The Experimental Research About Aggregate Size On Concrete Catastrophe Damage

Concrete is a kind of nonhomogeneous and anisotropic engineering compositematerial which is composed of cement mortar and coarse aggregate such as gravel.Because of its low cost, convenient transportation and construction and easy pouring intoany shape, but also has good frost resistance, corrosion resistance and high compressivestrength, so the concrete materials are widely used in industrial and civil buildings.However, in some major accidents such as housing collapse in earthquake, and prematurefailure events in some dam, people found that removal of individual factors suchas construction quality, concrete materials are not reached the design strength but theyhave been destroyed. Later, after the research people discovered that why the catastrophicfailure occurred when the buildings of the concrete structure are far lower than thedesign value, because people do not consider the influence of size effect.The previous scholars have studied a lot about concrete’s various size effects. Thispaper is mainly to study the effects on the specimens from the concrete specimensof different surfaces and different aggregate sizes these two kinds of size effects. Thisstudy is mainly by experiments. Different surfaces of specimens were dividedinto pouring surface specimens and the cut surface specimens.Pouring surface specimens are formed in test moulds. The outer of the specimensare mainly mortar layer. The cut surface specimens are cut by a chunk of concretespecimens. The outer of specimen revealed many coarse aggregate. In different aggregatesize of concrete samples, aggregate diameters were5-10mm,5-15mm,5-20mm.In this paper, we did the hydrostatic pressure test to the different specimens first. Wecompared the strength value and the strain values and we calculated the absorption energyof each specimen. By comparing these values, we found that both in strength value andstrain value or in the absorbed energy pouring surface specimens were higher than cutsurface specimen. In different surface states, different aggregate size specimens in thethree parameters have been consistent, namely the smaller maximum aggregatediameter specimens were greater than the larger maximum aggregate size specimens. Next we used white light speckle method to analysis the deformation field of the specimens’surface. We use strain field fluctuations to quantitative analysis the localization degree ofconcrete specimen strain field in this paper. After the test comparison, we found thatdifferent surface states and different aggregate size have obvious different effect onthe specimen’s strain fluctuation. The localization degrees are different andthe failure modes are different at last.In this paper, we made fatigue loading to the different types of specimens. In the test,we respectively used the white light speckle method, ultrasonic method, acousticemission and low field nuclear magnetic scanning method. By using the white lightspeckle method to collect and calculate, we obtained the different strain field duringloading process. By contrast, these two kinds of size effect also have evident influence onthe concrete displacement field, strain field and shear strain field evolution. Differentsamples have different failure. We use ultrasonic to conduct internal inspection forspecimens after fatigue. Then we found that if specimens’ surface states and aggregatesizes were different, damage amount of specimens were different. That is, size effectsproduce different effect to the internal damage of the specimen.In this paper, we used acoustic emission technique to collect the dates of staticloading and fatigue loading specimens. The test found that in the two kindsof load forms, the influence of size effect is roughly the same as the front tests’ laws.Finally, in order to further analyze the internal pore destruction of the differentspecimens, we conducted MRI scanning on the fatigue specimens. By analyzing T2spectra, pore size distribution, porosity and MRI imaging, we found that when differentsurface states and different aggregate sizes specimens were fatigue loaded,internal pore destruction features of concrete specimens are same as the front tests. Thisalso further confirmed the accuracy of influence law summarized in this paper about howtwo kinds of size effect affect the concrete destruction.