Classified Fly Ash contributes to world-class Gautrain Concrete. (part 1)
The following article was published in “Concrete Trends” (Official Journal of the Cement and Concrete Institute) VOL 12 No4 November 2009 edition. We thank Concrete Trends and the Cement and Concrete Institute for allowing us to republish it.
Part 1
It’s not easy to sum up an enormous, complex project such as Gautrain, the largest rail project currently under construction in the world. Work is forging ahead over the 80-km length of eventual rail tracks, involving 15 km of tunnels, 11 km of viaducts and bridges and 10 stations. Pervading every activity site is an air of pride as the various sections start to take shape. But nowhere, says Lauren Lonsdale, is pride and enthusiasm more apparent than when talking to the ‘concrete men’! Cyril Attwell, chief concrete technologist and research scientist for main contractor, Bombela Civils Joint Venture (BCJV), has applied his synergy philosophy to the fly ash concrete designs and achieved remarkable results. “We are fortunate to have some of the best fly ash in the world,” says Attwell, “Not only that, I believe South Africa can show the rest of the world a thing or two about concrete technology.”
Gautrain will change the face of Johannesburg and, in keeping with this, the parameters for the concrete work embraced a durability specification of a 100-year lifespan. The project will require in the region of 800 000 m³ of concrete and 116 000 m³ of grouts. BCJV are using DuraPozz®, the classified fly ash from Ash Resources, in virtually all of its mix designs.
“Fly ash is making a significant contribution in all the key parameters we focus on in our concrete work,” explains Attwell. “Essentially this refers to durability, workability, off-shutter finishes and even high early strength, not traditionally associated with fly ash. All this has been achieved cost efficiently. Our success in optimising the performance of our concrete mixes owes much to the excellent reactivity of the classified fly ash. This has more than compensated for the relatively low surface area and reactivity of the CEM I 42,5 cement we are using. Ash Resources’ DuraPozz® is rated at a reactivity of 0,8 compared with European material at typically 0,4. In addition, there is reliability: this fly ash has proven to be the most consistent in quality of all the materials supplied on this project – and that includes water!”
The Allandale precast yard
Where the Gautrain team’s innovative approach to concrete design has produced outstanding results is the Allandale precast yard at Midrand (see pic), the largest precast facility in the southern hemisphere. It produces a wide variety of castings, predominantly for the above-ground work. The most striking of these are the huge viaduct segments up to 54 m long and weighing between 45 and 65 tons each.
Traditionally, a high-productivity precast yard uses CEM I 52,5 or 42,5R cement. However, when the Gautrain project was ‘building up a head of steam’, the local construction industry was experiencing peak demand and there was insufficient availability of those grades, which in any case would not have been as cost-effective. After two months of trials using a foundation of advanced recrystallisation (ARC), a South African-developed technology attributed to Attwell, a 30-35% DuraPozz® concrete mix was used. Fly ash does not generally give high early strength but, by getting the other materials right, an accelerated mix was achieved that allowed the moulds to be turned around within 12–18 hours, obviating the need for steam.
Getting compressive strengths in the region of 30 MPa within 24 hours using 30-35% fly ash replacement with 300 kg/m³ of CEM I cement, 140 kg fly ash and approximately 200 litres of water is generally considered impossible. However, BCJV is achieving the 24-hour strength and averaging 28-day strengths of 74 MPa in winter! To match this, a precast business would normally expect to use 400 kg/m³ of a CEM I 52,5R cement. “Old concrete technology relied purely on water:cement ratios with very limited account taken of the other materials, even though they make up 60-70% of the mix,” says Attwell. “Our approach is to look at mechanical and chemical properties of the total ingredients: we consider voids content and how different sizes fit together, as well as the chemistry of how the fly ash interacts with dolomitic materials.
