具工作應力裂縫之鋼纖維混凝土梁鹽霧加速劣化後力學行為實驗設計

Abstract

鋼纖維混凝土於ACI318-08中已允許使用於RC結構物中，取代最少剪力鋼筋；惟台灣處於高溫高濕的環境，大氣中帶有鹽分，且結構物在使用年限中時常受工作載重產生裂縫，使得外界氯離子能直接侵入混凝土內部，降低建物耐久性，尤其當鋼纖維因為外界氯離子侵入而鏽蝕時，會加速構件劣化。 本研究透過不同水灰比及添加不同體積取代率的鋼纖維，設計相關實驗來探討混凝土水灰比高低及高強度鋼纖維混凝土梁受工作應力產生裂縫後之耐久性行為。其中實驗分成三大步驟，首先為預裂試驗，再來是鹽霧加速劣化試驗，最後是抗彎試驗及氯離子濃度檢測。預裂試驗模擬真實梁結構在受到工作應力下，在拉力區產生撓曲裂縫情形；鹽霧加速劣化試驗則模擬外界環境經過多年鹽害後腐蝕發展情況；而抗彎試驗則將經過鹽霧加速劣化之試體加載至極限載重，並觀測多項數據來評估劣化處理後梁試體的力學及韌性行為表現，並於試驗後將裂縫處混凝土取出進行氯離子濃度檢測。 由預裂試驗結果顯示，高強度未添加鋼纖維之梁試體在工作載重下之裂縫寬度遠大於普通強度混凝土試體，而在添加鋼纖維之後，裂縫寬度有明顯改善變小的情況。而在鹽霧加速劣化試驗中腐蝕電流密度的量測初步結果顯示，普通強度混凝土之試體有最高的腐蝕電流密度，其次為高強度未添加鋼纖維之試體，而添加鋼纖維之試體則有最低的腐蝕電流密度。然而由於梁試體之設計是模擬實際結構使用之尺寸，鋼筋號數較大且有符合規範之保護層厚度，因此要使其產生較明顯之腐蝕效果需要較長時間的試驗，因此本研究未進行最後之力學試驗，僅整理出未來力學試驗之評估指標，提供給後續學者繼續完成此實驗，以驗證具工作應力裂縫之高強度鋼纖維鋼筋混凝土之耐久性及腐蝕後力學行為。

As stated in ACI 318-08, the steel fiber is allowed to be used as minimum shear reinforcement in RC structures. However, the climate in Taiwan is toride and humid with high saltinity in the air and the working load usually induce the cracks in the service life of concrete structure. For these reasons, the chloride ion may penetrate into the concrete easier, and therefore reduce the durability of the structures. Further, the steel fibers corrode and weaken the concrete sturctures. This study investigates the water-cemnt ratio and the addition of the steel fibers which influence the durability of the normal and high strength concrete with working stress cracks. The experiment was divided into three parts, including pre-cracking test, salt spray test, and flexural test. Pre-cracking test is to simulate the formation of flexureal cracks which are caused by working stress. Salt spray test is to simulate the corrosion situation of cracked beam under the salt environment for many years. Flexural test is to evaluate the residual mechanical behaviors and toughness of corroded beam. After these tests, the concrete around the cracks will be removed from beam and starts the chloride penetration test. According to the results of pre-cracking test, the crack width of the high strength concrete without steel fiber is larger than normal strength concrete. With the addition of the steel fiber, the cracks width reduced apparently. According to the results of salt spray test, the corrosion current denstity (Icorr) of normal strength concrete is the highest. The second highest Icorr is high strength concrete without steel fiber and the lowest Icorr is the concrete with steel fiber. However, the beam specimems was deisgned following the guideline of ACI 318-14; thus, it takes more time for reinforcement to corrod severely. In this study, the flexural test isn’t conducted, but the evalution index used in the flexural test was provided for future study in verifying the durability and residual mechanical behavior of steel fiber high strength concrete with working stress cracks.