Performance of Lightweight Concrete with Expansive and Air-Entraining Admixtures in CFST Columns

Adequate load transfer in concrete-filled steel tubes (CFSTs) requires a close interaction between the steel walls and the concrete core. The present work analyzes the adhesion and confinement effects in steel tubes promoted by three types of lightweight concrete: without any admixture (reference), with an expansive agent (EA), and with an air-entraining admixture (AEA). The following tests were performed: expanding the potential of the admixtures, characterization of the hardened concretes, shear tests, axial compression with load applied to the concrete core, and axial compression applied to the mixed section. The results indicated that the dimensional variation generated by the EA induces a confinement prestress, which improves interface adhesion and, thus, the performance of the CFST. The concrete with AEA presented a lower modulus of elasticity and superficial irregularities that contributed to the manifestation of mechanic adhesion, adhesion by friction, and a high degree of confinement. Although the AEA-CFSTs presented compressive strength 2% lower than the reference, they were 10% lighter. On the other hand, the EA-CFST presented a similar density and an 8% increase in the compressive strength. In conclusion, the use of both admixtures contributed to a suitable performance of the filling cores.