509 POST-INSTALLED REBAR CONNECTIONS UNDER SEISMIC LOADING Isabelle Hofacker and Rolf Eligehausen Institute of Construction Materials, University of Stuttgart, Germany Abstract This paper presents results of monotonic and cyclic tests with post-installed reinforcing bars performed at the research laboratory of the University Stuttgart. The main objective was to study the behavior of post-installed rebar connections in uncracked concrete under cyclic loading. Varied was the type of mortar and the peak displacements during reversed cyclic loading between constant displacements. The results show that the behavior of post-installed rebars under cyclic loading depends on the failure mode under monotonic loading. If pullout is caused by a bond failure between mortar and bar, the cyclic behavior of post-installed rebars is much the same as for cast-in-place rebars. On the contrary if pullout is caused by a bond failure between mortar and concrete then the bond behavior of post-installed rebars under reversed cyclic excitations maybe rather poor. In the paper the tests and the evaluation of the results are presented. 1. Introduction 1.1. General The requirements in earthquake resistant structures usually lead to the need for large seismic energy input absorption and dissipation through large but controllable inelastic deformations of the structure. To meet these requirements, the sources of potential structural brittle failure must be eliminated and degradation of stiffness and strength under repeated loadings must be minimized or delayed long enough to allow sufficient energy to dissipate through stable hysteric behavior. In reinforced concrete, one of the sources of brittle failure is the sudden loss of bond between reinforcing bars and concrete in anchorage zones, which has been the cause of severe damage to, and even collapse of, many structures during recent strong earthquakes. Even if no anchorage failure occur, the hysteric behavior of reinforced concrete structures, subjected to severe seismic excitations, is highly dependent on the
Transcript
509
POST-INSTALLED REBAR CONNECTIONS UNDER SEISMIC LOADING
Isabelle Hofacker and Rolf Eligehausen Institute of Construction Materials, University of Stuttgart, Germany
Abstract This paper presents results of monotonic and cyclic tests with post-installed reinforcing bars performed at the research laboratory of the University Stuttgart. The main objective was to study the behavior of post-installed rebar connections in uncracked concrete under cyclic loading. Varied was the type of mortar and the peak displacements during reversed cyclic loading between constant displacements. The results show that the behavior of post-installed rebars under cyclic loading depends on the failure mode under monotonic loading. If pullout is caused by a bond failure between mortar and bar, the cyclic behavior of post-installed rebars is much the same as for cast-in-place rebars. On the contrary if pullout is caused by a bond failure between mortar and concrete then the bond behavior of post-installed rebars under reversed cyclic excitations maybe rather poor. In the paper the tests and the evaluation of the results are presented.
1. Introduction
1.1. General The requirements in earthquake resistant structures usually lead to the need for large seismic energy input absorption and dissipation through large but controllable inelastic deformations of the structure. To meet these requirements, the sources of potential structural brittle failure must be eliminated and degradation of stiffness and strength under repeated loadings must be minimized or delayed long enough to allow sufficient energy to dissipate through stable hysteric behavior. In reinforced concrete, one of the sources of brittle failure is the sudden loss of bond between reinforcing bars and concrete in anchorage zones, which has been the cause of severe damage to, and even collapse of, many structures during recent strong earthquakes. Even if no anchorage failure occur, the hysteric behavior of reinforced concrete structures, subjected to severe seismic excitations, is highly dependent on the
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interaction between steel and concrete (bond-stress-relationship) [2]. Therefore the behavior of cast-in place rebars under seismic excitations has been studied extensively [1] and design recommendations have been formulated in codes of practice. In practise many structures have to be strengthened to increase their seismic resistance. This is often done by deformed reinforcing bars, which are bonded by a special mortar into a predrilled hole.
Previous investigations with post-installed rebars in pull-out tests and overlap splices under monotonic loading have shown that the bond behavior of post-installed rebars is much the same as that of cast-in rebars provided the injection mortar is suitable [3], [4]. This statement is valid for the failure modes pullout and splitting.
In contrast, the behavior of post-installed rebars under cyclic loading representing seismic excitations is not known. However, this knowledge is needed so that post-installed rebars can safely be used in structures in seismic active areas. Therefore pull-out tests have been carried out to study the cyclic behavior of post-installed rebars. Varied were the type of injection mortar (product A and product B) and the peak displacement during reversed cyclic loading (smin=± 0.2 mm to smax=± 2.0 mm (product B) and smin=± 0.2 mm to smax=± 4.0 mm (product A)).
