CONTINUOUS CONCRETE PLACEMENT OF BRIDGE DECKS IN MULTISPAN STRUCTURES WITH ADDITIONAL RETARDER OVER SUPPORTS

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1 CONTINUOUS CONCRETE PLACEMENT OF BRIDGE DECKS IN MULTISPAN STRUCTURES WITH ADDITIONAL RETARDER OVER SUPPORTS Frank Gallo and Mike Rabkin Abstract Investigations of concrete decks in multispan bridges poured continuously even with retarder indicate that transverse cracks may be developed over the supports. Weak cross sectional properties of composite section over supports at early stages may be subjected to tensional forces caused by self weight of concrete placed at the same time in adjacent spans. Also change in curvatures and deflections may contribute to additional cracking of green concrete. Stage placements reduce cracking, however the construction joints used in stage placement of decks are a source of water penetration and structural damage. Revibration of retarded concrete for continuous bridge decks takes additional time and requires special screed design. Also the method is sensitive to the time of revibration. Abrasive resistance of riding concrete surface after revibration is most likely reduced. Self-repair concrete with adhesives in chemically inert tubes, fibers or beads cast within concrete is very interesting solution but is not cost effective. The authors proposed continuous placement of concrete with the use of additional retardation for concrete over the bridge supports. This method was successfully tested in stage I on approximately 75 Cubic M. of concrete placed for the New York City Department of Transportation in two spans of the Northern Boulevard Bridge over Cross Island Parkway, Queens, New York. 259

2 Fig. 1. General view of Northern Boulevard Bridge over Cross Island Parkway 1. Introduction The Northern Boulevard Bridge over the Cross Island Parkway (Fig. 1) consists of two 15 meters spans connecting Bayside and Douglaston. The bridge was built in 1937, and after 60 years of service it was decided to completely rehabilitate the structure. The Contract drawings called for the entire superstructure replacement and a major alteration of the existing substructure. The owner and the designer of the bridge is the New York City Department of Transportation. The General Contractor is Judlau Contracting, Inc., the Inspection and the Construction Support Services were conducted by Greenman- Pedersen, Inc., one of the leading consultants in the transportation field. Design manuals and other guidelines in bridge engineering often require sequential concrete placement for multispan bridges. The goal in such sequencing is that the positive moment areas of concrete deck should be placed first followed by placements in areas over the supports (so-called closure pours). Issa (1999) illustrated analysis of data for two bridges from Illinois DOT computing maximum curvatures and deflections and compares them with the maximum values that fresh concrete may resist before it cracks. The deflection values in concrete when the deck was cast from the left side of the bridge were significantly greater than in the placement where the pours in the positive moment regions were prior to the negative moment regions. 260

3 Concrete properties at early stages change fast. At the same time concrete in bridge decks become composite with the girders. As concrete starts setting, a composite section with weak concrete properties becomes sensitive to many external and internal factors including shrinkage, temperature and curvature changes, vibration, etc. Therefore, the bridge composite section with minimal concrete strength can not resist tensile forces over the supports in area of negative moments. For example, placement of the concrete in the second span of a multispan bridge creates the additional negative moment over adjacent pier where concrete was already placed (case of continuos placement from the left), however adequate strength is not achieved. In this case transverse cracks over the supports confirm that sequencing of concrete placement in multispan structures may be required. Hilsdorf and Lott (1970) proposed an effective method to close flexural cracks on the concrete surface using retarder in the mix and surface revibration with a vibrating screed approximately 2 hours after the concrete placed. The screed should be designed in such a way that its profile can be adjusted to mach the profile of the deck prior to revibration. Also provisions should be made such that during revibration the screed closely follow concrete surface. Retarders slow down the process of hydratation, however not all of them have the same chemical formula and may act differently. The prolongated finish of retarded concrete may, in fact, increase the duration of hydratation, which is not desirable. The ambient temperature effects the amount of retarder required in the mix, therefore an inadequate amount of retarder and late excessive revibration may increase the crack propagation The critical velocity level of vibration versus age of curing concrete was reported by Esteves (1978). It is important to note that concrete resistance to vibration reduces 40%- 50% between hours after placement, so it is important to avoid revibrating during this interval. The concrete revibration slightly improves the resistance of concrete against surface spalling, however increased spacing factor of air voids reduces the abrasive resistance of the surface. A slight reduction in compressive strength does not significantly affect the overall performance of the deck. Disadvantages of both methods of placement are summarizing in Table

