Rust due to the formation of condensation water preferably occurs when prefabrication components are cured in a heated chamber or under steam. Typical for this cause is rusting on the edges of concrete units, approx. at a distance of 40 to 60 cm from the clamps, Fig. 1.
Condensation water will form when there is a gap between concrete and steel. A slight uplift can be related to the lifting of the edges of the concrete component that occurs while the concrete is hardening, e.g. due to shrinkage processes. Because of the uplift the protective release agent layer is destroyed.The heat of hydration keeps the temperature in the concrete high. As soon as a gap has opened up, the temperature of the concrete is clearly higher than the temperature of the steel. The pure water condenses on the steel that is no longer protected by the release agent and can dissolve ions from the fine concrete dust, which may result in rust formation.
Among the technological causes regarding concrete that may promote rust, concrete bleeding is a major factor. When a concrete mix bleeds heavily, the bleeding water can run out around the clamps. Rust is especially prone to develop when the steel underneath the clamps is not protected over its entire surface by the release agent.
Concrete mixes less inclined to bleeding could be a remedy. Adjusting the grading curve in the fine particle range (less than 25 um) can also be helpful.
An appropriate way to reduce the bleeding tendency is the usage of limestone containing cement. The limestone particles fit into the aggregate grading and reduce the water demand and the bleeding tendency. Beside that the reduction of the w/c ratio with a plasticizer or addition of admixtures, like stabilizer have been reported to lower the rust problem.
Any of concrete's ingredients-cement, aggregate, addition, admixtures, and water-can influence rusting in prefabrication operations. It is important to look at the circumstances and to what extent concrete suppresses rust formation. To estimate the characteristics of concrete concerning its impact on the rust problem, a test method was developed. Therefore, bleeding water has to be collected form the concrete. Fifty to 70 droplets of this bleeding water are distributed over the entire surface of a steel plate, and the plates are subsequently stored at standard climate (68°F/65% relative humidity).Technological causes
Steel that is not protected by a release agent, in contact with the bleed water will almost always rust, so experience is required to assess the results. It was found that a slight change in the amount of trace components could change the characteristics of the concrete totally. It was, for example, found that a "high" water-soluble chromate content could completely suppress the rusting of the unsheltered steel. Three different concrete samples were tested which differed only in this chromate content:
- Concrete A: 0.0002 % per cement weight
- Concrete B: 0.001 % per cement weight
- Concrete A: 0.002 % per cement weight
It was found that, although the differences were extremely low, the higher the chromate content the lower the rusting of the steel. Besides chromate there could be several trace components that influence rust formation. Recommendations for prevention
Rust on steel formworks in prefabrication plants is greatly annoying and costly to manufacturers of fair-faced concrete units.
It was found that the rust pattern on a steel surface could give crucial evidence to the rusting reason. Due to the fact that steel formworks, compared with reinforcement, are not completely covered with concrete, the surface has to be sheltered with a release agent. Incomplete coverage will in all likelihood lead to rust spots. Formation of rust spots under uplifted concrete unit edges will appear when the release agent film is destroyed and passivation of the steel is incomplete. In this case a change to inhibiting release agent could be promising. Prefabrication plants should forbear from mechanically removing rust, as this will activate the surface. When the reason for rusting is identified and eliminated, rusting will vanish automatically during daily production load.Only an integral supervision of all possible causes will lead to a sustainable prevention of rust spots.
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