For each batch of concrete mixture, three 150-mm cubes were cast and cured in conditions similar to the CFSST test specimens. The average cube strengths (fcu) of the normal concrete and RAC at the time of testing were 65.4 and 64.9 MPa, respectively, and the measured Young’s moduli were 29,500 and 30,800 MPa, respec- tively. These results indicate that the replacement of 50% natural coarse aggregate by recycled concrete aggregate had no obvious influence on the mechanical properties of the  concrete.

Specimen Preparation

All the CFSST columns were designed to have the same length. In preparing the specimens, cold-formed stainless steel tubes were cut and machined to the length of 1,500 mm, and then were welded to a 16-mm-thick steel base plate at one end. Subsequently, SCC con- crete was poured into the steel tube in layers without any vibration. During curing, a very small amount of longitudinal shrinkage of around 0.3 mm occurred at the top of the column. A high-strength epoxy was used to fill this longitudinal gap so that the concrete surface was flush with the steel tube at the top. Finally, another 16-mm-thick steel base plate was welded to the treated top of the stainless steel tube.

Instrumentation and Test  Setup

The specimens were tested under combined constant axial load and cyclically increasing lateral load. The length of the specimens is 1,532 mm including two end plates. The specimens are an idealized representation of the column segment between the inflection points at upper floor and lower floor, and the interstory shear force in- duced by an earthquake is transferred from the floor to the column at the floor level, as shown in Fig. 3(a). The schematic diagram test setup is presented in Fig. 3(b). Prior to testing, the steel base plates of the specimen were attached to cylindrical bearings. This allowed the specimen to freely rotate in-plane, thus simulating pin–pin end conditions. The axial load (N0) was applied and maintained

High strength bolts

Fig. 3. Test set-up (size units: millimeters): (a) CFSST column model in a real building; (b) schematic diagram of test set-up; (c) photo of test set-up

constant by a hydraulic jack of 1,000 kN capacity. The applied axial load was monitored during the whole testing process by a hydraulic control system, and any deviation from the target axial load applied to the column would be automatically adjusted. Precautions  were

made to minimize any accidental eccentricity in applying the axial load by very careful alignment of the test setup. Prior to testing, the load eccentricities at the left and right ends of the specimen were measured, which were less than 0.5   mm.

A very-rigid stub with a width of 150 mm was installed at the midspan of the specimen in order to simulate the effect of rigid floor [Fig. 3(a)]. A hydraulic actuator from MTS Systems Corpo- ration with 250 kN capacity was connected to the rigid stub in order to impose a lateral load, shown in Fig. 3(b). This arrangement aimed to simulate the interstory shear force transferred from the floor to column in a real building under earthquake. The stub ar- rangement contained two separate halves of a box with a concentric hole that exactly fits the perimeter of the specimen (Han and Yang 2005). The two halves were connected to each other using eight high-strength bolts, and the bolts were tightened before testing to eliminate any gap between the specimen and rigid stub, as shown in Fig. 3(c). Two lateral braces were specifically designed to pre- vent the unexpected instability and lateral torsional buckling of the column specimen.

The lateral load was applied by the hydraulic actuator and moni- tored during the complete loading process. Three lateral displace- ment transducers were placed at 1/4L, 1/2L, and 3/4L along the length of the specimen. A series of strain gauges was employed to record the strains of the stainless steel tube during the whole loading process.