混凝土樑裂縫之音洩特性探討

A Study of Acoustic Emission Characteristics in Cracks in Concrete Beams

混凝土結構物，例如隧道、橋樑、樓版、連續壁與邊坡擋土牆等，於施工中發生倒崩塌等災害事故頻傳。傳統的監測方式，無法及時反映混凝土結構物發生異狀的警訊。因此本研究應用音洩技術，探討混凝土內部裂縫之聲音特性，以作為營造工程音波監測防災技術之開發依據。本研究在實驗室內規劃 32 根不同齡期、強度與最大粗骨材粒徑之混凝土樑試體，進行三點彎曲試驗，藉以探討混凝土樑裂縫之音洩特性。實驗結果發現：（1） 混凝土樑抵達極限載重的瞬間，混凝土內部裂縫的發展急遽增加，因此造成音洩計數急遽上升。（2） 音洩撞擊之反應歷程區分為三個階段，其中混凝土樑極限加載階段之音洩撞擊增加趨勢，可作為混凝土樑破壞前的警訊。（3） 混凝土於尖峰載重時產生的斷裂頻率，屬於較為低頻的聲音特性。（4） 混凝土樑齡期越長，內部裂縫的成長路徑越少，因此音洩訊號越小；混凝土樑抗壓強度越強，內部裂縫的成長路徑越多，因此音洩訊號越大；混凝土最大粗骨材粒徑越大，內部裂縫的成長路徑越多，因此音洩訊號越大。（5） 混凝土圓柱試體最大粗骨材粒徑超過 15 mm 時，其抗壓強度會有降低的趨勢。本研究成果可提供事業單位應用音洩技術於混凝土結構物之安全監測，亦可應用於混凝土結構物倒塌災害之預警。此外，本研究成果可提供混凝土裂縫安全監測所需之相關資訊，亦可對混凝土裂縫之早期監測與及時預警提供新的研究方向。

Accidents often happen when concrete structures such as tunnels, bridges, slabs, diaphragm walls, or retaining walls collapse while under construction. Traditional safety monitoring methods are incapable of providing alarms in time. The purpose of this study is to apply acoustic emission (AE) technology to detect the growth of cracks in concrete so as to monitor concrete structures and prevent them from collapsing. In this study thirty-two concrete specimens of different age, compressive strength, and maximum aggregate size distribution were wubjected to three-point-bending tests, and the growth of cracks in concrete and AE detection were identified. The test results show: (1) AE counts increase speedily when concrete beams reach the ultimate load, due to rapid interior crack growth. (2) The response of AE hits can be divided into three stages, and the increasing trend of AE hits in the ultimate loading stage can serve as a collapse warning for concrete beams. (3) The cracking frequency of concrete under the peak load is in the lowerfrequency range. (4) As the age of concrete beams increases, interior crack paths and AE signals decrease; and as the compressive strength of concrete beams becomes greater, the interior crack paths and AE signals increase. (5) The compressive strength of concrete decreases when the maximum aggregate size of the concrete is over 15 mm. Furthermore, as the maximum aggregate size of concrete beams becomes larger, the interior crack paths and AE signals increase. The results of this research can assist construction companies in applying AE technology to monitor the safety of concrete buildings, and they can also be used to provide early warning of collapse in structures under construction.