KOPUA FOOTBRIDGE REPLACEMENT, RAGLAN, NEW ZEALAND

JOHN MCNEIL1, DAVID MOCKETT1, ANDREW HISCOX2

1 Aurecon New Zealand Ltd 2 HEB Construction Ltd

SUMMARY The Kopua Footbridge Replacement involved the design and construction of a new footbridge across Opotoru Estuary in Raglan. The footbridge required replacement due to significant degradation of the existing structure which was built in the 1960’s. The new structure was designed and built to strict durability specifications to meet the requirements for a 100 year design life in a marine environment. INTRODUCTION Raglan is a world famous surfing destination which attracts a large number of tourists to the Waikato area each year. The Kopua Footbridge is an iconic feature of the town, and provides a link between the town centre and Te Kopua Domain which has sports and camping facilities, and is frequented by families who enjoy swimming and jumping off the bridge into the estuary during summer. The replacement of the existing bridge was required due to significant degradation of reinforcement to key structural members. The replacement structure consists of 6 equal 23.6m spans with an overall length of 140m. Each span is formed from two precast prestressed concrete single hollow core girders with a total width of 2.2m. The substructure was built using precast concrete headstocks and single, 900mm diameter piles. The bridge was opened by the Mayor in September 2011. This paper presents the following key issues:

Specialised concrete mixes – 50MPa concrete designed with micro silica and flyash, along with a low water to cement binder ratio was used to increase durability within a marine environment. The mix also included additional additives to further enhance durability, and account for the 50 minute transportation time between Hamilton to Raglan.

Shrinkage strain measurements – concrete was tested and found to have high shrinkage strain values. Materials engineers were used to undertake a desktop study to confirm the concrete durability was not compromised.

Constructability over water – significant environmental issues required the use of temporary staging, cofferdams and the minimisation of in-situ concrete works. Precast beams and headstocks were utilised and stressed on to piles / piers.

Transient Dynamic Pile Testing – piles were tested using this inexpensive technique. Anomalies were identified in 2 of the 7 piles, one of which required remedial work.

Finishes – significant work was undertaken to test and select appropriate concrete finishes for the project.

Community – significant consultation with key stakeholders and the general public during the design and consenting phase, and also during construction.

Figure 1. New Kopua Footbridge SPECIALISED CONCRETE MIXES The previous bridge suffered significant concrete degradation and had been repaired over a number of years by Waikato District Council. The new bridge required a robust concrete mix design to ensure a 100 year design life with minimal maintenance requirements. Mix Design The bridge is located within a ‘Tidal/ splash/ spray’ exposure environment as defined by NZS3101:2006 Concrete Structures Code. This gives a ‘C’ exposure classification which is defined as ‘Special Concrete’ by NZS3109:1997 (Amendment No. 1, August 2003 inclusive). In addition, the Concrete Structures Code specifies the following requirements for type ‘C’ Special Concrete:

Total binder content shall be equal to or greater than 350km/m3

Water to cement binder ratio shall not exceed 0.45

Chloride content limited to no greater than 0.1% by weight of chloride in chloride salts or chemical admixtures

Minimum of 45MPa concrete compressive strength when using 30% fly ash

Minimum of 50MPa concrete compressive strength when using 8% micro silica

Minimum of 50mm concrete cover

316 Stainless Steel cast-in fixings and fastenings In addition to code requirements, there was some concern that given the history of repeated underperforming concrete at the bridge site, further investigation was required. A materials engineer was engaged to review code requirements taking into consideration the structure’s environment and history. This led to the establishment of two distinct concrete mix designs:

Primary Mix: 50MPa 8% micro silica – used for the primary structural elements such as the piles and girders. The mix includes a water to cement binder ratio of 0.38, and a corrosion inhibiting admixture.

Secondary Mix: 50MPa 30% fly ash – used for the bridge in-situ topping, footpaths and miscellaneous concrete works out of the tidal zone. The mix includes a water to cement binder ratio of 0.4, and a corrosion inhibiting admixture.

There was also a recommendation that the engineer undertake a thorough review of the proposed concrete supplier, aggregate size and properties, and aggregate source. A testing regime was established for the concrete and included trial mixes, compressive strength testing, and tests of the specific shrinkage strain to check the extent of micro cracking expected.

