On July 25, 2021, a major construction bridge deck collapse occurred in Zhuhai, Guangdong Province.At 8 o'clock in the morning, the sun shone onto the sea, and construction work was underway on the Jinhai Bridge

On July 25, 2021, a major construction bridge deck collapse occurred in Zhuhai, Guangdong Province.

At 8 o'clock in the morning, the sun shone onto the sea, and construction work was underway on the Jinhai Bridge. The originally peaceful and peaceful scene suddenly erupted into a series of screams.

The box girder collapsed near the right span of the construction section from 165 # to 166 #, and the five construction workers who had no time to react fell into the seabed together with the collapsed bridge body in intense panic.

The huge collision sound and the brutal screams of the construction workers scared the nearby people to the point where they were weak. After a while, the engineers and passersby who reacted were calling the police station one after another.

This accident resulted in four deaths and one missing person. As the first highway and light rail dual purpose bridge erected at the mouth of the Pearl River Delta, such a thrilling scene occurred during construction. The reason behind it was that workers illegally removed the steel pipe support at the bottom of the box girder, causing the steel pipe column to collapse and causing the collapse of the entire support.

Coincidentally, there is a bridge in Shenzhen Bay that has been built for 12 years and almost collapsed. However, due to timely repair, it has avoided casualties and property damage.

The truth that caused the bridge to almost collapse is so outrageous. Let's walk into the unknown stories of the Shenzhen Bay Bridge together.

Blocked Bridge

In the early 1990s, with the call for reform and opening up, many people rushed from the mainland to coastal cities for development. With the deepening of reform and opening up, trade exchanges between cities in coastal areas have become increasingly frequent.

The economies of Shenzhen and Hong Kong are becoming increasingly close, and with the investment and promotion of the joint governments of both sides, a bridge spanning the Western Corridor and the two regions has gradually moved from imagination to reality.

Over the past four years, the two domains have achieved mutual exchange.

Although this bridge has stood tall and towering for over a decade, and has driven the rapid development of the respective economies of the two regions, the authorities, who have the primary responsibility for the safety of the bridge, dare not slack off at all.

The authorities have arranged for engineers from the Local Affairs Department to conduct regular inspections of the bridge, although the process is quite tedious and tedious, the routine inspections cannot be relaxed.

For more than a decade, the monotony was like a day, but I didn't want to be broken in an early morning phone call.

On February 15, 2019, an engineer from the Hong Kong Road Department discovered during a routine inspection that one of the six "external prestressed steel cables" inside the non navigable bridge section had broken.

The engineer reported this situation to the authorities, and they immediately realized that there was a huge danger hidden inside. I immediately contacted the person in charge of the Shenzhen section and relevant construction engineers.

On the one hand, urging relevant personnel to quickly block the entrance channels on both sides of the bridge to ensure that no vehicles enter; On the other hand, he rushed to the site with the engineer to conduct a survey.

As the staff arrived at the scene, after identification and inspection, we were able to see the original appearance.

The steel cable itself is cylindrical, but the appearance of this cable breaking seems to be a conical shape. The entire appearance seems to have been slashed diagonally, and some parts of the steel rope are as sharp as needles. At this location, there are still some small powders quietly adhering to it.

The engineer quickly provided temporary and safe survey results and a general method for repairing in detail. The authorities formed an investigation team to investigate the cause of the steel cable breakage.

However, the personnel responsible for the early construction of this Shenzhen Bay Bridge have long disappeared into the sea of people with the development of society. Even valuable information such as drawings has long been lost.

The investigation team carefully studied the situation where the steel cable broke and planned to reverse the cause. From the perspective of the mode of fracture, it can be determined that this belongs to ductile fracture.

There are two types of fracture modes for general items, brittle fracture and ductile fracture. The shape caused by brittle fracture is like a cracked biscuit, with a uniform surface; The shape of ductile fracture is like a stretched chewing gum, with thick ends and thin middle.

So the investigation team preliminarily determined that the steel cable was constantly stretched due to gravity, resulting in ductile fracture. But with it comes constant questions. Why hasn't this fracture phenomenon occurred in other steel cables, which are also "external prestressed steel cables"?

