Workplace 6 at Darling Island: A Six Greenstar Building

Tonkin (formerly known as TLB Engineers) was appointed to carry out the consulting structural engineering design and documentation of the $50m six storey office building, ‘Workplace 6’, in Pirrama Road opposite the Star City Casino in Darling Island, Pyrmont. In 2008 it was awarded the first Six Star rating for a NSW building from the Green Building Council of Australia.

An important element in achieving a Six Star rating was the requirement to reduce the Portland cement content and use recycled cementitious materials and recycled aggregates in the concrete. Usually the replacement of normal Portland cement with fly ash and slag reduces the early strength development of concrete (Ref. 1). As the floor slabs were to be post-tensioned, there was consequently a concern about an increase in the risk of concrete failure at the dead end and live end anchorage zones at the time of initial stressing, usually carried out 24 hours after pouring the concrete.

This report briefly describes the test programme carried out to check the early strength development of alternative green concrete mixes and to test the posttensioning dead end anchorages in model slabs at 24 hours, The aim was to reduce the risk of concrete failure when the initial stressing loads were applied.

The success of this Tonkin test programme resulted in the use of a ‘three star’ concrete for the post-tensioned floor slabs, which contributed to the achievement of a Six green Star rating for the building.

Construction commenced in March 2007 and the building was completed in 2008. It is a reinforced concrete framed structure supported on piles up to 15m deep to rock with post-tensioned one-way banded concrete floor slabs.
The column grid is generally 8.0m x 12.0m with a typical floor plan area of 3,600m2. 

The six story building has a gross floor area of 18,000m2.

The main structural element in contributing to the Six Star rating for the building was the concrete, which was to have a significantly reduced Portland cement content and a reduced natural aggregate content.
This was to be achieved using increased fly ash in the concrete mix as well as blast furnace slag.

The specified concrete strength was 32 MPa at 28 days, which is typical for posttensioned slabs, with 7 MPa (min.) required for the first stage stressing at one day of age, and 22 MPa (min.) required for the second (and final) stage stressing at five days of age.

In addition to the concern that the concrete may not achieve sufficiently high early strength to minimise the risk of slab failure under the applied prestressing load, there was also doubt about the use of standard laboratory cured concrete cylinder sample compression tests to adequately represent the strength of the concrete in the actual floor slabs.

Tonkin developed and supervised a test programme to be undertaken using three 32 MPa concrete mixes and six model slabs to assess the effects of recycled cementitious materials and recycled aggregates on the anchorage capacities of post-tensioned tendons at initial stressing. Each of the concrete mixes included a range of recycled cementitious and aggregate materials.

The test programme aimed at providing an indication of the comparative capacities of the three different concrete mixes to adequately hold an initial prestress at 24 hours, which is typically the time when the tendons are stressed to 25% of their final jacking load to minimise the risk of drying shrinkage cracking in unreinforced concrete elements.The three 32 MPa concretes tested were:

a. Standard mix with a cement content of 320 kg/m3.

b. 2 star concrete with 40% cement replacement with fly-ash and no recycled aggregates

c. 3 star concrete with 40% cement replacement with fly-ash and 20% recycled aggregates

A summary of the results is shown in Table 1 and Figure 1. For a description of the test programme and comments on the test results, please download Appendices 1 and 2.

At the test site it was found that the concrete mixes supplied had slumps of 95, 105 and 130mm which were in excess of the design requirement of 80mm. The 2 star and 3 star (ambient cure) cylinder sample compression tests did not meet the 24 hour requirement of 7 MPa, but they did meet the requirement of 22 MPa at five days.
For the ‘standard mix’ and the ‘3 star mix’, the application of stressing loads to the tendons caused no cracking of the model slabs and there was no evidence of slipping at the anchorages. The ‘2 star mix’ showed evidence of slippage in the dead end anchorage at 36% of the jacking load, which was below the 40% minimum requirement set out in AS 3600 Appendix B4 Prototype Testing.

The 3 star mix met the requirements of AS 3600 Appendix B ‘Prototype Testing’, and this concrete mix was adopted for use in the building.
As a result of the test programme, each of the floor slabs was poured using the 3 star concrete mix. Cylinder samples were taken and tested prior to initial stressing, which was carried out at no less than 24 hours and occasionally up to 48 hours after the concrete pour.

The post-tensioned concrete structure of the building was successfully completed without delays on site and without failure of any dead or live end anchorages.

The successful completion of the concrete test programme increased the level of confidence of the builder and the post-tensioning sub-contractors in the ability of the 3 star concrete mix to adequately sustain the initial post-tensioning loads, and this mix was successfully used in the floor slabs of the structure.

As a result, this portion of the building work satisfied the requirements for reduced Portland cement content and increased use of recycled aggregates, and this contributed to the award of a Six Star green star rating by the Green Building Council of Australia.

The model test programme developed and supervised by Tonkin was an essential part of this work to demonstrate that the replacement of cement by fly ash in the mix and early post-tensioning could be successfully achieved.

Ref. 1: Neville, A. M., “Properties of Concrete”, Longman, 4th ed., 1995 
Developers: GPT and Citta Property Group
Builders: Buildcorp Australia Pty Ltd
Architects: Nettleton Tribe Pty Ltd
Structural Engineers: Tonkin (formerly known as TLB) Engineers