Self-Compacting Concrete Enhances Tauranga's New Harbour Link

Self-Compacting Concrete for Superior Marine Durability
New Zealand’s new Tauranga Harbour Link

Len McSaveney
Golden Bay Cement, New Zealand

Michael Khrapko
CBE Consultancy Ltd, New Zealand

Frank Papworth
BCRC, Australia

ABSTRACT

Concrete made with a blend of Portland Cement, Microsilica and Fly Ash, has for some time been recognized as delivering superior long-term durability in marine structures. These triple-blend binders are increasingly being used for major structures internationally, but have only recently been tested with New Zealand-sourced materials. The ability to use local materials is of course one of the key sustainability attributes in favour of the use of high-performance concrete for major structures.

In addition to the normal concrete attributes that can so significantly influence durability, the fine particle size-grading achieved by the use of selected cement and pozzolanic fillers in such high-performance concrete mixes, means that the binder combination makes a perfect medium for self-compacting concrete. This enables the Contractor to take advantage of all the quality improvements; the environmental benefits of a clean, quiet site; and the cost-saving advantages that SCC can bring to a well-managed construction project. The boost in productivity can be quite outstanding and as vibration during casting is not required, durability risks such as displacement of reinforcement, over or under compaction, and honey-combing at regions of congested reinforcement are also virtually eliminated; which allows tighter tolerances on bar locations to be used in the durability modeling for a hundred-year design life.

The application of these insights to a recent “Design and Construct” contract for the Tauranga Harbour Link project, in New Zealand, has verified the cost advantages of smarter concrete technology - when it is backed by rigorous testing and detailed durability modeling.

1. INTRODUCTION

The Tauranga Harbour link duplicates an existing Harbour Bridge and includes approach ramps, and over-bridges (Figure 1), to carry traffic to the Port of Tauranga - connecting the coastal main highway, via Mt Maunganui, to the motorway routes through the city of Tauranga. The tender for construction was let as a Design-Build Contract in 2007.

Fig. 1 Tauranga Harbour Link

The winning bid was based on the use of high durability SCC; as permitted by NZS3101:2006, with specific durability modeling. Recent international research had indicated that a hundred year design life could be achieved with 40mm of concrete cover, but local New Zealand materials had not yet been tested.

A Triple-blend binder, consisting of HE Cement, Class C fly ash and natural geothermal microsilica makes a very good self-compacting concrete - at the binder content that was required. The contractor was able to set up the site-precasting yard and formwork to take full advantage of SCC and to evaluate its impact on costs.

The twin technologies of the high-durability triple-binder paste, and SCC, proved to be a winning combination. The cost advantage to the successful Design-Build team was 20% of the bid price: some $20 Million was left on the table, but the team had won a contract that they were confident would be profitable.

2. EXPOSURE CONDITIONS

The bridge is located within Tauranga Harbour on the East Coast of New Zealand (Figure 2). The height of most of the structure far exceeds the wave height in this protected location and design was generally based on a code exposure of no splashing but high wind-blown salts.

The surface chloride levels were discussed between the Client’s Engineers and the Design Engineers, to set the design basis for the 100-year life.  The project’s Durability Consultant, BCRC, had also by that stage identified an extreme durability condition, which was not included in NZS 3101; or in the Client’s Design Brief. This was the low soffit of precast concrete T-Roff beams that were within 300mm of extreme tides (Fig 3b). With wave splashing, droplets of water could hang on the soffit, giving extended time for chloride ions to soak in.

Figure 2: Exposure of Tauranga Bridge

a) Existing Bridge in Protected Harbour 

b) New Bridge Section Close to Seawater

3.  TYPICAL T-ROFF BEAM DETAILING

The T-ROFF Beam (Figure 3) has become a de facto Industry Standard in New Zealand and Australia. The cross-section copes easily with the typical span ranges of easily transported pre-tensioned, precast sections and simplifies the on-site deck construction. Moulds are simple to construct and easily adjusted for variable lengths and end skews.

Figure 3 : T-Roff Beam 

a) Reinforcement on High Performance Spacers

   

b) Concreted Unit

For this project, the ability to reduce the concrete cover reduced the self-weight of the beams, allowing the units to span further while remaining within the lifting capacity of available cranes. This resulted in a dramatic reduction in the cost of the deep pier foundations and was responsible for a significant part of the total cost saving with this alternative.

4. CONCRETE PROPERTIES

4.1 Mix Designs
An early decision was that Self-Compacting Concrete, with a 60 MPa, 28-day cylinder strength, would be the basis of the design. For the pretensioned T-Roff beams, the overnight strength after accelerated curing would need to be 35 MPa; while for the incrementally launched box girder, the required strength after two days of ambient curing was 30 MPa.

