This study aims to estimate the life of external wall tiles under a thermal cyclic load, and also to propose a guidance to develop adhesive mortar for the external wall tile construction in the future. In the first objective, failure mechanism of the external wall tile structure under the thermal cyclic load could be understood in terms of interfacial crack propagation during the increase of thermal loading cycles. After the first objective, the guidance to develop adhesive mortar for the external wall tile construction against the thermal loads was shown in terms of relationships between the interfacial thermal stress level and mortar properties.

Based on these objectives, the current research was divided into two main parts. The first one was the development of the analytical model based on a finite element method (FEM), and the other one was the application of the developed model. In other words, the current research developed the analytical model for quantitatively estimating the life of the external wall tile structure, and for proposing the guidance to develop a durable material for the external wall tile construction.

The external wall tile structure consists of three materials that are concrete, adhesive mortar, and tiles, and polymer-cement mortar (PCM) is commonly used as the adhesive material for tile construction. Between these material layers, bi-material interfaces are formed inevitably. From field observations, the interface between concrete and PCM was often found as the failure locations of the external wall tile structure, and most of the tile delamination areas were found at the building walls that face to sunshine directions. From this information, the interface between concrete and PCM was realized as a potentially weak location under solar radiation.

From the field observations, therefore, the interface element in FEM was developed for the thermal fatigue analysis in order to study the interfacial failure under the thermal load in this study. In order to develop the interface element for such a purpose, the interfacial resistance in terms of interface fracture toughness and S-N diagrams was investigated. To investigate the interface fracture toughness, the constitutive material model of the interface elements was proposed, and its corresponding values, having a relation to the interface fracture toughness, were calibrated with four experiments that have a different ratio of shear to tensile stress, which is defined as a phase angle. With the proposed method, the interface fracture toughness of the interface between concrete and PCM in the external wall tile structure was successfully obtained for the whole range of phase angles, and the constitutive material model of the interface element for stress analysis was obtained simultaneously. Regarding the interfacial resistance under the cyclic load, a stress-life approach was applied to construct the S-N diagrams of the interface between concrete and PCM. After the interfacial resistance investigation, the interface element was developed for the thermal stress analysis, and the obtained S-N diagram was modified for the constitutive material model of the interface element under the thermal cyclic load.

Then, the developed interface element was applied to analyze the interfacial crack propagation of the tile structure under both the thermal monotonic load and the thermal cyclic load. The experimental methods in a laboratory scale, namely the monotonic heating experiment and the cyclic heating experiment, were invented for reproducing the interfacial delamination of a tile structure under thermal loads, and for checking the validity of the developed interface element. The experimental results in terms of the length of interfacial crack propagation under a given thermal load were compared with the analysis results. With the proposed constitutive material model, the analysis results agreed with the experimental ones under thermal monotonic load, and it is good for rough estimation of the interfacial life under the thermal cyclic load.

In order to understand more in real situation, the developed interface element was, then, applied to analyze the external wall tile structure under solar radiation. The temperature on tile surfaces in the field was investigated, and modified to be the thermal boundary condition on the tile surfaces in the FE analysis. Three models, namely normal case, high tensile case, and high shear case, were simulated in FEM. Each model had different mechanical boundary conditions that might be possible in the reality.

In the normal case, there was no initial defect assumed in the analysis. In this analysis, low interfacial stresses were observed in both normal and shear directions of the interface element. The analysis results implied that the external wall tiles are not harmful to the external wall tiles under a normal condition that is assumed to have no initial defect in the structure. It is agreed with the reality that most of the external wall tiles have their life longer than 30 years.

For high tensile case, the initial defect was assumed at the interface between concrete and PCM in order to generate tensile stress concentration above the initial crack tip. In this case, the maximum tensile stress level was 2.3 times higher than the normal case, but the tensile stress level was not harmful to the structure in comparison with the tensile bonding strength of the PCM. In other words, although high tensile case presents in the reality, tensile bonding strength of the PCM is good enough to prevent the failure under the thermal load.

In the last case, the initial defect was assumed at the gout location between two tiles. As a result, the PCM could expand during heating in the shear direction of the interface element, while the concrete was fixed. The severity occurred in this high shear case, because the interfacial shear stress was high enough to cause a fatigue failure at the interface under thermal cyclic loads during the building's life.

In reality, such kind of defect that causes high shear stress condition is difficult to prevent, because it commonly occurs due to shrinkage of adhesive mortar or due to poor workmanship. In order to reduce the severity of that condition, suitable adhesive mortar can be used. Therefore, the analytical model was further utilized to study the effects from parameters, related with the properties of adhesive mortar, on the thermal stress at the interface in order to propose the guidance to develop the adhesive mortar in the future.

The effects on the thermal stress from Young's modulus, thermal expansion coefficient (TEC), and thermal conductivity were investigated. It has been found that Young's modulus and TEC were the parameters that significantly affect to the interfacial shear stress level in the external wall tile structure. The shear stress level at the interface between concrete and adhesive mortar under high shear stress condition could be significantly reduced by using suitable Young's modulus and TEC of mortar in the external wall tile construction. In other words, in order to reduce the severity of the external wall tile structure, such as high shear condition, Young's modulus and TEC are the keys to develop adhesive mortar in the future.

本論文は,外壁タイル構造が剥離を起こす原因を分析することと,外壁タイルの耐久性を向上させるために必要な条件を把握すること,の二点を目的としている.この目的を達成する為に,有限要素解析法による剥離解析モデルの構築と,構築された解析モデルによる各種条件下での剥離解析の実施と妥当性の検証を行っている.

剥離解析モデルの構築においては,外壁タイル構造における躯体コンクリートと接着モルタル間の剥離を表現するためのインターフェース要素の開発を行っている.剥離は異種材料間の界面破壊問題であるため,界面破壊力学を用いて,引張応力下,せん断応力下,そして引張とせん断の混合応力下の破壊実験結果に基づく,インターフェース要素の構成則の校正を行っている.同様に,疲労荷重下でも破壊実験を行い,インターフェース要素の疲労構成則の校正を行っている.さらに,外壁タイルの剥離が熱応力により引き起こされると考えられる為,応力-熱伝導連成解析に対しても構成則を拡張している.

こうして構築された解析モデルを用いて,各種実験によりその妥当性を検証している.まず,タイル貼りされたコンクリート柱の圧縮実験の解析により,力学的な荷重を受けた際の変形と剥離の再現ができることを確認している.次に,タイル表面に熱を与える単調熱荷重実験により,温度応力による剥離が生じることを確認すると共に,解析による剥離長が妥当であることを示している.最後に,タイル表面に繰り返し熱を与える疲労熱荷重実験でも,剥離の進展を確認し,解析による剥離進展長の妥当性を示している.また,後者二つの実験は独自に考案されたものである.

さらに,解析モデルを用いて,界面における初期欠陥の影響と接着モルタルの材料特性の影響についても検討しており,影響の大きい初期欠陥位置と応力方向を明らかにし,材料特性の影響度合を示している.

以上のように,本論文は,外壁タイル構造における躯体コンクリートと接着モルタル間の剥離進展を再現できる応力-熱伝導連成解析手法の構築を行い,独自の実験も含めて校正と妥当性の確認まで行っている.原因の分析と耐久性向上に必要な条件も示しており,本問題に関する有効な体系を構築したと考えられる.

よって本論文は博士(工学)の学位請求論文として合格と認められる。