A bridge over Yitong River in Changchun city, the capital of Jilin province in Northeast China, lays in ruins after collapsing on 29 May 2011. Two people were injured.
In Africa, the integrity of some bridges are questionable, particularly those built before 1970, due to structural designs, the quality of materials used in construction and general ageing. Today, bridge integrity is a concrete science, or is it?
In 1950, the world had 2.5 billion people. In 2005, the world had 6.5 billion people. And, on the 31 October 2011, we passed the seven billion mark. That’s an increase of 4.5 billion people, an almost tripling in 60 short years – people who need food, medicine, clothing, furniture and a host of other goods that need to be transported.
The point of mentioning these statistics is to bring focus to bear on the high correlation between increased population and increased traffic flow on major arterial routes. Also correlated to population growth is the increase in our knowledge over the same period. As such, we can safely say that concrete structures built forty or more years ago would not be of the same standard or quality of those built today.
And, forty and more years ago, trucks towing trailers, such as the modern Interlink we find on our roads today, did not carry loads of up to 56 tonnes per load. A load such as this causes stresses, deformations and displacements in bridge structures, which is intensified by the dynamic nature of the focal points as the truck moves.
Besides other vehicles crossing a bridge, two, or even more, such trucks crossing the bridge simultaneously multiplies these factors. Is this possible? Absolutely! The Swartkoppies Bridge over the R59, to and from the Alrode industrial area, regularly carries four or more large trucks, along with a couple of dozen cars. Bridges like these need to be capable of carrying this sort of load for 20, 30, 40 years and more.
Resonating vibrations, the harmonics of the bridge, is another factor to keep in mind as this happens on a continual basis. Then there is something the Australians call ‘concrete cancer’, a hidden danger that literally eats away the insides of a concrete bridge.
At the time of writing this article, the meeting of the 20 000 delegates attending the COP 17 climate change summit in Durban was underway. The purpose of this international gathering was to advance, in a balanced fashion, the implementation of the Convention on Climate Change and the Kyoto Protocol, as well as the Bali Action Plan, agreed at COP 13 in 2007, and the Cancun Agreements, reached at COP 16 last December.
But climate change should not have been the only thing on the agenda. Emissions affect other aspects of life too. Bridges, for example, which are quite strategic to every economy, as well as life itself! The very same emissions that affect climate change affect the integrity of bridges around the world. It is as important an issue as climate change because lives can and have been lost due to bridge integrity failure. The chemicals that make up acid rain are the very chemicals that find a way into reinforced concrete structures and attack the steel reinforcing bars.
It is this corrosion that is ‘concrete cancer’, the hidden danger, which compromises the integrity of concrete bridges.
Once this process has started it will continue to weaken the structure over time. It is important to realise that these failures are aggressive in nature and should not be left to eventually undermine the structural integrity of a bridge.
As RE Wilmot wrote in the Journal of the Southern African Institute of Mining and Metallurgy, the problem of corrosion, of reinforcement in concrete structures, is internationally recognised and represents the single biggest expenditure in the preservation of steel reinforced concrete structures.”
The problem chemicals are:
Carbon dioxide, a ubiquitous gas, is one of the most abundant gasses in the atmosphere. However, it is the continuous production of this gas, as well as carbon monoxide, through human activity and the combustion of organic matter, fossil fuels, petrol, diesel and ethanol, that concerns us. Mixed with water it dissolves slightly to form a weak acid called carbonic acid, according to the following reaction: CO2 + H2O H2CO3.
Human activity is responsible for 70% of the emissions into our atmosphere. Of the sulphur content in the air, we produce about 37%. Volcanoes contribute 7% of the emissions and 18% of the sulphur content, while biogenous activity contributes 23% of the emissions and 42% of the sulphur content.
