Self Compacting Concrete

In South Africa, self compacting concrete (SCC) is mainly utilised for specialised applications where the use of ordinary concrete is very difficult and often not possible.  The development of SCC in South Africa is still in its infancy and its current use is thus limited.

Presently there are two general mix design philosophies in South Africa.  One comprising high cementitious contents with no viscosity modifiers and the other where the cementitious material content is reduced and a viscosity modifier is included.  Research done using both approaches has generally yielded equally good results.
The first project on which SCC was used in South Africa was the Nelson Mandela Bridge, constructed in 2002.  The placement method used entailed pumping from the bottom up (another first in South Africa) and the height to which the concrete was pumped was also exceptional.
Other projects for which self compacting concrete was used include the deck of Bridge 2235 on the Bakwena Highway in 2002; a spiral staircase at a Pretoria residence in 2003 and, during 2004, on a number of relatively small projects.

Self-compacting concrete (SCC) is a relatively new product that sees the addition of superplasticiser and a stabiliser to the concrete mix to significantly increase the ease and rate of flow. By its very nature, SCC does not require vibration. It achieves compaction into every part of the mould or formwork simply by means of its own weight without any segregation of the coarse aggregate. Developed in Japan and Continental Europe, SCC is now being increa used in the South Africa where apart from health and safety benefits it offers faster construction times, increased workability and ease of flow around heavy reinforcement. Having no need for vibrating equipment spares workers from exposure to vibration. No vibration equipment also means quieter construction sites.

SCC is a generic term for mix designs that differ from traditional concretes at the molecular interface between the cement compounds and the admixture polymers. The fluidity of SCC ensures a high level of workability and durability whilst the rapid rate of placement provides an enhanced surface finish. SCC's high strengths overnight strengths typically reach 30-40N/mm2 and 2 day strengths can break the 100N/mm2 barrier enable easier and more reliable demoulding. SCC is certainly the way forward for both insitu and precast concrete construction. The health and safety benefits and the improved construction and performance results make it a very attractive solution.

CASE STUDIES

This bridge is the largest cable-stayed bridge in South Africa, connecting Braamfontein with Newtown, and spanning the Braamfontein shunting yards.  Newtown is the centre of the cultural precinct and the bridge provides access from the northern side of Johannesburg to this area. The bridge, opened by Nelson Mandela on 19 July 2003, was designed to become a landmark associated with Johannesburg the world over.(2)
                        Fig 1. The Nelson Mandela Bridge

The greatest challenge of the project was the placing of the concrete inside the pylons which were constructed from 20-mm (southern pylons) and 40-mm (northern pylons) thick steel plate rolled to produce 1,35-m diameter steel pipes which had to be filled with concrete to provide the required stiffness. The southern and northern pylons were respectively 31,1 m and 43,9 m high, creating difficulty with concrete lifting and placing (due the free fall limits), access constraints (due to operating railway lines) and stressing chambers at the top of the pylons.(2)

In addition, mechanical vibration was impossible due to limited access and external vibration was inappropriate because of the large amount of energy needed to overcome the pylon inertia.  To overcome the placing problems it was decided to pump SCC into the pylons from the bottom.  The concrete was pumped through a special pipe and valve arrangement at the bottom of each pylon as shown in Figure 2.
Each pylon was filled separately, taking 90 minutes to fill the first northern pylon and only 58 minutes for the second pylon.(3)  The filling volume for the southern pylons was 34 m3 and 52 m3 for the northern pylons.  Since the pumping process couldn’t be stopped at any stage and because this project was a first for South Africa, it was specified that 60% of the required concrete had to be on site and the balance dispatched and en route to site before the pumping process could commence.(2) 

Figure 2: Pumping and valve arrangement at the pylon base

Although the company policy of the concrete suppliers prohibited disclosure of the SCC mix design used in both the Mandela Bridge and Bridge 2235, they did indicate that the SCC included a CEM II A-M(S) 42,5N cement (15% fly ash, 30% GGBS), no viscosity modifier and a superplasticiser.  The aggregate used was 9,5 mm crushed andesite and andesite crusher sand in combination with a natural sand as a filler.  A slump flow (4) of 650 mm was reached and the 28-day cube strength was 64 MPa.

Project Team

Owner:                      Johannesburg Road Agency

Consulting Engineer    Nelson Mandela Bridge Consultants Consortium

Contractor                   Grinaker LTA-BCW Joint Venture

Readymix Supplier      Holcim

Other Firms:                 Gorba                       

Bridge 2235
Bridge 2235 forms part of an off ramp from the Bakwena highway, which extends from Pretoria to Botswana, and is part of the east-west link across the southern continent.

The bridge deck is a post-tensioned two-cell box girder type structure (Figure 3), unlike the conventional metal drum void formers used in similar bridges.  In the case of the latter design, the bottom slab and webs are cast first and must harden before the top slab can be cast.  To save time and labour costs, it was decided to cast the deck of Bridge 2235 in one operation.  Since compaction and placing was a problem in the reinforcing-congested bottom slab, SCC was chosen.

Click for an enlarged image

Figure 3: Deck cross section (5)

Before the deck was cast, a fully reinforced replica of the bottom slab (4,2 m x 1,2 m x 0,2 m) with two upstand edge beams (0,4 m wide and 0,75 m high) was cast alongside the bridge.  The bottom part of this replica was fully shuttered to represent the bottom deck slab of the bridge.  On the first trial, the concrete showed signs of segregation and too much slurry.  Adjustments to admixture/binder proportions were made and the trial was repeated next day. The second attempt was successful and the concrete stayed in suspension and flowed from the one upstand through the bottom slab shutter filling both upstands to their full height.(5)  The bridge deck was then cast successfully, with very few trapped air voids, using a 50-MPa SCC mix with a slump flow (4) of 600 mm.  Although this flow does not comply with the minimum of 650 mm specified in the EFNARC Specification,(4) the mix did not segregate and flowed successfully.

Spiral Staircase

Table 1: Mix design (Lafarge (SA))

This mix design differed from those of the preceding case studies in that two different superplasticisers were used.  Trial mixes indicated that the aggregate and cement varied too much for a sensitive superplasticiser.  Superplasticiser 1, which is the same as that used in the Nelson Mandela Bridge project, was a superplasticiser that is not too sensitive to over- or under-dosage.  However, this superplasticiser made the mix very cohesive and superplasticiser 2, one much more sensitive to dosage, was included for flowability.  Since only material to hand was used in the mix design, GGBS was included as an extender.  The 9,5 mm crushed dolomite was used for its relatively good particle shape and the preference given to this particle size in SCC.  A natural sand was used as a filler.

The actual slump flow(4) as measured on the day of casting was 750 mm.  The L-box, V-funnel and slump flow tests, in accordance with the EFNARC Specification,(4) and the Tattersall two-point test(7) were recently carried out with the same mix design and materials.  The results for these tests are given in Table 2.

Table 2: Workability results

Other projects
Projects undertaken in 2004 using SCC included:

CONCLUSIONS
These case studies demonstrate that although SCC is a relatively new technology in South Africa, its use thus far in specialised applications using locally available resources, has been very successful.

However, the only research done to date on SCC in South Africa has been specifically on workability and compressive strength. Nevertheless, with increased awareness of SCC and its advantages, more extensive use in South Africa is envisaged on projects such as Gautrain, the partially underground railway between Johannesburg and Pretoria.

Time and motion study.