Research About High Strength Admixture Of Non-autoclaved PHC Piles And Its Application

Autoclave curing and steam curing have been used in the past to produce Prestressed High strength Concrete(PHC) piles. However, drawbacks such as high energy consumption, insufficient durability, and high brittleness of PHC piles were observed using autoclave curing. In order to solve these defects, non-autoclaved curing method was studied by some domestic enterprises. But steam curing with the temperature range of 80-90 ℃ also brought a new problem, delayed ettringite formation(DEF) which leads cracks and spalling in concrete. To achieve high strength without autoclave curing, but also avoid DEF expansion, In this paper, high strength admixture(HSA) was prepared through mortar experiments. Application and micro-enhancement mechanism of HSA were discussed, and properties of non-autoclaved curing high strength concrete which made from HSA was also studied.The main work and contributions are drawn as follows:(1) Research and development of non-autoclave curing HSMA. The effects of fly ash, blast furnace slag, silica fume and calcareous materials on strength development of control mortar under the curing condition of 70℃ were studied. The mixed addition of calcareous materials and silica fume with the proportion of 6 to 5 was used to produce normal high strength admixture(NHSA). Further, mixed addition of calcareous materials and silica fume with the proportion of 6 to 15 was also employed to make ultra high strength admixture(UHSA). With the presence of NHSA and UHSA, compressive strength of mortar at 1 day can be reached 104.2 MPa and123.8MPa, respectively.(2) The preparation of non-autoclaved high strength concrete. It was found that the best sand to coarse aggregate ratio was 36%. The best dosage of NHSA and UHSA was 10% and 20%, respectively. Moreover, 1 day’s compressive strength of concrete mixed with NHSA and UHSA under 70 ℃ steam curing was 81.2MPa and92.6MPa, respectively. The compressive strength of NHSA concrete and UHSA concrete was 126.7% and 144.5% times higher than that of control concrete.(3) The effects of high strength mineral admixture on concrete properties. To compare with control concrete, mechanical properties and durability of concrete with the presence of HSA were greatly improved. Meanwhile, the increase of brittleness and drying shrinkage was also observed.(4) Enhancement mechanism of HSA. It was found that the content of Ca(OH)2decreased and fiber like typeⅠC-S-H increased with the addition of NHSA and UHSA by XRD, TG-DTA, pore structure and SEM analysis. The interweaving of typeⅠCSH and correspondingly filled in the internal porosity of cement resulted in the increase of gel pores and the decrease of most probable pore size, and then thestrength and durability of concrete was improved.(5) Applications of HSA. The compressive strength of concrete with NHSA was2-6 MPa lower than that of concrete with Japanese high strength admixture, and the compressive strength of 86.7MPa at 7 days can be reached by centrifugal molding. It was also found that the compressive strength of concrete with the addition of HSA was 10-26 MPa higher than that of control concrete from a field tests in a domestic pile factory, and even reached 100 MPa at 1 day. The enhancement effects of HSA on concrete under the curing condition of room temperature was also observed.