# Prediction Models for Mechanical Properties of Cement-Bound Aggregate with Waste Rubber
## 3 Results  
The results obtained for the dynamic modulus of elasticity (E\({}_{\rm dyn}\)), compressive strength (f\({}_{\rm c}\)) and static modulus of elasticity (E\({}_{\rm sd}\)) measurements are shown in Table 4. The presented results were calculated as the average value of the three tested specimens for each mixture and curing period. Values that deviated by over 20% from each other were discarded according to the standard EN 14227-1 [30]. The table contains the results of the mechanical characteristics for the curing periods, 7, 28 and 90 days, and express the standard deviation (St.dev.). The mixtures were divided into groups (columns) according to the amount of rubber and, additionally, the results were divided according to the proportion of cement in the mixture. For example, the third, fourth and fifth columns in the table show the  
Figure 3: The f\({}_{\rm c}\) and E\({}_{\rm st}\) testing. Figure 2: UPV testing. results of mixtures with 0% rubber and 3%, 5% and 7% cement, respectively. From the plotted results (Table 4), it can be concluded that an increase in strength occurs with an increase in cement and in the duration of the curing period. The highest strength values were reached for the 90-day curing period and with 7% cement for each rubber content. On the other hand, increasing the amount of rubber in the mixture causes a decrease in the compressive strength. An increase in the \(\mathrm{E_{dyn}}\) and \(\mathrm{E_{st}}\) accompanies the increase in strength. The decrease in \(\mathrm{E_{din}}\) and UPV with a decrease in \(\mathrm{f_{c}}\) due to rubber incorporation complies with the findings in [16]. The authors of that paper state that a decrease in UPV is correlated with a decrease in compressive strength due to the incorporation of fly ash instead of cement, which has less early-age pozzolanic activity. In this case, the rubber is the reason for the compressive strength decrease. Observing the modulus values, one can conclude that there is no rapid modulus-growth phase, as stated in [8], because of the use of rapid hardening cement. The rapid growth in the strength and modulus occurred in the first seven days of the specimen curing. The same can be concluded from Figure 4, which shows the development of the UPV over time for the mixture with the highest cement content. These mixtures are shown because they are expected to have the most significant influence on the development of cement stiffness. It can be seen in Figure 4 that the difference between the UPV for 7 and 28 days increases with the amount of rubber because the hydration slows down due to the reaction of the Zn from the rubber with C3S [46]. However, for mixtures with up to 20% rubber, the UPV increases almost linearly until the 90 th day of curing. It is impossible to determine the phase of the UPV's rapid growth as it is detected in rubberized mortars, i.e., the rapid growth phase occurs in the first 7 days. At the same time, the amount of 20% rubber was shown to reduce the initial development of the stiffness of the mixture. In general, the use of 20% rubber as a sand replacement reduces the rate of stiffness development and linearizes the stiffness development over time. This means that there is no sudden development of strength and stiffness and, consequently, no sudden development of internal stresses. Furthermore, by observing the static modulus of elasticity, it can be concluded that for reference mixtures, the \(\mathrm{E_{st}}\) values stagnate for longer curing periods, and that this is more pronounced with higher cement contents. On the other hand, with the incorporation of rubber, the \(\mathrm{E_{st}}\) develops with age and it is more pronounced with mixtures with higher cement contents. Greater changes in mixtures with higher proportions of cement and rubber directly indicate the interaction of rubber and cement.