【作者】 居贤春；

【导师】 张君；

【作者基本信息】 清华大学 ， 土木工程， 2012， 博士

【摘要】 PVA-ECC是一种高韧性的纤维增强水泥基复合材料。应力-裂纹口张开宽度关系是控制PVA-ECC性能的关键。本文旨在探讨如何设计材料的应力-裂纹口张开宽度关系以实现应变硬化和多点开裂。具有低收缩特性的PVA-ECC能与传统的混凝土复合用于结构中，提高结构的延性和耐久性。本文将对ECC-混凝土组合梁的抗弯性能做实验和理论研究。本文采用2种性能相似，断面直径不同的PVA纤维，4种具有低收缩特性的基材配比。对2种纤维增强材料的抗压和抗拉性能做了对比研究。实验结果显示：材料抗拉性能受纤维直径影响显著；在基材配比、纤维掺量均相同时，采用直径较大的纤维可获得应变硬化与多点开裂模式，而采用直径较小的纤维，复合材料却表现出明显的应变软化与单点开裂模式。当采用细的PVA纤维时，材料的抗拉强度有所提高。其主要原因是纤维的粗细影响纤维的桥接应力。保证纤维从水泥石中拔出而非断裂是优化纤维桥接性能的基本条件。同样采用上述的4种配合比和2种纤维，用已有的导出σ-w关系的方法，导出了2种低纤维掺量下的σ-w关系。实验结果显示：加入纤维后，σ-w曲线由单峰值变为双峰值。应力达到开裂强度后降低至最低点，然后上升至第二个峰值，呈现出应变硬化的特点。但加入纤维并不会影响第一个峰值的大小。第二个峰值随着纤维掺量的增大而提高，当第二个峰值高于第一个峰值时才可能实现应变硬化。时至今日，PVA-ECC材料的成本依旧比较高，经济因素阻碍了这种材料在建筑结构中的应用，为了使这种高性能材料能得到较好的应用，砼与ECC组合后形成一种复合材料是一种比较理想的提高性价比的方式。本文研究了将低收缩PVA-ECC应用于复合梁的实际效果。并建立了模型来预测梁的抗弯承载力。复合上ECC后，ECC层出现多条细密裂缝，裂缝上升至混凝土层后转变为单一的裂缝。当ECC层的强度较高时，梁的承载力随ECC层厚的增加而增大。当上部混凝土的强度较高时，梁的承载力随ECC层厚的变化不明显。模型能较好预测梁的承载力随ECC层厚的变化规律。更多还原

【Abstract】 PVA-ECC is a kind of high ductile fiber reinforced cementitious composites.Stress-crack opening relationship is the key to control the performance of PVA-ECC. Inorder to achieve strain-hardening and multiple cracking, stress-crack openingrelationship is studied.The PVA-ECC with low shrinkage characteristics can work withtraditional concrete, as well as increase the ductility and durability of structures. Thispaper is dedicated to the experimental and theoretical studies of ECC-concretecomposite beam with flexural performance.Two types of PVA fibers with different diameters and four kinds low-shrinkagecements are adopted. Comparison of compressive and tensile properties between twofiber-reinforced composites is studied. Experimental results show that the tensileperformance of the composites may greatly be influenced by the dimension of fibers. Asusing thicker fiber, the strain hardening and performances of multiple cracking can beobtained under tensile load. Whereas using thinner fiber leads to significant behaviorsof strain softening and single cracking. Some increase of tensile strength amongcomposites reinforced with thinner fibers is also observed, which is mainly due to theinfluence of fiber thickness upon its bridging capability. Ensuring that fibers pull out ofcement base instead of fracture is a fundamental requirement for optimizing its bridgingproperty.Using the above-mentioned four kinds of complexes and two kinds of fiber, we cansolve the stress-crack opening relationship with exiting methods. After adding polyvinylalcohol fibers, the stress-crack opening relationship of the composites changes to adouble peak mode. The second peak is greatly increased with increase of fiber content.The second peak stress larger than the cracking strength means strain-hardening andmultiple cracking performances can be expected under tension.Nowadays, the high price of PVA-ECC restrains its application in structures. Thecombination of ECC and ordinary concrete is a good way of reducing the cost. In thispaper, the practical effect of applying low-shrinkage PVA-ECC to composite beam wasstudied, and a corresponding model was established to predict flexural capacity of thebeam. Results shows that ECC could significantly improve the ductility of the beam.Observation was made that multiple fine cracking appeared in ECC layer, and whileascending to concrete layer, they transformed into one single cracking until failure. When the strength of ECC is relatively higher, beam capacity increased with thickerECC layer; Whereas when the strength of concrete layer is higher, influence of ECClayer thickness is not obvious. The corresponding model can be used to predict flexuralcapacity of the beam with different thickness of ECC layers.更多还原