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Carbonation treatment was recently developed to enhance the properties of recycle concrete aggregate and prepared concrete, but the durability of concrete with carbonated recycled aggregate have received little consideration in the previous studies. This study investigates the chloride permeability and the resulting steel corrosion of concrete with carbonated recycled fine and coarse aggregates, and the effects of recycled aggregate quality on chloride permeability are presented. The carbonization treatment was first applied to recycled aggregate to prepare carbonated recycled aggregate; then, the chloride permeability and steel corrosion experiments were conducted on concrete with carbonated recycled aggregate. The results exhibit that carbonation treatment improves the physical performance of recycled aggregate, which results in an increase in the compressive strength of preparing recycled concrete. Concrete with carbonated recycled aggregate has a lower chloride permeability than that with non-carbonated recycled aggregate, and the improvement in the chloride permeability resistance is more obvious for concrete with carbonated recycled fine and coarse aggregates. In particular, the improvement of chloride permeability resistance becomes more distinct with the reduction of recycled aggregate quality. The addition of carbonated recycled aggregate reduces the steel corrosion risk of reinforced recycled aggregate concrete, and the chloride permeability and steel corrosion risk follow a linear trend.

An innovative interlocking compressed earth block, called interlocking compressed earth block developed at Shanghai Jiao Tong University, was developed for structural masonry. The locking mechanism of the interlocking compressed earth block developed at Shanghai Jiao Tong University completely depends on the grout in the vertical holes. Therefore, there is no gap between the interlocking key and the blocks, which increases the wall stability and reduces the block manufacturing costs. Experimental studies on the mechanical behavior of the unit (the block) and the masonry (prism constructed with a dry interface) were performed in accordance with the related standards. Soil samples from the northern Gansu Province of China were collected and studied. Small cylindrical samples were tested to determine the compressive and splitting tensile strength. Subsequently, the compressive strength of the prisms with three dry-stack blocks and the shear behavior of the masonry through the triplet test were investigated. The results show that the compressive and shear strengths meet the related standards. This work may provide a valuable structural system for low-cost, eco-friendly dwelling in developing countries.

A simplified scour model for tripod foundation taking into account both local scour and global scour is proposed in this paper. The model is incorporated into a three-dimensional (3D) finite element model for analysis of a full-scale offshore wind turbine founded on a tripod structure using realistic structural properties. Applicability of the 3D finite element model is validated using full-scale load test data. Four different scour conditions under two wave situations are examined for the ultimate limit state (ULS), serviceability limit state (SLS) and fatigue limit state (FLS). The results show that scour has a minor effect on the natural frequency of the tripod-supported wind turbine but can significantly increase the maximum cross-sectional von Mises stress of piles under the ULS and increase the deflection of piles within nearly 20 m below the original seabed under the SLS. As for the fatigue life of the tripod structure, it can also be reduced by the effect of scour. These findings provide insights which are useful for development of safe and economic design of offshore wind turbines supported by tripod foundations.

In order to study the effect of typhoons on the accumulated deformation of monopile foundations for offshore wind turbines, a series of 1-g laboratory model tests with a geometrical scale of 1:100 were carried out. Through the horizontal static and cyclic loading tests of a stiff pile embedded in a medium dense sand deposit, the relationship between the accumulated rotation of the pile and the number of loading cycles under different loading conditions was obtained. The results show that the final accumulated rotation is mainly caused by the typhoon load series and is not affected by the loading sequence. Based on these results, a method is presented to predict the accumulated rotation of the monopile foundation during its service life, and a case study of a 6 MW wind turbine supported by a monopile at a water depth of 30 m in sand is conducted by using the method. The results show that the permanent accumulated rotation of the monopile throughout the design life is mainly contributed by cyclic loading induced by typhoons and the contribution of cyclic loading with small amplitudes can be ignored.

The driven monopile is the most common foundation type for all installed offshore wind turbines. These monopiles are often subject to scour phenomena, which cause a reduction in foundation strength and stiffness. In order to prevent the consequences of this phenomenon, scour protection systems are used to protect the seabed around a monopile structure against scour in practice. This paper presents a numerical model to investigate the effect of scour protection on the natural frequency and lateral responses of monopile-supported offshore wind turbines under the ultimate limit state (ULS), serviceability limit state (SLS) and fatigue limit state (FLS). This numerical model was validated through a series of model tests, and then applied to the National Renewable Energy Laboratory (NREL) 5-MW offshore wind turbine supported on an Offshore Code Comparison Collaboration (OC3) monopile with riprap rock scour protection system. The study concludes that the scour protection has a slight impact on the first natural frequency and monopile behavior under ULS and FLS. However, the scour protection significantly decreases the pile head rotation at mudline under SLS, which provides the opportunity for further optimization of monopile design by incorporating the contribution of scour protection, thereby leading to a more economical design.

Offshore wind turbines (OWT) are often supported on large-diameter monopiles and subjected to cyclic loading such as wind and wave actions. The cyclic loading can lead to an accumulated rotation of the monopile and a change in the foundation stiffness. This long-term effect is not yet well understood. This paper presents a three-dimensional finite element model for analyzing the long-term performance of offshore wind turbines on large-diameter monopiles in sand in a simple way. In this model the characteristics of pile-soil interaction under long-term cyclic lateral loading, observed from well-controlled laboratory model tests, are taken into account. A parametric study has been conducted for a full-scale wind turbine supported on a large-diameter monopile, with focus on the influence of several design parameters on the deformations of the monopile and the tower supporting the wind turbine. The study shows that under the serviceability limit state, the deflection and rotation at pile head in the case of considering the effect of long-term cyclic loading are notably greater than that computed in the case where this long-term effect is ignored. This significant difference suggests that the long-term cyclic loading effect cannot be overlooked in design and analysis.

