Structural health monitoring
Chapter 1 - Introduction to Structural Health Monitoring
1.1. Definition of Structural Health Monitoring (SHM)
Structural Health Monitoring (SHM) aims to give, at every moment during the life of a structure, a diagnosis of the ‘state' of the constituent materials, of the different parts, and of the full assembly of these parts constituting the structure as a whole. The state of the structure must remain in the domain specified in the design, although this can be altered by normal aging due to usage, by the action of the environment, and by accidental events . In short, the definition of SHM can be summarised by saying that the objective is ‘to check that the structures behave as intended'. It is basically an activity where actual data related to civil structures are observed/measured and registered. Civil structures most commonly requiring SHM are bridges, pipelines, dams, tunnels, building alongside many others.
1.2. Need for Structural Health Monitoring
When it comes to civil engineering infrastructure, use of, or need for Structural Health Monitoring becomes most evident if it can avoid human loss of life. Ensuring safety by preventing catastrophic failures in civil structures is a must do for any country. After some serious failures in the past such as i) ‘Silver Bridge' between Point Pleasant, W. Va, and Gallipolis, Ohio, USA, which resulted in the loss of 46 lives, ii) ‘Mianus River Bridge' Greenwich, Connecticut, USA, resulted in three deaths and three serious injuries , just comes to prove what can happen and shows necessity for ensuring safety of human lives. Structural ageing is an enormous problem within civil engineering field when it comes to bridge failures. A good demonstration of how big the problem is can be by looking at North Americas statistics by Federal Highway Administration which has classified 42% of the United States' 578,000 bridges as structurally or functionally deficient (some even said to be obsolete) . That means, 242,760 bridges are deficient by present day standards, and currying rehabilitation or replacement on these bridges would cost many billions of dollars for USA government.
The sooner the problem is identified the better it is for all stakeholder groups (i.e. operators, contractors, users, government etc.) as structural stability can be ensured, cost kept low, and from operational point of view, it can avoid major disruptions or accidents therefore ensuring serviceability to the community as well as safety. Most major highway bridges are expected to have a service life between 110-120 years only with routine maintenance, however many require extensive and expensive rehabilitation only after 10-20 years . This is due to numerous problems that bridges are exposed to which are identified in later chapter. It has to be mentioned however, that further complexity to the problems are added due to structural diversity. Ignoring other civil engineering structures and solely focusing on bridges it can comfortably be said that no two bridges will be identical. This would be due to many factors such as span of the bridge, type of bridge, materials used, geographical location therefore different environmental influences, different loads, soil properties and so forth. Hence, for each individual bridge structure separate monitoring has to be carried out since there is no guaranteed behavioural pattern or knowledge how the structure will behave, even if bridges in close proximity of the same type are used for comparison.
If a bridge is partly or completely out of service it still affects the general public by causing disruption of traffic as people have to either find alternative routes or wait in long traffic queues. The closure of the Williamsburg Bridge between Manhattan and Brooklyn, from April to August 1988, caused 100,000 drivers a day to find alternative routes . This can lead onto other problems, perhaps if alternative bridge is used for crossing it would be experiencing higher than normal traffic loads. Therefore, in this case it would be a good idea to have some sort of feedback on structural performance of the bridge in order to manage the traffic flow so it does not have damaging impact on alternative bridge. SHM is not only used during service of the structure, this is only one phase of structures life. Monitoring can be required during many stages such as during construction of a new structure(e.g. checking for premature cracking), after refurbishment, enlargement or strengthening, during testing (e.g. testing before service for safety reasons), during service and lastly during dismantling.
Structures that use SHM systems would like to have maintenance costs at constant level alongside good reliability, which is not the case for older structures that do not have SHM which actually seem to have increasing maintenance cost and decreasing reliability.
Chapter 2 - Basic bridge problems that compromise structural integrity
2.Basic bridge problems
As mentioned in the earlier chapter even in the early years when newly built bridge enters its service life it may need extensive and expensive rehabilitation. Older bridges built around or after WWII would definitely be more prone to problems that structures are faced with, perhaps because of materials used at the time and pressure to build quickly. In seismic areas, earthquakes would most likely be classified as the most severe threat to structural integrity of the bridge. However, there are also other problems and types of damage, degradation mechanisms. After earthquakes, corrosion of reinforcement in concrete structures can be deemed as second most important problem. There is also physical/mechanical damage that has to be taken into account, with both chemical and biological degradation also. Actions such as freezing and thawing can increase cracks within concrete, and increasing traffic volume needs to be appreciated in recent decades along with trucks carrying heavier loads.
