北台灣沿海地區氯鹽環境與混凝土耐久性質之研究

Abstract

本研究主要針對台灣北部沿海地區之氯鹽環境加以調查，並進行現地暴露試驗及老舊結構物之表面混凝土取樣並分析其氯離子含量，進而建立兩者之關聯性，經由試驗結果結合學理分析，提出鋼筋混凝土結構物鹽害之對策分區及保護層厚度設計之建議值。 沿海地區氯鹽量之調查在北台灣地區共設置了45個採集點，儀器係使用日本飛來鹽份採集器，並依據本研究所發展出來之採樣程序進行分析，試驗期間自95年12月至99年8月止。試驗結果顯示各地區除了氯鹽量分佈有很大的差異外，各季節受季風之影響亦有很大之變化，但每年各季節氯鹽量有很好之重現性，藉由有效風向、有效風速、有效雨量及臨海距離等環境因子，可建立出本土化氯鹽量之預測經驗式。 氯鹽環境下鋼筋混凝土耐久性質之探討，係藉由鋼筋混凝土暴露試驗及現地老舊結構物之取樣，分別探討混凝土材料耐久性質及鋼筋腐蝕行為，暴露地點分別為基隆海洋大學、桃園竹圍漁港及苗栗龍鳳漁港，試驗內容包含混凝土力學性質、氯離子滲入行為、表面氯離子濃度及鋼筋腐蝕速率等，試驗結果顯示力學性質與抵抗氯離子之能力隨著水膠比的減少而增加，而添加卜作嵐礦物摻料之配比可有效降低氯離子擴散係數，至於混凝土表面氯離子濃度與時間平方根有很好之線性關係，並可以指數方程式與海岸線距離建立起各配比之關係式。 經由上述之試驗結果，可建立混凝土表面氯離子濃度與大氣中氯鹽量之關係，並可依各地區有效雨量之不同，分別選擇總氯離量或附著氯鹽量之經驗式；此外，藉由暴露試驗所得到的氯離子擴散係數，可用以修正文獻中加速試驗所得到之擴散係數，代入氯離子擴散方程式中，得到各種配比在不同氯鹽環境下所需要之保護層厚度。 此外，本研究並依各地區氯鹽量之分佈情形，將鹽害區域分為極嚴重鹽害區(S)、嚴重鹽害區(Ⅰ)、中度鹽害區(Ⅱ)及輕度鹽害區(Ⅲ)等四區，並由所對應之氯鹽分佈情形訂定出各分區之範圍；由各鹽害分區之保護層厚度計算結果可知，台灣沿海地區之鹽害程度確實相當嚴重，在極嚴重鹽害區(S)及嚴重鹽害區(Ⅰ)的環境下，幾乎無法使用普通強度混凝土來達到抵抗鹽害的侵蝕，使用水灰比0.35之純水泥混凝土保護層厚度需達9~10 cm，而添加40%爐石粉之混凝土則可降至7.5 cm。 比較規範與本研究之建議值，顯示出規範對於較嚴重的鹽害區保護層厚度之考量似有不足之處，而對於輕微的鹽害區範圍之訂定則較為保守，而本研究依據實際環境調查資料所提出之建議值，可初步訂定出各縣市鹽害分區之範圍，並給予較合適之保護層設計值，有助於工程師於沿海環境下設計出抗鹽害且符合經濟效益之構造物。

This research focused on the study of air borne chloride in the coastal region of northern Taiwan, and experiments of on-site exposure tests and chloride content on the surface of old reinforcement concrete (RC) structures, to investigate the correlation between air borne chloride and chloride content of RC structures. Through the theoretical analysis and experiments, the partition of the salt injury to RC structures and the method to estimate the design thickness of covering were proposed in this research. There were 45 sampling sites to investigate the air borne chloride in the northern Taiwan. The instrument of Japanese air borne chloride sampler was used and the sampling and analysis procedure had followed the steps proposed in this study. The testing period started from December 2006 to August 2010. The testing results have shown the obvious difference of air borne chloride distribution among the testing sites, and the apparent influence of the seasonal monsoon. However, the tests also indicate the considerably well seasonal reproducibility of the air borne chloride content. Based on the environmental factors of effective wine direction, effective wind speed, effective precipitation, and distance from the shore, a domestic empirical prediction model for air borne chloride content was established. The study of the durability of RC structures in marine atmosphere had been conducted using RC exposure tests and on-site samplings, to investigate the durability of concrete and the behavior of corrosion of rebar. The exposure tests were performed at NTOU(Keelung), Chu-Wei Fishing Port(Taoyuan), and Lung-Fong Fishing Port( Miaoli). The tests included the mechanics performance of concrete, permeation of chloride, the chloride content on the concrete surface, and the corrosion rate of rebar. The results had shown that the mechanical properties and the resistance to chloride ions increased as the water binder ratio deceased. Furthermore, the addition of pozzolan mineral mixture had effectively reduced the diffusion coefficient of chloride. The chloride on the concrete surface and the square root of time had good linear relationship, and thus an exponential expression with the distance from the shore can be used to estimate the mixture proportion. By the test results, the relationship between the chloride content on the concrete surface and the air borne chloride had also been studied. Depending on the effective precipitation, an empirical equation can be applied based on either total air borne chloride (Cair) or adhesive air borne chloride (Cadh). Besides, the diffusion coefficient of chloride obtained from exposure tests can be used to correct the one from the indoor accelerated tests, and can be applied to the diffusion formula of chloride, to estimate the required thickness of covering in various conditions of mixing proportion and chloride content. The study results from this research can be used to initially define the distribution of salt injury in each county, and to provide adequate design thickness of covering for RC. The procedures and recommendations of this research can helpfully assist the engineers on designs of chloride-resistant and economic costal structures.