Replacement Ratio Of Demolished Concrete Lumps Research Articles

Shear strength of square concrete-filled steel tubular columns reinforced with internal steel stirrups

A concrete filled steel tubular (CFST) column internally reinforced with steel stirrups has significantly enhanced fire resistance over a conventional CFST column without stirrups. Resistance to shear may be a vital load case to be considered when designing CFST columns. However, there is comparatively little research into the shearing behaviors of CFST columns and the existing design equations do not adequately describe the contributions of different components to the total shear resistance. In this study, thirty-five square CFST columns, with and without stirrups were fabricated and tested to failure under combined shear load and axial compression. The portions of observed shear resistance contributed by the steel tube, concrete infill and stirrups were isolated. The concrete contained demolished concrete lumps (DCLs). The tube’s thickness, the ratio of shear span to depth, the axial load ratio, the replacement ratio of DCLs, the center-to-center spacing between stirrups, and the gap between steel tube and stirrups were varied in the experiments. The test results showed that: (1) the shear capacity was much greater at a smaller shear span-to-depth ratio, but the ratio had little effect on the contribution of the steel tube to shear resistance when ranging from 0.15 to 0.50; (2) the shear capacity gradually increased as the axial loading increased, but the shear contribution of the steel tube decreased; and (3) the internal stirrups contributed little to the shear resistance, and there was little difference in shear capacity as the gap between the stirrups and tube increased from 15 mm to 35 mm. A method to adequately capture the respective contribution of different components in the column was proposed. The new method generally gave more accurate predictions than the methods currently in use, and its predictions were generally conservative.

An experimental study of the axial compression performance of two-part precast concrete columns

Recycled lump concrete (RLC) made with demolished concrete lumps (DCLs) and fresh concrete (FC) provides a solution for effective waste concrete recycling. To promote the development of precast RLC structures, this study tested a new type of connection for precast concrete columns: connecting the upper and the lower halves of columns with bent longitudinal reinforcements and structural adhesive. In this work the behavior of precast RLC columns with the new connection was studied under axial compression. The axial compressive strength of nine two-part columns was tested. The effects of the degree of bending in the longitudinal reinforcement, the replacement ratio of DCLs and the stirrup spacing were investigated. Tests showed that: (1) the failure mode of precast concrete columns is different from that of cast-in-place columns; (2) when the strength of the waste concrete is close to that of the fresh material, there is no significant difference in the axial compression performance of either precast or cast-in-place columns; (3) the bent longitudinal reinforcement causes the axial load bearing capacity of precast concrete columns to be 4.2%–12.3% lower than that of a similar cast-in-place column; (4) reducing the stirrup spacing has little effect on a precast column’s axial load bearing capacity and ductility; (5) when using Chinese and American codes to predict the axial load bearing capacity of the column, the predicted value should be multiplied by a reduction factor.

Axial Compressive Behavior of Geopolymer Recycled Lump Concrete.

To reduce the environmental pollution from cement production and the damage to natural resources from aggregate mining in the concrete industry, a relatively new concrete, termed geopolymer recycled lump concrete (GRLC), which uses geopolymer as the binding material to replace traditional cement and uses large demolished concrete lumps (DCLs) to partly replace concrete, is prepared in this study. Cubic and cylindrical GRLC specimens containing fresh geopolymer concrete and DCLs were tested under axial compression with various parameters, including the compressive strength levels of both fresh geopolymer concrete and DCLs, and the replacement ratio of DCLs. The compressive behavior of the GRLC specimens was compared with traditional cement recycled lump concrete (CRLC) specimens, with test results showing that GRLC specimens possess higher compressive strength than CRLC specimens under the same experimental conditions, which is due to the strengthening effect that fresh geopolymer concrete has on the DCLs. From the scanning electron microscope pattern of the GRLC specimen, it is found that the geopolymer bonds well with the old mortar attached to DCLs. As the replacement ratio increases from 0% to 33%, the elastic modulus of GRLC increases by 5%–11% but Poisson’s ratio remains almost constant (in the 0.16–0.17 range). Based on the measured strength and the predicted results, which coincide with one another well, a modified method for predicting the compressive strength of GRLC cubic and cylindrical specimens is proposed.

