Key Engineering Materials Vols. 400-402

Abstract: In this project, a process was developed to cure concrete masonry blocks using CO2. It was found that the strength of concrete mixtures for block manufacture after CO2 curing was close to that after conventional steam curing. A dry pre-conditioning before CO2 curing was very critical to achieve high degree of CO2 curing. The specimen pre-conditioned in the moist environment hydrates more than those pre-conditioned in the dry environment during the pre-conditioning period. However, much less CaCO3 formed in the former that that in the latter after the CO2 curing. Thus, the CO2 curing is mainly contributed by the reactions between CO2 gas and cement clink minerals.

Abstract: The physical crystallization action of sulfate in cement based material, which often occurs by dry-wet cycle, capillary rising and evaporating action, can damage seriously cement-based materials as compared to chemical corrosion by sulfate. The deterioration mechanism of cement-based materials by sulfate crystallization attack, experimental investigations of the process of sodium sulfate crystallization and the factors affecting the process, and the invading track of salt solution by capillary effect are presented in this paper. Results show that the crystal type and crystallization velocity of sodium sulfate is influenced greatly by relative humidity and ambient temperature. There is a close relationship between invading depth and the porosity of concrete. Addition of mineral admixture to concrete can significantly reduce the invading depth of sodium sulfate solution and thereby enhance the resistance of concrete to sulfate crystallization attack.The profile of invading track of salt solution by capillary effect is similar to the shape of concave parabola.

Abstract: The current state of bridges in the United States calls for the implementation of a continuous bridge monitoring system that can aid in timely damage detection and help extend the service life of these structures. A typical monitoring system would be one which enables non-invasive, continuous monitoring of the structure. The passive nature of the acoustic emission (AE) evaluation technique makes it an ideal choice to serve this purpose. Although the technique has been successfully used for decades for damage detection in other fields, its potential in bridge monitoring has not yet been fully exploited. This paper presents experimental tests performed on four concrete beams in laboratory. The laboratory concrete beams were tested under different loading patterns. While acoustic emission monitoring of bridge structures is not a new vista, most evaluations conducted before are only qualitative. Though some quantitative methods have been proposed, they have not yet developed to be useful for actual field test of bridges. Therefore, an attempt was made in this study to use the intensity analysis technique for damage quantification. The use of intensity charts may help in better estimating the damage severity, although clearly marked zones of damage are not yet prescribed for certain materials like concrete and steel. All in all, the applications of AE in bridge monitoring reveal the potential of this technique’s versatility. The technological advances made in recent years have made the method more suitable for onsite monitoring situations. Although more research may be needed to implement the current ideas, the future looks promising for the application of this technology in efficient continuous bridge monitoring scenarios.

Abstract: Displacement-controlled design method is now being used by current guidelines such as TM5 and ASCE to design RC members against airblast load. If the maximum deflection of the designed member under airblast loads is less than the allowable deflection, the designed member is considered to be safe. Although the displacement-controlled design method is easy to use, it may not result in a design having maximum energy-absorption capacity against airblast loads, especially for a design of a reinforced ultra-high performance fibre concrete (RUHPFC) member which is of both high strength and high ductility, that is, high energy-absorption capacity. In this paper, a layered analysis model allowing for varying strain rates with time as well as along the depth of the member was used to calculate energy-absorption of a simple supported RUHPFC slab under airblast loads. An optimal reinforcement ratio of the slab was achieved by maximizing the energy absorption of the slab under different reinforcement ratios. The energy-controlled design method was validated by field blast tests. Using the validated design method, a designed slab with the optimal reinforcement ratio was also tested and the effectiveness of the design was demonstrated.

Abstract: Magnesium phosphate cement (MPC) paste is prepared by mixing dead burned magnesia powder and potassium di-hydrogen phosphate with a proper proportion of water. The superior properties such as fast setting, high early strength, low shrinkage and good bonding makes MPC suitable for applications in rapid repair of infrastructures. However, the setting time of MPC is sometimes too fast, only taking a few minutes, which causes difficulties in manipulation. How to retard and control the setting time of MPC becomes a challenging topic in both scientific and engineering fields. It was found that the setting of MPC was greatly influenced by the reactivity of magnesia, which could be determined by a quick, accurate and convenient test method. Two kinds of magnesia, namely, high-reactivity magnesia and low-reactivity magnesia were blended in order to control the setting time of MPC. Effects of magnesia to phosphate molar ratio, blending ratio and borax addition on setting time, workability and mechanical properties of MPC cement paste were investigated. Finally, the suitable ranges of the different constitutes of MPC based repair materials with satisfied workability and strength development were proposed for the practical engineering applications.

