04.chapter ii
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This document describes the construction process and technology used to cast concrete beams for Do Vang Bridge on scaffolding. Key steps include: 1) Constructing steel scaffolding and formworks to support the casting of each concrete beam segment. 2) Installing reinforcement cages, ducts for prestressing cables, and bearings before pouring concrete. 3) Segmentally constructing each beam by casting concrete sections supported by scaffolding, then moving the scaffolding to begin the next section.
04.chapter ii
- 1. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding CHAPTER II: TECHNOLOGY PROCESS I. CONSTRUCTION SEQUENCES Concrete beams of Do Vang Bridge are casted in place on the scaffolding. Construction scaffoldings are made up of shape steel including of: longitudinal beam H200 and horizontal beam and scaffolding system for supporting side formworks. Side formworks, bottom formworks, internal formworks with splint system, through bar must be stably, firmly throughout concreting process. After completion span 2 and span 5, formwork scaffolding will be transferred to construct the next spans. Concrete shall be provided from batching plan at Site with capacity 60m 3/h to the constructed position by truck mixer and concrete pump. • Construction sequences + Step 1: Land levelling at position of girder construction, filling sand and compacting to K95. Installing the concrete plank 1x1x0.2m. + Step 2: Installing the scafolding, longitudinal beam H200 and horizontal beam H200 + Step 3: Testing of scaffold. + Step 4: Install external formwork, side formwork + Step 5: Install reinforcing steel, pouring concrete + Step 6: Stretch prestressed cable. + Step 7: Removal of scaffolding, internal formwork, pushing scaffolding system. + Step 8: Connect projecting reinforcement couplers of diagraphm, reinforcement, diaphragm formwork. Placing concrete and stressing horizontally prestressed cable of diaphragm. + Step 9: Construction of the next span. II. TECHNOLOGY STEPS 1. Land levelling Land levelling at position of girder construction, filling sand and compacting to K95. Installing the concrete plank 1.2x1.2x0.18m2. Production and erection of scaffolding Scaffolding, formwork must be designed to resist any unfavouble displacement during superstructure construction process. Construction scaffolding is included: + Supporting scaffolding of side formwork + Installing the scafolding, longitudinal beam H200 and horizontal beam H200 5
- 2. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding Figure 1: Span structure’s construction scaffolding 3. Testing scaffolding This test aims to remove the settlement of scaffolding system, check the pressure bearing ability of scaffolding. Load test results must be reported immediately to The Engineer. 4. Install beam formwork • Concreting formwork design + Bottom formwork is combined by flat formwork 1x2m, placed directly on diaphragm steel shape H200.. + Side formwork and flange beam is combined by flat formwork and special formwork put on modules of steel shape H200x200, wihch are associated with diagram steel shape H200x200. + Core formworks shall be fabricated by modulus 0.25x1.0m, fitness, easy working, and convenient removing. The formwork is designed by the type of 6
- 3. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding modulus 1m including formworks- supporting bars- bands, ready erected on the ground and hoisted into its position. + Elevation of formwork at each section must be included design deflection of the bridge and deflection resulting from pre-stressing and deformation of scaffolding. • Install bottom formwork Erection sequence as follows: + Formwork boards are placed directly on the diaphragm steel shape H200x200 + Erection of formwork boards with the length 2m is in the direction of bridge and parallel to connecting centre line between two piers. + After completing bottom formwork installations, in a maximum distance of 4m between sections, measure the level to adjust the level of beam bottom. + Insert the steel wedge below the formwork to adjust the level of formwork surface (beam bottom). + Installation of a special formwork at pier position • Installation of external wall formwork Erection sequence as follows: + Installation of outside scaffolding system and protective handrail. + Completely erecting formworks modulus and supporting frame for beam web consisting of: web formworks, supporting frames. + Hoisting each formwork modulus and supporting frames into exact position. + Adjust level of formworks. + Tighten the connection bolts, fix geometric position of formwork segment and install the next segment until finish a span of beam. • Installation of core formwork The installing sequence shall be as follows: + Formwork system and supporting frame of core formwork shall be installed in every modulus at 4~6m length, suitable with the capacity of the hoist. + Hoisting each formwork modulus into exact position. 7
