Chemical attack on concrete take place either by leaching of calcium hydroxide or by ingress of harmful substances, such as sulfates or nitrates. Since Elkem Microsilica reduces the amount of soluble calcium hydroxide, leaching is reduced, and less is available for the harmful expansive reactions caused by sulfates and nitrates.
Microsilica also consumes alkalis, thus providing protection against alkali-silica attack. In Iceland all concrete produced since 1979 has included microsilica to negate serious ASR problems.
The addition of microsilica reduces the permeability of concrete. This retards the leaching and ingress of materials from, or to, the interior of the concrete. Consequently, concrete with Elkem Microsilica will possess superior resistance to chemical attack.
Highways E6 & E18, Norway
During the winter season cars with steel-studded tires have a severe wearing effect on asphalt pavement. Resurfacing is needed every year in heavy traffic areas. To combat this problem and gain experience using high-strength microsilica concrete, an area of 110 000 m2 was paved in 1989. The basic mix design was 390 kg/m3 cement, 5% microsilica (by weight of cement) and a water/binder ratio of 0.36-0.38. The characteristic 28 days cube strength was 85-90 MPa. This pavement has performed most satisfactorily.
Mayer Parry, London, UK
The Mayer Parry scrap metal reprocessing unit is one of the largest in the UK. Many thousands of tonnes of metal pass through the yard every week. The physical attack on the concrete ground slabs is severe - a 350 mm deep slab would previously lose 300 mm of depth in just over one year.
In 1992 the whole yard was reconstructed using Elkem Microsilica concrete (delivered by Tarmac Topmix under the tradename Toproc). After years of service the conclusion is that the microsilica concrete extends the life of the slab six- to eightfold.
Sainsbury's supermarket at Leigh, UK
In 1998 an 1800 m2 light-coloured slab was installed at the Sainsbury's supermarket at Leigh, UK. The concrete used was a proprietary microsilica concrete supplied by Tarmac Topmix. A white Elkem Microsilica powder was used to obtain the light colour. The increased strength of the slab concrete meant that the surface could be rapidly ground to provide a terrazzo type effect, more quickly and cheaply than the original terrazzo, which has to be laid on a conventional concrete slab.
Spruce Tree Parking Structure, Minnesota, USA
Almost every parking structure has about 20 to 25% of its construction cost set aside for a maintenance program to assure safe and adequate performance. Steel corrosion and freeze/thaw damage are both major causes of deterioration and failure of such concrete structures.
The use of microsilica concrete in this parking structure, in both the elevated slabs and columns, was the first in Minnesota. It was a new way to achieve low permeability and high durability concrete throughout the structure.
Microsilica will produce concrete with better workability for ease of placing and finishing. Microsilica will also reduce bleed water from the concrete surface and reduce the potential for spalling in the future. It also provides an improved surface texture for traffic and reduces surface abrasion.
The use of microsilica concrete was cost effective because the “very low” permeability concrete has eliminated the need for the usual sealer application every two to three years. This microsilica concrete will also reduce regular maintenance costs resulting from steel corrosion caused by de-icing salts.
General Mitchell International Airport Parking Addition, Wisconsin, USA
This project began in late 1990 and was completed in July of 1991. It consists of 56 000 m2 (2300 parking spaces) on six levels. The engineer/architect, Graef, Anhalt & Schloemer, designed the structure for the owner, Milwaukee County, as cast-in-place post-tensioned concrete, 180 to 250 mm in depth. Central Ready-Mix supplied the concrete, and the contractor was C.G. Schmidt Co. of Milwaukee.
The bid package presented to the contractors contained two alternates for long term protection of the concrete, microsilica or a traffic membrane. The Graef firm specified microsilica concrete for four major reasons:
- Low susceptibility to chloride-induced corrosion
- Fewer maintenance requirements
- Lower life cycle costs
- Increased compressive strength
Test slabs, pre-placement conferences, training of the crew and an open mind about new techniques resulted in a high quality parking structure, on time and within budget. The contractor has to date successfully completed projects with an additional 65 000 to 100 000 m2 of microsilica concrete. As the use of microsilica to protect concrete exposed to severe conditions becomes ever more popular, procedures and techniques outlined above will become more commonplace. This will guarantee a higher percentage of cost-effective, crisis-free projects.
U.S. Borax - USA
The U.S. Borax plant is located in Death Valley in California, one of the worlds most severe environments. The concrete slabs and drainage ditches that surround the plant are subjected to spillage of borax in combination with 5% sulphuric acid. The rate of degradation on the ordinary concrete meant that the slabs and ditches had to be replaced every two years.
In 1986, microsilica concrete was specified after a full scale test. The results showed that this concrete experienced only about one-fifth of the weight loss of a reference conventional concrete.
Fertiliser Storage Silos, Porsgrunn, Norway
Microsilica concrete was selected for this project to reduce the effects of chemical attack. The structure was checked after 20 years of service life, with no signs of deterioration.
Norsk Hydro, one of the world's leading manufacturers of fertiliser, conducted a lot of research into designing a highly durable concrete in the 1970s.The mechanism is that calcium nitrate reacts with the calcium hydroxide in the hydrated cement paste causing expansion and disruption of the concrete within a very short period. Microsilica consumes the calcium hydroxide and thereby prevents this form of attack.