Wavelet Power and Shannon Entropy Applied to Acoustic Emission Signals for Corrosion Detection and Evaluation of Reinforced Concrete

Acoustic emission (AE) signals detected from corrosion test on a steel reinforced concrete beam subjected to the coupling effects of corrosive wet-dry cycles and static load are analyzed by power spectral density, wavelet transform, and Shannon entropy. The degradation process of the corroded reinforced concrete beam can be divided into four stages on the basis of the accumulated event number (AEN). Due to the difference of material properties, steel reinforcement and concrete matrix have distinguished AE features. The time-frequency characteristics of AE signals can reflect the microstructural degradation mechanism of steel corrosion and concrete cracking. The corrosion evaluation entails investigating the evolution of the wavelet power mathematically by Shannon entropy. The frequency-entropy clearly exhibits the relative power distribution of AE signal in a certain frequency region. With the accumulation of steel corrosion and concrete deterioration, the increment of the overall entropy integration is considerably apparent. The variation of frequency-entropy curve reveals the corrosion revolution of the reinforced concrete members under static load, which is represented by a transforming from corrosion-induced micro cracking to load-induced localized cracking

Enhancement of Cement Paste with Carboxylated Carbon Nanotubes and Poly(Vinyl Alcohol)

Cement has been a major consumable material for construction in the world since its invention, but its low flexural strength is the main defect affecting the service life of structures. To adapt cement-based materials to a more stringent environment, carboxylated carbon nanotubes (CNTs-COOH) and poly(vinyl alcohol) (PVA) are proposed to enhance the mechanical properties of cement paste. This study systematically verifies the synergistic effect of CNTs-COOH/PVA on the performance of cement paste. First, UV-Vis spectroscopy and FTIR spectroscopy prove that CNTs-COOH can provide attachment sites for PVA and PVA can improve the dispersion and stability of CNTs-COOH in water, which demonstrates the feasibility of synergistically enhancing cement paste. When a 0.015% CNTs-COOH suspension with 0.1% PVA is added, the flexural strength of the cement paste increases by 73, 32, and 42% compared with control specimens at curing ages of 3, 7, and 28 days, respectively. The strength enhancement mechanism is revealed from the aspects of cement matrix enhancement and interface enhancement. Thermogravimetric (TG) analysis and mercury intrusion porosimetry (MIP) prove that CNTs-COOH can enhance the hydration degree of the cement matrix and fill the pores introduced by PVA. Based on the fact that PVA can improve the dispersibility and the nucleation site effect of CNTs-COOH in cement paste, molecular dynamics simulation confirms that PVA can bridge CNTs-COOH and C-S-H to enhance the interfacial bonding by 64.1%

Water and ions transport in calcium silicate hydrate: a molecular dynamics study

Transport properties of water and ions in calcium silicate hydrate (C-S-H) greatly affect the durability of cementitious materials. In this study, molecular dynamics (MD) technique is used to investigate the transport behaviors of NaCl solution in C-S-H nanopores with different sizes (from 0.5 nm to 5 nm), and the hindering effect of C-S-H on the diffusion of water molecules and Cl ions is further explored in the case of a 5 nm pore. Results show that the diffusion coefficients of water molecules and Cl ions in C-S-H nanopores increase with the expansion of nanopore. At the atomic scale, the Ca-rich C-S-H forms Ca-O and Ca-Cl clusters with water molecules and Cl ions, respectively, and the Si-O tetrahedra on silicate chains can also build hydrogen bonding interactions with water molecules, which constrain the transport behaviors of water and ions. From the molecular perspective, this study innovatively investigates the effect of C-S-H pore size on the diffusion capacity of water and ions, and reveals the chemical bonding mechanism between water molecules, Cl ions and C-S-H, which provides a theoretical basis for studying the resistance of concrete to ionic attack

Molecular Dynamics Study of Water and Ions Transported during the Nanopore Calcium Silicate Phase: Case Study of Jennite

Durability is an important property that determines the long-term behavior of cement-based materials. Water and ions are transported in nanopores of calcium-silicate-hydrate (C-S-H) gels, the main element in cement-based material, which significantly influences the durability of cement. Because of its structural similarity, jennite, an important mineral analog of C-S-H gel, is first taken to investigate the transport behavior at a molecular level. In this paper, structural and dynamical properties of the water/ions and the jennite interface are studied by the molecular dynamics (MD) simulation method. On the (001) surface of jennite, water molecules diffusing in the channel between silicate chains demonstrate the following structural water features: large density, good orientation preference, ordered interfacial organization, and low diffusion rate. The channel water molecules have more H-bonds connected with the neighboring water molecules and solid surface. As the distance from the channel increases, the structural and dynamical behavior of water molecules varies and gradually translates into bulk water properties at 10-15 angstrom from the liquid-solid interface. With respect to the interaction between jennite and the ions, the surface demonstrates Cl- repulsion and Na+ adsorption. With increased ion concentration, the jennite adsorption capability for Cl- is enhanced because Na+ and Cl- aggregate to form a cluster in the interfacial region. The simulation results, matching well with the Cl35 Nuclear Magnetic Resonance (NMR) studies and isotherm adsorption tests, give a molecular-scale interpretation of experimental studies