- 4. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding + Adjusting elevation and coordinate of formwork. + Tighten the connection bolts, fix geometric position of formwork segment and install the next formwork segment until finish a concreting segmentation. 5. Beam bearing installation The Contractor will install bearing before casting concrete. Detailed construction is follows: • Step 1 Cleanning reinforcement of bearing stone, installing formwork of bearing stone, casting non-shrinkage motar for bearing stone construction. • Step 2 When the motar of bearing stone meets requirements of strength, locating exactly bearing position, defining the horizontal and vertical position of bearing stone, installing the bearing. 6. Installation of reinforcement, duct and high strength cable • Installation of reinforcement Reinforcement is fabricated at yard according to the designed geometric dimension, numbered and bundled for hoisting into installed position. Sequence for installing rebar will be prepared by construction team and informed to foreman during installation of beam rebar with the below principles: + Fully installing bars in accordance with the size and location indicated in the design. + Steel structural members must be removed or fixed at designed position by using supporting bar, chock or tie to resist the excessive movement or bending during construction process. + The distance from formwork surface to steel structural member must be remained by using supporting bars and chocks. The chocks must be placed in a suitable distance to make the formwork be free from cracking or excessive deflection. 8
- 5. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding + The chocks must be enough hardness, strength and suitable shape to not form holes around them. + The position tolerance for internal reinforcement of concrete: Bar detail and bending dimension for the rebars must satisfy the requirement stipulated in Section 5.11 of Bridge Construction Instruction AASHTO LRFD 98. The installation tolerance for the super structure as follows: d ≤ 200 200 < d < 500 500 <d ≤ 2000 d>2000 Tolerance (mm) 5 10 20 30 In there: d may be: Dimension by measurement direction (mm) Height of cross section (mm) • Installation of Duct and high strength Cable Install holing duct: + The holing ducts for high strength tendon are put into designed position before concreting. The duct is installed segmentally (according to produced length) ducts are connected together with linking pipe and joint tape around to prevent motar from inner leakaging. + To fix ducts into reinforcement mesh, using D6 bars supporting below and binding into the mesh at each coordinates adjusting position of the strand according to design requirements. + The head of duct is inserted in the anchor head fixed into beam formwork. + The duct is associated to ventilation pipe and grouting chute at the lowest and the highest points to ensure the pumped grout will completely seal the duct, free from air bubble. 9
- 6. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding Figure 5 : The process of connection cladding Figure 6: Setting mortar Wrap joint tape to be sealed and free from grout leakage Figure 7: Field duct connection Install duct and high strength tendon: + Install duct and high strength tendon, using cupler to connect tendon. + High strength tendon will be cut up to designed length, bundled and inserted into the duct then put into place. + Fix cable into head of dead end, locate the dead end. Notes when installing duct and tendon: + Duct position must be installed in right designed place within allowed tolerance. + Curve of cable must be smooth when visually check. 10
- 7. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding + Duct must have enough tightness so that mortar can not run into there. 7. Placing concrete for beam • Concrete mix design Concrete of approach beam of Do Vang bridge has relatively dimension and quantity, thus mix design work greatly affects the concrete quality. The mix design shall be in accordance with requirements of Specification. Concrete shall ensure to be enough strength, waterproofing as requirement of design also meet demand of yield to ease the construction with minimum cement content. Minimum compressive strength of cylinder specimen at 28 days shall be 40Mpa to ensure concrete reach to strength 40Mpa as designed. In any case, the required slump of concrete shall be 14 ± 2 cm. It shall be rejected if it is not as requirement. Do not add water to fresh concrete. Concrete shall have enough adhesiveness and flexibility to ensure it is not segregated throughout concrete transporting and placing process. Water-cement ratio in concrete mix shall be less than 0.40. Concrete mortar shall have setting time over 6 hours. The parameters of beam concrete: + Grade: 40 Mpa + Slump: 14 ± 2 cm • Concrete materials Beam concrete of Do Vang Bridge as shown in the Specification of Project has cylinder strength 40MPa. Mix design shall conform to Specification of Project. Admixture of concrete is produced by Sika manufacturer. This is a super-plastically high-tech substance at the thirdly versioned polyme origin to push concrete harden. The admixture will affect on the below characteristics of concrete: + The ability of reducing water is so high. + To be deliveried in a great distance. 11
- 8. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding + The yield strength is high (decrease the works of placing and compacting) + Boost the development of strength + Improve the ability of creeping, shrinkage. • Construction plan layout + Currently, construction plan of the Do Vang Bridge has already leveled off material store yard, reinforcing yard to ready for installing, testing load and placing concrete for beam. + Night lighting system: The Contractor shall arrange fully lighting system along the service road, beam and all constructing positions throughout the concreting process. The bulbs used during construction have capacity 500W and shall be disposed at the distance of 5÷10m per bulb. The minimum illumination level for concreting is 30lux/m2. (When it comes to horizontal concrete plane – Refer to “Bridge constructing manual” composed by Pham Huy Chinh- Construction Publisher 2007). + Standby power system: The Contractor has standby power system 250KVA in case of power loss. This system shall be always maintained and checked to find out defects and treat immediately. • Construction Equipment No Equipment Unit Quantity Note 01 Mixing plant 60m3/h each 01 02 Mixer Truck each 03 03 Pumpcrete machine Puztmaiter each 02 04 Generator 250KVA each 01 05 Crane 40T each 01 06 Purchase tackle each 04 07 Jacking and oil pump house Set 02 08 Mortar mixer and pump Set 02 12
