Lightweight concrete application in construction field is growing rapidly in these recent years due to its advantages over ordinary concrete. In this paper, pumice breccia which can be found abundantly in Indonesia is proposed to be utilized as the coarse aggregate. In spite of its benefits, lightweight concrete exhibits more brittle characteristics and lower tensile strength compared with normal concrete. On the other hand, fiber addition into concrete has become widely used to improve its tensile properties. Furthermore, the utilization of hybrid fiber in a suitable combination may potentially improve the mechanical properties of concrete. This paper experimentally examines the effects of hybrid polypropylene-steel fiber addition on some hardened properties of pumice breccia aggregate lightweight concrete. Five groups of test specimens with fixed volume fraction of polypropylene fiber combined with different amounts of steel fiber were added in concrete to investigate the density, compressive strength, modulus of elasticity, splitting tensile strength, and the modulus of rupture of the concrete mixtures. Test results indicate that hybrid fiber addition leads to significant improvement to the compressive strength, modulus of elasticity, splitting tensile strength, and the modulus of rupture of the pumice breccia lightweight aggregate concrete and meet the specification for structural purposes. 1. Introduction 1.1. Background Lightweight concrete (LWC) application in construction field is growing rapidly in these recent years both for structural and nonstructural purposes due to its advantages over ordinary concrete. The demand for lightweight concrete in many applications of modern construction is increasing, owing to the advantage that lower density results in a significant benefit in terms of load bearing elements of smaller cross sections and a corresponding reduction in the size of the foundation. The maximum density of concrete which can be classified as lightweight concrete in some European construction codes is limited to 2000？kg/m3 [1], while the limitation in Indonesian National Standard is 1900？kg/m3 [2], compared with that of 2400？kg/m3 for normal weight concrete (NWC). Some techniques that are widely used for lightweight concrete development include utilization of natural lightweight aggregates such as pumice, diatomite, and volcanic cinders, or artificial byproducts such as perlite, expanded shale, clay, slate, and sintered pulverized fuel ash (PFA). Lightweight aggregate can be defined as mixture of uncrushed and/or crushed grains for
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