Introduction
The interfacial bonding properties between steel fibers and cementitious matrix under harsh marine environment are crucial for the mechanical properties and durability of UHPC. However, studies on the deterioration mechanism of the steel fiber-cementitious matrix interface in marine environments are insufficient. There is a lack of effective approaches to improve the interfacial bonding properties. This study provides novel approach to strengthen the interfacial transition zone and enhance the interfacial bonding properties using silane coupling agent (SCA). Based on molecular dynamics simulation and steel fiber pull-out test, the variation rules of bonding performance and debonding strength of steel fiber-matrix interface in UHPC under harsh marine environment were clarified. The microstructural variation, bonding network evolution and bonding trend of interfacial chemical bonds for the pristine and SCA-modified interfaces in the harsh marine environment were investigated. Based on this, the interfacial bonding degradation mechanism of the pristine and modified interfaces under marine chloride environment was revealed. This study is expected to provide theoretical basis and methodological support for optimizing and regulating the durability of UHPC in harsh marine environment.
Methods
Based on molecular dynamics simulation, the variation of interfacial interaction energy and initial contact area before and after SCA modification in the harsh marine environment was calculated and the images of interfacial configurations in the marine environment were captured. Using interface debonding simulation and steel fiber pull-out experiments, the load-displacement curves and interfacial fracture energy pristine and SCA-modified interfaces were investigated during the debonding process in the marine environment. Microscopic morphology of interfacial transition zones in the marine environment was observed by scanning electron microscopy (SEM) and backscattered electron imaging (BSE). Moreover, molecular dynamics simulation was used to statistically characterize the evolution of the hydrogen bonding network in the interfacial region and to calculate the formation probability of chemical bonds during the interfacial debonding process.
Results and discussion
The bonding property of the steel fiber-matrix interface under marine environment was severely damaged, where the initial contact area of the interface was significantly deteriorated. Compared to the dry environment, the initial contact areas of pristine and modified interfaces under the NaCl environment were reduced by 51.6% and 39.2%, respectively. Moreover, the interfacial transition zone around the SCA-modified steel fiber was dense. The interfacial bond strength and pull-out energy of the SCA-modified sample after seawater erosion were 23.6% and 21.3% higher than those of pristine sample, respectively. The SCA modification can effectively enhance the adsorption effect between interfacial molecules and improve the denseness of the interfacial transition zone, thus improving the interfacial debonding strength.The hydrogen bonding network within the pristine interface consists only of OCuO—HCSH hydrogen bonds, while that within the SCA-modified interface consists of OAPS—HCSH, OCSH—HAPS and NAPS—HCSH hydrogen bonds. The number of hydrogen bonds in the modified interface is over 2 times higher than that in the pristine interface. The high atomic compatibility between the cross-linked APS molecules and the C-S-H matrix leads to the formation of a variety of stable hydrogen-bonded connections in the interfacial region, which enhances interfacial bonding.Under the marine environment, the bonding probability between the C-S-H matrix and water molecules in the pristine interface is higher than that between matrix and steel fibers. The bonding probability between the Cl– and C-S-H matrix is much higher than that between the matrix and pristine steel fiber atoms in the dry environment. It is indicated that in marine environment, water molecules and aggressive ions are prone to intrude into the pristine interface, forming strong interactions with the interfacial materials and destroying the interfacial bonding between the steel fiber and matrix. Conversely, in the marine environment, the bonding probability between the modified steel fiber and the matrix is higher than that between the interfacial material and the water molecules. Specifically, in the NaCl environment, the bonding probability of silicon-oxygen covalent bonds between the cross-linked APS and the C-S-H matrix remains high. It is demonstrated that SCA modification facilitates the formation of a stable interfacial bonding network, enhancing the interfacial bonding properties and elevating the interfacial corrosion resistance to water molecules and aggressive ions.
Conclusions
The main conclusions of this paper are summarized as follows. 1) The marine chloride environment can seriously damage the integrity of the steel fiber-matrix interface and weaken the interaction energy between the steel fibers and matrix. The SCA modification can effectively enhance the adsorption effect for the interfacial molecules and improve the denseness of the interfacial transition zone, thus improving the interfacial debonding strength. 2) Strong interfacial bonding between the steel fiber and the matrix is achieved through the formation of hydrogen bonds. Water molecules in the erosive environment seize the initial hydrogen bonding bonding sites, generating Owater—HCSH, OCSH—Hwater hydrogen bonds, breaking the bonding network connection between the steel fiber and the matrix. Moreover, Na+ and Cl– are easy to accumulate on the surface of C-S-H matrix, cutting the covalent bond connection between steel fiber and matrix, expanding the invasion channel of water molecules, and eventually exacerbating the interfacial deterioration. 3) Due to the rich type of hydrogen bonding and large number of hydrogen bonds between SCA-modified steel fibers and the matrix, a strong hydrogen bonding network connection is formed within the interface, which improves its resistance to water molecules and aggressive ions. Furthermore, in the SCA-modified interface, the cross-linked APS molecules can develop stable silicon-oxygen covalent bonds with the matrix, and its good compatibility with the matrix promotes the crosslinking of silicate chains in the interfacial zone. This leads to enhanced interfacial bonding properties and ultimately improved interfacial corrosion resistance to the harsh marine environment.