- 9. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding 09 Surveyor's level each 01 10 Tachometer each 01 11 Vibrators each 15 12 Pumpcrete pipe m 250 13 Welding machine 400A each 04 14 Pump 60m3/h each 02 • Arrangement manpower No Equipment Unit Quantity Note 01 Engineering person 03 02 Investigation person 02 03 Woker drive crane person 04 04 Worker support crane person 06 05 Woker drive electric person 02 06 Woker drive mixing plant person 02 07 Worker drive shovel person 02 Worker drive pumpcrete 02 08 person machine 09 Woker weld person 15 10 Woker jack person 05 • Concrete mixing Use batching plants that can automatically measure and weigh at Site. The scales shall be checked accuracy, meet the Specification of Project as stipulated. Concrete of each batch shall be in accordance with Specification as required. The slump of concrete must conform to requirements. Therefore, before each concrete placing time, determine the material moisture to caculate suitable water content for 13
- 10. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding concrete mix. • Provide concrete Concrete shall be provided from batching plant to constructed position by truck mixer and concrete pump. Time from mixing, placing to compacting concrete is not over 90 minutes to ensure the works of concreting finish before concrete setting. Components and slump of concrete mix shall be tested and trial pumped to ensure the quality of concrete and construction conditions simutanously suitalbe with technical funtions of pump. When construction in hot weather, the external surface of concrete must be covered or whitened to restrict sunlight effect causing hot concrete. • Method for decrease concrete mix temperature The Contractor intends to cast concrete from 16 p.m to 10 a.m next morning. In case of daytime construction, the Contractor shall not construct on the day that temperature is over 35°C. Construction in nightime to reduce temperature, direct sunlight causing hot pump pipe and newly placed concrete. Concrete mix temperature from batching plant shall be controlled to not excess 25°C and when placing it is not over 35°C. For these purposes, beside construction in nightime to decrease temperature, the Contractor also use the following methods to decrease aggregate temperature: + Spray water on balast, gravel: Ballast, gravel in store yard shall be sprayed periodcally for keeping wet surface to create evaporating mechanism so that reducing agrregate temperature. + Use cool water to mix concrete, water tankers shall be covered from sunlight. • Pour concrete + Concrete is poured by mix truck and pumping machine 14
- 11. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding Concrete placing direction: Concrete shall be casted from the low head to the high head of beam and placed alternately on all beam webs, ensure that height defference between these webs is not over 50cm. Pour concrete vertically by each segment that splayed a 45 0 angle against to horizontal direction. Attention when pour concrete + Slump of concrete must be as required. Therefore, before each concreting batch, it is necessary to determine the moisture of material to calculate the suitable water content for concrete aggregate. + The height of the concrete to fall is not more than 1.5 m to avoid segregating. + Fresh concrete is poured into formwork by a small amount and using methods to remain rebar not movement during concreting process. + Mixxing speed must ensure that when pouring the above layer, the below layer has not yet cured to create perfect bonds between layers. • Tamping + In the concrete mix, the Contractor has used superplasticizer with high mobility of manufacturer Sika to decrease significantly placing and compacting work. + Fresh concrete must be placed into formwork and compacted for each layer from 250mm to 300mm thickness. By this time, compacting to the previous concrete layers. + During compaction, at the position near ducts, pay attention to advoid touching the duct to be broken. Do not use vibrator to push concrete. + Special attention to concrete quality at anchor heads. • Curing concrete 15
- 12. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding Curing for concrete beam casted on scaffolding is conducted immediately after finishing a surface area of concrete beams. Usually use wet sacks to cover on concrete surface around one hour or when the concrete surface will begin to dry. After finishing surface and until the surface can resist water (usually after 2 hours of concreting) must water regularly to the concrete surface which face is finished. When finishing pouring concrete for girder segments, watering to cure all concrete and wall formwork. After removing the formwork must water to the concrete regularly throughout day and nighttime. Water for curing process shall be fresh water used to mix concrete. Setting time must prolong at least 7 days from finishing of beam concrete. • Emegency occurrences that may happen and resolving method. Treatment method in case of work suspension because of Force Majeure. The concreting work shall not be executed in the following cases: hard rain, flood, storm, or the conditions for placing concrete still not ready… In the event that concreting process is suspended because of the interruption in concrete providing source (long-time traffic jam, accident…) the problem shall be resolved as below: + Use the standby mixing plants at the Site. + If the quantity of concrete required to place is further more than capacity of standby mixing plants, the construction must be suspended and the Contractor continuously compacts to finish the casted concrete. In the vertical direction, concrete shall be made by diagonal section bias an angle ≤ 450 against horizontal plane. This special construction joint shall be treated, roughed in accordance with procedure and specification for the next concreting stage. Measure to prevent honeycomb: 16
