424 publications from this institution
Most materials used in concrete repairs have a tendency to deform due to shrinkage, heat release at early age and ambient temperature change. The restraint of these deformations by substrate concrete induces stresses in repair systems. The stresses can lead to vertical cracking through the thickness of the repair material, peeling of the repair material from the substrate concrete and/ or delamination of the interface. ECC has been proposed to be one of the most promising repair materials. Unlike common cement-based materials ECC shows tensile strain-hardening behaviour with strain capacity in the range of 3-7%, which is hundreds of times of the strain capacity of common cement-based materials. Figure 1 shows the typical tensile stress-strain curve and the average crack width of ECC. The high ductility of ECC is achieved by multiple cracking. When ECC is used as repair material, the multiple cracking can release stresses in repair systems induced by differential volume changes. The risk of repair material in tension and interface delamination is therefore reduced. It can be expect that the use of ECC can enhance the durability of concrete repairs. This phenomenon has been demonstrated both in the laboratory and in the field. An analytical model was developed to calculate stresses in repair systems subjected todifferential volume changes. In this paper, this model will be further developed to estimate the performance of ECC repair systems under differential volume changes. The modelling results, with comparisons to experimental observations, are reported here.
The reduction of pH from ~12.5 to ~9 by carbonation of the pore solution of reinforced cementbased concrete structures results in the reinforcement corrosion. The rate of carbonation is an important input for design of the concrete cover depth and the service life prediction of reinforced concrete structures because the initiation of reinforcement corrosion is usually considered as the end of service life of concrete infrastructure. The information from the field carbonation of alkali activated concrete is in most cases limited and related to exposure shorter than 40 years. In this paper, a comparative study regarding accelerated and natural carbonation of alkali-activated concretes and cement-based concretes has been carried out. The pH and carbonation depths are periodically measured. The results show that, despite the low porosity of alkali-activated concrete with 50 wt. % slag, these concretes must have an appropriate curing in order to be used in exposure classes where carbonation is an issue, due to their lower carbonation resistance compared to cement-based concrete. Regardless the exposure conditions, the pH of carbonated alkali-activated concrete was maintained above 9. Finally, recommendations for alkali activated concrete applications and their improved carbonation resistance are given.
As cement paste is a fundamental matrix of all cementitious materials,it would be interesting to investigate the fracture processes in cement paste at micro scale.In this paper,two modeling schemes are proposed and applied to analyze the microstructure of cement paste,namely HymLat and ImgLat.In the HymLat scheme,the input microstructure has to be represented by sphere parti-cles,and this restriction is removed in the ImgLat scheme,where all the phases are described in terms of voxels.For both schemes,the microstructure of cement paste is obtained by the simulations using the HYMOSTRUC3D model and the fracture process is simulated by the code GLAK(Generalized Lattice Analysis Kernel).The comparison between these two modeling schemes were made to illustrate the differences in flexibility and computing efficiency.
This work was financially supported by project POCI-01-0145-FEDER-007457 – (CONSTRUCT - Institute of R&D in Structures and Construction) and by project POCI-01-0145-FEDER-007633 (ISISE) funded by FEDER funds through COMPETE2020 - Programa Operacional Competitividade e Internacionalizacao (POCI) – and by national funds through FCT - Fundacao para a Ciencia e a Tecnologia. FCT and FEDER (COMPETE2020) are acknowledged for the funding of the research project PTDC/ECM-EST/1056/2014 (POCI-01-0145-FEDER-016841). The first author also thanks the individual grant with reference number of SFRH/BPD/114754/2016 funded by FCT
Micronized sand, with high purity SiO2, has been selected as cement replacement. The service life of structures made of concrete blended with micronized sand was evaluated by using the DuraCrete model. In this model the end of service life was assumed to be reached when the chloride concentration at the reinforcement had reached a critical threshold value. The chloride profiles in concrete structure and the cover thickness of structures were evaluated for structures made of concrete blended with micronized sand as partial replacement of cement. For a required service life of 80 years, the chloride ion concentration over the cover depth of structures made with blended concrete was a bit higher than in reference OPC concrete. To ensure the required service life, the cover thickness of structures made with concrete blended with micronized sand has to be increased. The increase of the cover thickness was relatively small compared to the size of the structural elements.