The combustion of coal and other fossil fuels account for about 80% of human-generated sulphur dioxide in the atmosphere. Most of this is from coal-fired power stations. In addition, this same combustion also forms sulphur trioxide. Motor vehicle emissions account for only about 1% of the sulphur dioxide present in the atmosphere. Nonetheless, sulphur dioxide dissolved in water forms a weak acid solution called sulphurous acid, according to the following reaction: H2O + SO2 H2SO3. If sulphur trioxide is present and dissolves in water, sulphuric acid is formed by the following reaction: SO3 + H2O H2SO4.
Nitrogen dioxide (NO2) dissolved in water produces nitric acid in the following reaction: 3 NO2 + H2O → 2 HNO3 + NO. Nitric oxide, or nitrogen monoxide, which is produced through the combustion of fossil fuels and in internal combustion engines as a result of the reaction between oxygen and nitrogen at high temperatures, reacts with water to form a mixture of nitrous acid and nitric acid in the following reaction: 2NO2 + H2O HNO2 + HNO3.
Perhaps the most crucial aspect of concrete, especially in load bearing applications, is durability. This one factor determines not only the life span of a structure but how vulnerable it is to atmospheric and chemical attack. Choosing the right aggregates and removing air and concrete compaction are essential aspects of the process to prevent corrosion.
When cast, high alkalinity of the cement paste, approximately pH13, ‘passivates’ the steel surface and protects it against oxidation (corrosion). However, with the presence of chlorides, carbonation, acid attack, or a combination of these factors, the pH of concrete is reduced and the reinforcing bars start to corrode.
The hydration of calcium hydroxide (Lime) (Ca(OH)2) or calcium silicate (Ca2SiO4) in cement creates a highly-alkaline environment. This ‘passivates’ the steel to protect it against corrosion. As concrete ages, lime reacts with atmospheric carbon dioxide as follows: Ca(OH)2 + CO2 CaCO3 + H2O. This reaction reduces the pH of concrete to approximately pH 9.6, 9.5. At this level the passivation protection of the steel surface due to the alkalinity of cement paste disappears and steel starts to corrode.
With the introduction of acid rain, the reduction of the concrete’s pH is further exacerbated and, in a similar chemical reaction, diffuses over time (into the concrete).
This then makes the thickness of concrete cover, and the composition of the concrete, extremely important. The more concrete one has covering the steel, the longer it will take before the steel starts corroding. In terms of the composition, given the chemical reactions that do take place, the choice of aggregates and concrete compaction are vital aspects of bridge construction today.
Books to read
Every municipal engineer should read at least one of the three books illustrated below. These books provide insight into the planning, design, inspection, repair, strengthening, testing, load capacity evaluation and demolition of concrete bridges. These books offer a global overview of concrete bridge management based on the knowledge and experience of the authors, as well as presenting rational and objective criteria to aid decision-making. A classification system concerning defects, their causes, repair techniques and diagnostic methods is included in the Handbook of Concrete Bridge Management.
The monitoring of the structural health of bridges involves two distinct phases. The first phase is the risk-based assessment of the current condition of the bridge structures in question. In this process, non-destructive evaluation and load ratings assist the assessment of the physical condition of the bridges undergoing inspection. The second phase is to implement a risk management plan of identified safety-critical and security-critical bridges. This will include ongoing activities such as visual inspection, non-destructive evaluation, quantitative assessment, maintenance (at regular intervals), strengthening and protection (when necessary).
Ten years ago, the South African National Road Agency Ltd (SANRAL) commissioned an Afri-Coast Engineering joint venture project that undertook the inspection and condition assessment of 50 bridges on the N2 route in and around Port Elizabeth and, where necessary, structural rehabilitation designs were produced and implemented. The project included major bridge structures such as the Van Stadens River Bridge.
Unfortunately, not all roads and bridges fall under the jurisdiction of SANRAL. Most monitoring of provincial and municipal bridges, if done at all, is done by inspectors who rely on visual observations and personal experience (if adequately qualified) to judge a bridge’s structural integrity. Observation and experience, however, do not enable an inspector to know with certainty how a bridge structure is threatened by corrosion and thus affected by its dynamic loading.
