Strengthening of prestressed viaducts by means of a reinforced concrete overlay

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1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN Strengthening of prestressed viaducts by means of a reinforced concrete overlay R.W. Keesom BAM Infraconsult, Gouda, The Netherlands W.J. Bouwmeester van den Bos BAM Infraconsult, Gouda, The Netherlands, University of Technology, Faculty of Civil Engineering and Geosciences, Delft, The Netherlands M. van Kaam BAM Infraconsult, Gouda, The Netherlands A.Q.C. van der Horst BAM Infraconsult, Gouda, The Netherlands, University of Technology, Faculty of Civil Engineering and Geosciences, Delft, The Netherlands ABSTRACT: Many viaducts in the Netherlands have been designed for traffic loads that are smaller and less intense than required by the present day standards. This means that for maintaining the structure s primary function, recalculation and often strengthening is required to guarantee safety and durability. For the viaducts in highway A9 near Amsterdam strengthening by a reinforced concrete overlay is used. This paper describes material testing of the existing structure, FEM-modeling of the existing and overlay structures and some aspects of the detailed design of the concrete overlay. 1 INTRODUCTION Recalculation of the structural integrity of seven viaducts in highway A9 has been conducted as part of a large-scale road renovating contract called KOS- MOS, by Rijkswaterstaat (the Dutch Public Roads and Water Works Authority), which was awarded to the combination of BAM Civil and BAM Roads. Although (visible) damage to the viaducts was negligible, assessment of strength and fatigue behavior was required, due to the heavier and more intense traffic loads than anticipated during the design 35 years earlier. The philosophy of the Client was that the Contractor should either proof a surplus service life of minimum 15 years according to Dutch standards and the Rijkswaterstaat regulations (partly specifically developed for existing structures), or to strengthen the structure to enable a surplus service life of minimum 100 years. As the traffic on working days is intense, the renovation and strengthening measures needed to be completed with minimum traffic disruptions. Elaborate phasing of work was required to fulfill this requirement. For the same reason the project was executed during the summer holidays and continued 7 24 hr during 6 weeks. BAM Infraconsult, the in-house engineering company of BAM Infra, advised on material testing and in-situ testing of concrete mix and concrete strength development, conducted all required FEMcalculations and performed the detailed design for the strengthening measures. Figure 1. View of typical viaduct. 685

2 2 GENERAL DESCRIPTION OF VIADUCTS The viaducts are all designed and built around They typically consist of two separate decks (one per direction of travel) and three to four spans per deck, see Figure 1. The viaducts are build using prestressed girders and a reinforced cast in place deck. The cast in place deck is designed for the negative bending moments at the supports, caused by the statically undetermined system. The structural dimensions and reinforcement steel quantities were derived from as-built drawings. The thicknesses of the cast in place deck and the asphalt layer were mainly taken from the drawings and subsequently verified in-situ. 3 DESIGN APPROACH The design approach was based on four main principles: Assessment of the actual material strength of girders and cast in place deck in order to utilize the strength increase over time; FEM-calculations of the decks with ESA PT to accurately predict the structure s response to traffic loads as realistic as possible; The requirement to minimize traffic disruption let to tight phasing of the required strengthening works, see Figure 2; A fit-for-purpose concrete mix design and on-site testing of overlay properties and anchor strength were incorporated to prove the effectiveness of the proposed overlay. 4 MATERIAL INVESTIGATION The viaducts have been designed and built around The design concrete strength is K300 for the compressive layer and K600 for the girders. This can be compared with present concrete classes according to NEN-EN of successively C20/25 (f ck = 25 N/mm 2 ) and C40/50 (f ck = 50 N/mm 2 ). For the recalculation of the decks, the actual material strength is of mayor importance. The design concrete strength is determined after 28 days of hardening. Due to ongoing hydration the strength increases over time, especially when blast furnace cement is used. For the assessment of the actual material strength concrete cores were drilled and tested for both compressive and tensile strength. For the evaluation of the compressive strength class, use is made of NEN-EN 13791, see table 1. The actual material strength proves to be much higher than the design concrete strength. For practical reasons, drilling of cores is minimized and viaducts of same type and construction period are combined. Table 1. Results testing compressive strength of cores and related concrete class. Construction Number f ck,is Concrete Viaduct part cores (N/mm 2 ) class KW157 girder 9 81 C80/95 KW158 cast in place 9 69 C60/75 KW159 deck KW161 girder 3 96 C70/85 cast in place 3 49 C45/55 deck KW174 girder 9 74 C70/85 KW175 cast in place 9 55 C45/55 KW181 deck characteristic compressive strength in-situ determined according to NEN-EN concrete class according to NEN-EN in combination with NEN As the relation between compressive strength and tensile strength is not generally accepted for high strength concrete and aged concrete, cores were drilled for testing on tensile strength as well. At first, the tensile strength was determined by the direct tension test. This test method is used in case of suspicion of expansive reactions like ASR. The result of this test is not very reliable, because direct application of a pure tension force, free of eccentricity, is very difficult (Neville). Therefore, during the design phase, extra cores were drilled and tested by the tensile splitting test. The average of the tensile splitting results was compared with the calculated average tensile strength (f bm ) according to the relation to the characteristic compressive strength (f ck ) as given in NEN 6720: In table 2 the results are presented, inclusive a remark whether the in-situ tensile strength is higher (+) of lower( ) than the calculated tensile strength. All results are in the same order (97%) or higher. For the recalculation of the decks, the actual compressive strength class is used, inclusive the corresponding tensile strength. In NEN 6720, the shear stress capacity of concrete is a function of the tensile strength. 5 FEM-CALCULATIONS The influence of the construction sequence is investigated by super positioning of the statically determined state (loads: dead weight of girders and cast in place deck) and the statically undetermined state (loads: asphalt and live loads). It is concluded that, due to creep of the concrete after approximately 35 years of 686