2. Experimental Program
2.1. Test Specimen Pullout tests were performed with deformed reinforced bars (ds= 20 mm, fy= 900 MPa) installed in a concrete slab (h= 400 mm). The embedment depth was hef = 10ds. In the tests the tested rebars were produced from one lot. To avoid concrete splitting large edge distances (c≥ 250 mm) and spacing (s≥ 500 mm) were used. Two concrete slabs were made having a concrete compressive strength of about fcc~ 30 MPa (measured on cubes with a side length of 200 mm).
2.2. Injection Systems Two types of injection systems were used in the tests. Product A is a hybrid system which employs styrene-free vinylester and cement as binding material. For the cleaning of the holes newly developed special equipment was used. First the hole was cleaned 3 times by compressed air using a special lance. Then the hole was brushed 3 times by a special steel wire brush which was installed in the drilling machine. Afterwards the hole was again air lanced 3 times with compressed air.
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Product B uses unsaturated polyester as binding material. The hole was also cleaned by 3 times blowing, 3 times brushing and 3 times blowing. However the blowing was done using a hand pump and the brushing was done by hand with a steel wire brush. With both products the components of the mortar (binding material, hardener, supplement) are separately preserved. While injecting the mortar the components are automatically mixed in the mixing nozzle.
2.3. Rebar installation Holes (d0= 25 mm) were drilled by rotary hammer drilling. After cleaning them carefully according to manufacturers recommendations (see Section 2.2) the mortar was injected using an injection tool, the holes were filled from the bottom of the hole up to about 2/3 of the embedment depth. Afterwards the rebars were installed under slight turning with the required embedment depth. All tests were performed 3 hours after rebar installation. The curing time was larger than the curing time required by the manufacturer. The temperature of the specimen was about 19° C. Different methods were used to install the rebar in concrete. All specimen tested in monotonic loading were installed in a hole with a depth of h0= 200 mm (Figure 2).
Figure 1. Cleaning process
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The results of the cyclic tests should be compared with the results of tests described in [1]. In these tests a contact pressure at the bar end under compression loading was excluded. To achieve the same condition in the cyclic tests with post-installed rebars, the hole was first drilled through the entire depth of the slab. After cleaning
the hole a wire sleeve (length 80 mm) with the closed end first was installed from the lower end of the slab (Figure 3). Then the hole was injected with mortar and the bar was installed. In this way the correct embedment depth (hef =200 mm) was ensured and no contact pressure could build up. After the tests the wire sleeve, which could easily be removed from the hole, showed indentations resulting from the displacements of the rebar under cyclic compression loading.
2.4. Experimental Setup and Testing Procedure Each specimen was attached to a specially designed testing apparatus and was loaded by a hydraulic servo-controlled cylinder. The tests were run under displacement control by subjecting the free end of the bar to the force needed to induce the desired displacement. The displacement was simultaneously measured on either side of the load application using two LVDTs; the average of the two displacements was used to control the loading. It was necessary to prestress the loading frame to the ground to apply a compressive load to the test specimen during cyclic loading (see Figure 4). No upwards movement of the test setup was observed during the tests. For monotonic tests, the specimens were tested in tension so no prestressing was applied. In all other respects the testing apparatus for monotonic and cyclic tests was the same.
Figure 2. Test specimen (monotonic loading)
Figure 3. Test specimen (cyclic loading)
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2.5. Test Program The program for the monotonic tests is given in Table 1. Besides post-installed rebars cast-in place rebars were tested for comparison. The program for cyclic tests is given in Table 2. Only tests with post-installed rebars were performed. Five identical tests were carried out in each test series to account for the inevitable scatter of results. The main parameters studied in the tests are as follows:
(1) Type of mortar. Two different types of mortar were used with the corresponding cleaning method.
(2) Loading history in the cyclic loading tests. The main parameters were the peak displacement values ∆s between the peak values of displacement between which the specimen was cyclically loaded (∆s1=±0.2 mm, ∆s2=±0.4 mm, ∆s3=±0.8 mm, ∆s4=±2.0 mm and ∆s5=±4.0 mm (only product A)). The number of cycles was 10. After cyclic loading a monotonic tension test was performed.
