Foamed bitumen and cement stabilized mixes

Considering the rising concerns of the transportation sector in case of negative environmental effects from the construction or rehabilitation activities and more efficient use of the resources, different solutions and approaches have gained attention. Stabilization technology is one of the methods that can be adopted to produce materials with enhanced characteristics, lower production energy, higher rates of recycled products, and therefore to decrease the negative environmental effects. By utilizing foamed bitumen and cement as the binders, a composite product can be produced from different types of granular parent materials. Foamed bitumen and cement stabilized material (FCSM) has a higher bearing capacity than its parent material, lower moisture sensitivity with a balance between flexibility (resulted from bitumen) and rigidity (resulted from cement). It can be integrated into the pavement section with the aim of faster construction, higher share of recycled aggregates, lower production temperature and therefore lower emissions. Understanding the material’s behavior is necessary to be able to get the best out of mentioned advantages and deliver optimum characteristics based on the requirements of each project. In the meantime, the utilization of the material is limited in Germany mainly due to the lack of national behavioral data. Among other parameters, the amount of two binding agents (foamed bitumen and cement) plays a big role in the characteristics of the resulting material. This research tried to take a deeper look into this material and assess the effect of these two binders on its mechanical and performance characteristics and integrate them into the existing national pavement design analytical approach. The research approach was to apply as much as possible the existing methods and available knowledge in Germany. In the first part, samples were produced with different combinations of bitumen and cement content but the same parent material and mix gradation. Indirect tensile tests in static and cyclic modes, at different temperatures and frequencies, were performed to assess the strength, stiffness and response of the mixes to cyclic load (fatigue). Comparing the stiffness master curves of different mix combinations together showed that the effect of cement on increasing the stiffness is much more than the bitumen. It was shown that the Poisson’s ratio amount is an important factor in case of applying indirect tensile mode for the stiffness tests. Poisson’s ratio was determined at different temperatures for all mix combinations and its effect on the stiffness master curves was assessed. Comparing the master curves with the reference hot mix asphalt showed a lower temperature dependency of these mixes. Results of the multi-step stiffness tests showed that besides the temperature and loading rate, material’s response is also affected by the level of stress (or strain) in the test. Results of multi-round stiffness tests revealed that after the first round, a state of stiffness resiliency forms in which the stiffness is only temperature-dependent and is valid till the maximum experienced strain level. These results showed that the material behavior is a combination of a bituminous bond and granular unbound materials. Fatigue tests results showed a good correlation between the ratio of cement to bitumen and the slope of the fatigue line. It is recommended to keep the cement content low (normally 1% and not more than 1.5% of the dry mass of aggregates). Based on the test’s results of this research, bitumen amounts higher than 3% can lead to a resulting material that the fatigue can be taken as the primary failure mode. In the second part, the gained knowledge was used for developing an analytical structural design for pavements with FSCM layer. A model was developed to consider both behaviors of the material. Its parameters can be determined by using the stiffness master curve and
multi-step stiffness test (one temperature and frequency) results. The model was applied to determine the stiffness at different temperatures and horizontal strain levels as the input parameter into a linear elastic pavement design software. A shift factor was also determined to link the laboratory fatigue results to the pavement life. By considering the results of structural designs with different material models, a three-level structural design method was developed based on the level of the available material data and the required accuracy from the design. By utilizing the findings and developed methods in this research, it is now possible to produce FCSM mixes, prepare specimens, test them, determine the needed input parameters and perform the structural design of the pavements with this construction type in Germany. These are the foundation for further research and developments on this material and construction type in Germany.