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Denka CSA is being used in many sections of construction and by a variety of concrete products manufacturers. The advantages and merits of Denka CSA have been well proven in over 30 years of use in New Zealand.
Denka CSA cement allows for larger placements as the number of joints are greatly reduced and cracks are largely eliminated. This is particularly useful in large warehouses with high level racking. The result is greater durability and less damage from forklift traffic. Joint free concrete slabs up to 3080m2 (65m x 60m) have been constructed in New Zealand.
Denka CSA is always mixed with Type GP portland cement and it is called Denka CSA cement or expansive cement. When mixed in the proper proportions with cement, a cement mortar or concrete expands during the early stages of hydration. This expansion, when restrained, makes it possible to minimise drying shrinkage. Moreover, when the ratio of Denka CSA to cement is increased its high expansion qualities make it ideal for use in prestressed concrete. Denka CSA works as follows. As the concrete sets and bonds to the steel, and while the concrete is curing and gaining strength, the expansive reactions cause a slight controlled elongation to the concrete. The bond to the steel causes it to be stretched slightly and places the steel in tension. The equal and opposite law is still at work and the concrete is put in slight compression. It is "precompressed," but at a level of magnitude which is much less than that of conventional prestressing. This controlled expansion is complete in the first few days of the concrete's life. When the expansive concrete begins to dry out, it shrinks as does normal concrete, but unlike normal concrete, the shrinkage is accompanied by the relief of a slight compression built into it - not by tensile stresses being set up. Tensile stresses imposed by the base can largely be eliminated by good preparation and the use of two layers of polythene. When all the drying shrinkage has taken place, Denka CSA concrete will be relieved of the problems that contribute to drying shrinkage cracking because, ideally, it will never go into tension.
Denka CSA cement is not guaranteed as a remedy for all the problems of cracks in concrete, however it will assist in the elimination of drying shrinkage cracks and allow for very large placements without control joints. Denka CSA should not be used as a crutch to cover bad construction practices, poor subgrade preparation, or the addition of too much water on site. Note: Lime based expansive agents have different expansion and shrinkage characteristics. Use of these agents can place limits on slab dimensions due to rapid expansion during the first 24 hours of concrete hardening, when bonding to reinforcing steel may be insufficient. Specifications and reinforcing guidelines are available on request. |
Datasheets |
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Raw materials used in the manufacture of Denka CSA (Calcium Sulpho Aluminate) include quick-lime, gypsum, bauxite. The mineral composition of Denka CSA is CaO, CaSO4, 3CaO.3Al2O3.CaSO4. Calcium sulphoaluminate, anhydrous in form, hydrates to form ettringite. Ettringite occupies a greater volume than the water and anhydrous materials that make it up. Its formulation is accompanied by an increase in absolute volume and therefore a net expansion.
Denka CSA should not be confused with shrinkage reducing agents. Shrinkage reducing agents do not compensate for shrinkage by expansion and therefore cannot introduce prestress to the concrete. 