The Experimental Study On Strengthening Axial Compression Columns By Prestressing

Enlarging concrete section, enveloping iron and strengthening with carbon fiber sheet are the widely-used methods to strengthen concrete column under axial compression, although these methods have their respective defects: wet-work in construction sit, long time for curing and deformation of appearance, the aging of pouring compounds and so on.The strengthened structure is the structure subjected to twice-loading. The old structures have worked and produced stress and strain before strengthening; the new parts just begin to work only under new loading. When the whole structure is loaded twice, the strain of new parts lag behind that of the old, so that the new parts cannot reach ultimate state when damaged.The strengthening by prestressing may resolve the above-mentioned problems. In this way, unloading and strengthening can be achieved synchronously and strain lagging can be cleared up partly or fully. But the method for structure under axial compression is rarely used in our country because of its difficult manipulation. In this test, a special bolt-jack is used to exent of prestressing force to brace angles as the whole structure is under dead-load. In this state brace angles withstand partly load before twice-load and the stress and strain of concrete are decreased. In this way the aim of removal of load and strengthening can be reached.The test data demonstrate that the column strengthening by the prestressing force can be removed load and strengthened synchronously and the lagging of strain of the new parts compared that of the old can be cleared up when twice-loaded. The bearing capacity of whole construction is increased remarkably and the material of both the new parts and the old reach ultimate states. The construction execution of prestressing work has many advantages such as: dry-work, rapidity, no organic adhesion agent, small space to be taken, the high efficiency of strengthening, so this way can be applied in the structures which request to improve bearing capacity largely and take small space afterstrengthening as well as other structures which will work in long time and in high temperature.The key making full use of strengthening by prestressing is to control prestressing. So estimation of stress and strain of the old structure is necessary and to estimate accurately the stress and strain of the old structure should be researched furtherly.In this test a new method was adopted to check and test the behaviors of structures by using optical fiber Bragg grating sensors which realize the synchronous measurements with electricity-resistance gauges. The results show that optical fiber Bragg grating sensors has boundless prospects for checking in Civil Engineering.