Construction Requirements The project technical specification provided clear guidelines of what was required from the concrete supplier. This included:

Proposed admixtures for shrinkage reduction, corrosion prevention and water reduction

GP cement and micro silica certification

Fly ash certification

Fine and coarse aggregate properties

Crushing strength and alkali aggregate reactivity

Trial mixes were prepared and tested for the mix designs with additional testing required due to the primary mix being produced in Hamilton and Tauranga from different concrete suppliers.

The following data was provided to the engineer as a result of testing.

Slump at manufacturing plant, and slump when on site

7 and 28 day concrete compressive strength

56 day drying shrinkage strain

Water to cement binder ratio before addition of admixtures

Water to cement binder ratio after addition of admixtures

Timing of the addition of the admixtures to the concrete

Actual quantities versus target quantities of the mix

Concrete delivery dockets Placement The primary mix (50MPa 8% micro silica) was easier to handle, work and finish so was used for the majority of the in-situ concrete works on site. Superplasticiser was added on site as required under the direction of the engineer and technical specification. SHRINKAGE STRAIN MEASUREMENTS The footbridge is located within a salt water environment in which there is a history of underperforming concrete bridge structures. It was therefore important that the actual concrete shrinkage strain was measured and compared to code and specification requirements, and modelling assumptions. This testing allowed the potential of micro cracking to be assessed, and the subsequent impact on the required 100 year design durability. Code Requirements New Zealand concrete standards do not define shrinkage strain well. For the design of locked in stresses from creep and shrinkage reference was made to AS5100 “Bridge design” and RRU70 “creep and shrinkage in concrete bridges”. Reference was also made to the NSW RTA Bridge Concrete Specification B80 which specifies a limit of between 560 – 760 micro strain at 56 days for a class C exposure classification. The project specification was written to require testing of the 56 day shrinkage strain with approval by the Engineer for values over 600 microstrain. Testing and Results Samples of the primary and secondary mixes were tested and gave results of 750 and 790 microstrain respectively. As this was not within the maximum 600 microstrain limit specified for exposure classification C, additional work was undertaken to assess if this would compromise the design.

Aggregates The shrinkage strain results above 600 microstrain are typical of concrete mixes using aggregates sourced from the North Island. Surface moisture content of crushed aggregate has an influence on shrinkage, and information in published literature suggests that the higher the moisture present in aggregate, the higher the microstrain shrinkage. The concrete shrinkage is also influenced by the actual surface area and morphology of the crushed aggregate relative to available surface water. Both concrete mixes tested (with 30% flyash and with 8% microsilica) showed similar shrinkage strain results. Further investigation showed that the mixes had an almost identical mass and aggregate grading distribution which suggests that the aggregates are likely the mechanism associated with higher microstrain values. Durability The higher shrinkage values were considered not to be significant and unlikely to cause significantly increased micro cracking. Shrinkage as a time dependent strain involves chemical (including hydration) and drying shrinkage mechanisms that can lead to micro cracking. Hydration is a dominant contributor of micro cracking which is associated with the propagation of cracks and hence increased risk of corrosion. On conclusion of a study of the issues the test results were considered to be acceptable and not compromise the durability or performance of the structure. CONSTRUCTABILITY OVER WATER The bridge spans over a 120m wide tidal estuary which is frequented by recreational fishing boats and pedestrians. The marine habitat is sensitive and is home to several endangered marine species including seahorses. This required special consideration of construction methods to ensure the safety of works whilst preserving the current ecological environment. Environmental Issues The following potential construction-related environmental issues were identified and addressed by the project team:

Working near and over a sensitive marine environment

Dust

Noise

Cement works

Refuelling of machinery on site These issues were eliminated or minimised at the design phase, and addressed by the project Environmental Management Plan. This included the establishment of silt fencing, concrete wash out pit, rubbish skips, spill kits and a construction methodology that minimised working with wet concrete over the water. These issues were monitored constantly and the Environmental Management Plan was amended to suit the specific task being undertaken and the site conditions. Minimisation of In-situ Works A safety in design workshop was undertaken during the design phase to identify any safety risks associated with the construction and operation of the footbridge. This process identified that in-situ concrete works should be minimised to avoid working at height over water for long periods of time. There was also economic benefits due to the minimisation of temporary works, and a decrease in the timeframe required to construct various elements.