4 Table 1: Disadvantages in two conventional methods of continuous deck placement STAGING PLACEMENT CONTINUOUS PLACEMENT WITH REVIBRATION Construction joints are required Special screed design is required Concrete placement is time consuming Retarders formula may be different Interference of curing water with sequential pours Lack of time and revibration may destroy concrete Hard to make joints on large skew Revibration reduces concrete abrasive resistance Dry (1999) proposed to cast adhesives in chemically inert tubes, fibers or beads within the concrete. At the onset of cracking, the walls of the encapsulated tubes fracture, allowing the adhesive to exit and penetrate the developing cracks. The effect of this self repair technique is dependent on the type of adhesive and position of the capsules within the concrete matrix. Capsules containing strong adhesives should be placed just below the surface in areas of tension caused by bending, i.e. at the top of the sections over supports. The method is expensive and does not guarantees filling of the crack above the capsule. The authors proposed to add additional retarder to HP (High Performance) Concrete placed in the deck over the intermediate pier in two spans of the Northern Boulevard Bridge over Cross Island Parkway. It allowed a continuous pour without any observation of cracks after curing. 2. Approval of continuous Placement The General Contractor made a written request to the New York City Department of Transportation, Roadway Bridges to eliminate the designed sequence of deck placement (Fig. 2) and allow him to continuously pour from the left with additional retarder for the entire concrete. C. L. BRGS. C. L. BRGS. C. L. BRGS. W. ABUTMENT CENTER PIER E. ABUTMENT POUR # 1 POUR # 2 POUR # m 4.0 m 4.0 m m Fig. 2. Sequence of deck placement suggested by Designer 262

5 The change of two staged construction of the deck was proposed when employed one stage continuously placed with the following steps: 1. Set up the screed Gomaco C-450 at east side of the structure and progress placement heading west. 2. Use a concrete pump set up on the west side of the bridge 3. Place concrete at a rate of 20 cub. m/hr minimum. This would translate into one concrete truck every 20 minutes, or an approximate placement time of 3 hours total for the bridge. 4. Use approved class HP concrete. This contains a Set Retarder/Water Reducer (SR/WR) in the mix. Adjust the dosage rate of the SR/WR depending on the temperature to attain a 3-4 hour range on the set delay. Also the Contractor stated that some of the advantages of placing concrete continuously from one end of the bridge to the other included: A. Elimination of unnecessary transverse construction joints, B. Allowed the most effective use of the Gomaco finishing machine. If placed in the contractor s proposed sequence, the machine, which runs normal to the curb line, is effectively used for 30 linear meters of deck. If placed in the sequence suggested by the Designer, the machine is effectively used on only 21 meters. This will substantially reduce the amount of hand finishing usually required near bulkheads, and will further ensure a superior finished product. C. Eliminated curing water problems, often encountered with sequential placement, since the entire operation is one directional. D. Significantly reduced the placement time of the concrete by eliminating the stripping time needed to remove the bulkheads prior to placement over the intermediate support. Placement of retarded concrete without revibration was tested by the same Contractor earlier on the Reconstruction of 9th Avenue Bridge over the New York Transit Authority Yard, Brooklyn, New York. Five spans continuous structure were loaded with class H concrete from the left with additional retarder. Due to complications with the screed, it was impossible to revibrate the concrete at finish. Two years later the authors investigated the conditions of the concrete placed continuously and found transverse cracks over the piers (see Fig. 3) 263

6 Fig. 3. New York City DOT 9 th. Avenue Bridge over the New York City Transit Yard, Brooklyn, New York. General condition of concrete deck over the piers It was decided to reject the Contractor s request for the continuous placement of the concrete deck with uniform retardation, however another alternative of the continuous placement was proposed by the Consultant which included that additional retarder be placed over the intermediate support: 1. It was calculated that 9 trucks would be needed for the entire continuous placement of the deck. 2. The sequence of placement was approved as proposed by the Contractor. 264

