# Waste Rubber-Modified Cement-Bound Base Course: Laboratory Characterisation and Field Application
## 4 Large-Scale Production  
Large-scale CBC production for field applications, as part of road construction projects, is usually undertaken in plants for conventional concrete production. In such plants, CBC is usually produced with the same types of aggregate and cement used for concrete mixtures in order to reduce the costs and labour related to ordering new materials, testing properties, creating and testing new-mixture recipes, and swapping and storing all of those materials in the aggregate storage boxes and various parts of the mixing plants' mechanisms. This is particularly the case if only a small amount of CBC is needed, compared to the volumes of concrete that are produced every day. Therefore, the two concrete mixing plants in this part of Croatia, which produce all of the concrete (in large volumes) and the majority of all CBC for road construction, were tested for the technological transfer of WR-modified CBC. However, these two plants have different technologies, and use different aggregate sizes and cement volumes in their regular CBC production. Within the first trial mix production, crushed stone (0-16 mm) and cement were used. These had a strength of 42.5 N (CEM II A-M(S-V) 42.5 N) and comprised 0.8% of the total aggregate mass. During the trial production, 1.5 m\({}^{3}\) of the mixture was produced, with WR comprising 2% of the total aggregate mass. Mass ratio was used, rather than volume, to simplify the production process. The other components of the standard cement-bound aggregate usually produced by this plant were not corrected. The weighed amount of rubber was added directly to the mixer during the process of mixing the aggregate and cement in order to obtain a homogeneous mixture. Material sampling was carried out immediately after the production to make samples, test the mixtures in laboratory conditions, and compare WR mixtures with conventional CBC mixtures from these plants. Proctor samples (cylinders with a diameter of 10 cm and a height of 12 cm) were made using a vibrating hammer, according to the EN 13286-51 standard [27], as in previous laboratory tests. The samples were cured in a climate chamber, with controlled humidity and temperature conditions, for 7 and 28 days at a temperature of 20 \({}^{\circ}\)C and an air humidity of 90%. They were wrapped in cling film until testing. The laboratory research determined compressive strength and F/T resistance in plant-generated mixtures, as these are the properties required by the technical conditions for CBC and are regularly tested as part of quality control during the field construction of CBC layers. The compressive strength (f\({}_{\rm c}\)) was determined according to the EN 13286-41 standard [28] (3 identical samples for each curing period) and resistance to freeze/thaw cycles (RTF) was tested according to the EN 13286-54 standard [29]. The results are shown in Table 3. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mixture** & **f\({}_{\rm c,7}\) [MPa]** & **f\({}_{\rm c,28}\) [MPa]** & **RTF [\%]** \\ \hline Reference & 2.7 & 3.1 & 89 \\ WR mix & 2.0 & 2.4 & 99 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mechanical properties of the first trial mix production. The results show a decrease in compressive strength of about 25% but an improvement in resistance to freeze/thaw cycles of about 10%. However, during the production process, excessive humidity was observed in both mixtures, possibly because of a high aggregate humidity (held in open boxes), which potentially caused the lower compressive strengths. A second trial mixture was produced from crushed stone (0-30 mm) with 1.5% cement by weight of aggregate and a strength of 42.5 N (CEM II A-M(S-V) 42.5 N). WR comprised 2% of the mass of the aggregate. The other components of the standard cement-bound aggregate usually produced by this plant were not corrected. The second concrete plant had a more modern design and did not have direct access to the mixer itself.