on the original status of the materials in service in order to evaluate the opportunity of rehabilitating old reinforced ..... hardness methods, TC43-CND. 1116.
Concrete Repair, Rehabilitation and Retrofitting – Alexander (eds.) © 2006 Taylor & Francis Group, London, ISBN 0 415 39654 9
Characterizing old reinforced-concrete structures for compliance with new standard requirements V. Corinaldesi, F.M. Liberatore, F. Pascucci & G. Moriconi Technical University of Marche, Ancona, Italy
ABSTRACT: The rehabilitation of old reinforced-concrete structures raises the problem of satisfying new standard requirements, characterized by a more conservative approach with respect to that of standards in force at the time of construction. In addition, the quality of construction materials has to be evaluated in order to assess their actual state with respect to the original one. In this paper, a procedure is suggested to recover reliable information on the original status of the materials in service in order to evaluate the opportunity of rehabilitating old reinforced concrete structures to meet new standard requirements. This procedure is articulated into various phases, from widespread non-destructive analysis on working materials, evaluated referring to few pointwise mechanical characterizations, to calculations developed for the most significant structural frames. The same procedure was applied to a 50-year old building in order to evaluate the effectiveness of this method.
1
INTRODUCTION
Laws controlling urban development, beyond environmental issues, make new constructions more and more difficult and impose the rehabilitation of old buildings by upgrading and fitting their structures to new standards and new acting stresses. For this reason, the type of intervention depends on the status of the working materials, the damage prognosis, the risk assessment and the service life prediction with respect to new requirements. Obviously, no plan of intervention can be arranged without an accurate analysis of the actual acting stresses, without proper in situ testing in order to assess the actual materials’ status as well as their residual performance and, finally, to outline critical situations. However, this outline cannot be sufficient when the original materials’ performance is unknown. In this paper, a procedure is suggested to recover reliable information on the original status of the materials in service in order to make the life prediction of reinforced concrete structures safe. This procedure was applied to a 50-year old building with a bearing structure in reinforced concrete and later-concrete floors cast in field. Due to the presence of a structural joint, the body of the construction is divided into two adjoining buildings. The first is characterized by a ‘C’ shaped plan inscribed in a rectangle with dimensions of about 21 by 31 metres. The second is characterized by a rectangular shaped plan with dimensions of about 29 by 31 metres. In this work, attention was focused on the ‘C’ shaped building. Its structure consists of a basement
plan and of five other floors. The last floor is mainly a terrace structure. A typical plan and a section of the whole building are reported in Figure 1 and Figure 2 respectively.
Figure 1.
Typical plan of the building.
Figure 2.
Section of the building.
1111
Taking into account the construction date, the building has already completed 50-year service, period which is commonly indicated as the reference life span for reinforced concrete structures. The aim of this study was to carry out the health assessment of the structure and to verify the possibility of changing the building use. 2
PROCEDURE FOR SAFETY ASSESSMENT
Testing on structure was divided into four subsequent stages. The first step was the acquisition of data related to design, which was supplemented by geometrical survey in order to revise the floor framing. The second step was a visual inspection of the materials by removing plaster surrounding columns and, in some areas, also the concrete cover for the examination of both dimension and quality of the steel reinforcement. The third step was devoted to the characterization of materials in general and concrete in particular. At this purpose, concrete was tested by means of rebound hammer in order to determine its hardness, by means of ultrasonic pulse velocity measurements in order to evaluate its dynamic elastic modulus, by means of core sampling for estimating its compressive strength in laboratory. The last step was dedicated to the safety assessment. 2.1
Acquisition of design documents
The main part of the original design documents has been found. Information related to many columns, beams and floors are available for the whole building. In the original structural drawings the main reinforcements are reported while stirrups are not indicated except for columns. As enclosures of the structural drawings, load analysis and the calculus of reinforcements are reported besides information about geometry of the structural elements. Unfortunately, information about the materials’ mechanical properties could not be ascertained. Moreover, a visual inspection was carried out in-situ for verifying the correspondence between structural drawings and the real structure and also for measuring dimensions of the structural elements which were not completely sure. 2.2
Testing materials in-situ
In choosing the structural elements to be tested, columns have been favored with respect to beams because of structural reasons. However, in-situ both non-destructive and destructive testing was carried out. The non-destructive tests were performed on 10 columns for each floor: 5 chosen between those more
loaded and 5 chosen between those less loaded for a total number of 50 monitored columns. The level of stress on columns was evaluated in advance by means of finite-element analysis as is much deeply described below in § 2.5. The attention was focused on the two extreme loading conditions in order to evaluate the ageing effect on concrete due to a high loading rate (4.0÷4.5 MPa, see Figure 8 below) with respect to almost unloaded concrete (0.5÷1.0 MPa). On each structural element under testing, the following tests have been carried out: – – – –
covermeter or magnetometer tests; rebound measures; ultrasonic pulse velocity measurements; carbonation depth.
