News Date:2015-09-14 15:03:48
GGBS proportions
On its own, ground granulated blast furnace slag (GGBS) hardens very slowly and, for use in concrete, it needs to be activated by combining it with Portland cement. A typical combination is 50%GGBS with 50%Portland cement, but percentages of GGBS anywhere between 20 and 80%are commonly used. The greater the percentage of GGBS, the greater will be the effect on concrete properties.
Setting times
The setting time of concrete is influenced by many factors, in particular temperature and water/cement ratio. With GGBS, the setting time will be extended slightly, perhaps by about 30 minutes. The effect will be more pronounced at high levels of GGBS and/or low temperatures. An extended setting time is advantageous in that the concrete will remain workable longer and there will be less risk of cold joints. This is particularly useful in warm weather.
Water demand
The differences in rheological behaviour between GGBS and Portland cement may enable a small reduction in water content to achieve equivalent consistence class.
Consistence (Slump)
While concretes containing GGBS have a similar, or slightly improved consistence to equivalent Portland cement concretes, fresh concrete containing GGBS tends to require less energy for movement. This makes it easier to place and compact, especially when pumping or using mechanical vibration. In addition, it will retain its workability for longer.
Early age temperature rise
The reactions involved in the setting and hardening of concrete generate significant heat and can produce large temperature rises, particularly in thick-section pours. This can result in thermal cracking. Replacing Portland cement with GGBS reduces the temperature rise and helps to avoid early-age thermal cracking. The greater the percentage of GGBS, the lower will be the rate at which heat is developed and the smaller the maximum temperature rise.
Strength gain in GGBS concrete
With the same content of cementitious material (the total weight of Portland cement plus GGBS), similar 28-day strengths to Portland cement will normally be achieved when using up to 50%GGBS. At higher GGBS percentages the cementitious content may need to be increased to achieve equivalent 28-day strength. GGBS concrete gains strength more steadily than equivalent concrete made with Portland cement. For the same 28-day strength, a GGBS concrete will have lower strength at early ages but its long-term strength will be greater. The reduction in early-strength will be most noticeable at high GGBS levels and low temperatures. Typically a Portland cement concrete will achieve about 75 per cent of its 28-day strength at seven days, with a small increase of five to 10 per cent between 28 and 90 days. By comparison, a 50%GGBS concrete will typically achieve about 45 to 55%of its 28-day strength at seven days, with a gain of between 10 and 20%from 28 to 90 days. At 70%GGBS, the seven-day strength would be typically around 40 to 50%of the 28-day strength, with a continued strength gain of 15 to 30%from 28 to 90 days. Under normal circumstances, the striking times for concretes containing up to 50% GGBS, do not increase sufficiently to significantly affect the construction programme. However, concretes with higher levels of GGBS will not always achieve sufficient strength after one day to allow removal of vertical formwork, particularly at lower temperatures, lower cementitious contents and in thinner sections.
Colour
Ground granulated blast furnace slag is off-white in colour and substantially lighter than Portland cement. This whiter colour is also seen in concrete made with GGBS, especially at addition rates of 50%cent and above. The more aesthetically pleasing appearance of GGBS concrete can help soften the visual impact of large structures such as bridges and retaining walls. For coloured concrete, the pigment requirements are often reduced with GGBS and the colours are brighter.
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