Key Engineering Materials Vols. 629-630

Abstract: Silane, as an extremely hydrophobic material, changes concrete surface from hydrophilic state to hydrophobic state by spraying on the surface of concrete. It has the ability to block the external corrosive medium to improve the durability of concrete. Three types of strength grade of concrete (C30, C40, C50) were designed, after spraying with silane on the surface of concrete, to measure the penetration depth of silane and the influence on water-absorption rate, liquid-absorption rate, chloride ion diffusion coefficient and salt frost resistance. The test result shows that: by spraying silane on the surface of concrete,the one hour water-absorption rate of concrete was reduced by more than 98%, the 48 hours water-absorption rate was cut down above 91%, and the 48 hours liquid-absorption (3% NaCl solution) rate was decreased above 94%. The penetration depth for C30 and C50 has the maximum value (6.3mm) and minimum value (3.9mm) respectively. Absorption reduction rate of chloride ion overall was decreased by above 93%, and chloride ion diffusion coefficient was reduced by 42% ~ 48%. The freeze-thaw resistance was improved by 5 times. Key words: silane, the salt frost resistance, liquid-absorption rate, chloride ion diffusion coefficient

Abstract: Concrete has a high performance corrosion resistance, with the choice of the thickness of the active alkali aggregate,grain shape structured,well-graded,small porosity. Superfine cement with mineral powder instead of 30% to 50%, mineral superfine powder participate hydration, solve concrete interface structure Ca (OH) ₂ enrichment and directional crystallization problems, improve the coagulation. The internal structure of the soil, improving the strength and density of concrete. Meanwhile ultra-fine powder mixed with minerals, reducing the C₃A content in the powder and the concrete in the late ettringite hazards greatly. Improve the corrosion resistance of concrete, with also improve the internal structure of concrete and the strength of concrete, compactness and corrosion resistance. Composite type superplasticizer, reducing water consumption unilateral concrete, air-entraining agent is added to improve the density of concrete, the concrete has good impermeability and frost resistance. On the use of ordinary portland cement, it gets high performance, by the ratio test, developed the corrosion and frost resistance of concrete for the chloride, sulfate alone or jointly under the action of the environment concrete structures in corrosive environments, such as coastal areas, inland lakes, underground sewage or chemical zones, there are concrete structure durability requirements, with good economic and promotional value.

Abstract: Quality of concrete cover is a key factor for concrete durability. In fact, the quality of concrete cover has close relationship with mixture design and curing condition. Present study adopted transient test of air permeability to study on affecting factors of air permeability of concrete. Results dedicated that curing condition was the most important factor to affect transient air permeability of concrete. Adequate curing is required to decrease effectively the air permeability, and low ratio of W/B is helpful to reduce air permeability. Air permeability of concrete samples, added with fly ash or slag powder, was more sensitive to dry curing than standard curing. Besides, CaO based expansive agent effectively decreased coefficient of air permeability at the adequate curing condition

Abstract: Curing techniques and curing conditions have crucial effects to the strength and durability of concrete. The objective of this experimental study is to examine the strength and resistance to chloride ion permeability of high performance concrete under various curing methods in different simulated arid and large diurnal temperature variation climates. Laboratory experiments were conducted to investigate the strength and electric flux when different curing methods were used. Three curing methods and three simulated climates were applied to concrete specimens. The results show that the difference of measured experiment data are not significant under standard curing (SC), moisture insulation curing (MIC) and thermal and moisture insulation curing (TMIC) in an arid climate. However, the compressive strength and resistance to chloride ion permeability are worse under MIC and TMIC compared with SC in an arid and large diurnal temperature variation due to frost heaving and thermal stress. Moreover, compressive strength and resistance to chloride ion permeability decrease significantly when the large diurnal temperature variation increases and the lowest temperature reduces. The insulation material can effectively reduce concrete temperature variation between near surface and core region. Therefore, TMIC is a better curing method than MIC in an arid and large diurnal temperature variation climate.

Abstract: Heat-exposed High-Performance Concrete (HPC) has been the subject of relatively few studies focused on the relationship between the chemo-physical processes occurring in concrete constituents (microscopic level) and concrete mechanical properties (macroscopic level). In order to investigate the correlation between the intrinsic damage and the mechanical damage induced by heat exposure, eleven concrete mixes have been investigated, differing for: compressive strength (f_cm,cube ≥ 45, 70, 95 MPa), aggregate type, fiber type (polymeric and metallic) and fiber content. The microstructural and mechanical characterization was carried out on concrete specimens before and after being exposed to 105, 250, 500 and 750°C. The results show that using different experimental techniques at the microstructural level allows to monitor the development of microcracks, whose size is similar to that of macropores. This behavior at the nano, micro-level is reflected by the post-peak branch of the stress-strain curve at the macro-level. This extensive investigation allows to understand concrete mechanical decay due to heating on the basis of microstructural observations, paving the way to mix optimization for high temperature and fire.