After inspection, it was found that the quality of the steel cable was not a problem. Setting aside internal factors, only external forces can cause such results.

The possibility of steel cable breakage being caused by human factors is relatively low. The entire Shenzhen Bay Bridge has a total length of 5.5 kilometers. For the convenience of transportation, the road was designed according to the high-speed road.

It is almost impossible to stop by the road and cut the steel cable. Even if feasible, depending on the area of the steel cable, cutting tools are required to achieve this. It is also difficult to achieve oblique cutting surfaces with tools. Will someone spend time and effort doing some "boring" things?

This conjecture is almost untenable, and human factors are excluded. So it is caused by natural external forces, and according to experience, there is only one reason that can quietly cause this situation in steel cables: corrosion.

Tracing back to the ultimate cause

Before the construction of the Shenzhen Bay Bridge, it was claimed that the expected service life of the bridge would be 120 years!

The sudden lockdown news caused panic. The Director of Hong Kong, Chan Pai ming, explained that after this situation occurred on the same day, relevant personnel were immediately dispatched to conduct a survey and reassess the safety of the bridge, while the reserved safety margin was sufficient to cope with daily traffic in both places.

Some engineers commented on the internet that the service life of steel cables can generally be as long as 50 to 60 years, but it is extremely rare for this bridge to experience cable breakage after only 12 years of operation.

At the subsequent press conference, the Hong Kong Highways Department also acknowledged that the fracture of the steel cable this time was very rare, but emphasized that no abnormalities were found during previous routine inspections.

Despite constant ridicule from the outside world, what needs to be investigated still needs to be investigated.

By observing the condition of the steel strand at the fracture of the steel cable, the investigation team took a sample of the white powder attached to it and brought it back for inspection.

After testing, the white powder is composed of Calcium hydroxide. It is the crystallization left by the evaporation of water on the surface of cement after hydration. This phenomenon is called cementation alkali and is very common in construction sites.

It is reasonable if Calcium hydroxide corrodes the steel strand in a period of up to 12 years and then exerts tensile force.

The problem is that the probability of such a situation occurring in steel cables is basically zero.

The periphery of the steel strand is usually wrapped in cement, and in order to avoid chemical reactions with the air, any project involves covering the steel reinforcement with cement.

So, there is only one possibility - this part of the steel strand is not filled with cement.

Due to the loss of core data, the progress of the investigation team has been extremely slow, but the pressure of public opinion cannot prevent them from working overtime to investigate clearly.

Finally, hard work pays off for those who have a heart, and after extensive research, we finally found a similar case.

As early as 1992, the Haiyin Bridge in Guangzhou experienced a collapse, and the cause of the event was also due to the corrosion of the steel strands, which led to their fracture.

By comparing the information released by Haiyin Bridge, the investigation team can fully determine that this steel cable was not fully wrapped during cement pouring, resulting in the cementation of the edges and corrosion of the steel strand.

The Hong Kong section of the bridge has five piers, P1 to P5, with steel cables connected at the interface of each pier, also known as the anchor head.

Usually, there is an exhaust pipe at the interface to be used in conjunction with cement sealing. To maintain consistent air pressure for each steel cable, it is necessary to insert the exhaust pipe into the exhaust pipe, then pour cement, until the exhaust pipe begins to emit cement slurry outward, and the entire sealing process is considered complete.

In order to locate the problem, the investigation team brought the answer to the source and began to inspect the steel cable at number P1. As expected, the exhaust pipe here was completely grouted with cement, and there was no similar situation as numbered P5.

After checking one by one, the possibility of similar situations occurring at other bridge pier interfaces was also ruled out.

This method of drawing gourds and making ladles quickly helped the investigation team grasp effective information. After a series of conjectures and verifications, the progress of the investigation team was finally at the end.

Based on the survey data, the initial cause can be clearly and truly restored. But to understand the initial reasons, two points have to be mentioned: the position structure of the steel cable and the surge effect.

The broken steel cable is located at the pier of the Hong Kong section numbered P5, while the port where it was poured is located at the position numbered P2. The "W" shaped structure of the steel cable from P2 to P5. The leftmost upper interface is located at P2, and the rightmost upper interface is located at P5.