2.2.1 Covermeter or magnetometer tests A preliminary monitoring was carried out by means of the covermeter in order to point out the exact reinforcement location. A low frequency alternating magnetic field was applied on the surface of the concrete; the presence of embedded reinforcements altered this field and measurements of this change provided information on the reinforcement location and their diameter (Raiak, 2001). 2.2.2 Rebound number (ASTM C 805) The determination of the rebound numbers (see Figure 3) has been carried out according to ACI specifications for in-place methods to estimate concrete strength (ACI Committee 228, 1995). The mean value and the standard deviation of 32 experimental data obtained on each column are reported in Table 1. 2.2.3
Ultrasonic pulse velocity tests (ASTM C 597) This method for estimating concrete mechanical properties is based on the principle that ultrasonic pulse velocity passing through concrete depends on its microstructure; the test consists on measuring the time taken for the waves to travel a fixed distance. This test cannot give a quantitative evaluation of the mechanical strength of concrete but it is quite effective for a qualitative and comparative analysis of the material (ACI Committee 228, 1998). Two or four measurements were performed following the through-transmission method (that is with transmitter and receiver placed at the opposite sides of the structural member), the number of tests depending on the dispersion of data around the mean value. The mean value and the standard deviation of the experimental results obtained on the various columns are also reported in Table 1.
1112
Figure 4. Result of the phenolphthalein test on a concrete core.
Figure 3.
Typical grid of the rebound points.
Table 1.
Experimental results obtained in situ. Mean value
Rebound number
Ultrasonic pulse velocity (m/s)
High-loaded columns Low-loaded columns High-loaded columns Low-loaded columns
Standard deviation
51.0
1.1
51.7
1.1
3561
174
3565
135 Figure 5.
2.2.4 Carbonation depth This colorimetric test was carried out by spraying an ethyl alcohol solution of phenolphthalein on concrete in order to verify the depth of carbonation. In this work, the phenolphthalein test was performed on both the beams after the concrete cover removal and the concrete cores just after their extraction from the columns. In Figure 4 is shown the typical result of this test highlighting the carbonation thickness (light colored) in contrast with the unreacted inner section of the core (pink colored). 2.3
Laboratory testing of materials
The destructive tests were carried out on six columns in all: three chosen between those more loaded and three chosen between those less loaded. These columns are a selection of those previously tested by means of
Image of the coring machine.
both rebound hammer and ultrasonic pulse velocity measurements. In order to extract concrete cores, a continuous coring was performed with diamonded crown and circulating water. The coring machine used in this work is represented in the picture of Figure 5. Concrete cores were characterized by a diameter of 98 mm and a length of at least 200 mm. After the extraction, the left holes were suddenly filled with highstrength shrinkage compensating mortars, in order to restore the previous tensional status. 2.3.1 Compression test on concrete cores Compressive strength of concrete cores extracted from the structure was evaluated. The cores were previously cut in order to obtain a height to diameter ratio of 2:1 and subsequently they were rectified, before undergoing compression test (see Figure 6).
1113
Table 2.
Results of compression test on concrete cores.
Cylindrical compressive strength (MPa)
High-loaded columns Low-loaded columns
Mean value
Standard deviation
16.9
0.6
21.2
0.7
Table 3. Comparison between the experimental results on the steel reinforcement collected in situ and the acceptance limits according to Italian Ministerial Decree D.M. 9 January 1996. Figure 6.