There is no linear trend; each mix must be engineered separately. Our 15-MPa design has the best synergy of all our mixes: after numerous development revisions, it is currently using 30% DuraPozz® and achieving 40 MPa at 28 days. There are other sources of fly ash but they are not as reactive or consistent as this product, and only Ash Resources could guarantee product availability, which is vital for a project of this size.” The Allandale yard produces all castings for Gautrain apart from the tunnel lining segments. (These are produced by Southern Pipeline Contractors in Wadeville to meet the logistical demands of supplying the tunnels, which are all in the south section of the rail network.) By the end of 2009, the Allandale yard will have produced 120 000 m³ of precast units, the major portion being 3 208 viaducts, which were all cast ahead of schedule by May 2009 and 27 245 Motorway beams (M-beams) totalling 27 km in length, which are 90% complete. “The viaducts are by far the most difficult casting,” says Fanie van Tonder, BCJV’s precast yard manager. “The 28 to 54-m spans are produced to a tolerance of ≠20 mm using a long-bed production technique, which has been an interesting challenge, with the need to continually ‘leapfrog’ the shutters over each part section as soon as it has set. You do match casting with a steel mould at each end and although we had the basic technology from France it really only related to short-bed casting, which is very different.
A major challenge is that every viaduct span has a different design depending on the loading at the top. It means the reinforcing changes, the stressing ducts and stressing cables change together with the positions of the stressing anchors. The precast yard was established in March 2007 with production of viaducts and M-beams beginning in June. There was supply pressure on Allandale from Day One as they supply the whole project: all the project sites wanted to get off to a good start and needed castings simultaneously. “We needed special concrete and Cyril Attwell has developed exactly that,” comments Van Tonder. “Using a quality fly ash and Attwell’s synergy approach has achieved the demanding requirements of our high-productivity precast yard. We can pump or place the mix for up to ninety minutes and then strip the formwork the next day. People do not believe that we can pour a major structure like a viaduct, strip it at 15 MPa and take the formwork away.” Van Tonder minimises the inherent risk in early stripping by using a ‘maturity test’ to monitor the concrete. This involves placing thermocouples in the freshly poured casting and monitoring the temperature trend the following day as a guide to when the formwork can safely be removed. “In effect, we monitor the heat being generated in the concrete and it has proved to be an accurate indicator of strength development,” he says. The technique is also being used to monitor the M-beams, which cannot be detensioned until they reach 43 MPa. Rigorous quality monitoring of mixes is carried out with a programme of cube testing for 28-day strength.
BCJV have a laboratory on site and an independent laboratory facility provided by SNA Laboratories close by. In addition, all mix designs have to be approved by the consulting engineers, the Province and the independent certifier, Arup, who have their own technologists and a certified independent laboratory. Siting the massive precast yard at Allandale stems from the geology of the terrain between Johannesburg and Pretoria. Originally, to minimise the environmental impact, Gautrain was intended to run in tunnels to Pretoria (like the south section) but the ground rock is predominantly unstable dolomitic limestone. Existing cavities in the rock have been weakened by dewatering inducing sinkholes and dolines. Despite opting to route the rail track above ground, this is posing continual challenges with constructing the piers and columns. “One test hole will pick up foundation rock at 4 m, the next one will go to 20 m, while the following one might reach 60 m and still find nothing,” says Van Tonder. “It means the engineers have to deal with each section as they find it, either putting in a soil raft, doing a casing operation or stabilising the area by grouting. It is another area where large volumes of fly ash are being used to help make a pumpable grout mix.
Fly ash also makes concrete denser and more impermeable which helps to protect against acidic water.” Concrete production is done on the Allandale site using two Liebherr pan mixers and a boiler to heat up the water during winter months. “Although South African normal practice is to do dry batching, the trend on large projects is to revert to pan mixers,” comments Van Tonder. “On our highly sophisticated computerised units, the mixing cycle is between 30 and 40 seconds per mix, so the production rate is much higher and the concrete consistency much better. We program the recipe and after mixing the materials, the system gives us a printout of the actual mix produced. Quality control using the dry batching route would have been very difficult on a site like this where the truck mixers only have short journeys. You have to allow at least two minutes spin time and give the foreman a chance to check the mix. We need absolute consistency and cannot have drivers randomly adding water because their mix looks dry.”
|
