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CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 REPORT ON COUNTERMEASURE OF CRACKING OF SUPER T GIRDER I. General After first girder casting finished, through process of monitoring, we found that there are some cracks and breaking concrete at locations as follows: Location: 1. Crack at contiguous location of void and solid section; 2. Location: Crack at angle of contiguous location of void and solid section; 3. Location: Crack of angle on girder end; 4. Location: Traverse crack of girder end; 5. Location: Breaking angle of bottom of girder; 6. Location: Breaking of partition of girder. Reasons: During process of construction, cracks may be appear: because of Creation of formwork, sequence of disassemble formwork, cut PC strands…. are not suitable which cause local stress in concrete of girder over allowable limitation, simultaneously some steel re-bars of girder end is short of length comparing with technical design drawing. Reference of some projects constructed (Ring Road No. 3) also happened situation as the above mentioned. After applied some solutions to treat, it reached good result, therefore we would like to propose some solutions as follows: Page 1 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 II. Countermeasure: 1. Location: Crack at contiguous location of void and solid section: (See picture in Appendix 1) + Countermeasure: Add more 6 rebars A1-D12, L=1000 mm, @200mm at the connection between void and solid of the girder Page 2 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 2. Location: Crack at angle of contiguous location of void and solid section: (See picture in Appendix 2) + Countermeasure: Add a re-bar A2-D12, L=3421 mm and a re-bar A3-D12, L=2928 mm Page 3 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 3. Location: Crack of angle on girder end (See picture in Appendix 3) + Countermeasure: Add 2 re-bars A4-D12, L=720 mm and add 2 re-bars A5-D12, L=610 mm, @50 mm. And cut 1.3m formwork of girder flange so that when cut cable we can disassemble formwork Page 4 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 4. Location: Traverse crack of girder end (See picture in Appendix 4) + Countermeasure: Extend bar NY10-D16 and bar NY11-D16 with length 1329 mm into length 1440 mm. A B Page 5 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 5. Location: Breaking angle of bottom of girder (See picture in Appendix 5) + Countermeasure: - Cutting bottom formwork of girder end and change by rubber cushion (panel). - Be careful during compacting concrete 6. Location: Breaking of partition of girder (See picture in Appendix 6) + Countermeasure: - Surface grinding of angle of internal formwork, cleaning carefully before installing - When disassemble internal formwork, using 6 jacks to lift internal formwork up (replace the use 4 jacks as used on site) after that using gantry crane to crane internal formwork out of. Page 6 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 7. We propose change sequence to cut cable as follows: - The procedure of cutting strands as follows: from strand No. 18→ 6→ 30→ 31→ 17→ 19→ 5→ 7→ 29→ 32→ 16→ 20→ 4→ 8 → 28→ 33→ 15→ 21→ 27→ 34→ 14→ 22→ 38→ 39→ 26→ 35→ 13→ 23→ 37→ 40→ 25→ 36 → 12→ 24→ 41→42 → 3→ 9→ 2→10 → 1→11. Page 7 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 Appendix 1 Crack at contiguous location of void and solid section Page 8 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 Appendix 2 Crack at angle of contiguous location of void and solid section: Page 9 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 Appendix 3 Crack of angle on top of girder end Page 10 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 Appendix 4 Traverse crack of girder end Page 11 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 Appendix 5 Breaking angle of bottom of girder Page 12 of 13 CIVIL ENGINEERING CONSTRUCTION CORPORATION No.4 PROJECT MANAGEMENT BOARD – NHAT TAN P-3 Appendix 6 Breaking of partition of girder Page 13 of 13 photographs of super t production 1 Contents: 2 Cleaning formwork, Installation of rebar and PC strand 3 Contents: Contents: 4 Tensioning of PC strand 5 Installation of inner formwork and casting concrete Pic No. 1 & 2 Pic No. 3 & 4 6 Pic No. 5 & 6 photographs of super t production Contents: Installation of inner formwork and casting concrete 7 Contents: 8 Checking temperature and slump of concrete 9 Contents: Pic No. 5 & 6 Pic No. 7 & 8 10 Cutting of PC strand 11 Pic No. 9 & 10 12 Nhat Tan Bridge Construction Project Contract Package 3 Tokyu Construction Co., Ltd Report on casting figure, crack happen and Construction countermeasure to prevent reoccurrence TABLE OF CONTENTS 1. General Description ......................................................................................................................... 2 2. Report of Casting Figure.................................................................................................................. 2 2.1 Location Map ..................................................................................................................... 2 2.2 Casting time ....................................................................................................................... 2 2.3 Casting sequence................................................................................................................ 