3 Figure 2. Principle of phasing for one of four shifts. Table 2. Results testing splitting tensile strength of cores and related tensile strength according concrete class. Construction Number f bm(n),is f bm Viaduct part cores N/mm 2 N/mm 2 KW157 girder 3 5,7 5,7 + KW158 cast in place 3 5,0 4,7 + KW159 deck KW161 girder 2 5,0 5,2 cast in place 4 4,1 3,8 + deck KW174 girder 3 5,3 5,2 + KW175 cast in place 5 4,6 4,0 + KW181 deck average tensile strength in-situ, determined on basis of cores. average tensile strength calculated according to NEN service life, approx. 42% of the stress state is statically determined and approx. 58% is statically undetermined. Thus, the stress state in the decks had been shifting considerably to the statically undetermined state. Structural assessment of these type of deck structures is commonly performed by modeling a deck as a 2D orthotropic slab with different stiffnesses in longitudinal and transversal directions. For the A9 viaducts, the chosen model is a 2D orthotropic cast in place deck combined with eccentric girders attached to the slab. This approach was chosen because of the more realistic modeling of the structural behavior and it was anticipated that optimizations could be achieved in strengthening measures. The choice of model type caused considerably longer calculation times (up to 1 hour) and produced Figure 3. Longitudinal bending moments in the cast in place deck; tension at the bottom of the slab. some results that are not normally encountered when applying the standard orthotropic model, especially: Torsional moments in the girders. Bending moments in the cast in place deck in longitudinal direction due to the distribution of the wheel forces, see Figure 3. For the torsional effects in the ribs in de model, two approaches are valid. Either the torsional stiffness is neglected and the therefore increased bending moments in the girders are assessed, or the torsional stiffness is limited to 40% of the uncracked stiffness (as allowed in the ROBK standard) and the combination of torsional and shear stresses in the girder is assessed. In the latter case, the bending moments are reduced due to higher distribution between the girders. 687

4 Compared to the transversal bending moments in the cast in place deck as derived from the conventional orthotropic slab modeling, the transversal bending moments in the chosen model were considerably lower. This can partly be explained by the torsional stiffness of the girders which contra-acts the bending curvatures of the cast in place deck. Also, the actual transversal bending moments proved to be less than the usually assumed sum of the distribution function of the slab and the local moments due to wheel loads between the girders. The maximum moments caused by the distribution function of the slab did not coincide with the maximum local bending moments. In assessing the slab bending moments in this way, the strengthening of some viaducts could be avoided. The outer girders proved to yield the highest bending moments and shear forces. These are considered for the checks in the ULS and the SLS. For the checks on fatigue strength, the actual lane layout on the viaduct (with an emergency lane at the edge) has been taken into account. 6 PRE-STRESS IN THE GIRDER By calculating the effects of construction sequence, pre-stress, creep and shrinkage of the girders and the cast in place deck, it could be demonstrated that the pre-stress in the girder will not significantly distribute over the combined cross-section. Due to creep and shrinkage of the young cast in place deck on the (already older) girders, the pres-stress force remains almost completely in the girder. This enables large calculation benefits, because of the design rules of NEN 6720 (design of concrete structures) where the permissible tension stress in the girder due to bending is related to the average prestress in the girder. A higher average pre-stress leads to a higher compression zone and thus to cracks of smaller depth in the girder. 7 STRENGTHENING OPTIONS Three of the seven investigated viaducts proved to have adequate strength for a surplus service life of minimum 15 years, thus needed no strengthening. Four viaducts had structural shortcomings such as insufficient reinforcement in the cast in place deck at the supports, high tension stresses in the girders at mid-span or insufficient transversal reinforcement in the cast in place deck. All of these shortcomings can be solved using a reinforced concrete overlay of approximately 100 mm thickness. Strengthening by applying carbon sheets has been considered as well, but the capacity to reduce tension stresses in the SLS in pre-stressed girders is limited Figure 4. Detail of design of concrete overlay. and strengthening of the bottom side of the cast in place deck is evidently impossible. 8 DESIGN OF CONCRETE OVERLAY The specifications for the overlay were partly prescribed by the Client, e.g.: Concrete grade C53/65. Minimum bonding strength 1.5 N/mm 2. Minimum number of steel bar anchors 4 per m 2 and 10 per m 2 at the edges. The FEM-calculations showed that the 4 anchors per m 2 in the middle of the overlay were not strictly required, as the adhesive capacity of the overlay was more than sufficient. The 10 anchors per m 2 at the edges, however, were required to withstand the curling-up forces caused by shrinkage of the overlay after casting. The thickness of the overlay was minimized to approximately 100 mm to accommodate two layers of reinforcement, sufficient concrete cover (40 mm) and enough space to apply the (fine) concrete mix. Minimizing the overlay thickness was necessary to avoid substantial higher dead-weight loads on the substructure and foundations of the viaducts. The increase of dead weight will raise the foundation forces at the pile toes up to approximately 10%. This is considered acceptable, because of the relative large safety factors applied in designing the foundation. 9 TESTS ON MIXTURE The mix design of the overlay is based on the following specifications: Compressive strength class: C53/65 (Dutch: NEN- EN in combination with NEN 8005). Minimum bonding strength: 1.5 N/mm