This design methodology was used for the footbridge headstocks in which precast units were used instead of in-situ concrete works. This enabled fast placement and erection of the headstock, and enabled manufacturing to take place in parallel with other on site activities such as piling. The construction sequence used stress bars as shown below.

Step 1: Complete pile with cast in stress bars.

Step 2: Place headstock, stress bars and grout.

Step 3: Place girders and pour in-situ topping.

Step 4: Install balustrade.

Figure 2. Precast headstock and girder construction sequence On the eastern side of the bridge is a splayed concrete span which is used to create an open connection with the approach walkway. Originally, the slab was designed as an in-situ span due to the size and un-usual shape of the element. The contractor reviewed this methodology and decided there was significant safety, economic and programme advantages, and constructed this element as a single precast slab. Minor changes were made to the slab design to add additional reinforcement for lifting. Because of the tight tolerances between the pier cap and walkway abutment it was decided to wait until these were installed before undertaking a detailed as-built survey, preparing the fabrication drawings and pre-casting off site. The splay slab also had a longitudinal fall with the bridge and crossfall to tie into the existing walkways so it was crucial that the survey and fabrication drawings were accurate.

Figure 3. Splay slab temporary formwork and steel placement Due to the capacity of the 70T crawler crane used on site and the radius of the proposed lift, it was decided to pour the slab 100mm lower than the finished level to keep the weight of the splay slab under 20 tonnes. This presented difficulties in finishing the concrete to the required level below the top reinforcing mat. Reid Construction Systems carried out a detailed lifting design which included 4 lifting points and the use of a spreader beam. Two of the lifting chains had to be shortened so that the slab could be lifted level. This also prevented the possibility of the slab “rolling” when being lifted due to its narrow width. The chains and spreader beam were lift-trialled in our Mt Maunganui yard and trucked over to site together with the slab.

Figure 4. Splay slab placement and final result with in-situ topping Cofferdam Arrangement The design required the piles above sea bed level to have an F5 finish in accordance with NZS3114:1987 Specification for Concrete Surface Finishes. In order to achieve this, the pile steel casings were cut off at sea bed level after they had been driven to depth and a muff mould installed to allow the top section of the pile to be cast as in-situ. A 2.5m diameter steel casing was designed and constructed as a temporary cofferdam. It was installed over, and after, the permanent piles were installed. It was driven to a required depth as determined by the temporary works to ensure it was water tight and stable under strong tidal flows. Once installed the water inside the coffer dam was pumped out allowing us to cut off the pile steel casing below the estuary water level and install a muff mould. All work inside the cofferdam was classed as confined space and required the appropriate staff safety training and equipment. The reinforcing cage, stressing bars and Drossbach ducts were then installed before pouring the piles with a concrete pump and tremmie tube. The muff mould was removed after curing after which the coffer dam pulled out.

Figure 5. Pile cofferdam and muff mould arrangement

The cofferdam arrangement worked well and produced a very good finish on the piles which met the requirements of the project specification. TRANSIENT DYNAMIC PILE TESTING Integrity testing involves applying an instrumented hammer blow to the top of a pile, and recording the response using a geophone (accelerometer). The blow sends a compression wave down the length of the pile, which in a “perfect” pile is reflected off the toe and recorded by the accelerometer. If variations (i.e. possible defects) exist along the length of the pile, a tension wave will be reflected from the variation, which will be recorded by the geophone at the top of the pile. By recording the wave signal returned from the pile, and analysing the frequency response, an assessment of the integrity of the pile is able to be determined. Testing Regime The contract required integrity testing of all 7 piles completed using the Transient Dynamic Response (TDR) method in accordance with AS 2159:2009.

TDR Test Results The analysis and test results showed no significant or apparent reductions in pile diameter and concrete quality for 5 of the 7 piles tested. An anomaly was identified in the pile 2 at approximately 10m from the top of the pile, and a smaller anomaly in the pile 3 at approximately 9m from the top of the pile.