2 2.4 Curing concrete.................................................................................................................. 2 2.5 Removal of inner formwork............................................................................................... 3 2.6 Compressive strength of concrete ................................................................................. … 4 3. Cutting PC strand and monitoring camber and cracks…………………………………………… . 4 4. 3.1 Cutting PC strand…………………………………………………………………………..4 3.2 Monitoring camber……………………………………………………………………… ..4 3.3 Monitoring cracks………………………………………………………………………….5 3.4 The reasons of incident……………………………………………………………………..6 Countermeasure to prevent reoccurence……………………………………………………… ... 11 4.1 Summary of crack locations………………………………………………………………. 11 4.2 Countermeasure………………………………………………………………………….…11 4.2.1 Crack at contiguous location of void and solid section (at girder web)………………..11 4.2.2 Crack at contiguous location of void and solid section (at girder wing)…………………13 4.2.3 Crack of angle on girder end (wing part)…………………………………………………14 4.2.4 Tranverse crack at girder end……………………………………………………………..16 4.2.5 Breaking at the bottom of girder end…………………………………………………..…17 4.2.6 Breaking of partition of girder…………………………………………………...……..18 5. Changing of cutting sequence………………………………………………………………..... .18 6. Appendix 1 ……………………………………………………………………………………….19 7. Appendix 2 …………………………………………………………………………………….....21 8. Appendix 3 …………………………………………………………………………………….....23 Total: 24 pages 1 Nhat Tan Bridge Construction Project Contract Package 3 Tokyu Construction Co., Ltd 1 Report on casting figure, crack happen and Construction countermeasure to prevent reoccurrence General Description This report describes actual casting figure of first super T girder of Thiep River bridge, mention the reasons of the cracks happened after cutting PC strand and cranning girder out of formwork and propose the coutermeasure to prevent re-occurrence. 2 Report of casting Figure 2.1 Location Map Super T girder production for Thiep River Bridge was carried out at Vinh Ngoc Temporary Facilities ( appendix 1). Girder No. G-1-12 2.2 Casting time : Casting date : 28th , May 2011 Casting duration : 17h30’ to 22h27’ It toke 5 hours to finish casting concrete, this is unexpected time. The reasonable countermeasure shall be propose to shorten the casting time. ( See Appendix 2 –Inspection sheet for placing concrete) 2.3 Casting sequence. 2.3.1. Supplying concrete. Concrete grade : C50. Design Slump : 15 ± 2.5 cm. 2.3.2. Work procedure. Concrete was mixed at site Batching Plant ( 60m3/h) and delivered to casting yard by mixer truck. Temperature and slump of concrete was checked prior to casting concrete to ensure the temperature is always less than 32oC and the slump is 15 ± 2.5 cm. Concrete was poured directly to girder formwork by concrete chute from mixer truck.. Detail ( Appendix 3). 2.4 Curing concrete. Using wet jute sheet to cover on wing of girder after casting concrete 6 hours and supplying water to keep the jute in wet condition. Curing layout 2 Nhat Tan Bridge Construction Project Contract Package 3 Tokyu Construction Co., Ltd Report on casting figure, crack happen and Construction countermeasure to prevent reoccurrence H.1 2.5 Removal of inner formwork. Inner formwork were removed on 30th May, 2011. Gantry crane was used to crane formwork out with support of 04 hydraulic jacks. H.2 2.6 Compressive strength of concrete 15 cylinder specimens were casted for 01st super T girder with the Engineer’s approval. 06 specimens 3 Nhat Tan Bridge Construction Project Contract Package 3 Tokyu Construction Co., Ltd Report on casting figure, crack happen and Construction countermeasure to prevent reoccurrence were cured on site at the same curing condition of girder, strength test shall be carried out at 03 and 04 days after casting and compressive strength of these specimens will be used to confirm the time for cutting PC strand, remaining specimens were cured at site Laboratory, strength Test shall be carried out at 07 and 28 days after casting. Compressive strength at 03 days: 41.3 MPa ( average) Compressive strength at 04 days: 43.8 Mpa ( average) Compressive strength at 07 days: 54.45 Mpa ( average) 3. Cutting PC strand and monitoring camber and cracks. 3.1 Cutting PC Strand. Cutting PC strand was carried out when compressive strength of concrete is not less than 42.5 Mpa ( 85% design strength). Date of cutting : 02rd June, 2011. Concrete strength at 04 days : 43.8 MPa. PC strand were cut individually and simultaneously at two end of girder, cutiing procedure was shown as below sketch. H.3 3.2 Monitoring camber. - Camber after cutting PC strand : 34 mm 3.3 Monitoring cracks. a) When removing inner formwork, 4 Nhat Tan Bridge Construction Project Contract Package 3 Tokyu Construction Co., Ltd Report on casting figure, crack happen and Construction countermeasure to prevent reoccurrence H4 b) After cutting PC strand and cranning girder out of formwork, there were some crack appear at the end of girder.( see the sketch below). H.5 c) After cutting PC strand and move girder to stock yard (after 03 days), there were some crack appear at the end, the web and the wing of girder .( see the sketch below). + At girder web and wing: 5