混凝土於單軸壓下表面裂縫特徵與軸壓行為分析研究

Abstract

混凝土為土木工程最常見的材料之一，且因為混凝土有耐久、耐火，容易造型且經濟的特性，故廣泛用於房屋建築、橋梁等構造物。混凝土結構在內外應力作用下，往往會有裂縫的產生，進而影響結構物的耐久性與承載能力；在工程實務上，混凝土表面裂縫是唯一能經由目視，且直接測量的指標。因此若能連結混凝土表面裂縫特徵與構件殘餘強度等相關力學性質關係，將能提供混凝土構件之安全性評估；唯過去受侷於裂縫量測技術的限制，較難量化包括裂縫長度、寬度及密度等裂縫特徵。 本研究利用數位影像量測的方式，針對不同水灰比、粗粒料粒徑大小及粗粒料含量，快速建立混凝土軸向應變、側向應變、體積應變與軸向應力之間力學行為關係，且偵測並量化混凝土受壓過程中的表面裂縫特徵，包含裂縫密度、平均裂縫長度、平均裂縫寬度和最大裂縫寬度，探討混凝土應力應變曲線與裂縫特徵的關聯，可提供混凝土在受單軸壓下，其表面裂縫特徵所對應的力學行為，並推估其殘餘力學強度。由試驗結果顯示，混凝土受單軸壓過程會受到幾何性質和邊界條件影響，是一綜合材料與結構的力學表現。另外，由表面裂縫特徵發現，最大裂縫寬度和應力損失(∆SLE)以及線彈性段折減係數(RLE)之間呈較佳的線性行為，亦即在同樣的水灰比之下，有相同的裂縫寬度，就有相同的應力損失和折減係數，和國際間利用最大裂縫寬度指標去評估建築物殘餘強度的應用相似。在破壞能方面，其和裂縫密度之間呈現良好的對數遞增關係，但無論水灰比的高低以及粗粒料尺寸、數量的變化，在相同的裂縫密度下，均有相近的破壞能。

Concrete is one of the widely used construction material in infrastructures owing to its high durability, low cost and economic advantages. However, cracks tend to propagate under internal and external stresses in concrete, thus affecting its strength and durability. Surface cracks of concrete are the only indexes that can be observed with naked eyes and easily identified. This study involves monitoring and quantifying surface cracks of concrete to conduct the connection between surface crack characteristics and mechanical properties of concrete under uniaxial compressive loading. Digital image measurements are applied in this research to measure the crack characteristics and to establish the relationship between axial strain, lateral strain, volumetric strain and axial stress. The crack characteristics include crack density, average crack length and maximum crack width, etc. With the data of uniaxial compression tests of concrete, regression analysis is also accomplished to obtain the correlation between concrete strength and crack characteristics. The experimental results show that the concrete subjected to uniaxial compression is affected by geometric properties and boundary conditions. Moreover, maximum crack width and the stress loss are linearly increasing, while crack density and the compressive fracture energy are logarithmically increasing. The regression results in this study can be further applied to estimate residual strength of concrete with certain crack characteristics.