tailieunhanh - Precast concrete materials, manufacture, properties and usage - Chapter 6

PRODUCTION—SPECIFIC PROCESSES - Chương này nên thu hút sự chú ý đáng kể và người đọc giáo điều sẽ tìm kiếm các con số để sử dụng và chỉ định. Nếu này là của người đọc triết lý tác giả gợi ý rằng chương này nên được cho một bỏ lỡ bởi vì công nghệ, "thuật giả kim 'và' bí quyết 'vai trò chơi tất cả các quan trọng trong sản xuất, và ý định của phần này của cuốn sách là để chỉ nơi phạm vi mục tiêu và các điều kiện biên. Tại chỗ hoặc đúc sẵn,. | 6 PRODUCTION-SPECIFIC PROCESSES This chapter should attract considerable attention and the dogmatic reader will search for numbers to use and specify. If this is the reader s philosophy the author suggests that this chapter should be given a miss because technology alchemy and know-how all play important roles in production and the intention of this part of the book is to indicate where the target-range is and what the boundary conditions are. Concrete production in situ or precast is not a case of working to maximum and or minimum limits in any respect it is a case of working within a range of ideas where there is a maximum and a minimum for each variable at one s disposal. The know-how is knowing where these limits are and the alchemy the kitchen routine of ringing the correct changes to get the materials to produce what one wants from the plant one has. The technology is relatively simple because although strength is specified at a specific age or at delivery the early handling or durability requirements for precast concrete generally result in the specified strength being achieved before it is required. Three things control the performance of concrete Design Materials Workmanship Materials are seldom the cause of faulty products and even when marginal or suspect materials are used any potential trouble can more often than not be designed out by careful thought. Before dealing with specifics a general comment about mix design should be included so that the numbers suggested as bases for design are understood. Many times a minimum cement content is specified . 350 kg m3 and this is easy to understand and implement. It is when the Copyright Applied Science Publishers Ltd 1982 maximum water cement ratio W C is specified that confusion arises because there are two water cement ratios in mix design Total water cement ratio Effective or free water cement ratio The second term is the only meaningful one because the difference between the two ratios is the extra .