The beam cross sections were not particularly complex (Figure 2), so an average slump flow of 680 mm was selected, with a T500 value not exceeding 3 seconds. The selected coarse aggregate was local crushed greywacke of 19 mm maximum size. Sand was a mixture of greywacke manufactured; and marine quartz, sands. A Triple Blend Binder consisting of  High Early Strength Portland cement, Class C fly ash, and natural geothermal microsilica (Microsilica 600R); a 62/30/8 ratio was used.

A comprehensive development program was set out at Golden Bay Cement’s laboratory at the Portland cement manufacturing plant. A testing procedure for each successful laboratory batch of concrete consisted of the following tests:

• Slump flow test
• Time for 500 mm slump flow spread
• L-Box, for passing ability
• Air content and wet concrete density
• Compressive strength in 16 hours (cured for 10 hours at 650C), 1day and 28-day.

After enough test data was collected for both types of SCC, three best performing mixes were selected for each type SCC for further testing. At the end of the mix developing program, two SCC mixes for different exposure conditions were offered to the contractor with the following characteristics:

Characteristics	Unit	SCC 1	SCC 2
Slump Flow	mm	710±30	700±30
T500	sec	2.84 (2÷3)	2.52 (2÷3)
L-Box	ratio	0.85 (≥0.80)	0.88 (≥0.80)
Slump Flow 60 min Delayed	mm	660±20	630±20
Air Content	%	1.5±0.5	1.7±0.5
Wet Concrete Density	kg/m3	2350±30	2406±30
Compressive strength:      16-hr (accel.)	MPa	38.5±3.5	35.5±3.5
1-day	MPa	22.0	15.8
28-day	MPa	87.0 (≥70)	71.5 (≥70)

Table 1 – SCC Characteristics

To achieve the strength requirements a target w/c ratio of 0.32 was set. The accuracy of batching meant that the maximum w/c ratio could be 0.34. Durability analysis indicated that the design life reduced by 20% when the w/c ratio increased from 0.32 to 0.34 hence the durability design was based on a maximum w/c ratio of 0.34. The concrete mix finally selected, following trials of various mixes, is referred to in this paper as “Mix M”.

4.1 Curing
Concrete; particularly that incorporating fly ash and microsilica, is highly sensitive to curing. Often 14 days curing is stipulated for fly ash concrete. In this case 7 day curing was proposed and a small allowance on predicted concrete properties was made. This allowance was based on research into the effect of curing on concrete strength (Haque 1990) & then related back to an effective w/c ratio.

4.2 Chloride Ion Permeability
Three heat-cured representative samples of Mix M were tested by BCRC (using the NT Build 443 Method and gave a diffusion coefficient of 1.37 x 10-12 m3/sec. For chloride diffusion tests, two months was allowed for curing before commencing chloride exposure as experience suggested that poor early results would be obtained due to the slow hydration of fly ash if 28 day curing was adopted. Three months is generally allowed for exposure prior to chloride profile testing when using high performance concrete to ensure that the chloride profile is sufficiently established. The resultant five month lead time was untenable from a construction perspective. Hence modeling based on expected chloride diffusion results was undertaken and the diffusion values were subsequently confirmed by testing.

4.3 Sorptivity
Where concrete is subject to wetting and drying, water (and whatever is dissolved in it) is drawn into the concrete by capillary action. As initial water ingress is relatively fast (i.e. a few millimeters in a few minutes with normal concrete) with seawater splashing, chloride penetration into the sorption layer is rapid. Between splashes water dries out but the chlorides remain. Subsequent splashes provide further doses of chlorides to the sorption layer. As this build-up of chlorides in the sorption layer occurs quickly it must be deducted from the cover when doing the diffusion analysis. The thickness of the sorption layer is a function of the concrete sorptivity and the time of wetting. On vertical surfaces water runs off, so a wet time of 15 minutes was used in the analysis. On horizontal surfaces water hangs on, so on “low soffits” a wet time of 200 minutes was used. The significance of these parameters is seen in Table 2. For the low-soffit exposure condition, and in the splash zone on the beam sides, this parameter was critical. Fortunately, recent research conducted by Professor Peter Bartos (Wenzhong 2003) had discovered that the undisturbed external structure of self-compacting concrete will give amazingly low sorptivity values. Tests at BCRC confirmed a sorptivity value of 0.036 mm/min0.5 for Mix M and the “Design for Life” model was adjusted to accept this low number.

