Pier Cap

Sequence work for pier cap

3.2.1.1 Scaffolding

Scaffolding is a temporary structure used to support people and material in the construction or repair of buildings and other large structures. It is usually a modular system of metal pipes or tubes. The key elements of a scaffold are standards, ledgers and transoms. The standards, also called uprights, are the vertical tubes that transfer the entire mass of the structure to the ground where they rest on a square concrete base plate to spread the load. The base plate has a shank in

Ledgers are horizontal tubes which connect between the standards.

Transoms rest upon the ledgers at right angles. Main transoms are placed next to the standards. The height of strands and ledger available are 0.5m, 1m, 1.5m, 2m, 2.5m, 3m, 4m etc.

Fig.3.2: Scaffolding for pier cap 3.2.1.2 Level transferred to pier

After completion of pier, the levels as per drawings are transferred to the pier with the help of auto level (see appendix IV) the bottom level of pier cap is marked on the pier and scaffolding is erected up to that level, some space is left for bottom shutter plat and IS Medium Beam 125 to support the dead load of pier cap before it attains its full strength. Adjustable stirrup head are placed between the ISMB 125 and top of scaffolding to adjust the top level of bottom shutter plate.

Fig 3.3: Arrangement of shutter plate, ISMB, Adjustable stirrup head

After fixing of bottom shutter plate the top level of bottom shutter plate are checked by auto level (see appendix IV) the top surface of shutter plate should be in a same level, so that the bottom of pier cap is to be casted should be smooth.

3.2.1.3 Reinforcement

The reinforcement bar is to be placed as per drawings and bar bending schedule. Rebar cages are fabricated either on the project site commonly with the help of hydraulic benders and shears, however for small or custom work a tool known as a Hickey or hand rebar bender, is sufficient. The rebars are placed by rod busters or concrete reinforcing ironworkers with bar supports separating the rebar from the concrete forms to establish concrete cover and ensure that proper embedment is achieved. The rebars in the cages are connected by welding or tying wires. Welding can reduce the fatigue life of the rebar, and as a result rebar cages are normally tied together with wire.

Besides fatigue concerns welding rebar has become less common in developed countries due to the high labor costs of certified welders.

There are different types of ties used for securing rebars. It is better to use two twisted strands of annealed 0.9 to 1.6 mm diameter wires.

3.2.1.4 Concreting

Concreting at a higher altitude is a difficult task, so concrete pump is required concrete pump is attached to a truck. It is known as a trailer-mounted boom concrete pump because it uses a remote-controlled articulating robotic arm (called a boom) to place concrete with pinpoint accuracy. Boom pumps are used on most of the larger construction projects as they are capable of pumping at very high volumes and because of the labour saving nature of the placing boom. They are a revolutionary alternative to truck-mounted concrete pumps. The bends in the pipes conveying concrete from the pump should be minimal in order to avoid losses. In addition, these should not be sharp. Each 10^o bend is equivalent to an extra length of pipe of 1 m. The pipe diameter should be at least 3 times the maximum aggregate size. Large aggregates can especially tend to get blocked near the bends.

The economy of pumping depends on the number of interruptions.

Each time, the priming of the pipes using mortar is required (0.25 m³/100 m of 6 inch pipe), and the pipe also has to be cleaned.

Aluminum pipes should be avoided, as the Al reacts with alkalis in the cement, and leads to the evolution of hydrogen gas. These gases tend to introduce voids in the concrete, which reduce the efficiency of pumping.

Pumping enables concreting of inaccessible areas. Moreover, the direct conveyance of concrete from the truck to formwork can avoid double handling of the concrete.

Requirements for pumped concrete

 Concrete mixture should neither be too harsh nor too sticky;

also, neither too dry nor too wet.

 A slump between 50 and 150 mm is recommended (note that pumping induces partial compaction, so the slump at delivery point may be decreased).

 If the water content in the mixture is low, the coarse particles would exert pressure on the pipe walls. Friction is minimized at the correct water contents. The presence of a lubricating film of mortar at the walls of the pipe also greatly reduces the friction.

 High cement content in concrete is generally beneficial for pumping.

 Water is the only pump able component in the concrete, and transmits the pressure on to the other components.

Two types of blockage to efficient pumping could occur:

 Water can escape from the mixture if the voids are not small enough; this implies that closely packed fines would be needed in the mixture to avoid any segregation. The pressure at which segregation occurs must be greater than that needed to pump concrete.

 When the fines content is too high, there could be too much frictional resistance offered by the pipe. The first type of blockage occurs in irregular or gap-graded normal strength mixtures, while the second type occurs in high strength mixtures with fillers. In order to avoid these two types of failure, the mixture should be proportioned appropriately.

