Precast concrete materials, manufacture, properties and usage - Chapter 2

ADMIXTURES - Chương này bao gồm phạm vi của các phụ gia được sử dụng trong các sản phẩm bê tông đúc sẵn và súng cối được sử dụng với khối xây bê tông đúc sẵn. Họ là tất cả được sử dụng ở nồng độ tương đối thấp ( -5% w / w xi măng) cho mục đích sửa đổi các thuộc tính và tươi / hoặc vật liệu cứng. Chương này không bao gồm các chất phụ gia như xỉ mặt đất lò cao và tro nhiên liệu nghiền thành bột (tro bay), sau này được bao gồm. | 2 ADMIXTURES This chapter covers the range of admixtures used in precast concrete products and mortars used with precast masonry. They are all used at relatively low concentrations 0-01-5 w w cement for the purpose of modifying the properties of the fresh and or the hardened material. The chapter excludes additives such as ground blast furnace slag and pulverised fuel ash fly ash the latter being covered in Chapter 4. Pigments although in the admixtures group are covered in the next chapter. It is often stated that well-designed compacted and cured concrete should not need an admixture. However when all the practical aspects of a situation are considered such dogmatism can seldom be justified. The author s opinion is that with the complexity of castings the chance to reduce water contents for the same workability and with the many other advantages that admixtures can bestow there are few cases where their non-use can be justified. However one point that needs to be emphasised is that admixtures are used at relatively low concentrations and over or under dosage can lead to potentially disastrous results. Control of the basic ingredients of aggregate cement and water must be strict because if there is any doubt about this there is no point in using admixtures. The admixtures are used to make good concrete products better not to make poor or mediocre concrete good. Having said all this one can now proceed to discuss admixtures under performance headings. ACCELERATORS The purpose of these is to accelerate the setting and hardening rates of cement for the main purpose in precast work of getting a faster turnover Copyright Applied Science Publishers Ltd 1982 rate in demoulding and stacking and faster delivery times. The most common and the cheapest accelerator is calcium chloride. Calcium formate sodium nitrite and others are not so effective as calcium chloride and are extremely sensitive in their performance to the chemistry of the Portland cement. This is .