However, technology advances in structural health monitoring, undertaken to survey, evaluate and assess bridges, has reached a level of sophistication, robustness and reliability that makes the task easy, convenient and effective. Bridge health monitoring uses an array of inexpensive, spatially distributed, wirelessly-powered, wirelessly-networked, embedded sensing devices supporting frequent and on-demand acquisition of real-time information about the loading and environmental effects, structural characteristics and responses of a bridge. The only challenge, in South Africa, is to prevent the theft of equipment. This, however, should not be used as an excuse for not installing the necessary monitoring equipment when and where it is required.
Moving abnormal loads
When it comes to moving abnormal loads, it is not simply a case of ‘load up, drive and deliver’. It’s a case of careful route planning and investigation. In this case study, Vela VKE was approached to assess four bridges along the N2 route to ensure that the C3 Splitter super-loads could safely cross the bridges. The C3 Splitter was to be transported in three units. The heaviest was the lower unit, with a weight of 720 t. With its two trailers and four tractors, it was 122 m long, 8.4 m wide and 11.1 m high, taking up most of the road width. This vehicle was 17% heavier than the largest load that South African bridges are designed for – the NC30x5x40 super-load (610 t vehicle, 5 m wide and up to 65 m long). The other two super-load units were lighter at 490 t and 340 t. As the super-load exceeded the design load, the bridges had to be re-analysed. To save time and reduce cost, conservative calculations were initially done to identify potential capacity problems, followed by a more detailed examination.
1. Mkuze River Bridge: This bridge consists of a five-span, simply supported, double voided box girder. The spans are 27.4 m long. It was originally designed in 1971 to the old Ministry of Transport (MOT) loading standard. In 1977, Vela VKE designed the strengthening of the bridge to carry current design loads. As the spans are short relative to the length of the super-load, it was not greatly affected by the C3 Splitter. Although various components were overloaded by up to 10%, this was within the capacity of the sections.
2. Mfolozi River Bridge: The original bridge was washed away during Hurricane Demoina in 1984. It was replaced by a Vela VKE-designed balanced cantilever bridge with a main span of 102 m and two side spans of 52 m. As the C3 Splitter could fit entirely onto the main span, the super-load had a significant effect on the bridge. The assessment showed that the C3 Splitter exceeded the design, bending in the deck under traffic. Fortunately, the bending in the critical sections of the deck was higher during construction, so the deck had the additional capacity required to carry these bending loads. Shear in the deck was also exceeded in two places, but as this was less than 5%, this temporary overload was acceptable. The axial loads on the piers and piles increased significantly – up to 25% higher. As these components were designed for flood conditions, they could safely carry the load under normal conditions. However, the instruction was issued not to cross the bridge during a flood!
3. Vaal River Bridge: This 60 m long, three-span bridge was also strengthened in 1977. As both the bridge and its spans were short, it could safely carry the C3 Splitter.
4. Assegaai River Bridge: This 116 m long, five-span bridge has spans up to 30.5 m long. The abutments were strengthened in 1977. Although the C3 Splitter exceeded design loads by up to 7%, this was within the capacity of the bridge.
One can only imagine what would have happened, in moving this abnormal load, if just one of these bridges had a hidden problem or a weakened structure due to corrosion, and the assessment was not done at all or because the expertise was unavailable. A life, or lives, would have been lost. One of which may have been yours.
It is not too difficult to spot bridges that are suffering from corrosion. The ‘Kilarney Bridge’, a component of Johannesburg’s M1 highway system, is a classic example. The concern is the lack of planned maintenance, especially since the ‘cancer’ is aggressive in nature and, as mentioned earlier, should not be left to undermine the structural integrity of the bridge.
The development of effective municipal strategies for the inspection and monitoring of local bridges is necessary, and critical, due to aging, increased traffic loads, changing environmental conditions and advanced deterioration. Not to do so would be extremely short-sighted and an invitation for disaster. An ‘it will not happen to us’ attitude will result in the death or injury of one or more people. Besides the loss of life or limb, in today’s world of litigation, can really we afford it?