Abstract: In this paper, preparation and mechanical properties of Alite-calcium barium sulphoaluminate (Alite-C2.75B1.25A3 ) cement concrete were studied. The results showed the compressive strength of Alite-C2.75B1.25A3 cement concrete was much higher than that of Portland cement concrete, especially the early-age compressive strength. The 24-hour compressive strength of Alite-C2.75B1.25A3 cement concrete could reach 22.81Mpa for w/c=0.45, 17.29Mpa for w/c=0.50 and 17.04Mpa for w/c=0.55 respectively. They were about 50 to 65 percent higher than those of Portland cement concrete. The 7-day compressive strength could reach about 80 to 90 percent of 28-day strength for Alite-C2.75B1.25A3 cement concrete. The 28-day strength could reach 55.85Mpa for w/c=0.45, 48.01Mpa for w/c=0.50 and 44.21Mpa for w/c=0.55 respectively. The results of SEM showed the interfaces between the hardened cement paste and aggregates in Alite-C2.75B1.25A3 cement concrete were more compact than those in Portland cement concrete. Distribution of particulate bulk was more uniformity and a majority of clinker particles was wrapped by hydrated gel in Alite-C2.75B1.25A3 concrete. And, the structure of Alite-C2.75B1.25A3 cement concrete was much more compact than that of Portland cement concrete.

Abstract: 0-3 cement based piezoelectric composites were fabricated using sulphoaluminate cement as matrix by compressing technique method. The effects of strontium ferrite content on the piezoelectric properties, dielectric properties and acoustic impedance of the composites were studied. The results show that the piezoelectric strain constant d33 and piezoelectric voltage constant g33 of the composites increase gradually with increaing strontium ferrite content. When the strontium ferrite mass fraction is 0.4%, both of the piezoelectric strain constant d33 and piezoelectric voltage constant g33 of the composite have the maximum value, and the values are 16.6pC•N-1 and 31.4mV•m•N-1, which are 35% and 19% larger than that of the composite without strontium ferrite doped, respectively. With the strontium ferrite content increase, both of the dielectric constant εr and the dielectric loss tanδ of the composites increase. With the addition of strontium ferrite the acoustic impedance Z increases, but there is no obvious relation between the strontium ferrite content and Z.

Abstract: ASTM (American Standard Testing and Materials) C 618 prohibits use of biomass ash in concrete. This paper systematically investigates mitigation of Alkali Silica Reaction (ASR) expansion of concrete by three biomass ashes (cement: biomass ash = 65: 35 by weight), and the ASR expansion is triggered by high alkali cement and opal (1-9% weight of quartz replacement). The three biomass ashes come from switchgrass or sawdust cofired with Powder River Basin coal and they cut the ASR expansion significantly below 50% of the control level; however, all three biomass ashes doubled the available alkali of the cement they replaced. Therefore, the exclusion of biomass ash in concrete by ASTM C 618 seems impropriate and more quality research on its role in mitigating ASRs expansion should be conducted.

Abstract: Determination of a realistic model for the estimation of autogenous shrinkage in plain cement mixtures has been an ongoing research among researchers in high performance concrete. While no standard test method exists for the determination of autogenous shrinkage, various researchers have designed different test methods for measurement of autogenous shrinkage. Current study involved the experimental determination of autogenous shrinkage using the test method developed by O.M.Jensen and co-workers, complimented with non-contact eddy current sensors. Measurements were conducted from as early as 1.5 hours from the time of casting. The samples were placed in a constant temperature chamber and the temperature of the sample was also monitored using a thermocouple. The study was carried out on plain cement mixtures at three water cement ratios of 0.25, 0.32 and 0.38. Measurements were also conducted on simple sealed prismatic samples but these measurements could only be collected after 24 hours of casting. The work is supplemented with CEMHYD3D simulations of the samples at similar water-cement ratios under sealed conditions so as to understand the development of the microstructure of the cement responsible for autogenous shrinkage. While experimental determination of internal relative humidity is quite difficult, data regarding chemical shrinkage, amount of water left and the development of the discontinuous capillary network from the simulations help to understand the determined experimental values of autogenous shrinkage. A detailed explanation on the causes of autogenous shrinkage and the basic mechanism responsible for it has been presented.

Abstract: Water repellent treatment (WRT) has proved to be effective to prevent porous materials from water penetration and consequently can improve the durability of structures. In this contribution, three types of mortar, which are pre-carbonated and non-carbonated, had been water repellent treated by silane gel with usage of 400 g/m2, to investigate into the influence of WRT on carbonated and non-carbonated mortar. Results indicate that carbonation reaction increased the compressive strength of all three types of mortar. Silane gel penetrated to a comparatively higher depth for the carbonated mortar. WRT reduced the absorbed water and capillary absorption coefficient greatly, both for non-carbonated mortar to 2 % ~ 7 % and for carbonated mortar to 22 % ~ 66 %, compared with non-treated ones. WRT is still a feasible method for porous materials with some carbonation to prevent from water penetration. However, the efficiency of WRT on reducing capillary absorption became much lower compared with the non-carbonated mortar. Capillary absorption curves of non-WRT mortar, both carbonated and non-carbonated, could be well fitted by a hyperbolic function of square root of time. For the treated mortar, the absorption curves could be described as a linear equation before carbonation, but an exponential function when the mortar carbonated.