The inhibition effects of L-tryptophane (Trp) and its synergistic effect with KI on the corrosion of mild steel in 1.0 M HCl solution have been investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements.The results show that the inhibition efficiency increases with the concentration of Trp and increases further when KI exists.The synergistic effect between Trp and KI was discussed by calculating the synergism parameter, which suggests that there is a cooperative mechanism between iodide anion and Trp cation.Finally, Monte Carlo simulation was utilized to search for the equilibrium configurations of inhibitors/Fe(110) adsorption systems in the presence of water molecules.
In engineering practise, interior relative humidity (RH) of concrete significantly affects the transport properties and thus the service life of concrete structures. In this paper, the development of RH due to self-desiccation in blended cement pastes was studied from 1 day to 1.5 years. The pore structure and non-evaporable water content at same ages were determined by mercury intrusion porosimetry and thermogravimetric analysis, respectively. The results revealed that interior RH was significantly reduced at the first 105 days’ curing and falls off slightly afterwards, regardless of water to binder ratios and type of blends. Compared to ordinary Portland cement (OPC) paste, the OPC paste blended with slag shows much lower interior RH, whereas the addition of fly ash slightly increases the interior RH. Minor amount of limestone addition i.e., 5% wt. greatly increases the RH in ternary system consisting of OPC, slag and limestone, whilst slightly decreases the RH in OPC paste blended with fly ash. In the presence of blends, high total porosity corresponds to low interior RH. In case of self-desiccation, it is concluded that interior RH is mainly controlled by average pore size in the cement-based materials.
Based on simulated annealing (SA) and reinforcement learning (RL) algorithm, a hybrid intelligent controller is proposed to ship steering. The SA algorithm is a powerful way to solve hard combinatorial optimization problems, which is used to adjust the parameters of the controller in this paper. The RL algorithm shows its particular superiority in ship steering, which just needs simple fuzzy information. With the advantages of the two algorithms, the controller can overcome the influence of the wind, wave and flow, the limitation that data are not exactly accurate. At last, the results of the simulation show that the ship course can be properly controlled when changeable wind, wave, and measure error exists.
Because the ship steering control is uncertain, nonlinear and time-varying. A predictive control algorithm based on RBF neural network is adopted to the ship steering control. Recursive k-means clustering algorithm and recursive least squares algorithm are used to adjust the RBF neural network. And clonal selection algorithm is used in predictive control algorithm to ensure the global optimal solution. The simulation results show that the predictive control algorithm based on RBF neural network possesses good control performance and strong robustness.
During the carbonation of cement paste blended with supplementary cementitious materials, the C-S-H with different Ca/Si ratios, formed from the hydration and pozzolanic reactions, are the major calcium-bearing phases which react with CO2. Therefore, it’s important to study the carbonation mechanism of different C-S-H phases. In this paper, the pure C-S-H phases (Ca/Si ratio: 0.66 to 2.0) were synthesized and used for the accelerated carbonation test. Synthesized C-S-H phases with different Ca/Si ratio were identified by X-ray diffraction and 29Si nuclear magnetic resonance. The carbonation rate and products of different C-S-H phases are also determined. The results show that pure C-S-H phases with different Ca/Si ratio (lower than 1.40) can be synthesized in the lab. The structure of synthesized C-S-H is similar to the C-S-H(I) reported by Taylor. The mean chain length of the C-S-H decreases dramatically when the Ca/Si ratio increases from 0.66 to 1.40, then it keeps no change. The portlandite appears in the products when the designed Ca/Si ratio is over 1.40. The C-S-H with lower Ca/Si ratio is decomposed faster than that with a higher Ca/Si ratio. All the C-S-H phases are fully decomposed to CaCO3 and silica gel after 3 days’ accelerated carbonation.
