Abstract

Results presentation of an experimental study of concrete deformation caused by the creep of bend stressed and pressured specimens. The measured results were evaluated by comparison of the creep coefficients of bend stressed and centrally pressured concrete specimens. The results that were processed graphically show that the creep of bend stressed elements is significantly larger than that of pressured specimens. This result provides a tool allowing evaluating the stress, deformation or cracking onset and development.

1 Introduction

When evaluating the effect of direct and indirect long-term stress on concrete structures, it is commonly assumed that the creep progress is the same for both tension and compression. However, results of many observations show that the creep of concrete subjected to the tensile stress is more intense. Results of most experiments carried out [1] show that the bending tensile creep of concrete is larger than the compression creep by a factor of almost l.5. Papers dealing with this issue usually state that the reliability of knowledge on creep of concrete subjected to tension, bend or shear stress is still too low for us to assess this phenomenon unambiguously.

2 Long-term measurement of creep

Measurements of concrete deformations on bend and pressure stressed elements were carried out for more than 400 days at a testing laboratory of the Department of Building Testing, Faculty of Civil Engineering Brno, University of Technology. In parallel, shrinkage of unstressed elements was measured. The results of this study demonstrate that the difference between the bend stress and pressure induce creep values, which be in some cases even larger than the ones stated in the valid standards.

The concrete C 20/25 consisted of 350 kg cement CEM II/B - S 325, 835 kg of 0/4 mm fine and 980 kg of 8/16 mm coarse aggregates. The effective water quantity was 175 l for workability 15 s VeBe. Beams of dimensions 100/100/1000 mm and prisms of dimensions 100/100/400 mm were manufactured from this concrete. The elements were removed from the mould 2 days after. Curing in a wet environment took 14 days. 28 days cube strength was 28,6 N mm^-2, the volume weight 2320 kg.m^-3.

The beams 100/100/1000 mm (Sign I) were positioned in couples as single beams of 900 mm span and loaded by a couple of loads placed symmetrically in distance 250 mm. Loading cast iron ingots were positioned on a platform carried by suspension stirrups, whose cross bars loaded the beams from above. The load level was selected so that besides the beam's dead weight a tensile tension s_tequal to 0.4 multiple of the bending tensile strength of concrete of the same age was acting. The beams deflections f in the span centre were measured by a couple of dial gauges of accuracy of 0.001 mm.

The deformation of 100/100/400 mm prisms was measured on the basis of their shrinkage after their removal from the mould. After 28 days of maturing, the prism was placed in a special fixture and loaded by an axial pressure force corresponding to 0,4 multiple of the concrete strength, which was determined for similar prisms after 28 days of maturing. Further prisms were subjected to measurements intended to eliminate the effect spontaneous shrinkage on calculation of the creep-induced deformation. The deformation magnitude was measured by means of type deformation meter fitted with dial gauges allowing the accuracy of 0.001 mm. The compression deformation values e_crp, were calculated from the measured deformation of the given level by deducting the initial load induced deformation e_in, and the respective value of the shrinkage deformation at given time.

3 Evaluation of results

The creep coefficients j_b (t) were determined as f_cr/fin and e_cr/e_in ratios, where f_cr is deflection in time t, f_in is initial elastic deflection, e_cr is compression of prisms in time t and e_in is initial elastic deformations of prism. In the diagram of Fig. 1 the creep coefficient versus time plot are calculated and illustrated for selected values of t. It is evident that the creep coefficient
j_b (t) for beams I reached after 370 days a value that was more then 2.0 times that recorded for a same age prism II pressed. In the initial deformation phases, the ratio of the respective values is still larger.

Fig 1: Time relationship of creep coefficients on bended and pressed specimens.

4 Conclusion

The creep of concrete loaded by tension, bending, shear or torsion require further studies, particularly from the viewpoint of the effect of the strain magnitude, time of loading, effect of long-term slowly growing load, etc. In this way, more accurate basic data would be acquired to be used, for example, to evaluate the strain deformation or crack development in structures, which are indirectly loaded by limited shrinkage or to evaluate the long-term deflections of elements, where the tensile effect is more distinct for low reinforcement degrees.

Acknowledgement

This research has been supported by projects of Grants Agency CR No 103/02/0749, GACR No 103/03/0295 and Ministery of Education of Czech Republic under contracts No CEZ MSM 261100007.

References

1. Hájek J.: Creep of concrete structures (in Slovak), Veda, Slovak Academy of Sciences,1994