7 3. The Set Retarder/Water Reducer for trucks 1-3 and 7-9 was calculated depending on the weather condition, to delay the setting for hours. The SR/WR for trucks 4-6 (between points of contraflecture) was designed to delay the setting for hours. 4. DARATARD-17, a water reducing and retarding admixture was approved, which delayed set time and temporarly suppressed the hydratation reaction. Dosage Rate of Retardar vs. Set Delay is shown on Fig. 4 which was furnished by the supplier. DOSAGE RATE, (gram/100 kg of cement) SET DELAY, HOURS 21 deg. C 27 deg. C 32 deg. C Fig. 4. Dosage Rate of Dratard-17 vs. Set Delay Calculations for SR/WR were performed in the following way: for truck No. 1 at 21 deg.c with 3.5 hours of set delay: 260 x 3.77 = 979 gram/cub.m., where 377 kg is the amount of cement and fly ash, as per approved design mix formula for class HP concrete; For truck No. 5 at 27 deg.c with 6 hours of set delay : 380 x 3.77 = 1427gram/cub.m. The calculations of SR/WR for each truck are combined in Table 2 Dosage of SR/WR in continuous concrete placement for the Northern Boulevard Bridge over Cross Island Parkway 265

8 Table 2 Dosage of Self Retardar/Water Reducer TRUCK # QUANT CUB. M. ASSU- MED DELAY, HOURS CALCUL BATCH TIME SR/WR GR/CUB.M CALC. FINISH TIME TEMP : : : : : : : : : : : : : : : : : :00 3. Deck Placement Deck placement was successfully completed on September 20, Truck, placement and finish information are exhibited in Table 3 Deck Placement Information Table 3 Deck Placement Information TRUCK # QUANT. CUB. M. OUTSIDE TEMP. C MIX TEMP. C END OF DISCHAR. END OF FINISH :07 12: :39 13: :46 13: :00 15: :45 16: :15 16: :35 14: :48 14: :15 15: :27 15:10 Notes: * 9.3 Cub. M. of concrete were rejected during the placement due to overrun of 90 minutes from the time of batching to time of discharge ** Some places on the deck were still fresh when brooming was performed Excessive retarder over the support (trucks 4-6) offset setting time of concrete and extended the period available for finishing operation. When concrete in spans had become composite with the steel girders, the tensional forces were introduced in the upper mat of the deck reinforcement over the support. The concrete over the support at 266

9 this time was still fresh, therefore the cracks from the surface to the reinforcement couldn t be formed. Even in case of forming minor cracks/voids, the brooming operation at the end would eliminate them. Fig. 5. Newly poured deck on Northern Boulevard Bridge over Cross Island Expressway Preliminary investigation of HP concrete performance indicated higher early compressive strength of class HP in comparison with similar class E concrete. The authors believe that possibility of forming of shrinkage cracks is proportional to the early compressive strength. Latest publications ( Schmitt and Darwin, 1999) confirmed this assumption, therefore it was proposed to the NYCDOT use the upper limit of the air content in the mix. Curing of the concrete was conducted in full compliance with NYSDOT Specifications. Continuous wetting burlap was placed over the entire deck and maintained for seven full days. Also, as per Consultant s recommendations, the burlap was left on the deck for another three days in order to avoid rapid drying shrinkage. After the burlap was removed, no visual defects were observed on the deck (see Fig. 5) 267

10 4. Conclusions The following conclusions are made based on the results described in this article: 1. For multispan monolithic bridge decks, excessive retarder in tensional zones reduces the possibility of transverse cracks, if concrete in spans sets before concrete over the supports. 2. Final finishing/ brooming of concrete with additional retarder over the support is effective even before initial setting and can close small voids/cracks. 3. Results from non-uniform retardation of concrete mix depend on the chemical composition of the retarder and may vary significantly. 4. The described method may be combined with concrete revibration over the supports. 5. References [1] Hilsdorf, H., Lott, J. (1970), Revibration of retarded concrete for continuous bridge decks NCHRP Rep. No. 106, Highway Research Board, Washington, D.C. [2] Esteves, J. M. (1978), Control of Vibration Caused by Blasting, Memoria 409, Laboratorio de Engenharia Civil, Ministerio da Habitacao e Obras Publicas, Lisbon, Portugal, p. 11. [3] Issa, M. (1999), Investigation of Cracking in Concrete Bridge Decks at Early Ages Journal of Bridge Engineering, ASCE, May 1999, pp [4] Schmitt, T. R., Darwin, D. (1999), Effect of Material Properties on Cracking in Bridge Decks, Journal of Bridge Engineering, ASCE, February 1999, pp [5] Dry, C. (1999), Structural Crack Repair in Large-Scale Concrete Bridge Decks,. Proc. 8th International Conference Structural Faults + Repair - 99, London, UK,