混凝土內部之裂縫會損害其抗壓強度，然而裂縫寬度的增加會減少混凝土中的熱傳導性。本研究主要目的在探討水泥漿、水泥砂漿及混凝土受火害所造成寬度約在0.11 mm以下之微裂縫特性。
　　在水泥漿及水泥砂漿部份，採用了常溫、100℃、200℃、300℃、400℃、500℃、600℃、700℃及800℃等九種溫度，此外，在水泥漿體中採用了延時1hr、2hr及3hr等三種延時；而在普通混凝土及自充填混凝土部份，則採用了常溫、400℃、600℃及800℃等四種溫度。試驗方法是利用掃描式電子顯微鏡(SEM)之觀測及PIA(Power Image Analysis)影像分析軟體來進行微裂縫之分析。
　　本研究以裂縫面積比來探討裂縫數量及裂縫寬度的關係，由試驗結果顯示：水泥漿體受200℃~300℃火害後，裂縫面積比的增加主要由於裂縫數量之增多所造成，在400℃~500℃時，則主要由於裂縫寬度之增大所造成，而在700℃~800℃時，是由於裂縫數量增多及裂縫寬度增大所造成。從300℃以後，延時對水泥漿體中裂縫數量及寬度之影響主要在延時2小時以前。水泥漿體中約略0.001 mm至0.002 mm以下的更微小裂縫在約300℃時就明顯呈現，從約500℃起數量開始增多。水泥砂漿受400℃~600℃火害後，裂縫面積比增加最顯著，其中500℃~600℃時的增加主要由於骨材界面裂縫之寬度增大所造成，在約200℃時，漿體內的微小裂縫開始生成，到600℃時，微小裂縫之寬度增大，達800℃時，微小裂縫之數量明顯增多。普通混凝土在約400℃時，已有明顯的微小裂縫，達800℃時，微小裂縫之數量增多且寬度增大，與砂漿中之微小裂縫情形類似；自充填混凝土之微小裂縫發展形態與普通混凝土相似，但微小裂縫的數量較普通混凝土少。

　　Crackings are deleterious to the compressive strength of concrete, however, the increase of crack width decreases the thermal conductivity of concrete. The purpose of this study is to investigate the characteristics of micro-cracks, smaller than 0.11 mm, in paste, mortar and concrete after fire damage.
　　Nine test temperatures, i.e., ambient, 100℃, 200℃, 300℃, 400℃, 500℃, 600℃, 700℃, and 800℃, were adopted to investigate the paste and mortar properties after fire damaged. In addition, in paste, the effect of fire duration for 1hr, 2hr, and 3hr were tested. Four test temperatures, i.e., ambient, 400℃, 600℃, and 800℃ were used for the normal (NC) and self-consolidating concrete (SCC). The scanning electron microscope (SEM) and image analysis software (Power Image Analysis, PIA) were used to observe and analyze the properties of micro-cracks.
　　Crack-area ratio was employed to study the relations of crack-number and crack-width. Test results revealed that, in paste, the increase of crack-area ratio within 200℃-300℃ is mainly due to the increase of the amounts of cracks. When the temperature is within 400℃-500℃, it is mainly due to the increase of the crack width. As the temperature increases up to 700℃-800℃, the ratio increase is primarily due to the increase of both the amounts of cracks and the crack width. For temperatures increased over approximately 300℃, in paste, the effect of fire duration on the amounts of cracks and the crack width is more significant in the first 2 hours period. The smaller cracks in paste, less than about 0.001 mm to 0.002 mm, can be observed clearly at approximately 300℃ and the amounts begin to increase at around 500℃. In mortar, the crack-area ratio increases more significantly between 400℃ and 600℃, and the number increase within 500℃-600℃ is primarily resulted from the increase of the crack width at the aggregate interface. The smaller cracks begin to appear in paste at approximately 200℃ and the crack width increases more significantly at 600℃. The amounts of smaller cracks increase obviously at 800℃. The smaller cracks of NC can be observed clearly at approximately 400℃ and both the amounts of smaller cracks and the crack width increase significantly at 800℃. The SCC have similar crack characteristics as NC except that the amounts of smaller cracks in SCC are slightly less than those in NC.

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