Concrete core under compression test.
Steel in situ
Fe B 32 k
Fe B 44 k
10.5 476.2 755.0 23.1
5÷30 315 490 23
5÷26 430 540 12
The mean value and the standard deviation of the experimental results obtained on both triplets (one relative to cores taken from low-loaded columns and the other from high-loaded columns) are reported in Table 2.
Bar diameter (mm) Yield stress (MPa) Maximum stress (MPa) Ultimate elongation (%)
2.3.2 Tension test on steel reinforcements A section of steel reinforcing bar was removed from a relatively low loaded beam (the less loaded of the whole building) for testing its tensile strength and for comparing it with the design strength of the reinforcement. The results obtained are reported in Table 3, where they are compared with the acceptance limits fixed by the Italian Ministerial Decree D.M. 9 January 1996 (Italian Public Works Ministry, 1996) for two kinds of steel bars for concrete reinforcement (one smooth ‘Fe B 32 k’ and one ribbed ‘Fe B 44 k’). The yield stress measured on the steel reinforcing bar section resulted particularly high with respect to typical yield stress of steel used in the last Forties, probably due to ageing phenomena. However, in terms of ultimate elongation the measured value shows a sufficient residual ductility.
On the basis of data reported in Table 1 and Table 2, it can be noticed that the quality of concrete of lowloaded columns did not undergone any significant change in time, while a strength loss can be detected for the concrete of high-loaded columns, which, at sight, could be ascribed to ageing phenomena. Indeed, this result is not confirmed by some experimental data collected on concrete specimens prepared in laboratory and submitted to low cyclic loads before rupture (Bennet, 1967; Ballatore, 1997; Berra, 1993; Bocca, 2001). For this reason research was planned to verify the influence of increasing cyclic loading on mechanical response of concrete. Under this loading action, the concrete mechanical strength turned higher as well as the concrete stiffness (Bennet, 1967; Ballatore, 1997; Bocca, 2001; Corinaldesi, 2005). These preliminary results justify the development of new research on the effects of long-term loading on the concrete performance, since unexpected behavior could be found. In particular, the mechanical strength of concrete could be affected by the loading direction, meaning that it could be underestimated when measured on cores from columns, currently cored horizontally.
2.4
Discussion of experimental results
The results of the rebound number is not significant because of the carbonation effect which causes a general hardening of the surface of concrete (see Figure 4). Therefore, any evaluation of the concrete strength based on both ultrasonic pulse velocity and rebound number, according to a well-known relationship (RILEM, 1984), cannot be carried out in this work. In the absence of any information about design strength class of concrete, its likely value has been estimated to be not lower than 20 MPa, according to results of compression tests (Table 2) and also keeping into account the routine procedures in use at the time of the construction.
2.5
Modeling and safety assessment
As a first step, a three-dimensional finite-element model of the building was realized with a suitable software (see Figure 7). The stiffening effect of the brick walls has not taken into account and floors have been considered infinitely stiff on their plane. Then, the stress distribution on the building elements was calculated keeping into account all the vertical
1114
Table 4. Typical periods of vibration (T) and participating mass ratios (Mx, My) of the structure.
Figure 7.
No.
T (sec)
Mx (%)
My (%)
1 2 3 4 5 6 7 8 9 10
1.91 1.78 1.57 1.12 0.81 0.74 0.64 0.62 0.54 0.39
68.02 0.21 1.80 8.42 0.00 0.02 1.53 10.49 0.05 0.27
0.45 34.54 40.48 0.02 0.22 9.53 0.39 0.00 5.39 0.22
Finite-element model of the building.
Figure 9.
Figure 10.
Figure 8. Compressive stress on columns’ concrete of the second floor.
loads without any earthquake or wind action. As an example, the maximum compressive stresses obtained on the columns of the second floor are reported in Figure 8 where they are represented by means of a chromatic scale. On the basis of the stress distribution, the structural elements were verified under tension-flexural stresses according to Eurocode 2 (EN 1992, 2004).
First mode of vibration.
Second mode of vibration.
