Lightweight aggregate concrete (LWAC) has been applied in marine structures in recent years due to its lower density compared to conventional concrete. The main durability issue is the corrosion of steel reinforcement induced by the chloride ion ingress and subsequent spalling due to volume expansion of steel rust. Besides, a damaged surface is prone to water absorption and loss of effective buoyancy in the case of floating structures. This paper presents two chloride resistance tests and a long-term water absorption test on LWACs cast using two types of coarse lightweight aggregates and various percentage of silica fume. The test results show that, with the same water to cement ratio and silica fume percentage, LWAC cast using expanded slate has a better chloride resistance than those using expanded shale. Besides, increasing the percentage of silica fume could yield a better durability performance of LWAC.
Lightweight aggregate concrete (LWAC) has been widely used in construction of high-rise buildings and long-span bridge decks (Chandra and Berntsson, 2002). The structural weight is significantly reduced compared to conventional concrete, which allows for higher live load capacity. In recent years, the application of LWAC is seeing an increasing popularity in floating marine structures, such as floating bridges, concrete vessels and floating offshore production platforms. Being the world's first concrete tensioned leg platform (TLP), Heidrun TLP was constructed by LWAC with a mean density of 1943 kg/m3 and 28-day strength of 79 MPa (fib, 2000). In order to achieve higher turnover, its design service life was more than 60 years. When it comes to reinforced concrete floating structure, the major durability issue is the chloride-induced corrosion of steel reinforcement due to the abundance of chloride ions in marine environment. Chloride ions are not detrimental to concrete itself, rather, through concrete's intrinsic porosity, it can ingress into the concrete cover and cause corrosion of the steel reinforcement. Subsequent expansion in volume of steel rust will result in spalling of the concrete cover. This not only affects the structural integrity, but also exposes the concrete interior, which is prone to water absorption and loss of effective buoyancy over time.
Chia and Zhang (2002) found that the chloride resistance of LWAC is comparable to normal weight concrete (NWC) with the same water to cementitious material (w/c) ratio, and a lower w/c ratio would benefit the chloride resistance for both. Liu et al. (2011) reported that LWAC exhibited higher resistance to chloride penetration compared to NWC with similar 28-day compressive strength. These findings outperform the conventional views that LWAC would exhibit a poor durability performance due to the high porosity of lightweight aggregates. This can be attributed to the better quality of the interfacial transition zone (ITZ) due to an improved mechanical interlocking between lightweight aggregates and cement paste (Zhang and GjØrv, 1989). In terms of long-term performance, Thomas and Bremner (2012) tested the chloride resistance of LWAC after being placed in a harsh marine environment for 25 years. They showed that the chloride diffusion in a real marine environment is much less than the measured value in an accelerated laboratory test. However, to conduct such a long-term test would be difficult and time consuming, thus the more conservative accelerated experimental programs are still preferable and widely used. Justnes et al. (2016) proposed a methodology relating the chloride diffusion coefficient with concrete cover thickness and the presented benchmark can be used for structural service life design or evaluation.