This paper investigates the mechanical properties of two different types of recycled concrete, which use wood and rubber, relative to those characteristics of pure concrete, in terms of maximum load and natural frequencies. This paper contributes to the state of the art in this area in a number of ways. Firstly, the paper provides furtherance to the progressively growing literature in the field of recycled concrete and mechanical properties of materials. Secondly, the paper investigates the mechanical properties of two different types of recycled concrete by means of investigating the natural frequency of the samples, which is a new contribution. Lastly, the results from predicting the natural frequencies of concrete using fuzzy logic have been effectively assessed and compared with the analytical results. Results from the study show that the pure concrete samples produced maximum natural frequency, then concrete samples with wood, and lastly, concrete samples with rubber. The tolerance between the lab test results and fuzzy logic is approximately 5%. These results could have significant implications for furthering recycled concrete research and for designing machine foundations. Evidence of the applicability of fuzzy logic as a predictive and analysis tool for the mechanical properties of recycled concrete are discussed.

The adoption of limit state design according to Eurocode 5 (EC5) has brought about design strength optimisation in design practices worldwide. However, the implementation of EC5 may not be suitable for Malaysian tropical timber species as the design strength data in European Standard (EN) 338:2016 is based on softwood and temperate hardwood species. EC5 has a well-established 1/k value of 0.2 for softwood and temperate hardwood with characteristic densities below 700 kg/m³. The value of 0.2 is still uncertain for tropical hardwood timber and it is predicted that the 1⁄k value for characteristic density above 700kg/m³ will be different. Therefore, in this study, the application of Weibull's theory is being used to determine the 1/k value for selected species namely, Balau (Shorea spp.), Kempas (Koompassia malaccensis), Kapur (Dryobalanops spp.), Keruing (Dipterocarpus spp.), Geronggang (Cratoxylon arborescens), and Light Red Meranti (Shorea spp.) with density ranges from 300 kg/m³ to 1000 kg/m³. Experimental bending data from previous researchers were analysed and verified using this theory. From the theoretical prediction, the calculated 1/k value for the selected species ranges from 0.158 to 0.204 which is close to the established value of 0.2. This study provides the actual k value as well as the true depth modification factors for tropical hardwood which important for safe and economical structural timber design.