Paste System	Sorptivity (mm/min0.5)	Calculated Sorption Layer Thickness (mm)      15min wet time             200 min wet time
GP cement, w/c = 0.50	0.38	15	54
GP cement, w/c = 0.40	0.28	11	39
8% Microsilica w/c = 0.40	0.07	3	9
8% Microsilica w/c = 0.32	0.05	2	7
Mix M	0.036	1	5

Table 2 : Sorption Layer Thickness as a Function of Mix and Wet Time

4.4 Resistivity
Modeling of reinforcement corrosion includes two phases:
- Initiation is the time for chlorides to reach the steel in sufficient quantity to initiate corrosion
- Propagation is time from corrosion initiation, to failure.

fib Bulletin 34 deals only with time to initiation. Other models (Life 365 & Concrete Society) make a set allowance for the propagation, regardless of the mix.

The “Design for Life” model recognises that concrete resistivity varies significantly with w/c ratio and cement system and notes it is inappropriate to treat concrete with very different resistivities in the same fashion, when assessing the propagation phase. Hence, the concrete resistivity was used to calculate the time to spalling and a high safety factor was applied to allow for the uncertainties in the calculation method.

Saturated concrete samples were tested using a four probe Wenner method to give a worst case resistivity. The tight internal structure of the triple-binder, low w/c SCC gave a very high resistivity of 45.6 kohm.cm

4.5 Bar Spacers
A durability weak point exists where spacers form an ingress plane, or have high chloride penetrability. Grout and mortar spacers are often made using GP cement and a w/c ratio no lower than the surrounding concrete. In such cases the cement system and high paste volume can give a spacer with far lower performance than the surrounding concrete. High performance concrete bar spacers of at least comparable durability to the adjacent concrete were used; positioned in compliance with BS 7973, to achieve a minimum concrete cover of spacer height, minus 2mm. Table 3 sets out the design cover requirements for the various parts of the structures, with the placing tolerance deducted.

Location	Exposure	Concrete Cover
T-Roff Soffit	Low soffit in splash zone - rebar Low soffit in splash zone - prestress	43mm  55mm
T-Roff Webs	Near to vertical - aplash zone	38mm
T-Roff Internal	Drained and vented, but no access	25mm

Table 3 – Cover Requirements

 

5. CONCRETE MANUFACTURE, BEAMS PRODUCTION AND QUALITY CONTROL

5.1 Concrete Manufacture
After completion of the mix development program in Golden Bay Cement’s laboratory, a series of full scale trial mixes were conducted at a concrete plant located in the precasting yard. This was to evaluate the effects of variation in quality of raw materials and production variations (batching accuracy, weather condition, etc.) on the quality and consistency of the SCC. Only minor adjustments to the mix designs were required. For ease of batching and for rapid truck-mixing, the Concrete Supplier (Firth Industries) chose to use the slurry form of Microsilica 600R.
 
5.2 Beam Production
The bridge beam production facility had been set up to maximize the production savings available with SCC: the concrete ready-mix trucks drove along an elevated ramp, discharging their loads directly into the formwork. The time to discharge a 4.0 cubic metre load of concrete was typically 15 to 20 minutes. This made it possible to reduce the required open time from 45 to 25 minutes, presenting another opportunity for more cost savings. Both SCC mixes exhibited very good flowing characteristics. Molds were quickly filled, without vibration, and with minimal site labour. Surface finishes were excellent; requiring very little remedial work.

5.3 Quality Control
A dedicated concrete plant was built at the precast yard and started producing conventional concrete a few months prior to the commencement of SCC supply for the beam production. This allowed for a number of SCC trials to be conducted and also for certification of the plant in accordance with NZS 3104:2003 Specification for Concrete Production. The certification would warrant the plant’s capability of producing quality concrete, within standard tolerances.

Every batch of SCC was checked for slump flow at the batching plant and then at the precast yard before discharging concrete. All tests were within the specified tolerances.

Part of the beam production quality control was to monitor concrete curing temperatures and release strengths. Temperature was monitored by a series of temperature probes and controlled by a hot water running through the rectangular pipes attached to the side shutter of the forms.

6. DURABILITY MODELING AND TEST RESULTS

The design life was calculated as time to corrosion intuition plus the time from initiation to spalling. Time to initiation was calculated using the fib Bulletin 34 formula:

Where
a              =   minimum concrete cover [mm] i.e. nominal cover, less placing tolerance
Dx            =   sorption layer thickness (see section 4.3)
t                =   exposure time [years]
C(x=a,t)   =   chloride level at the bar at time t, = cl- threshold for time to initiation
C0            =   initial chloride content [wt.-%/c], i.e. maximum permitted at time of delivery
Cs,Dx      =   chloride content at surface & through sorption layer, Dx [wt.-%/c]
x               =   Depth with corresponding chloride content, C(x,t), [mm]
Dc            =   Diffusion coefficient at each time interval Dc=Dit-m
m             =   a factor relating diffusion at any time, to initial diffusion

The chloride threshold level was taken as 0.77% by weight of cement, for passive reinforcement. Although high compared to some standard threshold values quoted, this includes a significant safety factor based on threshold levels found by Pettersson 1998 and Polder 1996 for low w/c ratio concrete. The activation level for prestressing steel was taken as 0.4% by weight of cement.