The safety of the columns is always verified also when the actual (i.e. measured) performances of concrete are considered. Moreover, the flexural stresses on beam elements are never higher than the allowable values. Also concerning with shear stresses, the situation on beam is rather safe. Finally, the seismic behavior of the building was studied by means of a dynamic analysis. The results obtained for the first ten modes of vibration are reported in Table 4 and a visual simulation of the first three modes of vibration are shown in Figures 9–11.
1115
Figure 11.
Third mode of vibration.
In this case, as it could be expected, verification criteria according to Eurocode 2 (EN 1992, 2004) of structural elements under tension-flexural stresses were not met: in fact, the seismic action had not been considered in the building design.
3
CONCLUSIONS
In this paper, a procedure is suggested to recover reliable information on the original status of the materials in service in order to evaluate the opportunity of rehabilitating old reinforced concrete structures to meet new standard requirements. This procedure is articulated into various phases, from widespread nondestructive analysis on working materials, evaluated in relation to few pointwise mechanical characterizations, to calculations developed for the most significant structural frames. This procedure was applied to a 50-year old building in order to evaluate the effectiveness of this method. The knowledge of the materials properties achieved by means of destructive testing was adequate for the safety assessment. In addition, the long-term loading of construction materials does not seem to weaken their mechanical properties excessively. In particular, it seems that the lower mechanical strength values detected for the cores horizontally extracted from the high-loaded columns could be attributed to concrete ‘orthotropy’ caused because of the vertical loading. In fact, it seems that the presence of vertical loading positively affects the strength of concrete along the loading direction, likely due to compaction of the material under load, and negatively affects the strength of concrete along the coring orthogonal direction. As a future
development of this research, the effects of long-term loading on the concrete performance, depending on the loading direction, is going to be widely investigated. Concerning with non-destructive testing, it does not appear to be an adequate tool for old concrete structure analysis due to concrete carbonation which strongly influences the rebound number values. The main problems concerning the possibility of fitting new standards requirements are related to the seismic aspect: from this point of view the intervention for seismic compliance appears to be a difficult proposition, without modifying the dynamic behavior of the structure through the insertion of base isolation devices, able to strongly reduce the seismic action on the structure and to make local strengthening interventions really effective.
REFERENCES ACI Committee 228, 1995. In-place Methods to Estimate Concrete Strength, ACI-SP-228.1R. ACI Committee 228, 1998. Nondestructive Test Methods for Evaluation of Concrete in Structures, ACI-SP-228.2R. EN 1992, 2004. Eurocode 2: Design of concrete structures, European Committee for Standardization, CEN/TC 250. Ballatore, E., Bocca, P., 1997. Variations in the mechanical properties of concrete subjected to low cyclic loads. Cement and Concrete Research, 27: 453–462. Bennet, E.W., Muir, S.E., 1967. Some fatigue tests of highstrength concrete in axial compression. Magazine Concrete Research, 19: 113–117. Berra, M., Bocca, P. Thermoelastic stress analysis temperaturestrain relationship in concrete and mortar. Materials and Structures, 26: 395–404. Bocca, P., Crotti, M., 2001. Mechanicl and thermal behavior of concrete preloaded by few cyclic loads (in Italian). L’Industria Italiana del Cemento, 761: 58–63. Corinaldesi, V., Liberatore, F., Pascucci, F., Moriconi, G., 2005. Unpublished results. Italian Public Works Ministry, 1996. Technical Regulations for Calculation, Execution and Testing of Reinforced Concrete Structures, Ordinary and Prestressed, and of Metal Structures (in Italian), Decreto 9 Gennaio 1996, Supplemento ordinario alla Gazzetta Ufficiale, Serie generale, n.29, Rome, Italy. Raikar, C.R., 2001. N.D.T. Techniques: Application in Repairs and Rehabilitation of Civil Engineering Structures. In P.A.M. Basheer (ed.), Proc. Technical session on ‘Non-destructive Testing of Concrete’, Fifth CANMET/ACI intern. conf., Singapore, 29 July–1 August, 2001. RILEM, 1984. Recommendations for testing concrete by hardness methods, TC43-CND.
1116