Abstract: It used conventional techniques and materials prepared high strength fiber reinforced concrete whose strength class is above C100 and it studied the effect of fiber content on the mechanical properties and elastic modulus. It also studied the fire resistance of fiber reinforced concrete. Results suggest that the strength of 28d concrete is above 100MPa and the highest strength is 126.4MPa. Under the same ratio conditions, the greater the volume content of steel fiber concrete flexural strength, the splitting tensile strength is higher. The steel fiber volume only affect elastic modulus of concrete little. When it heats to 300 °C, the no fiber concrete comminuted burst while the fiber concrete does not damaged at elevated temperatures up to 300 °C and continue to heat up, the crushing damage occurs at about 460 °C. Has not been damaged concrete specimens at 300 °C, the quality have emerged about 3% decline, while the compressive strength increased by 35%-52%, the highest strength reached 180.3MPa.

Abstract: An experimental investigation was conducted on behavior of high performance steel-fiber concrete subjected to high temperature, in terms of explosive spalling and permeability. A series of concretes incorporated steel fiber at various dosages were prepared, and further processed to have a series of moisture contents. Explosive spalling tests were conducted on control plain concrete and steel fiber concrete. After explosive spalling tests, each of the specimens that didn’t encounter spalling was sawn into two pieces. Crack observations and permeability tests were conducted on the sawn surfaces. The results prove that steel fiber is efficient to avoid spalling concrete under high temperature. The permeability increases significantly after thermal exposure, while it also exhibits an ascending trend with the increase of moisture content. Therefore it is concluded that steel fiber can play a positive effect on explosive spalling of high performance concrete under high temperature, as well as on permeability after thermal exposure.

Abstract: An experimental investigation was conducted to study residual mechanical properties of Ultra-High-Strength concrete with different dosages of glassified micro-bubble after exposure to high temperature. After exposure to different target temperatures (room temperature, 200 °C, 400 °C, 600 °C,800 °C), residual mechanical properties (residual compressive strength, residual tensile splitting strength, residual fracture energy) of Ultra-High-Strength concrete under different conditions including 1 water-binder ratios (0.18), 3 different contents of glassified micro-bubble (0%, 40%, 60%) were all investigated. The effect of different dosage of glassified micro-bubble was studied on residual mechanical properties of Ultra-High-Strength concrete after exposure to high temperature. The results indicate that the variations of different kinds of Ultra-High-Strength concrete with different dosage of glassified micro-bubble are basically the same. With the increase of temperature, the residual mechanical properties increase at first, then decrease. The residual mechanical properties decrease after exposure to high temperature of 800 °C.

Abstract: High performance concrete (hereafter, HPC) is well known by its high compressive strength, strong resistance to deformation and excellent durability. Whereas, HPC is prone to spall when exposed to high temperatures and it probably results in sharp reduction of the fire resistance and loading capacity of HPC elements and structures. This paper presents a summary of research achievements on fire-resistance behavior of HPC in the past 10 years including the mechanical behavior degradation, analysis of spalling mechanism, effect of various types of fiber and other factors influencing the post-fire properties of HPC material as well as structural behavior of HPC elements. Studies on micro-structure of HPC have been carried out, which will help build a more sophisticated recognition of its performance under high temperatures. In spite of the large number of research results, more improvement on HPC material and HPC structures are still needed because of the devastating consequences caused by strength degradation or spalling-to-collapse. Thus in this paper a new idea of HPC-composite structures is proposed, expected to decrease the probability of spalling.

Abstract: A reliability analysis was conducted on high-strength concrete (HSC) columns during a fire. The influences of fire’s randomness and explosive spalling of concrete were investigated. The fire resistance for axial loading capacity of HSC columns was in terms of steel yield strength and concrete compressive strength with considering the effect of elevated temperatures. The load random variables included dead load and sustained live load. The JC method was applied to calculate the reliability index of the fire resistance of axially loaded HSC columns. It was found that the randomness of fire and explosive spalling of concrete had a significant influence on reliability of HSC columns.