The surface chloride level was taken as 7% by weight of cement for the low soffit and 5.5% by weight of cement for vertical surfaces in the splash zone based on the use of fly ash which tends to have a higher surface chloride level than GP cement. The very high surface chloride level for low soffits was based on the Durability Consultants experience, while the lower level for vertical surfaces is in keeping with published data (Bamforth 1996) for splash zones. The diffusion coefficient was adjusted to allow for the in situ concrete temperature based on the Nernst-Einstein equation. The reduction in chloride diffusion with time (m) was based on Bamforth’s 0.62 value, for fly ash. This is a common value used in durability modeling around the world due to the scarcity of long term data. However, the “m” value is a very significant factor in the life calculation and there was significant concern that local materials may prove to be less effective in the long term than those used in Bamforth’s research. Application of a high safety factor led to an actual “m” value of 0.43 being used for the Tauranga Bridge.

For passive reinforcement, the propagation phase was calculated from the corrosion current being the potential difference between anode and cathode divided by the resistance between anode and cathode. Macro cell corrosion between the top and bottom mats of reinforcement was assumed. The results gave corrosion rates consistent with Polder (1996) however recognizing the uncertainty in many aspects of the calculations, a factor of 3 was applied to the calculated propagation phase. The propagation time for prestressing steel was taken as zero. The calculated lives are given in years, in Table 4. The models are of course not so precise, but considering the conservatism built in, it was agreed that these may be taken as the minimum lives to be expected.

Durability Item	Time to Activation (Yrs)	Time to Spalling (Yrs)	Design Life (Yrs)
Low Soffit 43mm min cover	113	39	152
Low Soffit 38mm Cover	68	35	103
Low Soffit prestress 55mm min cover	156	-	156
Web Splash 38mm min cover	92	35	127

Table 4: Calculated Lives for Tauranga Bridge

A risk assessment was undertaken using the AS 4360 risk matrix. The consequence of corrosion of reinforcement was found to be extreme and the likelihood “unlikely”. The associated resultant risk was “high” and unacceptable. Installation of corrosion monitoring probes and designing the bridge for future Cathodic Protection meant the consequence of failure of the durability design became “moderate” and the risk became “low”

7. CONCLUSIONS

The successful completion of this project, on time and within budget, has demonstrated that Self-Compacting Concrete technology can deliver significant cost savings to a high-performance concrete project of this type. The technology is ideally suited to a Design and Construct type of project, were the bid team can evaluate each of the cost-benefit attributes of the technology.

The project was the first detailed appraisal of the use of SCC for marine durability undertaken in New Zealand. The cost advantages, while no surprise to the Concrete Technologists, amazed the Bridge Designers and the Contractor: conventionally vibrated concrete was very quickly ruled out as a viable option for the critical spans in this project.

Proving the durability performance of this SCC “Mix M” has enabled it to be used as a “Standard” high-durability mix, in other applications with similar exposure.

8. REFERENCES

1. Bamforth, P.B. (1996) “Definition of exposure classes and concrete mix requirements for chloride contaminated environments” Corrosion of Reinforcement in Concrete Construction. The Royal Society of Chemistry, Cambridge 1996
2. Wenzhong Z., Bartos P.J.M. 2003 “Permeation  properties of self-compacting concrete” Cement and Concrete Research, Volume 33, Issue 6, June 2003, Pages 921-926
3. Haque et al (1990) "Strength of Fly Ash Concretes in Various Curing Regimes" International Conference on the use of Fly Ash, Slag, Silica Fume and other Siliceous Materials in Concrete. Leura. Australia.
4. Polder R.B., 1996 “Laboratory testing of five concrete types for durability in a marine environment” Proceedings of the fourth international symposium on “Corrosion of reinforcement in concrete construction” Cambridge, UK..
5. Petterson 1998 “Service life design of concrete structures including the propagation time” Proceedings of the second international conference on concrete under severe conditions. Tromso, Norway.
6. NZS 3101:2006. Concrete Structures Standard.  Standards NZ, Wellington.
7. Model Code for Service Life Design. fib, Lausanne, 2006. 

Print this page