Papers by Author: Jian Jun Yan

Abstract: The microstructure of complex binder pastes was investigated by means of XRD and SEM-EDS. The results show that adding fly ash means decline of cement content in cementitious materials, which leads to the decrease of Ca (OH)₂ content in complex binder pastes. The Ca (OH)₂ crystal still exists in the complex binder pastes at the age of 360 days. By replacing part of cement with fly ash, the early microstructure of complex binder pastes becomes loosened. With the extension of curing ages, a great quantity of low CaO/SiO₂ ratio of C-S-H gel is generated, which makes complex binder pastes compact gradually.

Abstract: The microstructure characteristics of complex binders under two curing conditions were studied by observing the micro-morphology, analyzing the pore character and means of spectrum test. The results showed that high temperature curing improved the pore structure formed in the early hydration period and reduced the porosity and the coarse pores in pastes. More hydration products were generated on the surface of fly ash particles that made the microstructure of pastes more denser than cured in room temperature. However, high temperature curing had insignificant influence on the CaO/SiO₂ ratio of C-S-H gel in pastes and contributed little to the improvement of the later hydration degree and microstrcture of fly ash-cement pastes.

Abstract: C40R60 High volume fly ash (HVFA) concrete were adopted for the continuously casting massive foundation slab of Tianjin Tower, which has a volume of about 20 000 m3, to decrease the risk of cracking during the construction process. Suitable raw materials and mix proportion were chosen. The properties of hardened concrete cured under different conditions were investigated. The temperature development in the core of structure was measured. The results show that the temperature rise in the core is relatively low and the compressive strength curing under the standard condition for 60 days is 54.3MPa. HVFA concrete is much suitable to the massive concrete structure in which elevated temperature would be kept for a long time.

Abstract: Using self-designed indoor detecting methods, the performance test of shotcrete doped compound admixture and crude fiber were carried out. The results showed that compound admixture and crude fiber can improve the compressive strength, flexural toughness and crack resistance of shotcrete. Axial bond strengths of shotcrete with rock were more than 1.0MPa, and that can be increased by combined-doped compound admixture and crude fiber. Rebound rates of shotcrete were 23% and 20%, layer thickness in one shot cycle were 22cm and 25cm, and combined-doped compound admixture and fiber can increase the cohesiveness and reduce rebound rate. Self-designed methods can be used to compare construction performance of different shotcretes.

Abstract: Influence of fly ash on alkalinity of pore solution and microstructure characteristics of hardened cement pastes were investigated by means of ex-situ leaching (ESL), MIP and SEM. The results reveal that the decline of alkalinity of pore solution is not endless, even though the contents of fly ash in fly ash-cement(FC) pastes is very high(65%).The incorporation of fly ash in proper ratios could increase the ratio of harmless pores and improve the microstructure of the FC pastes. Ca(OH)2 was not consumed heavily or exhausted when cementitious material was hydrated due to the low reaction degree of fly ash.

Abstract: Concrete cracking sensitivity made from different aggregates was compared with employment of advanced temperature and stress test machine by tracking thermal and stress development of concrete with age and elements distribution in interfacial transition zone of each concrete was explored by SEM and EDS. Test results indicated that compressive strength of concrete made from basalt and limestone was greater than diorite and natural aggregate and crack resistance of concrete based on cracking sensitivity comparison in terms of cracking temperature varying from minus 0.3°C to minus 2.5°C as diorite, basalt, limestone and natural aggregate. Elements Ca and Mg were apparently accumulated in vicinity of ITZ and some reaction between limestone and cement matrix might have taken place which blurred the boundary between aggregates and cement matrix.

Abstract: The advanced Temperature and Stress Test Machine was introduced to evaluate the cracking resistance of concrete with inclusion of light-burnt MgO under full restraint by tracking thermal and deformation development. Results showed that light-burnt MgO being incorporated ranging between 4 per cent and 6 per cent by mass of cementitious materials was beneficial to increase the maximum compressive stress and cracking stress of concrete by 0.37MPa and 0.2MPa on average respectively. The maximum temperature was slightly reduced from 59.8°C to 66.2°C while cracking temperature was significantly decreased from 0.8°C to minus 5.6°C. Sensitive anti-cracking coefficient F was forwarded to assess the early cracking tendency of concrete and in general, inclusion of 4 per cent light-burnt MgO with activity of 109s was rated the best in crack resistance.