CRC 528-D4: Strengthening of normal and torsion force exposed members with TRC
Table of contents
Project data
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Report in the yearbook 2010
Column strengthening with TRC
Deformation measurement at the prop head
Research on differently shaped column heads has shown the considerable strengthening effect of textile reinforced concrete (TRC). Now we aim to verify the validation of the developed calculation models in large scale tests.
On the one hand the increase of the load-bearing capacity is partly based on the additional concrete covering. On the other hand the confining effect of the textile reinforcement is also essential especially within the load introduction area of beams. The confinement results in a tri-axial state of stress in the concrete core and thus augments the ultimate load of the whole column. With a new developed calculation model we could predict the increase of the ultimate load of short strengthened columns. How accurate the predictions of the model for reinforced large scale columns were was examined by large scale testings at three reinforced concrete columns with round cross sections. They were 2 m high and each had a diameter of 300 mm. One column stayed un-strengthened and served as a reference. Two more were strengthened with 5 layers of TRC over their whole length while one of them received 2 more layers of reinforcement at its head and base.
The bearing capacity of the TRC strengthened columns was nearly twice as high as the reference’s. The achieved ultimate loads had been predicted very well by the developed confinement model. The normal force part of the fine-grained concrete mainly affects the reachable ultimate load raise until the failure of the column core. After this failure it probably comes to the creation of a frictional bond between fine-grained concrete layer and the core concrete due to the augmenting lateral strain. Thus the tied up part of the fine-grained concrete covering is being activated again and more longitudinal forces can be carried off through the TRC layer. This mechanism further contributes to the confinement of the core concrete to the load bearing raise.
Besides an increase of the ultimate load the textile reinforcement also led to a considerably improved prior warning behaviour. During the load increase fine and later well visible and, homogeneously distributed cracks occurred. Approaching the ultimate load the concrete cover predicted this by flaking. All in all we have reached a high ductility of the reinforced columns.
Report in the yearbook 2009
Normal force and torsion strengthening
Test specimen in a compression test machine
Considerable improvements of the load-bearing behavior of RC structures, as shown for flexural and shear strengthening with textile reinforced concrete (TRC), has prompted current tests to assess normal and torsional strengthening where TRC is used. The following introduces this research and its results.
Studies of laboratory produced quadratic and circular columns and beams of reinforced concrete strengthened with TRC were conducted. The specimens’ surfaces were roughened before pre-wetting and subsequently applying the TRC strengthening. Several variations of the test specimens were taken into consideration and studied; these included: type of reinforcing material (i. e., alkali-resistant glass or carbon fibre reinforced TRC); amount of reinforcement applied (i. e., 1…6 layers of textile reinforcement); and the radii of curvature of quadratic section. Textile reinforcement was aligned at a 45 ° angle in test specimens loaded with torsional forces since this is the preferred angle for torsion loads.
A compression test machine and a specially designed torsion test set-up were used to conduct the load tests. Load dependent experimental deformations were measured by the use of: strain gauges, linear variable differential transformers (LVDT), and photogrammetric methods.
Normal force strengthening. The ultimate load increase results from the enlargement of the cross-section due to the added concrete layer and the confinement effect of the textile reinforcement. Results of the examinations of circular columns show a considerable confinement effect, while quadratic columns only show an increasing capacity only with the increasing size of edge radii. Furthermore, a growing amount of reinforcement also augments the load capacity. The confinement creates a tri-axial stress state in the concrete core of the column and leads to an increase in the load carrying capacity.
Torsion strengthening. Since textile reinforcement can be aligned parallel to tensile forces in the structure (at an angle of about 45° in this case), it is not only possible to achieve an increase in the load capacity but also in the stiffeness and, therefore, the serviceability. The enlargement of the cross-section further increases the load-carrying capacity of the compression strut. The capacity of the test specimens could be enlarged by 300 %. Considerable decreasing crack widths and spacing also resulted from textile reinforcement concrete strengthening, which greatly enhances the serviceability, as well.