Structural behavior of steel-concrete partially encased composite columns containing demolished concrete lumps under axial compression

Steel-concrete composite members incorporating demolished concrete lumps (DCLs) have been devised to provide an innovative alternative means of recycling old concrete. Field applications have demonstrated those members’ potential. In this study the aforementioned recycling method is applied to steel-concrete partially encased composite (PEC) columns. To evaluate the viability of this new solution, a comparative experimental campaign was implemented to investigate the behavior of eleven large-scale PEC columns containing different content of DCLs under axial compression. The key parameters considered were replacement ratio of DCLs, steel flange thickness, spacing of transverse links, the source of DCLs.Overall, the initial stiffness and axial load-carrying capacity of the PEC columns containing up to 33% DCLs were found to be comparable to those of the columns cast with new concrete alone. Nevertheless, a more rapid load drop right after the peak was observed for the columns containing DCLs when the link spacing equaled or exceeded half the column section depth. To remedy this, a maximum link spacing limit of 0.3d (d = the column section depth) is recommended for the columns containing DCLs. An analytical model is then developed to reproduce the full-range axial response of the PEC columns. The model accounts for the gain in strength due to concrete encasement and the loss due to flange buckling. It also attempts to capture the columns’ descending branch with recourse to the concept of compression fracture energy. Applicable to both types of PEC columns (i.e. containing DCLs or not), the proposed model exhibits a good accuracy, evidenced by comparing the prediction results against a set of test data compiled in this study.

Compressive fatigue behavior of compound concrete containing demolished concrete lumps

This research focused on the uniaxial compressive fatigue behavior of compound concrete (CC) made of fresh concrete (FC) and demolished concrete lumps (DCLs) distinctly larger than conventional recycled aggregates. Sixty cylindrical and three planar specimens were tested under constant-amplitude sinusoidal loading. Digital image correlation was employed to monitor the initiation and propagation of cracks on the specimens. The variances in the dissipated energy, Poisson’s ratio, fatigue modulus and longitudinal residual strain of the cylindrical specimens with the number of cycles are discussed. Linear formulas describing the relationship between secondary strain rate and fatigue life are presented. The results indicate that there is no distinct difference in fatigue life between CC and conventional concrete, though the lateral expansion of CC under such loading is greater. The replacement ratio of DCLs can hardly influence the secondary strain rate of CC. The interfaces between DCLs and FC are not evident weak zones in CC.

Shape effect on compressive mechanical properties of compound concrete containing demolished concrete lumps

The viability of directly using coarsely crushed demolished concrete lumps (DCLs) to replace a portion of the new pouring concrete in structural members has been demonstrated in previous research and in practice. However, there is no information focused on the shape effect on the compressive mechanical properties of the compound concrete now. Meanwhile, the lateral deformation characteristic (e.g., volume dilatancy) of the compound concrete made of DCLs and fresh concrete still remains not very clear. To deepen the knowledge related with this concrete type, a total of 93 specimens containing DCLs including cubes, cylinders, and prisms were fabricated and axially loaded up to failure. The replacement ratio of DCLs considered in this study covered a relatively wide range (i.e., 0%, 20%, 30% and 40%). The test results indicate that the compressive strength of cubic compound concrete samples is about 1.30 that of similar cylindrical samples, and is 8.3% larger than that for conventional concrete. The compressive strength of the compound concrete measured with prismatic samples is about 1.08 times that measured using cylindrical samples, while this ratio is about 1.0 for conventional concrete. A formula has been established to predict the compressive strength of the cubic, cylindrical, and prismatic compound concrete specimens with a lateral dimension of 150–300 mm, and the predicted strength is usually within ±10% of the measured value. The ratio of the critical load (i.e., the load at which the volume of the concrete starts to increase) to the ultimate load decreases slightly with increasing DCL replacement percentages.

