# Prediction Models for Mechanical Properties of Cement-Bound Aggregate with Waste Rubber
## 5 Conclusions  
This research includes the testing of the compressive strength (f\({}_{\text{c}}\)) and dynamic (E\({}_{\text{dyn}}\)) and static (E\({}_{\text{st}}\)) moduli of elasticity of cement-bound aggregate modified with waste rubber to determine the inter-relationships of these characteristics and their time dependence. Reliable results were obtained through laboratory research, enabling the development of the prediction model. These are the main contributions of this research. Furthermore, a detailed statistical analysis of nonlinear relationships, which was not found in the available research for these materials, also contributes significantly to the non-destructive testing of pavement materials. Two models were developed: one for the estimation of the f\({}_{\text{c}}\) and the other for estimating the E\({}_{\text{st}}\) based on the measured UPV and the duration of specimen curing. From the results obtained, the following conclusions can be drawn:  
* Increases in the amount of cement and in the curing period positively affect the f\({}_{\text{c}}\), E\({}_{\text{dyn}}\) and E\({}_{\text{st}}\). The addition of rubber decreases these mechanical characteristics.
* The reliability of the modulus of elasticity results obtained by the two methods is supported with a strong linear correlation (R\({}^{2}\) = 0.88).
* A detailed statistical analysis of the obtained data resulted in two simple linear prediction models. One of these models serves for the estimation of the f\({}_{\text{c}}\) based on UPV, while the other serves for the E\({}_{\text{st}}\) estimation based on the UPV and curing period.
* An inter-relationship between rubber and cement was observed, especially in the mixtures with higher proportions of cement. The recommendation for further research is to analyze this influence through more mechanical properties and on a chemical and micro level.
* The increase in the UPV in the first 7 days and its linearization for a longer period of time indicates that the CBA stiffness passes through certain phases that cannot be precisely determined from the obtained results. To determine the stage of development of the stiffness, it is recommended to carry out tests in short time intervals between the first and seventh day of care.  
The presented prediction models were developed on limited data and are valid only for the tested materials. As a recommendation for further research, the verification the developed models on a more significant number of specimens and other materials is indicated to prove their general applicability. It is recommended to limit the application of these models to gravel materials, considering the shape of the grains and the manner of their entrapment.  
Conceptualization, M.Z. and M.B.; methodology, M.Z. and M.B.; software, M.B.; validation, M.B., I.B. and T.D.; investigation, M.Z., I.B. and T.D.; writing--original draft preparation, M.Z.; writing--review and editing, M.B., I.B. and T.D.; funding acquisition, I.B. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Croatian Science Foundation (HRZZ), UIP-2019-04-8195, \"Cement stabilized base courses with waste rubber for sustainable pavements--RubSuPave\".  
Not applicable.  
Data available on request due to restrictions eg privacy or ethical. The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ongoing research financed by HRZZ here].