- 13. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding Honeycomb at concrete is easy to happen because of special shape of box girder. Positions where honeycomb can happen and method which is carried out in process of pouring concrete to prevent honeycomb is showed in below table: No. Position where honeycomb can Reason Preventative measure happen 1 The lower chamfer of web Concrete pouring to Need to guide workers web is easy to flow not hesitate when to bottom slab compact concrete Workers hesitate when compact concrete because concrete flow to bottom slab The lower chamfer The vibrating tampers of web can’t be are often associated compacted well with a burning because outside web bamboo or a straight isn’t straight pipe to pass thread along shape of diagonal web Projector flashlight into the side of web These parts will be hammered handwork by wooden hammer It is difficult to Vibrator will be compact by vibrator associated with a 2 The angle of bottom slab straight bar to compact that part. 17
- 14. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding The claddings will Pour concrete slowly 3 The lower part of cladding prevent compacting and compact concrete concrete below them more carefully 4 The anchorage zone Much reinforcement Compact carefully Beam casting formwork No. Problem Resolve Load capacity of formwork Before fabricating the formwork shall be caculated, designed to ensure enough load 1 capacity. Checking their quality and geometrical dimension before leaving workshop. Formwork that is bad The formworks shall be checked, taken-over in quality, defected during quality and geometrical dimension to 2 transportation immediately discover and substitute the defected boards. The camber of formwork During installation, the formwork elevation shall resulting from designed be adjusted to be suitable with the camber. 3 camber of the bridge, stressing prestressed cable, scaffolding deformation 8. Prestressed cable tension • Preparation work It is necessary to make sure that before constructing all persons attending directly in stretching works, especially technicants and stretching team leaders must have full knowledge of stretch controlling steps to ensure the technical quality and safety regulation. Check concrete strength (taking sample before stressing). The strength shall reach to 90%Rtk (90%*40Mpa = 36 for cylinder sample). Check jack, pump, jack bracket, chain hoist and work platform. 18
- 15. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding + Jack, pump for stressing shall be checked and approved before using. + Jack and pump shall be inspected by specialist authority. + Jack shall be inspected in the following moments: • Before using to avoid accident or occurrence. • Change jack set. + Anchor test: • Anchor using for Project must be in accordance with Specification and inspected to pass testing criterium and approved by Engineer. • Anchor using for installation shall be free from crack, warp, and the chap must be sealed with anchor. Show stressing steps on the visble positions for the stressing team leader. Fill up enough points stipulated in stressing note. • Stressing equipment. Prestressed Cable High strength cable shall use kind of tendon as 7 lowly saged plies 12 strands 15.2m bundled cable Grade 270 according to ASTM A416 – 90A standard. No Item Value 1 Nominal diameter 15.2 mm 2 Area of cross section 140 mm2 3 Ultimate strength 1860 Mpa 4 Yield point stress 1670 MPa Prestressed cable shall be protected in store that has screen, be dry, apart 30cm from the ground. 19
- 16. Hanoi – Hochiminh city railway line Do Vang Bridge’s construction bridges safety improvement project technology of beam casted on scaffolding Pretressed cable must have Certificate of Producer approved by Employer and Engineer. Test for tendon in accordance with Design Document and criterium: breaking force, elastic modulus, yeild strength, elongation, dimension of each ply in tendon, condition of external surface. Any change that not be in accordance with Design Document shall be approved in writing by Design Unit, Employer, Engineer beforce constructing. Anchor Use imported anchor of OVM manufacturer for cable 15,2mm . Imported anchor must have Quality Certificate of Manufacturer. Jack All of jacks, oil pumps shall be inspected before using. Jack Area Strain Sroke Type of No pittong max pittong Note Name Mark Pump (cm2) (KN) (mm) Horizontally 1 YCQ500Q 0707 ZB4 - 500 1000 5000 200 stretching Horizontally 2 YCQ500Q 1001 ZB4 - 500 1000 5000 200 stretching • Prestressed cable tension Use one-head tensioning method. Using jack 500T applying for tendon as 7 lowly saged plies 12 strands 15.2m bundled cable. Stretch every twin cables symmetrically through centreline of beam in the following sequence: + From centerline to outside. + From neutral axis to outside. Sequence of stressing tendons on each segment: Stressing longitudinally then stretching horizontally prestressed cable of bridge. 20