2.1. Corrosion of reinforcement - damage to concrete
When looking at deterioration problem of concrete, it needs to be kept in mind that deterioration my happen due to several causes combined. It also has to be noted that corrosion of reinforcement is an area that goes into great depth, therefore in this paper only basic problems will be outlined. Greatest number of defects which affect reinforced concrete are associated with the corrosion of reinforcement . There are five mechanisms that can influence corrosion, with four of them said to be relatively rare in occurrence. These are, ‘concentration cell corrosion', ‘differential aeration cell corrosion', ‘galvanic cell corrosion', ‘stray current corrosion' and ‘bacterial corrosion'.
In order for corrosion to actually happen, water, oxygen and damage to passivation layer need to be present. Passivation layer is what protects steel from corrosion. Reaction that occurs between steel and surrounding concrete forms a relatively thin layer which is insoluble material known as ‘passivation layer'. Carbonation process can damage this ‘passivity layer' and therefore expose reinforcement in concrete to corrosion. This is a natural process where calcium hydroxide within concrete changes to calcium carbonate by the action of dissolved carbon dioxide. Concrete that is very well compacted should not suffer from carbonation, however certain locations such as corners within column or bridge deck will always be susceptible to this problem. If proper quality control procedures have not been implemented during building of the structure, it leaves concrete prone to deeper penetration of water, oxygen and especially prone to chloride contamination. Examples of poor quality control are excessive water content, or cement content, inaccurate quantities of ingredients that perhaps have not been mixed properly. Road salt (sodium chloride) is one of chlorides that can attack reinforced concrete. It can cause very fast corrosion of concrete reinforcement especially with presence of oxygen and water (moisture at least), although it does not have any affect on concrete itself. Corrosion in reinforcement is expansive process, which results in generation of radial pressure that through time will be big enough to cause surrounding concrete to break. This leads to spalling of the concrete, delamination of reinforcement which in turn results in loss of strength in the member that is reinforced.
2.2. Frost damage
Frost damage can occur due to daily and seasonal thermal cycles. Bridges whose concrete is relatively dry (i.e. interconnected pores having small amount of water) are said to be resistant to frost action. When water in the interconnected pores of wet concrete freezes, there is a 12% volume increase . Since water that is trapped expands, it can produce forces large enough to crack the concrete and cause spalling which in turn as mentioned in earlier chapter can lead to loss in strength of a structural member. This is certainly problematic in areas where water will stay in frozen state for long time (i.e. temperatures below zero). Problem of freezing and thawing is most influential in concrete of higher permeability. Improper drainage of the structure, or clogging up of drainage system can lead to over spilling of water on the sides, which only helps the matter further.
For thermal expansion in reinforced concrete, it has to be noted that concrete and steel have very similar coefficients of thermal expansion, hence, differential expansion stresses are small and thus cannot cause stress failure.
2.3. Traffic Volume
Traffic volume, either static or dynamic can have impacting damage on bridges critical members and components. Damage that can be observed are displacement (i.e. vertical displacement of deck beam), strains (i.e. in beams) and deflections. Loading happens constantly during the service day of a bridge (highway bridge in this context) and undergoes different loads at different times due to volume of vehicles passing through. Figure 1PB, shows how traffic volume varies during typical day of service for ‘Queen Elizabeth II Bridge' in Dartford, London. As expected, traffic volume is highest during rush hours of 8am and 6pm, with its peaks exceeding practical operating capacity of 5000 vehicles. Dynamic loads are imposed through moving traffic, however, if congestion is very bad it can bring traffic to a standstill, which would impose static loading on the bridge.
Going back to what was mentioned earlier that in recent decades traffic has been increasing, good example is the case of ‘Rio-Niteroi Bridge' (official name ‘President Costa e Silva Bridge'). When opened for service in 1974 estimate of traffic increase was at 1.5% to 3.0%, however, actual increase as seen on Figure 2PBwas at an average of 8.7% . Although designers have anticipated increase in traffic volume, questions arise when estimated figure is well below the actual one. The main question would be whether or not the structure can cope with new traffic increase and does it speed up deterioration.
Referring back to Figure 1PB, with recent approval of widening M25 ring motorway around London, UK from three lanes to four, brings possibility of traffic flow increasing over Queen Elizabeth II Bridge. This could perhaps see traffic volume peaking over longer period of time. However, extra lane does not mean more traffic