Two-Scale Modeling of Transport Properties of Cement Paste: Formation Factor, Electrical Conductivity and Chloride Diffusivity

Predicting transport properties of cement-based materials directly from the microstructure is very challenging, due to the problems of bridging length scales and the difficulties of realistically representing the microstructure. Based on a two-scale representation of the microstructure, a scheme is proposed in this paper to model the transport properties of cement paste through two-scale random walk simulation. A random walk algorithm is firstly applied at the sub-micro-scale to determine the diffusion tortuosity of the outer C-S-H layer. This is then up-scaled to the micro-scale to compute the diffusion tortuosity of cement paste. Based on physical laws, the diffusion tortuosity is transformed into the formation factor, and further into the electrical conductivity and the chloride diffusion coefficient of cement paste, and subsequently validated. It is proven that a more realistic representation of the microstructure makes it possible to derive transport properties of cement paste, directly and accurately, from the microstructure

Morphology of Calcium Silicate Hydrate (C-S-H) Gel: A Molecular Dynamic Study

Due to its complexity at nanoscale, calcium silicate hydrate (C-S-H), the dominant binding phase in cement hydrates, is not yet completely understood. In this study, molecular dynamics was employed to simulate the hydration products at low and high calcium/silicon ratios. It was found that two morphologies of calcium silicate hydrate gels can be distinguished - a branched structure at low calcium/silicon ratios and an ellipsoid particle structure at high calcium/silicon ratios. Using virtual X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and small-angle neutron scattering (SANS) techniques, the simulated structures were characterised, confirming that they show features of calcium silicate hydrate as revealed by experimental approaches. The short-range structures of calcium and silicon atoms and the distorted calcium tetrahedrons resemble the features of silicate glasses obtained from experiments, implying the amorphous nature of the local structure in calcium silicate hydrate gel. Furthermore, formation mechanisms for the two morphologies are proposed. In the hydration process, calcium ions play roles in depolymerising the silicate structure and preventing the amorphous network formation. Therefore, at low calcium/silicon ratios, the reaction is governed by silicate skeleton growth, but at high calcium/silicon ratio, aggregations of calcium ions and short silicate chains dominate

Multi-Scale Study Water and Ions Transport in the Cement-Based Materials:from Molecular Dynamics to Random Walk

The transport properties of water and ions play a key role in the durability of cement-based materials. However, little work has been done to study the diffusion behaviors of water and ions in a multi-scale view. Therefore, the current paper presented a multi-scale molecular dynamics fed random walk (MD-RW) simulation scheme for the purpose of obtaining the diffusion coefficients of water and ions in the calcium-silicate-hydrate (C-S-H) gel. At nanoscale, MD simulations were used to obtain the diffusion coefficients of water and ions in the C-S-H with nanopore size ranging from 0.5 nm to 5 nm since pore size has a crucial effect on the diffusion behavior of water and ions. Then these results were utilized as the input parameters for random walk simulation to scale the confining effect up and derive the effective diffusion coefficients of water and ions in the C-S-H gel at mesoscale. The results acquired from mean square displacement, atomic intensity distribution function and radial distribution function show that water molecules and Cl ions would form Ca-Cl, Ca-O ion-clustering and hydrogen bonds with C-S-H substrate which will restrict the transport of water molecules and Cl ions. In addition, with the size of gel pore increases from 0.5 nm to 5 nm, the diffusion coefficient of water increases from 0.016 to 1.85 × 10-9 m2 s-1 and the diffusion coefficient of Cl ions increases from 0.002 to 0.75 × 10-9 m2 s-1. In the random walk simulation, the diffusion coefficients of water and Cl ion were derived as 1.01 × 10-12 m2 s-1 and 1.2 × 10-13 m2 s-1 in the FCC packing model and 1.55 × 10-11 m2 s-1 and 1.77 × 10-12 m2 s-1 in the BCC packing model respectively, which reaches reasonable agreement with the previous studies results. Unlike traditional reverse methods which back-calculate these coefficients from macroscopic experimental results and hypothesized microstructure, this study presents the first bottom-up method deriving the transport properties of C-S-H gel directly from the physicochemical characteristics and microstructure, which has relevant significance for the study of concrete resistance to ion erosion and provides a basis for designing durable concrete