Abstract
In this work,ef fects of nano-SiO2(NS)and carbon nanotubes(CNT)addition on interfacial moisture transport properties of recycled concrete were evaluated.The results indicate that adding NS improved resistance to moisture transport due to its filling effect,of which the transport coef ficient decreased by up to 32.0%.While incorporating CNT exerted a negative effect with a significant increase in coefficient could be found at all dosages.This could be explained by the bridge effect,which increased the overall connectivity within the matrix.
Recycled concrete can meet the needs for sustainable and environment-friendly development in construction industry.Interfacial transition zone(ITZ)plays a decisive role in determining the performance of the recycled concrete.Nano-materials are used as a filler in recycled concrete to ameliorate its interfacial weak zone,which proved to be highly beneficial to mechanical properties,durability and microstructure[1-3].Du et al.[4]demonstrated that adding 0.9 wt% NS could contribute to densification of concrete pore structure by reducing large capillary pores(greater than 50 nm)by about 15% and increasing medium capillary pores(10～50 nm)by about 32%compared with the reference sample.While few literature has been published concerning the interfacial moisture transport property,which is responsible for deterioration of concrete durability.In this work,effects of nano-SiO2(NS)and carbon nanotubes(CNT)addition on interfacial moisture transport properties of recycled concrete were evaluated.
Ordinary Portland cement type 42.5R which conforms to Chinese standard GB 175—2007 was employed as the binder material.NS was incorporated into concrete at a dosage ranging from 0.2 wt% to 3 wt% and CNT at a dosage ranging from 0.05 wt%to 1 wt%.A fivephase parallel model of recycled concrete was proposed to measure the transport coefficient of ITZ,as shown in Figure 1.Water transport test was performed according to“Standard Test Methods for Water Vapor Transmission of Materials”[5].Average moisture transport coefficient of new ITZ can be determined using Eq.(1)
where DOM,DOI,DNM,DNI represent transport coefficient of old mortar,old ITZ,new mortar and new ITZ,respectively.VOM,VOI,VNM,VNI stand for volume fraction of old mortar,old ITZ,new mortar and new ITZ,respectively.
Figure 1 Five-phase parallel model of recycled concrete
Figure 2 shows typical moisture absorption plot of old mortar(OM)as an example.Initially,the mass of the specimens varied nonlinearly with the moisture absorption time.Subsequently,the weight gain curve tended to be linear,indicating that the transport process had reached a steady state at this time.The mass change rate can be characterized by the slope(k)obtained by linear fitting of 10 data points in this stage.
Figure 2 Typical moisture absorption plot with linear regression of old mortar
Moisture transport coefficients(relative value)of new ITZ of recycled concrete modified by NS and CNT are graphically presented in Figure 3.It can be found that samples with NS addition exhibited a lower transport coefficient compared with control sample.Adding 3 wt% NS exhibited an optimum effect regarding water absorption resistance,of which the coefficient decreased by 32.0%.The decreased coefficient probably arise from the filling effect of NS particles,which leads to densification of the microstructure and refinement of pore size distribution,especially in the interface transition zone,as a result,NSadded concrete samples exhibited a better resistance to water migration.The decreased coefficient can also be attributed to the reduced pore volume as well as more disconnected and tortuous transport channels within the matrix.Li et al.[6]indicated that the tortuosity factor may have a more significant effect on concrete permeability than porosity when the porosity is in excess of the percolation threshold,above which all capillary pores in the matrix are accessible to the transport process.
Figure 3 Moisture transport coefficient of new ITZ of recycled concrete modified by NS and CNT
It can be also found from Figure 3 that CNT modified recycled concrete exhibited a higher moisture transport coefficient than control sample at all dosages,suggesting a weaker resistance to moisture transport.This could be related to the bridging effect of CNT,which enhanced the overall connectivity of the matrix.Rhee et al.[7]stated that recycled concrete modified by carbon nanotubes presented a more loose interface transition structure,of which the bonding capability between new and old mortars was weak,thereby leading to more larger capillary pores.