Prestressed Carbon Fiber Plates

Project Overview

Bridge Overview

The bridge is located on the Zhangshi Expressway in Hebei Province. It is a four-span cast-in-place continuous prestressed concrete box girder bridge on a township road diagonally across the Zhangshi Expressway. The designed cross-section of the bridge is single-box and double-chamber, with a span of (16+20 +20+16) m.

Damage process

The bridge straddles the Zhangshi Expressway. The bottom of the beams of the Zhangshi Expressway has been seriously damaged due to the multiple impacts of ultra-high vehicles on the bottom of the beam.

Damage assessment

Two strands of the prestressed steel strands at the bottom left of the box girder of the second span of the bridge were broken, and some steel bars were also damaged. The damage range was 1.5m×0.9m. There is also a large pit with a depth of 10cm in the mid-span of the span, but the steel bars are not damaged, as shown in Figure 1.

After the bridge was appraised, it was concluded that the clearance under the bridge needs to be increased to meet the clearance requirements of Zhangshi Expressway vehicles, but the bridge cannot continue to pass due to large vehicles such as trucks on the damaged bridge.

After the appraisal, in October 2011, an engineering design and development company carried out the overall improvement of the bridge, and the average height of the improvement was 51cm. It is possible to allow vehicles on the Zhangshi Expressway to pass normally, but the passage of vehicles on the bridge is still restricted.

In 2012, according to the reinforcement design and calculation, the engineering design and development company carried out the reinforcement of the damaged side box girder body and the bridge deck.

Reinforcement design principle

From the economic comparison of reconstruction and reinforcement of the bridge, the reinforcement treatment plan is dominant. The reinforcement design is still based on the principle that the position of the reinforced prestressed carbon fiber plate coincides with the peaks and troughs of the bending moment diagram of the beam body, and the positive bending moment part of the bottom side of the box girder is externally tensioned by the prestressed carbon fiber. Carbon fiber tension reinforcement measures in the negative bending moment area of the bridge deck pavement. The number of carbon fiber plates is determined based on the replacement of damaged steel strands with equal strength. The amount of load on the bridge that each bundle of carbon fiber plates can withstand after the reinforcement construction needs to be calculated and corrected through experiments.

Reinforcement design plan

Since the damaged prestressed strands are the 2 strands on the left side of the bottom of the bridge, 7 strands of carbon fiber sheets are symmetrically stretched outside the left half of the box girder at the maximum positive bending moment per span. The specifications of the carbon fiber sheets are CFP-50-1.4.

Seven bundles of carbon fiber plates are symmetrically stretched on the bridge deck with the negative bending moment on the left half of the pier top of the box girder deck. The specifications of the carbon fiber plates are CFP-50-1.4, and the center line of the symmetrical pier column.

Reinforcement construction plan

The reinforcement construction process follows the design drawings and related specifications, and strictly abides by the principle of recovering as much as possible after the reinforcement is temporarily cut off due to the reinforcement construction. Based on the improvement of the maximum bearing capacity of the bridge, each process is strictly constructed in accordance with the relevant technical standards. The bridge reinforcement is divided into the external carbon fiber plate tension of the positive bending moment part under the bridge and the internal carbon fiber plate tension of the negative bending moment part of the bridge (Figure 2). The specific construction plan is as follows.

The reinforcement process of the box girder bottom plate is

Measure and pay off → slot the working anchor plate area → drill holes and implant bolts → reinforce and paste the first layer of working anchor plate → install tool anchor → install carbon fiber plate → apply steel glue to the carbon fiber plate → tensile carbon fiber plate Paste and fix the second layer of working anchor plate → supplement the fixed limit plate → remove the tool anchor series → fill the pores of the working anchor → brush the anti-corrosion material layer.

The process flow of bridge deck reinforcement is:

Measuring and laying out the concrete range → chiseling off the bridge deck concrete and steel bars → measuring and laying out → slotting the working anchor plate area → drilling and implanting bolts → strengthening and pasting the first layer of working anchor plate → install the tool anchor → install the carbon fiber plate → apply steel glue on the carbon fiber plate → stretch the carbon fiber plate → paste and fix the second layer of working anchor plate → supplement the fixed limit plate → removal of the tool anchor series→filling the pores of the working anchor→brushing the anti-corrosion material layer→restoring the bridge deck steel mesh→pouring the bridge deck concrete→the bridge deck concrete curing.

Concluding remarks

In this construction process, some procedures can be promoted during construction

1) The prestressed carbon fiber plate makes full use of the characteristics of carbon fiber, high strength, low deformation and corrosion resistance;

2) Combination of work anchor and tool anchor series, easy processing operation and perfect function;

3) The permanent prestressed part occupies less headroom and does not affect the headroom under the bridge.

4) The construction period is short and the road is basically uninterrupted, which is suitable for areas with heavy traffic.