## 6 Conclusions  
In this paper, the BFRP laminates with the thickness of \(h=1\), \(h=2\), and \(h=4\) mm were fabricated by wet-layup method and the influence of thickness on their water absorption and tensile properties were experimentally studied under hygrothermal environment as well as degradation mechanism. A specimen thickness-dependent accelerated ageing method was proposed. The following conclusions can be drawn from the testing results and discussions in this study:  
The long-term properties of BFFRP laminates were greatly affected under hygrothermal environment. The water absorption trend of BFRP laminates soaked in both 60 \({}^{\circ}\)C deionized water and alkaline solution increased first before reaching their peak water absorption and then decreased with the increase of immersion duration, which was caused by the hydrolysis of the epoxy matrix. The tensile properties of BFRP laminates degraded apparently after ageing, especially in alkaline solution. SEM images show basalt fiber deteriorated due to the solution immersion.  
Specimen thickness had a significant influence on the water absorption and tensile strength of BFRP laminates after ageing. When the BFRP laminates with different thicknesses were immersed in the water or alkaline solution for the same ageing time, the water absorption decreased in early stage of immersion and then increased in late stage of immersion as the specimen thickness increased, while the tensile strength retention kept increased during the whole ageing process. The reason is that the ratio of aged area to the total area in the thinner specimens was larger than that of the thicker specimens, leading to the more severe ageing degree of the thinner specimens.  
An innovative thickness-dependent accelerated ageing method for water absorption and tensile strength retention of BFRP laminates was proposed, in which the accelerated factors were theoretically deduced based on specimen thickness. The proposed method is in good agreement with test results. Compared with the traditional accelerated ageing method based on temperature, the proposed method is much easier to be conducted and has the potential to obtain a greater accelerated factor.  
Conceptualization, Y.W.; methodology, Y.W. and X.Z.; software, W.Z.; validation, B.W., W.Z.; formal analysis, G.C.; investigation, G.C. and Y.W.; resources, Y.W.; data curation, W.Z.; writing-original draft preparation, W.Z.; writing-review and editing, Y.W., X.Z. and B.W.; visualization, G.C.; supervision, Y.W., X.Z. and G.C.; project administration, Y.W.; funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.  
The authors are grateful for the financial support from the National Key Research and Development Program of China (Project No. 2017 YFC0703000), the National Natural Science Foundation of China (Project No. 51778102 and 51978126), the Fundamental Research Funds for the Central Universities of China (Project No. DUT18 LK35), and the Natural Science Foundation of Liaoning Province of China (Project No. 20180550763). The authors declare no conflict of interest.  
FRP Fiber-reinforced polymer BFRP Basalt FRP Carbon FRP Glass FRP AFRP Aramid FRP reinforced concrete reinforced concrete VARI vacuum assistant resin infusion SEM scanning electron microscopy AF accelerated factor