Series A Series B Series C Type Post-Installed Rebar Post-Installed Rebar Cast-In-Place Rebar Product/ Cleaning Method
Product A/ Machine Cleaning
Product B/ Hand Cleaning
-
Number of Tests 5 5 5
Table 1. Test program for tests in monotonic loading
Figure 4. Test setup
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Series A Series B Type Post-Installed Rebar Post-Installed Rebar Product/ Cleaning Method
Product A/ Machine Cleaning
Product B/ Hand Cleaning
Peak Slip ∆s [mm] 0.2 0.4 0.8 2.0 4.0 0.2 0.4 0.8 2.0 Number of Cycles 10 10 10 10 10 10 10 10 10 Number of Tests 5 5 5 5 5 5 5 5 5
Table 2. Test program for tests with reverse cyclic loading
3. Experimental Results
For reason of clarity only the averaged bond stress-displacement curves are given in the following diagrams. Each series was averaged by a computer program named Origin. The bond strength τ was calculated according to Equation (1):
3.1. Monotonic Loading The test results for monotonic loading are plotted in Figure 5. It shows the average bond stress-displacement curves of post-installed rebars using product A, product B and of cast-in-place rebars. In the pullout tests with rebars post-installed with product A the bond failure occurred in the interface between rebar and mortar. In contrast to that rebars post-installed with product B failed in the interface between mortar and concrete. The bond stress-displacement curve of the cast-in place rebar is similar to that of the rebar post-installed with product A. The rebar post-installed with product A reached an approximately 15% higher maximum bond stress. The stiffness of the ascending
0
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0 2 4 6 8 10 12s [mm]
τ [N/mm2]
post-installed bar (Product A)
post-installed bar (Product B)
cast-in-place bar
Figure 5. Bond stress–displacement diagram for cast-in-place rebars and post-installed rebars in monotonic loading.
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branch decreased gradually from its initial large value to zero when approaching the maximum bond resistance at a displacement value of approximately 1.4 mm (cast-in-place rebar) and 2.0 mm (post-installed rebar). After passing τmax, the bond resistance decreased slowly and almost linearly until it levelled off at a slip of s≈ 11 to 12 mm. This value is almost identical to the clear distance between the lugs of the bars used in the tests. The bond behavior of rebars post-installed with product B is significantly different from those of rebars post-installed with product A and of cast-in-place rebars. The initial stiffness of the bond stress-displacement relationship of rebars post-installed with product B is the same as that of the other bars. However, with product B at a rather low bond stress and corresponding small displacement the stiffness of the bond stress-displacement and the bond strength is reached at very large displacement values (s~15 to 18 mm). The bond strength is about 60% or 45% lower than for rebars post-installed with product A or for cast-in rebars. The bond behavior of the rebars post-installed with product B can be explained as follows. The adhesion between mortar and wall of the hole overcomes at low bond stress values and the load transfer at larger displacement values is mainly due to friction because of pulling rebar with mortar through the hole with relatively rough wall.
3.2. Cyclic Loading The influence of reversed loading on the local bond stress-slip relationship of cast-in rebars has been studied intensively in [1]. These results are compared with the results of the present tests. The results of the cyclic loading tests with post-installed rebars are plotted in Figure 6a)-6e) (product A) and Figure 8a)-8d) (using product B). In these bond stress-displacement diagrams, the first and the 10th cycle and the curves valid for loading to failure after 10 load reversals are plotted. Each Figure is valid for one peak displacement during reversed cyclic loading. Figure 7 (product A) and Figure 9 (product B) show the bond stress-displacement curves after cycling 10 times for all peak slip values. In all diagrams, the corresponding bond stress-displacement relationship for monotonic loadings are shown for comparison.
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Post-Installed Bar (Product A)
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Figure 6a)-e). Bond stress-displacement relationship for monotonic and reversed cyclic loading (product A). Only the first and the 10th cycle with subsequent loading to failure are shown.
Figure 7. Bond stress-displacement relationship for monotonic loading after 10 load reversals (product A).
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Figure 8a)-d). Bond stress-displacement relationship for monotonic and reversed cyclic loading (product B). Only the first and the 10th cycle with subsequent loading to failure are shown.
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Figure 9. Bond stress-displacement relationship for monotonic loading after 10 load reversals (product B).
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Note the different scale for the bond stresses in Figure 8a)-8d) compared to Figure 6a)-6e) for the bond strength which is ±8 N/mm2 (vertical axis) and ±3 N/mm2 (horizontal axis). If cyclic loading is performed between small peak displacement values (∆s≤±0.4 mm) the bond stress-displacement curves of post-installed rebars reach the monotonic envelope for displacement values larger than the peak displacement during previous cycling. If the rebars are cycled between peak displacement values ∆s≥±0.8 mm than the monotonic envelope is not reached again. This behavior is valid for rebars post-installed with product A and product B.
