140 / 2022-04-29 09:47:09

Development of low-alkalinity seawater sea sand concrete for BFRP bar reinforced marine infrastructure

Alkalinity; BFRP bar; SWSSC; Durability; Marine

During the recent decade, the use of FRP bar reinforced seawater sea sand concrete (SWSSC) as alternative to traditional steel bar reinforced concrete for the construction of marine infrastructures has been increasingly investigated and promoted. Compared to normal RC, FRP-SWSSC are believed more durable under the seawater invasion because FRP bar is highly resistant to chloride attack. The use of seawater and sea sand for concrete production avoids the consumption of river sand fresh water, which are two resources in shortage. Seawater and sea sand are also easily available for coastal and island construction. However, the durability problem of FRP bars in strong alkaline environment of concrete is still open to be addressed, especially for the newly developed BFRP bar. It has been demonstrated that the degradation rate of BFRP bars can be significantly mitigated as the alkalinity of the pore solution decreases (Fig. 1).



According to the improved durability of BFRP bar in low-alkalinity environment, low-alkalinity binder comprising of cement, silica fume and fly ash was been designed. The heat output during hydration (Fig. 2) of the low-alkalinity binder is lower than ordinary Portland cement and the hydration product, portlandite, is completely depleted (Fig. 3 and 4).



Based on the formulation of the above low-alkalinity binder, low-alkalinity SWSSCs were developed. The measured pH of low-alkalinity SWSSCs (11.6-11.7) was one unit lower than that of normal SWSSC (~12.5). After 180 days accelerated aging in 55℃ artificial seawater, the tensile strength degradation of BFRP bars embedded in the low-alkalinity SWSSCs was significantly reduced compared to their counterparts in normal SWSSC. The respective retentions of specimens wrapped by SWSSC with high (H) and low (L) compressive strength (C100SF0FA0-H and C100SF0FA0-L) were 16.66 % and 23.56 % (Fig. 5). In contrast, BFRP bars embedded in the two low-alkalinity SWSSCs (C65SF35FA0-H/L and C35SF15FA50 H/L) retained about 70% or more of their tensile strength regardless of the compressive strength grade of the concrete.



The recorded internal relative humidity of the two low-alkalinity SWSSCs are always lower than normal SWSSC, especially during early exposure period (Fig. 6). The reduced alkalinity and humidity in low-alkalinity SWSSC contributed to the mitigated the deterioration of BFRP bars. Low-alkalinity SWSSC was recommended to construct BFRP bar reinforced concrete structures with better durability in marine environment.