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Bridge Top slab

Once the curing of bottom slab and web completed. The construction of the top slab will begin. Top slab usually has a small width, therefore using cup-lock as a supportive system (falsework) will be enough. Top slab construction is simple and can be done in a shorter time compared to web and bottoms slab. The reinforcement for the top slab usually consists of four layers of reinforcement — two layers in each direction. The reinforcement in short direction will be more. As we know, the loads will be transferred in a short direction.

After the erection of falsework system. Primary beams will be installed at intervals in the short direction. Intervals between the primary beam shall be matching the calculation by the designer.
Above the primary beam and in the longitudinal direction, the secondary beam will be fixed to support the formwork for top slab.

Figure 2

Marine plywood will be nailed to secondary beams. The final shape, level, and dimension should be identical to the approved drawing. The surveyor should check all levels and dimension. The used plywood should be in good condition, so the resulted concrete surface met the minimum requirements. The location of joining bottom slab to web and top slab to the web should be hunched. The hunched shape will provide a better distribution of stresses compared to the sharp joint.

Figure 3

Before placing reinforcement. Existing concrete should be cleaned, roughened and all unsound concrete should be removed. This very important, and it should be done correctly to ensure strong bonding with the next pour of concrete. The scrabbling machine will be used to roughen the concrete surface. 

Top slab reinforcement:

  • Fix top slab reinforcement as per drawing. 
  • Use enough cover block to maintain minimum cover requirement.
  • Use enough chairs to keep your reinforcement stiff and to prevent movement during casting.
  • Bind reinforcement bars properly. Bind should be provided at each bar, or at least half of bar intersection should be tied. If the reinforcement bar is rusted, remove the rust before placing the reinforcement.

After completion of reinforcement fix the rails for screed vibrator. The surveyor shall check rails level. Usually, steel pipes are used as rails for screed vibrator. The figure no:5 showing screed vibrator rails. Those rails should be adequately tied to top reinforcement mesh. The rails should be rigid and should not move during the casting. The profile of the bridge will be affected adversely if the rails are not rigid. Increasing or decreasing the level of the concrete top surface will affect the bridge surfacing. Difficulties will be faced during the laying of wearing course. Also, the minimum thickness cant maintained if the concrete surface is high.

Figure 5

Concrete should be finished to the required level and cross-section. Concrete surface will be textured with a broom after consolidating concrete with screed vibrator. The broom finishing will allow better bonding between concrete and wearing course.


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