08.22: Axially compressive behaviours of steel tubes filled with post‐pouring concrete and precast segments containing DCLs and FC

ABSTRACTTo improve the construction efficiency of steel tubes filled with demolished concrete lumps (DCLs) and fresh concrete (FC) at construction site, a new fabrication method is proposed in this paper, and the axially compressive behaviours of four columns prepared using this method have been experimentally investigated and compared with those of the control column. According to the proposed fabrication method, the segments containing DCLs and FC are precast first, and then are placed into the steel tube. Finally, post‐pouring concrete is employed to fill the space between the steel tube and precast segments. In this way, the processing of DCLs is not involved at the construction site, and thus the efficiency of construction can be enhanced significantly. The parameters considered in the compressive tests included the number of the precast segments in the steel tube (S1=one segment, S2=two segments, and S3=three segments), and the cross‐sectional dimensions of the precast segment (C1=331 mm×331 mm, and C2=303 mm×303 mm). The outer diameter and thickness of the steel tubes were, respectively, 500 mm and 5.7 mm. The 150 mm cubic compressive strength of the post‐pouring concrete was 32.8 MPa. The replacement ratio of DCLs in the precast segments was 40%, and the 150 mm cubic compressive strengths of the demolished concrete and fresh concrete used in the precast segments were 31.5 MPa and 47.4 MPa, respectively. The control column was a steel tube filled with DCLs and the post‐pouring concrete simultaneously. The five columns had the same height as 1800 mm, and the weight of DCLs adopted in the control column was the same as that employed in other four columns. In this paper, failure patterns of the five specimens are presented, and influences of the aforementioned parameters on the specimens’ axially compressive behaviours have been discussed. It is found that the columns prepared using the new fabrication method have similar failure modes as compared with the control column, and the number and sectional size of the precast segments have different impacts on the column's axially compressive performance.

Cyclic behavior of thin-walled square steel tubular columns filled with demolished concrete lumps and fresh concrete

This study is part of a comprehensive project concerned with research and advocacy of the structural elements containing demolished concrete lumps (DCLs) or demolished concrete segments (DCSs) with a distinctly larger size than the recycled aggregates. Previous studies showed that the steel tubular columns filled with DCLs/DCSs and fresh concrete (FC), recycling demolished concrete in a simplified manner, have properties similar to the conventional concrete filled steel tubular columns. This research is aiming to study the cyclic behavior and to evaluate the strength and ductility of the thin-walled square steel tubular column filled with DCLs and FC. Fifteen specimens, including 10 columns filled with DCLs and FC and 5 reference columns filled with FC alone, were tested under constant axial compression and cyclic lateral loading. The main variables are: replacement ratio of DCLs, thickness of steel tube, and axial load ratio. The design codes are employed to predict the lateral strengths of the specimens. The findings indicate that: (1) the cyclic behavior of thin-walled square steel tubular columns filled with DCLs and FC is similar with that of the reference columns filled with FC alone; (2) the lateral strength of the columns filled with DCLs and FC is slightly lower than those filled with FC alone; and (3) the thin-walled columns filled with DCLs and FC exhibit acceptable deformation and energy dissipation capacities.

Seismic Tests on Square Thin-Walled Steel Tubular Columns Filled with Demolished Concrete Lumps

This paper presents a most recent experimental work for evaluating the seismic behaviors of square thin-walled steel tubular columns filled with demolished concrete lumps (DCLs). Fifteen specimens with 300 × 300 mm square section, including 10 columns filled with DCLs and 5 reference columns without DCLs, were tested under combined axial compression and lateral cyclic loading. The parameters considered in the tests are: replacement ratio of DCLs, thickness of steel tube, and axial load ratio. Research findings indicate that the seismic performances of square thin-walled steel tubular columns filled with DCLs are similar to the benchmark columns without DCLs, and the square thin-walled steel tubular columns filled with DCLs also exhibit acceptable ductilities and energy dissipation abilities.

Tests on Seismic Behaviors of Double Thin Skin Hybrid Walls Filled with Demolished Concrete Lumps

Double thin skin hybrid walls filled with demolished concrete lumps (DCLs) are composed of two steel plate skin connected by rib plates, with the space between them filled with new concrete and demolished concrete lumps. In order to investigate the seismic behavior of double thin skin hybrid walls filled with DCLs, quasi-static tests of seven specimens were conducted. The parameters considered in the tests are: replacement ratio of DCLs, thickness of steel plate, axial load ratio, rib plate spacing and height-width ratio. Test results show that the seismic performances of double thin skin hybrid walls filled with DCLs are similar to the walls without DCLs, and the double thin skin hybrid walls filled with DCLs also exhibit acceptable ductility and energy dissipation ability.