Strength and Deformational Characteristics of Foamed Bitumen Mixes under Suboptimal Conditions

The effects of foamed bitumen contents on the strength and deformational behavior of foamed bitumen mixes used for road pavements is very complex and not fully understood yet. While some writers report an increase in strength using one type of laboratory test, other writers report either only a small increase or even a decrease in strength using other types of tests, thus detracting foamed bitumen from being implemented as a cold-recycling technique for road pavement rehabilitation. This paper presents a laboratory study carried out on a specific granular material from New Zealand containing 1% cement and different foamed bitumen contents using indirect tensile strength (ITS), monotonic load triaxial (MLT), and repeat load triaxial (RLT) tests. The curing procedure, loading regime, and moisture contents were selected to simulate construction practice and suboptimal conditions normally found in New Zealand pavements. The results from these tests showed that an increase in foamed bitumen content up to an “optimum” content, increases the ITS but, at the same time, decreases both the permanent deformation resistance measured in RLT tests and the peak strength in MLT tests. In order to systematically examine the results from these tests, a general stress analysis was conducted, in which the stress paths applied in laboratory tests were plotted in I1−J1/22I1−J21/2 stress diagrams. The stress analysis showed that adding foamed bitumen results in a reduction of the compressive strength of the mixes and a simultaneous increase in the tensile strength, which explains the apparently “contradictory” effects of foamed bitumen reported in the literature depending on the type of test used.