Figure 6. TDR results for pile 2 showing a an anomily at 10m below ground level

Remedial Measures and Re-Testing Following the TDR test results both piles with anomalies were cored to the depth where the results were showing defects. The cores for pile 2 showed an area of aggregate without cement fines at the location of the anomaly. This may have been caused by the tremmie being lifted above the level of concrete as the pile was being filled with concrete. The cores for pile 3 showed nothing unusual at the location of the anomaly.

Figure 7. Concrete core of the pile 2 showing anomaly area The following remedial methodology was prepared and agreed for pile 2:

A suction pipe 10m long was lowered down the cored hole into the pile. The suction pipe was then connected to a sucker truck with a clear hose.

A high pressure water blaster (5000 – 10000psi) was also lowered into the cored hole, with a rotating, jetting head. This ensured that the reinforcing wouldn’t be damaged while jetting with high pressure. A trial above ground was carried out to prove this.

The sucker truck and water blaster were operated together to remove loose material. This operation continued until only clean water was flowing through the clear hose.

The water blaster hose was then removed and water sucked from the cored hole until it was dry.

The cored hole was then filled with Sika Monotop Micro-concrete to finished level of pile.

A second pile integrity test was completed which then showed zero defects in the pile. FINISHES

There were a number of factors to consider when finalising the surface finish of the bridge deck. A non-slip surface was required that was not harsh on bare feet but was still suitable for cyclists and mobility vehicles. It also had to look aesthetically good and not pond water. Surface Finish Testing 3 test panels were prepared with different aggregates and finish solutions. These were tested for a range of qualities.

Acid etched Exposed (crushed rock) Exposed (round stones) Figure 8. Test panels 1 to 3 The test panels consisted of the following. Panel 1 Acid etched – result was a smooth non slip surface, which aesthetically blended

with the colour of other concrete on site.

Panel 2 Exposed aggregate (crushed rock) - crushed rock and a high cement content in the mix gave a coarse no slip surface.

Panel 3 Exposed aggregate (round stones) – result was a smooth, non slip surface,

However, this mix had not been used previously on site and would have required trial mixes and shrinkage tests to be carried out to verify its properties.

A decision was made to use the acid etched finish which looks good and performs well.

COMMUNITY Prior to commencement of the design, Waikato District Council had undertaken significant public consultation in 2009 presenting several footbridge options. Feedback from this process identified the need for ongoing consultation and development of a solution supported by the community. Pre-Consent Consultation The design team was engaged after an initial phase of public consultation by the client in which the community were firmly against the bridge replacement. The project had received negative attention by local and national media. The design team accounted for community feedback and consulted with local community groups, Iwi, leaders, the deputy mayor, and members of the client team. Through this process a unique innovative solution was

developed to meet social and cultural requirements. The project has now been received by the community with a sense of pride and ownership. The project team consulted with a number of key stakeholders which included:

Ngati Mahanga and Tainui Awhiro

Waikato District Council Parks

Environment Waikato

Department of Conservation

Coastguard, Raglan Harbourmaster and Maritime New Zealand

Whaingaroa Harbour Care

New Zealand Fish and Game

Historic Places Trust

Utility service providers The project team worked with Iwi to develop the colour scheme for the bridge which is inspired by the marine landscape and the bright colours of Paua shell. The team worked with sculptors from local Hapu to incorporate pou into the design located as markers to the entrance of the bridge. Through the consenting process, the project was the catalyst of strengthening the relationship between local Iwi, Ngati Mahaanga, and Waikato District Council. The parties worked collaboratively on the project, and created a foundation for Waikato District Council to partner with local Iwi on future projects. The site has a100 years of history with the new bridge being the third to span over the estuary. Through the design process this history and importance was understood and the new bridge respects this history and incorporates details of the hand rails, alignment and span arrangement into the new structure to reflect the historical bridges. Construction Consultation During construction the community was always interested in the construction progress. We dealt openly with interested members of the public on site, and kept the community fully informed through the local newspaper and Harbourmaster. ACKNOWLEDGEMENTS Client: Waikato District Council Detailed design and project management: Aurecon New Zealand Ltd. Project Architect: Bossley Architects Ltd. Construction: HEB Construction Ltd.