3.5, NHL2) were updated resulting in a new version of the standard EN 459-1:2010 (CEN 2010). Thus, the different categories are differentiated in a more substantiated manner than in previous years; however, the new standard leaves room for improvement, as also proposed by Figueiredo et al (2016a,b). In particular, according to the updated standard, natural hydraulic limes which provide at least 5MPa compressive strength at 28 days (tolerance values:5-15MPa) and a Ca(OH) 2 content ≥ 15% are classified as NHL5, natural hydraulic limes which provide at least 3.5MPa compressive strength at 28 days (tolerance values:3.5-10MPa) and a Ca(OH) 2 content ≥ 25% are classified as NHL3.5, while natural hydraulic limes which provide at least 2MPa compressive strength at 28 days (tolerance values:2-7MPa) and a Ca(OH) 2 content ≥ 35% are classified as NHL2. Although many researchers have been involved with the study of natural hydraulic lime mortars, the results of each study is focused in a limited span of the range of mortar mix parameters. This is natural, as it is very difficult to produce mortars covering the whole span of possible values for the mix parameters involved in the production of a mortar and which influences the compressive strength values at different mortar specimen ages. Thus, the utilization of soft computing techniques in order to enhance our understanding in relation to the effect of mortar mix parameters on the compressive strength is highly alluring. The use of soft computing techniques for the prediction of the compressive strength has already been the center of research for cement mortars, as can be seen in Akkurt et al 2003 and Eskandari-Naddaf and Kazemi 2017, among others, blended cement mortars, as illustrated by Saridemir 2009 and Topcu et al 2008 and concrete materials, as studied by Ozcan at al 2009, Bilim et al 2009 and Duan et al 2013. In addition, the use of soft computing techniques has been highlighted in many studies in field of civil engineering (Armaghani et al 2014, 2017, Abad et al 2018, Koopialipoor et al 2018, Pham et al 2017a,b,c&d, and Pham et al 2018a,b). In the present study, a preliminary attempt is made to incorporate all available data from published research related to natural hydraulic lime mortars into one database, and to reveal the influence of natural hydraulic lime mortars’ mix parameters on the compressive strength as well as to predict the compressive strength of different mortar mixes at different ages for different mix design parameter values, using soft computing techniques namely artificial neural networks (ANNs).