This paper compares the seismic load of a 5-MW wind turbine supported by a 100-m-high prestressed concrete tower calculated via time history analysis and response spectrum analysis using elastic acceleration spectrum provided by the China Aseismic Code for Buildings. With 5 % damping ratio, the fixed-base multi-degree of freedom model and finite element model considering soil structure interaction are used for response spectrum analysis and time history analysis, respectively. The results indicated that the seismic load calculated by response spectrum analysis is significantly larger than those obtained using the time history analysis method. It implies that the seismic load determined from common building code procedures along with other loads for wind turbine foundation design is too conservative. Within this paper, the effects of damping ratio, horizontal acceleration amplitude, spring stiffness, and damping coefficient of foundation on the seismic load of the prestressed concrete wind turbine tower are discussed. It is shown that the seismic load with mode damping ratio for the prestressed concrete wind turbine tower is not significant when compared with traditional tubular steel tower designs. The maximum moment demand at the base of the tower may be controlled by earthquake loading as the seismic fortification intensity lever is more than seven. The foundation spring stiffness has an immense impact on the base bending moment and the natural frequency. Finally, seismic load should be considered more in detail when designing wind turbines that are supported by concrete towers, particularly for turbines over 100 m height and located in seismically active zones.

A CO2-curing treatment is frequently used to modify the properties of recycled aggregate (RA); however, the carbonation behavior of concrete with CO2-curing recycled aggregate (CRA) has not been investigated in previous investigations. This research focuses on investigating the carbonation behavior of concrete containing CRA when exposed to various environments. The results show that the CO2-curing treatment improves RA property, and there is an obvious reduction in water absorption of CRA. The CRA concrete has higher compressive strength and a lower carbonation depth than RA concrete; in addition, the decrease in the carbonation depth is more obvious when the CO2-curing recycled fine and coarse aggregates are added; for example, the maximum carbonation depth of CRA concrete is approximately 42.5 % lower than that of RA concrete. The carbonation depth increases linearly with the square root of carbonation time. When subjected to the freeze-thaw conditions, incorporating CRA improves the freezing-thawing resistance of concrete, and CRA concrete has better carbonation resistance than RA concrete after undergoing the same number of freeze-thaw cycles. An induced initial crack deteriorates the carbonation resistance of CRA concrete, but CRA concrete still has better carbonation resistance than RA concrete when the induced initial crack and carbonation duration are the same.

This paper presents a hybrid monopile foundation for offshore wind turbines. It is an ultra-high performance concrete-filled double skin steel tubular structure (CFDST) used as a replacement of the conventional steel tube between the water level and the mudline so as to reduce the monopile diameter and thereby reducing the wave loads on the pile. To study the feasibility of this monopile, the NREL 5 MW wind turbine supported by a conventional monopile is selected as a reference and a three-dimensional (3D) finite element model is developed. The natural frequency, the various responses under the serviceability limit state (SLS) and the ultimate limit state (ULS) of the hybrid monopile are presented. Particularly, the effect of varying outer diameter of the CFDST on the structural performance is investigated. By applying a consistent accumulated rotation at the mudline under the SLS, the natural frequency is found to be within a desired range, and an optimized embedded length of the hybrid monopile is determined. The results indicate that the proposed hybrid monopile is able to meet the design requirements for both SLS and ULS and the optimization of pile embedded length leads to an efficient and economic monopile foundation for offshore wind turbines.

As concrete wind-turbine towers are increasingly being used in wind-farm construction, there is a growing need to understand the behavior of concrete wind-turbine towers. In particular, experimental evaluations of concrete wind-turbine towers are necessary to demonstrate the dynamic characteristics and load-carrying capacity of such towers. This paper describes a model test of a prestressed concrete wind-turbine tower that examines the dynamic characteristics and load-carrying performance of the tower. Additionally, a numerical model is presented and used to verify the design approach. The test results indicate that the first natural frequency of the prestressed concrete wind turbine tower is 0.395 Hz which lies between frequencies 1P and 3P (0.25–0.51 Hz). The damper ratio is 3.3%. The maximum concrete compression stresses are less than the concrete design compression strength, the maximum tensile stresses are less than zero and the prestressed strand stresses are less than the design strength under both the serviceability and ultimate limit state loads. The maximum displacement of the tower top are 331 mm and 648 mm for the serviceability limit state and ultimate limit state, respectively, which is less than L/100 = 1000 mm. Compared with traditional tall wind-turbine steel towers, the prestressed concrete tower has better material damping properties, potential lower maintenance cost, and lower construction costs. Thus, the prestressed concrete wind-turbine tower could be an innovative engineering solution for multi-megawatt wind turbine towers, in particular those that are taller than 100 m.

Wind energy is a clean and renewable energy for which technology has developed rapidly in recent years. Wind turbines are commonly supported on tubular steel towers. As the turbine size is growing and the towers are rising in height, steel towers are required to be sufficiently strong and stiff, consequently leading to high construction costs. To tackle this problem, a new type of prestressed concrete tower was designed employing a novel tower concept having a regular octagon cross section with interior ribs on each side, which was optimized by comparing the natural frequency and stress difference under the same lateral load in different directions of the tower. The designed tower features a tapered profile that reduces the area subjected to wind; the tapered profile reduces the total weight, applied moment and the capital cost. An optimization method was developed employing ABAQUS software and a genetic algorithm. A target function was defined on the basis of the minimum cost of the concrete and prestressed tendon used, and constraints were applied by accounting for the stress, displacements and natural frequency of the tower. Employing the method, a 100 m prestressed concrete tower system for a 5 MW turbine was optimized and designed under wind and earthquake loads. The paper also reports a systematic design procedure incorporating the finite element method and the optimization method for the prestressed concrete wind-turbine towers.