 Other mixture factors that could affect pumping are the cement content, shape of aggregate, presence of admixtures such as

pumping aids or air entrainment. Air entrainment is helpful in moderate amounts, but too much air can make pumping very inefficient.

 When flowing concrete is being pumped, an over-cohesive mixture with high sand content is recommended. For lightweight aggregate concrete, pumping can fill up the voids in the aggregate with water, making the mixture dry.

Fig.3.4: Boom placer concreting pier cap

3.2.1.5 Compaction

It is important that concrete be vibrated at the correct frequency to fluidise the mix, to coat the aggregate with cement paste and to release trapped air. The operating frequency of internal vibrators may

be less than specified values, which may have been measured with the vibrator operating in air. A reduction in frequency results in an energy reduction, which in turn reduces the effective compaction area. It is important to introduce the vibrator in a systematic way, so that the compaction areas overlap and all the concrete is compacted. An internal vibrator with an electric motor and electronic speed control has been developed. This gives controlled energy input and has the added benefit of a lighter, more flexible cable.

Fig.3.5: Needle vibrator for Compaction

3.2.1.7 Finishing

When the concrete compaction and screeding is done, the slab is roughly floated with a trowel to give a smooth surface. After floating, slab is left to set hard. Free water (bleed water) will rise to the surface of the slab after it is leveled. Wait until the surface water dries before doing the final float or trowel finishing. On a cold day the bleed water may have to be dragged off by pulling a rope or hose over the surface.

Never spread dry cement or sand over the slab to absorb the bleed water as this will make the finished surface weak and dusty. Wood or steel hand-floats and trowels do a good job too; the whole surface

girder only to the standard needed for the type of finish to be used, the top surface is finished smooth.

3.2.1.7 Curing

Approved curing compounds may be used in lieu of moist curing with the permission of the engineer-in charge. Such compounds shall be applied to all exposed surfaces of the concrete as soon as possible after the concrete has set. Water covering closely the concrete surface may also be used to provide effective barrier against evaporation. For the concrete containing portland cement, portland slag cement or mineral admixture, period of curing may be increased.

Fig.3.6: curing of pier cap

3.2.2 Bearings

POT bearings for incrementally launched bridges have a dual function.

First, they provide low friction sliding surfaces over piers as the deck is launched during construction. Thereafter, they become permanent bearings for the completed bridge. A POT bearing serving both functions is shown in the picture above. During construction, a fixing device avoids relative movement between sliding plate and pot cylinder. POT bearing for bridge the sliding plate is supplied with a

second stainless steel sheet on top. Inserting neoprene-teflon pads between deck and bearings allows the launching operation to be carried out. Pads, second stainless steel sheet and fixing device are removed after launching. To achieve this, the deck is lifted by means of hydraulic jacks placed on top of the piers. Once this operation is achieved, the deck is lowered to its final position, the jacks are removed and the sliding plate is connected to a previously embedded steel plate in the deck. Finally, the fixing devices used for transportation are released, thus the bearing is in its final service position. For the correct design of these bearings, it is very important to know the loads during launching, because, they have major influence in the actual length and thickness of the sliding plates.

Although the cost of this type of bearing is higher than the standard ones, their use represents a saving for the job because:

 Temporary launching bearings are not required

 Demolition and replacement of the temporary bearings by permanent ones, is costly and time consuming therefore avoided

Site Installation

3.2.2.1 Preparation of the piers

Build the piers leaving on them the required recesses according to the dimensions indicated on the drawings. Pedestal reinforcement is anchored during the formation of mesh of reinforcement of pier cap.

Fig. 3.8: Reinforcement for pedestal

3.2.2.2 Levelling of bearings

Fig. 3.9: Plan view Fig. 3.10: Elevation

Place the pot bearing in its position levelling it with Steel wedges. It is important to ensure that the X-axis of the bearing is aligned in the longitudinal direction of the bridge and that the X and Y directions are accurately horizontal. For bearings allowing horizontal displacements it should be checked that the arrow painted on the slide plate is pointing in the correct direction.

Install the form for grouting the space between pier and pot bearing.

Grout the space between pier and pot bearing. Fill in the recesses checking that the level is the correct.

3.2.2.3. Formwork for diaphragm wall

The formwork of the deck is placed embedding the upper dowels of the bearing

Fig. 3.11: Formwork for Diaphragm wall

3.2.2.4 Removal of fixing plates

Once the formwork has been removed, the bearing is definitively installed. Remove the lateral fixing plates of the bearing in order to allow its free movement.

Fig. 3.12: Removal of fixing plates

3.2.2.5 Types of Bearing for joints of girder on pier cap

Fixed joint

Fig. 3.13: Bearing for fixed joint

Expansion joint

Fig. 3.14: Bearing for expansion joint

Free joint

Fig.3.15: Bearing for free joint