Introduction
Bridge is an essential part of infrastructural development. As such, the twin bridges in Grafton is meant to improve efficiency in road transportation between the two Grafton areas:  Grafton and south Grafton.  Moreover, the bridge will improve the safety that should be accorded to all the road users, from pedestrian to cyclists and moorists.The project was initiated in the year 2016 under the Woolgoolga to baliba project of upgrading the highways. Nevertheless, the bridge is about 81 m long and is made up of 8 decks, 4 in each of the single bridge As such, the northbound bridge has 4 decks while the southbound bridge has an equal number.  Moreover, there are 3 locations of piers in each of the bridge. These piers are located inside the sea waters to support the decks.
Moreover, the bridge is designed so that it can protect the surrounding the surrounding areas from the flood. Flood has been witnessed around the locality and its prevention is imperative. Moreover, the floodplain data indicates that there are locations wher structural elements mat be subjected to attacks by either sulphates.chloriodes or acids and alkalis.
Considering the importance of the bridge to the location, it is important to conduct a durability assessment. This report indicates the durability assessment methods used to ensure the bridge satisfies the design. The design life in this case should be at least 100 years,
The twin bridge
An overview of the bridge and recent developments
The construction of the bridge was initiated in the year 2016 by the New South Wales government. It was meant to connect the two aforementioned locations and to cross over the Clarence River.  The two rivers are Grafton and South Grafton. Nevertheless, the two main factors that led to the development of the bridge was the increased traffic congestion and the need for connectivity between the two locations. Moreover, the safety of the road users was aklso imperative in its development/
In essence, it was built at a distance of 70m from a bridge that was present. In essence, it was under a project estimated at 4.36 billion. It was an initiative to upgrade the highway. The two main project stakeholders are the Roads and Maritime sector and the NSW government. However, the cost of the bridge was estimated as $ 240 million while the deadline for completion was 2019.
As per the initial design, it was projected that the total length would be about 525m. On the other hand, it was proposed that the construction would involve both sea and land based pylon. However, the number of pylons that ere to be located on the land were estimated to 15 while those which were located on sea were 10. Nevertheless, the road was to be constructed of two lanes and there were future plans to increase this number to four.
The main contractors of the project, according to the development plans, were Bellwether contractors and Fulton Hogan, However, Fulton Hogan was proposed as the main contractor of the bridge. This was on the month of August the year 2017. On the other hand, Bellwether was proposed as the support team.
The initial project details.
The need to construct a crossing over the Clarence River was observed by the New South Wales government in the year 2002. However, it was not until the full results were released in 2003 that this need became so apparent.  Regardless of the study results, it was the observation by both government and local agencies that bridge that was present in the year 2008 was short of the capacity. In essence, the proposal to initiate the construction pf these additional carriageway was proposed in the year 2012.
 The scope of durability project
As stated, durability plans a vital in ensuring that a project serves its design life and there is proper and efficient monitoring. Nevertheless, the durability plan, should be done in the following stages to ensure efficient service delivery

  1. To begin with, there needs to be an outline durability plan that is usually created in order to ensure that the tender presents a more detailed approach to the various teams involved in the project. The details to be included in this case includes the exposure, the mechanisms that the structure will deteriorate among others. This enable the tendering to ensure that the team is well aware of the client’s requirements in regards to durability and the price contained therein.
  2. The development of the full durability plan is thereafter done after two mo0nths of the award of the contract. Nevertheless, the full durability plan is a joint venture between the design and the construction team. Working from the same platform will ensure that they are able to relate the documents plan to the actual structure. This will in essence determine the extent of durability that may be achieved. Nevertheless, this is usually dependent on the type as well as the size of the project.
  3. There is a need to alter the durability requirements due to the actual project development. Development in the project is bound to bring about significant design changes which need to be adjusted in the actual durability plan. Nevertheless, the most important addition to the durability plan may be the maintenance e planning. All this covers the additional exposure of the building.
  4. Ensuring that the durability requirements are incorporated in the actual bridge structure is an essential part of achieving the requirements of th4 durability design report. As such, there is counter checking of the actual specifications throughout the construction process. Moreover, there is the durability consultant checking that ensures all the durability requirements are implemented and verified. As such, it determines the quality of the durability measures that have been put in plan. However, it is also required that there be a non-conformance plan as well as a quality plan which will ensure that all the non-conformances are rectified.
  5. Finally, there should be a durability assessment report. This is usually done by the construction team. Nevertheless, this is only done at the end of the project and is meant to outline the methods that have been used to ensure that the structure conforms to the durability requirements.

Following the development of the stages, the actual durability plan is outlined into the following sequence of activities. Nevertheless, these stages usually present, the steps through the whole construction and design phases.

  • To begin with, the project will be divided into different sections. These categories are usually dependent on the level of exposure. There are various exposure categories that can be described from As3600, AS2159, AS3735 and AS 2312 among others.
  • Secondly, the mechanisms that can cause deterioration in each of the elements is considered for assessment. It in turn determines the durability that can be expected.
  • Thirdly, there is an analysis of the compatibility of the materials used for the design with the reinforcement as well as the method applied. However, the most appropriate method is the crack width analysis as well as the thermal analysis.
  • Risks that are associated with the failure of the durability measures, the requirements for the materials testing, the requirements for monitoring as well as the measurements for testing are updated into the durability plan.
  • Finally, the requirements of the performance, the workman ship required as well as the prescription requirements are put into different classes and for each element. This, in turn means that a report will need to be developed at this final stage.

The design life
It has been defined as the period of time when the bridge will perform satisfactory. Nonetheless, this is usually a careful estimation that takes in consideration all the factors likely to increase the rate of deterioration. As with this twin bridge, the design life is taken as 100 years. Nevertheless, this is for all the bridge components such as piers, abutment. However, the durability plan should be appropriately implemented to ensure that the piers and sections in contact with the water are able to be operational throughout the design life. Regardless, the design life of the bridge is designed as per AS 5100.1-2004
The protection from salt damage of the substructure and the pre stressed sections
Salt has the effect of increasing the rate of deterioration and as such, may result in expenses associated with repair. In extreme cases, this may result to the construction of a new bridge. However, this is very common in bridges in coastal areas and seas. As a matter of fact, the phenomenon is common in concrete bridges as well as sections that are usually pre-stressed.
To determine the effects of the salty water on the bridges, a preliminary investigation of the bridges surrounding the area need to be conducted.  Understanding the effect of the salt water on bridges that may be about 10 years just gives a benchmark from where the basis of corrosion can be based. One factor that can be estimated is the salt penetration into the concrete. Penetration of the salt into the concrete may reduce the strength of steel which in turn may lead to shorter design life. The coefficient of chloride iron may also be used to understand the effect of salt on the structure.