March 12, 2009
ONTARIO MINISTRY OF TRANSPORTATION
The MTO has had good success on some of its jobs using self-consolidating concrete in structural patches and resurfacing applications.
Pearson airport among recent major applications
Self-consolidating concrete (SCC) has been on the construction scene for about 30 years, but is just now making a mark in North America, delegates at the Ontario Road Builders' Association annual meeting were told recently.
"There's some myths around it and we want to debunk that, because there is no mystery," said Bill Lambros, project manager at Lafarge.
"It's simply concrete but the difference is that (because it is a more liquid form) it flows differently and moves differently and is placed differently."
It's those properties which make it a good choice for road and bridge patchwork and repair, he said, with the exception that it does not do well on sloped surfaces because of its fluid nature. However, used with tight fitting forms it can be pumped through ports as small as two inches and used in vertical applications since it should be poured or placed from the bottom.
SCC first came on the scene in the 1980s in Japan, Lambros said, and was developed to solve issues of noise on construction sites created by the vibrators used to tamp the air out of concrete after pouring There was also a skilled labour shortage the densely populated urban environment placed restrictions on what times construction activity could go on.
It's been in use in North America since about the 1990s said Lambros, and has in fact been used on several high-profile projects such as The Lester B. Pearson International Airport Terminal 3 project by Dufferin Construction where it was used in 180 steel-composite columns in 2000. CBM is also using 17,000 metres for the 50-storey building at the Bay Adelaide centre while the Four Seasons Residences will use 42,000 metres, making it one of the biggest projects yet here.
Still, SCC needs more buy-in from the construction industry, said Lambros and the concrete sector needs to do more outreach and education to explain the merits.
"Many engineers and architects have still never heard of it," he said.
The advantages of SCC are that is requires less skilled labour and doesn't require tamping with a vibrator to remove air. Because of its highly fluid nature, it perfectly mimics any form, right down to the scratches, he said, and surface finishing requires minimal effort.
Though it pumps from the bottom, it can be pumped up to 60 feet high and is flexible enough to use in several applications from elevated slabs, to filling in abandoned pipelines and even underwater since it resists wash-out. It also hydrates slowly and generates less heat and requires no cooling in some circumstances.
It does come with a 10 to 30 per cent price premium, Lambros said, depending on application and market conditions.
The only other downside is that there has to be some more planning and preparation before application, he said, but that is more than offset by the lower demand for skilled labour.
The Ontario Ministry of Transportation's experience with SCC on road and bridge work has been generally good, said Melissa Titherington of the MTO's Concrete Section Materials and Research Office.
She highlighted several projects, including a structural patch for resurfacing a pier girder and deck soffit abutment walls in which the contractor proposed SCC at no extra cost and maintained MTO standards.
The key so far has been to police the mix with a pre-approval process at the site before placement begins, she said.
What the MTO is looking for is a mix with aggregate well distributed and not separating from the paste. This is determined by a visual inspection and a slump test in which a set amount of the mix is poured onto a board. The result should be a patty with a smooth rounded edge in which aggregate is uniformly distributed right to the outside. Too much water and there will be issues, she said.
"There are two areas which need improvement and that's segregation and bug holes," she said, the later referring to the myriad of surface holes left after the forms are pulled away.
So far, that seems to be linked to the type of form surface, whether it's wood with some oil-based release formulate or metal, said Lambros.
Some forms pull away leaving an almost mirror-like white surface while others can be grey with bug holes. Pumping from the bottom and avoiding drop pours also eliminates getting air trapped in the mix which manifests itself as bug holes, he said.
Nenhum comentário:
Postar um comentário