In normal construction operations, pouring cement into steel strands is done in this way. It can greatly reduce the steps of operation, but the problem also arises here.

The cement poured from the P2 interface will lead to P3, P4, and P5. According to the predetermined plan, cement will flow out from the interface of P5 and represent the completion of cement pouring.

However, due to the surge effect, the cement encounters resistance during the upward flow towards P5. The cement above is squeezed down by pressure, causing the cement to flow out. Only the lower part of the steel strand comes into contact with the cement, but the upper part is still exposed to the air.

The surge effect is a phenomenon of airflow pulsation that typically affects gas chromatography analysis. When using a thermal conductivity detector and using nitrogen and other gases as carriers, a pulsation is generated, which generates a negative signal on the baseline.

According to the surge effect, this phenomenon is explained by the resistance that causes the shape of the cement to change. During the rise process, the steel cable is exposed, and after the cement hydration, it evaporates into corrosive particles, which are white powder.

So the result of the restoration is that when the construction worker saw the cement flowing out from the P5 interface during the cement pouring process, and did not inspect it, it was considered that the pouring was completed.

The exhaust pipe that is usually sealed with cement is a straight line, and even if cement is poured from bottom to top, it can perfectly seal the exhaust pipe.

Due to the wavy shape of this exhaust pipe, the surge effect was inevitably triggered during cement pouring, resulting in resistance to the exhaust pipe at the P5 interface.

At the same time as the overall speed of cement pouring decreases, the contact surface between the steel strand at the final interface and the cement is not large.

In fact, when checking the cement condition of the P5 exhaust pipe, it was found that there was not much cement in the exhaust pipe of this steel cable.

After experiencing the surge effect, the cement at that time did not fully wrap the steel strand. In subsequent inspections, as there were no issues with the steel cables and no differences in the location of this small area were found, the exposed steel strands were continuously corroded in the white powder after alkali treatment.

Due to the influence of gravity on one end of the steel cable, it underwent ductile fracture under the action of both, and was finally inspected by engineers one day 12 years later.

After several days of hard research, the truth finally came to light. But no one expected that the biggest threat to the bridge, which claims to be able to use for 120 years, would be some inconspicuous tiny particles.

Reflection on Bridge Engineering

Although this result is ironic, often things happen in these inconspicuous details.

Regarding the 1992 Haiyin Bridge, its highest level cable-stayed cable was corroded and fractured, resulting in collapse.

In the sixth year after the Haiyin Bridge was built, the exposed rust was enough to corrode hundreds of high-strength steel cables. The defect of cement slurry being prone to segregation results in the height of the cement slurry at the top of the stay cable not being full, and the ion induced oxidation effect in the cement additive greatly exacerbates the corrosion of the steel bars.

Is it worth reflecting on our building technology and testing methods as the same problem has occurred twice in different time and space?

The application of grouting technology in the field of construction engineering has a history of over 200 years, starting in 1802 when Charles de France used a wooden impact pump to inject clay and lime slurry to reinforce the masonry lining.

Afterwards, it went through two major stages and finally formed the modern grouting stage with cement slurry as the main raw material in 1969.

Since 1969, our approach to grouting has completely relied on past experience. Although the grouting techniques include high-pressure grouting method and static pressure grouting method, the raw materials for grouting have not changed.

In the 21st century, modern technology is in a highly developed trend, but incidents related to bridge collapses still occur frequently. Not only bridges, but also houses and buildings are all using the same technology.

In the relevant reports on the Shenzhen Bay Bridge, many professionals have also put forward their own opinions.

Some people say that bridge construction is different from other buildings, especially when building a 5.5 kilometer long bridge over the sea. If such a thing happens, the leader responsible for infusion should be held accountable.

Some people also expressed doubts about the loss of data. As usual, all construction drawings will be kept in the archives,80,,

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References:

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Analysis of five typical diseases of bridge cables - Bridge network

Fracture mechanics and Its Engineering Application - Harbin Engineering Press

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Author: Ru Wish

Editor: Lin Yan

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