5 Figure 5. Trial casting. Figure 6. Detail of roughened slab face. At one spot rather strange circular castings were visible after milling. A required surplus service life of minimum 100 year. Concerning the intense traffic, the granted working and hardening periods are limited. The last mentioned specification is translated into three strength time requirements: During the hardening of concrete, traffic vibrations has to be minimized. No traffic is allowed on the deck where the overlay is cast until a compressive strength of 12 N/mm 2 is reached (Ansell & Silfwerbrand). The available time span is 29 hours (including casting). The asphalt application, which is planned after 45 hours, is allowed after a compressive strength of 20 N/mm 2 is reached. 6 days after casting, the overlay has to be ready for use. The mix design is developed is close cooperation with the concrete supplier. Due to the specification for the service life the use of CEM III/B is preferred. A trial casting is performed to tackle possible problems and delays on site, see Figure 5. During the trial casting: The strength development is documented and verified: the strength requirements were easily met within the available time spans, even with the use of CEM III/B. A bonding strength of 1.5 N/mm 2 after 6 days is reached provided the preparation of the underlying surface has been done by milling and water jetting at high pressures. Several curing methods (with and without plastic foil, shaded) were tested: no significant effect was concluded. Based on the trial casting the final mix design is determined and the working method is established, including quality control measures to verify the execution. 10 EXECUTION OF STRENGTHENING The tight working schedule and contractual fines on delays required a thorough planning and effective execution of the works. The tight schedule was primarily met due to: The choice of reliable suppliers. Designing for constructability including high working speed. Developing method statements independent of weather conditions. Pre-construction testing and a high level of quality assurance of the product in order to avoid later improvements or replacements. Site-engineering support available at all times during construction. The chosen approach has been successful, as the works were finished in time and only minor difficulties occurred during execution. In order to ensure proper bonding between the cast in place deck and the overlay, the surface of the existing slab was roughened by milling the very top of the slab. As the top of reinforcement bars became visible in some cases, it became clear that milling of the concrete slab was done excessively in the past already. As the reduced thickness of the cast in place deck was not considered in the design phase, additional calculations were performed to check the impact of the finding.the thickness of the overlay was enlarged to compensate for the reduced cast in place deck thickness. Directly after milling, holes were drilled for the anchors to connect the existing and new concrete, see Figure 7. The reinforcement layers were prefabricated 689

6 Figure 7. Drilling of holes for anchors. 11 LESSONS LEARNED/CONCLUSIONS The design and execution of strengthening measures at four viaducts near Amsterdam, The Netherlands, has provided a better understanding of both the structural behavior of viaducts consisting of pre-stressed girders and reinforced cast in place decks as well as the possibility of enhancing the strength and durability of the structure by applying a reinforced concrete overlay. The most interesting concluding remarks are: Modeling of the deck with an orthotropic 2D-plate with eccentric girders yields more realistic results than an overall orthotropic deck and can lead to a reduction of strengthening measures; The actual cast in place deck thickness and thickness of asphalt layers have to be assessed prior to recalculation; Material technology (concrete core testing and testing of concrete mix properties) has led to great benefits for the project; The concrete overlay proved to be an effective way of strengthening of these kinds of structures. Overall can be said that constructions made within time and quality requirements can only be done by close cooperation of design, technology and execution. Figure 8. Installation of prefabricated reinforcement units. REFERENCES NEN-EN (en), 2007, Assessment of in-situ compressive strength in structures and precast concrete components, Nederlands Normalisatie-instituut NEN 6720/A4 (nl), 2007, Regulations for concrete TGB 1990 Structural requirements and calculation methods, Nederlands Normalisatie-instituut NEN-EN (en), 2001, Concrete Part 1: Specification, performance, production and conformity, Nederlands Normalisatie-instituut Neville, A.M., 1996, Properties of concrete, Pearson Education Limited, Harlow. Ansell, A & Silfwerbrand, 2003, The vibration resistance of young and early age concrete, Structural Concrete, Volume 4, Issue 3, page Figure 9. Ready-to-cast overlay with anchors and reinforcement. as much as possible to increase working speed, see Figure 8. The anchors were glued-in after installation of the reinforcement. Figure 9 shows the prepared overlay ready to cast. 690