Confined concrete and reinforced concrete members undermonotonic, repeated and reversed loading

Abstract

Ductility is of considerable importance in the case of reinforced concrete structures subjected to both static and dynamic loading. The required large ductility of a structure can be achieved by designing the members of the structure to have enough rotation capacity at critical zones. This can be achieved by confining concrete in the compression zone with steel circular spirals. Studies on reinforced concrete specimens with circular spiral confinement are very few.

Investigations on rectangular sections with circular spiral confinement in flexure combined with axial compression are also scarce.

The necessity for a ductile inelastic design is greater in the case of reinforced concrete structures subjected to seismic forces. Under the influence of strong winds, hurricanes, and earthquakes, the members of a structure are subjected to repeated and/or cyclic reversals of forces. Out of the large number of cycles the structure will experience during these actions, only a few cycles will be of severe nature. Therefore, a study of the behavior of reinforced concrete members under a few cycles of overload will be helpful in developing suitable design procedures for structures subjected to earthquakes and hurricanes. Noting the beneficial effects induced by circular spiral confinement in reinforced concrete members under monotonic loading, it is desirable to study its influence on confined reinforced concrete members subjected to repeated and cyclic reversed loading.

In view of the above, the present investigation was undertaken to study: (i) Properties of concrete confined in circular steel spirals and the reinforcing steel under monotonic and cyclic loading. (ii) Behavior of reinforced concrete sections with circular spiral confinement under monotonic, repeated, and cyclic reversals of flexure. (iii) Behavior of reinforced concrete sections with circular spiral confinement under combined axial compression and monotonic or cyclic reversals of flexure.

The experimental and analytical parts of the investigation, along with a review of the literature available on related aspects, are presented in the thesis in three parts.

Part I (Chapters 2 to 5): Properties of Confined Concrete and Reinforcing Steel under Monotonic and Cyclic Loading Properties of confined concrete under monotonic and repeated loading are studied with reference to the axial compression tests conducted on 38 confined concrete cylinders under monotonic loading and 28 cylinders under repeated loading. Based on the test results under monotonic loading and also on results available from literature, a single equation has been proposed for the complete stress-strain curve for confined concrete under monotonic loading. Stress-strain equations are also proposed, based on repeated load tests, for the envelope, unloading, and reloading curves of confined concrete.

Tests were conducted on 31 specimens of reinforcing steel to examine the stress-strain characteristics of high-strength deformed bars and mild steel bars under monotonic, repeated, and cyclic reversed loading. For high-strength deformed bars-which do not have a well-defined yield plateau-stress-strain relations under (i) monotonic loading and (ii) reversed loading have been proposed in the form of the Ramberg-Osgood function, fixing the bounds of the function from the test results. Test results of mild steel specimens under reversed loading have been compared with analytical models proposed by other investigators.

Part II (Chapters 6 to 9): Reinforced Concrete Sections with Circular Spiral Confinement under Flexure The strength and behavior of reinforced concrete sections with circular spiral confinement under (i) monotonic, (ii) repeated, and (iii) cyclic reversals of flexure are studied with reference to tests conducted on 50 simply supported beams under two-point symmetric loading. Out of these, 18 beams were tested under monotonic loading, 20 under repeated loading, and 12 under cyclic reversed loading. The variables in the study are the steel ratios and the degree of confinement.

Test results of monotonically loaded beams indicate that while the increase in strength of confined specimens over unconfined specimens is marginal (2 to 20%), there is a considerably large increase in ultimate curvature. The maximum increase is as high as about 620%.

Test results of beams under repeated loading indicate that a few cycles (3 to 8) of repeated loading near the ultimate load do not adversely affect the ultimate moment and curvature of beams. The moment-curvature plots under monotonic loading can represent the envelope of moment-curvature plots under repeated loading. The increase in ultimate moment and curvature of confined repeatedly loaded specimens over unconfined specimens is similar to that observed under monotonic loading. Stiffness of the specimens decreases with increasing residual deformation and stabilizes after a certain value of residual deformation is reached.

Experimental results of beams tested under cyclic reversed loading indicate that a few cycles (4 to 7) near the ultimate load do not affect the ultimate moment. However, the stiffness of the specimens reduces considerably with an increasing number of cycles. Confinement plays an important role in preventing concrete disintegration and reducing the tendency of rod buckling in compression during load reversals.

Theoretical moment-curvature relations are derived for reinforced concrete sections with circular spiral confinement under monotonic, repeated, and cyclic reversals of flexure using stress-strain relations for confined concrete and steel under monotonic and cyclic loading proposed in Part I. In view of the complicated stress distribution during cyclic loading, a discrete layer technique (dividing the section into multiple horizontal elements and considering stress-strain relations of each layer separately) is used for analysis. Moment-curvature plots obtained from the analysis are compared with test results, and a satisfactory agreement is observed. Test results of 24 confined beams from literature were also analyzed with the proposed method, and satisfactory agreement of ultimate strength was obtained.

Part III (Chapters 10 to 12): Reinforced Concrete Sections with Circular Spiral Confinement under Combined Axial Compression and Monotonic or Reversals of Flexure Studies under combined axial compression and flexure are carried out with reference to tests conducted on: (i) Eccentrically loaded short columns of rectangular cross-section with circular spiral confinement under monotonic loading. (ii) 12 similar specimens under cyclic loading with alternating eccentricities.

Pitch of the confining spirals and the eccentricity were varied.

Test results of monotonically loaded specimens indicate that while the increase in ultimate moment of confined specimens compared to unconfined specimens is small, there is a large increase (up to 812% in some cases) in ultimate curvature. This is due to the effect of confinement in increasing the strain capacity of the concrete and reducing the unsupported length of steel rods, delaying buckling.

Test results of cyclically loaded columns indicate that specimens with confinement withstand more cycles at larger deformations compared to unconfined specimens.

Theoretical moment-curvature relations are derived for confined reinforced concrete sections under these loadings and compared with test results. Satisfactory agreement is observed. The analysis for monotonically loaded sections is extended to study the interaction phenomena of axial loads, moments, and curvatures at ultimate, as well as ductility factors. The analysis for reversed loading is extended to examine the effect of different levels of axial load and confinement on moment-curvature relations after the first reversal of flexure.

The present investigation provides information on the strength and ductility of reinforced concrete structural elements with circular spiral confinement subjected to repeated and reversed loading, which will help in designing reinforced concrete structures to withstand such loading.