探討高分子纖維抑制混凝土塑性收縮裂縫之效果

An Investigation into the effect of plastic fibre on plastic shrinkage cracking of concrete

施克陵(Ke-Ling Shih)

指導教授 ： 詹穎雯

國立臺灣大學/工學院/土木工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201803225

摘要

新拌混凝土於澆置的數小時內會發生塑性收縮，而此種收縮會導致裂縫的產生，亦即塑性收縮裂縫。通常較容易發生在表面積比體積大很多的混凝土表面，而該裂縫的產生容易使有害物質侵入混凝土中，侵蝕混凝土以及加速內部鋼筋的腐蝕，影響混凝土結構的美觀、耐久性、使用性以及力學性質。而影響塑性收縮裂縫發生的因素有很多種，包括混凝土的配比、泌水量、初凝時間、環境條件以及束制條件等等，而這些因素之間會互相影響，因此塑性收縮裂縫的發生機制其實是複雜的。
為了防範塑性收縮裂縫的發生，通常使用的方法為灑水養護，讓混凝土表面保持濕潤的狀態，避免表面顆粒間的毛細管壓力上升，導致塑性收縮裂縫的發生。而添加高分子纖維於混凝土中也是另一種預防的方式，利用纖維與混凝土漿體間的介面剪切黏結應力，提高混凝土於塑性狀態下的抗拉強度，減少塑性收縮裂縫的形成。隨著環保意識抬頭，將回收材料製作成高分子纖維並應用於混凝土中，未來會逐漸地朝這個方向進行，而本研究將針對高分子纖維對於塑性收縮裂縫面積之間的關係進行探討。
本研究進行了泌水試驗、蒸發量的量測、初終凝試驗以及塑性收縮裂縫試驗，探討不同水灰比、環境條件以及束制條件對混凝土塑性收縮裂縫的影響，並選擇適當的條件進行低纖維體積取代率混凝土的塑性收縮裂縫試驗。利用上述所有的試驗結果求出塑性收縮裂縫風險值，並將該數值與最終塑性收縮裂縫面積套入塑性收縮裂縫模型中，比較混凝土塑性收縮裂縫的嚴重程度。
最後將本研究所使用的高分子纖維種類所拌合出來的纖維混凝土其最終塑性收縮裂縫面積與塑性收縮裂縫風險值與一般混凝土的數值進行比較。最終得到本研究所使用的高分子纖維其於不同體積取代率下的纖維折減值，並應用於塑性收縮裂縫風險值的公式中。

關鍵字

高分子纖維 ； 塑性收縮裂縫 ； 低纖維體積取代率混凝土 ； 介面剪切黏結應力 ； 纖維風險折減值

並列摘要

Plastic shrinkage occurs within several hours after casting and this type of shrinkage causes early age cracking which is known as plastic shrinkage cracking. Concrete with a large surface area compared to its volume are usually tend to form plastic shrinkage cracking. Aggressive substance may enter the cracks, which accelerate the deterioration of concrete and it also induces the exposure of reinforcement causing it to corrode faster. These phenomena will have an impact on the aesthetic value, durability, serviceability and mechanical properties of the concrete structures. There are many factors influencing plastic shrinkage cracking such as mix proportion of concrete, bleeding, initial setting time, environment conditions and restraint. These factors influence each other and change continuously with time, so they complicate the mechanism of plastic shrinkage cracking.
To prevent plastic shrinkage cracking, curing the concrete surface directly after casting is a method that usually adopted. It can keep the surface of concrete wet and avoid capillary pressure to build up causing plastic shrinkage cracking. Adding plastic fibers into concrete is another preventive method. The interfacial shear bond stress between fibers and concrete paste is used to increase the tensile strength of concrete under plastic state and reduce the formation of plastic shrinkage cracking. As the awareness of environmental protection rises, plastic fibers made of recycled materials are applied to concrete, which will gradually proceed in the future. This study will discuss the relationship between plastic fibers and plastic shrinkage cracking area.
In this study, the bleeding test, the measurement of evaporation, the setting time test and the plastic shrinkage cracking test were carried out to investigate the effects on the plastic shrinkage cracking of concrete with different water-cement ratios, environmental conditions and restraints. The appropriate conditions were selected to carry out the plastic shrinkage cracking test of concrete with low volume of fibers. The crack prediction value was obtained by using all the above test results, and the value as well as the final crack area were put into the crack prediction model to compare the severity of plastic shrinkage cracking.
Finally, the final crack area and the crack prediction values of fiber concrete mixed with the plastic fibers used in this study are compared to the values of general concrete. The fiber risk reduction values of plastic fibers used in this study with different volume substitution were obtained and applied to the equation of crack prediction value.

並列關鍵字

plastic fiber ； plastic shrinkage cracking ； low volume fiber reinforcement concrete ； interfacial shear bond stress ； fiber risk reduction

參考文獻

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