In Figure 12 and Figure 13 the bond stresses after n=2 to 10 cycles at peak slip value related to the bond stress when reaching smax at the first cycle (see Figure 11) are plotted as a number of load cycles. Figure 12 shows the results using product A, Figure 13 those for product B. For comparison Figure 10 shows the corresponding results for cast-in rebars according to [1]. For rebars post-installed with product A the bond deterioration during reversed cyclic loading is much the same as for cast-in-place rebars (compare Figure 12 with Figure 10) if cyclic is done between approximately the same values ∆s. In contrast to that the bond deterioration of rebars
post-installed with product B is much more pronounced than that for cast-in-place rebars (compare Figure 13 with Figure 10).
Figure 10. Deterioration of bond resistance at peak slip as a function of number of cycles (ds=25.4 mm, fcc~ 30 MPa)[1]
Figure 11. Graphics
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Figure 12. Deterioration of bond resistance at peak slip as a function of number of cycles (product A)
Figure 13. Deterioration of bond resistance at peak slip as a function of number of cycles (product B)
The different bond behavior during monotonic and cyclic loading of the bars installed with product A and product B can be explained as follows. Rebars installed with product A overcome the bond resistance at the interface between rebar and mortar. Therefore these rebars behave similar to cast-in-place rebars. The higher bond strength of post-installed rebars is caused by the higher compressive strength of the mortar compared to the compressive strength of the concrete. The failure of bars installed with product B occurs at the interface between mortar and concrete. After overcoming the adhesion strength at relatively small slip values (s~±0.2 mm), the load transfer is dominated by friction between mortar and concrete. This friction is reduced significantly by cyclic loading. The bond resistance of rebars post-installed with product B at small displacement values is rather low and the relatively low bond strength is reached at displacement values which in general can not be used in reinforced concrete structures. Furthermore the bond deterioration during cyclic loading is much more pronounced than for cast-in-place rebars. Therefore this product is not well suited for post-installed rebars under monotonic and cyclic loading.
4. Conclusions
From the results obtained in this study, the following main observations can be made for the local bond behavior of post-installed rebars under monotonic and cyclic loading. The results show that the behavior of mortared-in bars under cyclic loading depends on the failure mode under monotonic loading. According to the test results the bond failure of deformed rebars post-installed with product A failed by overcoming the bond strength at the interface between rebar and mortar (shearing of the mortar between the lugs). Therefore the bond behavior during monotonic and cyclic loading is very similar to the bond behavior of cast-in-place rebars. On the contrary to that rebars post-installed with product B failed at the interface
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between mortar and wall of the hole at relatively low bond stress values and corresponding low displacements. The bond behavior of these rebars under monotonic and cyclic loading was much inferior to the bond behavior of cast-in-place rebars. Therefore product B should not be used to post-installed rebars subjected to monotonic or cyclic loading. Many post-installed rebars may fail by a combination of a bond failure at the interface between rebar and mortar over a part of the embedment length and mortar and concrete over the rest of the bond length. The behavior of these rebars under monotonic loading and cyclic loading will lay in between the two extremes shown above. This behavior should be investigated in tests. The mode of failure of post-installed rebars might change with rebar diameter. Therefore with a given injection mortar, the influence of the diameter on the bond behavior should be checked by tests. In the tests described above failure occurred by pullout of the rebars. In many applications rebars will be installed with a small concrete cover and they might fail by a splitting failure. The behavior of post-installed rebars under cyclic loading in case of splitting failure should also be investigated.
5. Acknowledgement
Funding for this work was made available through the Institute of Construction Materials, University of Stuttgart, Germany. Special thanks to E. Schiebelbein and F. Stockert for their encouragement in data preparation. Thanks to M. Hoehler for improving the English.
6. References
[1] Eligehausen, R.; Popov, E.P.; Bertero V.V.: Local Bond Stress-Slip Relationships of Deformed Bars under Generalized Excitations, Report No. UCB/EERC-83/23, University of California, Berkeley, California, USA, 1983.
[2] Popov, E.P.: Mechanical Characteristics and Bond of Reinforcing Steel under
Seismic Loading, Workshop on Earthquake Resistant Reinforced Concrete Building Construction, University of California, Berkeley, 1977.
[3] Eligehausen, R.; Spieth, H.A.: Anschlüsse mit nachträglich eingemörtelten Bewehrungsstäben. Der Prüfingenieur. April 2000.
