# Waste Rubber-Modified Cement-Bound Base Course: Laboratory Characterisation and Field Application
## 3 Laboratory Test Results  
The results of the laboratory tests are presented in Table 2 and Figure 4. The mechanical characteristic was defined as the average value of three samples of a certain kind. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**mix** & **OMC (\%)** & **MDD (g/cm\({}^{3}\))** & **f\({}_{\text{c,7}}\) (MPa)** & **f\({}_{\text{c,28}}\) (MPa)** & **RTF (\%)** \\ \hline C3R0 & 4.91 & 2.08 & 1.73 & 2.69 & 98.68 \\ C5R0 & 5.35 & 2.12 & 4.11 & 6.45 & 88.98 \\ C7R0 & 5.53 & 2.15 & 6.82 & 8.89 & 94.09 \\ \hline C3R10 & 5.54 & 2.07 & 1.31 & 2.07 & 99.72 \\ C3R20 & 5.64 & 2.04 & 0.94 & 1.15 & 89.37 \\ C3R30 & 5.57 & 2.01 & 0.60 & 0.85 & 65.08 \\ C3R40 & 5.59 & 1.95 & 0.51 & 0.59 & 63.05 \\ \hline C5R10 & 5.27 & 2.13 & 3.54 & 3.99 & 98.61 \\ C5R20 & 5.32 & 2.10 & 2.20 & 3.01 & 98.60 \\ C5R30 & 5.58 & 2.04 & 1.56 & 1.94 & 91.22 \\ C5R40 & 5.92 & 2.00 & 1.32 & 1.51 & 89.63 \\ \hline C7R10 & 4.97 & 2.15 & 6.04 & 7.81 & 80.37 \\ C7R20 & 5.12 & 2.11 & 3.69 & 4.36 & 71.94 \\ C7R30 & 5.28 & 2.05 & 2.84 & 3.32 & 70.96 \\ C7R40 & 5.16 & 1.99 & 1.09 & 1.66 & 67.91 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Laboratory test results. Figure 3: Samples for compressive strength and RTF determination. The results of the compaction characteristics, optimal moisture content (OMC), and maximum dry density (MDD) indicate that the influence of adding WR to CBC can be neglected and CBC may be designed and constructed using reference values (natural aggregate mixtures) of OMC and MDD for small rubber contents (up to 20%). This presents significant time savings, since no additional tests are needed to determine the compaction characteristics of WR-modified mixtures, and it is necessary to correct the water content in the CBC recipe used in standard production. The results of testing mechanical characteristics indicate that only mixtures with 5% cement content gained satisfactory strength and RTF values. For most pavement purposes, the 7-day compressive strength and 28-day compressive strength should be 2.0-5.5 MPa and 2.5-6.0 MPa, respectively, with RTF \(\geq\) 80%. Mixtures with 3% cement content reached excessively low compressive strength (less than 2 MPa for a 28-day curing period) and RTF values (less than 80%), while 7% cement content mixtures reached excessively high compressive strength (more than 6 MPa for a 28-day curing period) and low RTF values (less than 80%), making them unsuitable for pavement applications. The negative impact of rubber on mechanical properties could be attributed to several phenomena: WR's low density compared to aggregate, poor internal bonds between the rubber and cement matrix, and WR's high elasticity and non-load-bearing nature [8; 30; 31]. The obtained results show that compressive strength increases with the increase in curing time but decreases with the addition of low-density material, such as waste rubber (WR). These results are expected. The addition of WR impacts the decrease in compressive strength by about 50%. Due to the stiffness of this material and the phenomenon of crack development under the influence of dynamic loading and frost, a decrease in the stiffness and strength of this material is desirable to some extent. As can be seen from the results presented, a decrease in compressive strength is accompanied by an increase in frost resistance when adding WR. This finding makes the tested mixtures potentially better for use in pavement construction when exposed to real-life conditions. Figure 4: Mechanical characteristics: 7 days (**a**); 28 days (**b**); compressive strength and RTF (**c**); red lines present limit values. Figure 4 shows that mixtures with 10-20%vol sand replacement by WR (corresponding to 2-3% of a total mixture mass) and 5% cement content indicate improved freeze-thaw resistance whilst maintaining satisfactory compressive strength values.