Influence of axial loads on CO2 and Cl− transport in concrete phases: Paste, mortar and ITZ

Abstract

In most studies concerning the influence of external loads on transport properties of concrete, 12concrete is assumed to be homogeneous and hence, differences in the transport properties of its components, 13which have various physicochemical properties, are not examined. The differences would be further changed, 14when there is load on the concrete. Therefore, the results obtained can seldom explain changes in the transport 15properties due to the applied loads and neither can they further enhance existing transport knowledge. An 16experimental study on carbonation and Clingress in concrete components, viz., paste, mortar and the interface 17 transition zone (ITZ), subjected to different levels of non-destructive axial tensile and compressive loads, was 18 carried out. The results obtained indicate that there was an approximately linear relationship between the non-19 destructive load and the depth of carbonation. The external load had a significant effect on the CO2 and 20 chloride transport process in the ITZ, where a large amount of calcium hydroxide crystals can be found; for 21 the mortar and ITZ samples, the influence of load on the depth of Clingress was more significant than on the 22 carbonation, whereas the influence of the load on the ingress of CO2 and Clwas similar in the interface-free 23 paste sample. It is also found that comparing to tensile loads, compressive loads inhibited the transport of CO2 24and Clat the later stage of exposure. 25 26

The Effect of Loading on the Diffusivity of Chlorides in Mortar

“…Thus, no detrimental effect was detected in case the load was maintained at the 30%F u but a larger diffusivity was obtained when the load remains at 60%F u , in agreement with the predictions provided by other authors [8,33]. A recent publication has revealed the opposite result, concluding a lower diffusivity under longer-term (up to eight weeks) compressive loads below 40%F u [16]. This apparent contradiction is likely related to an enhanced chemical interaction between the chloride ions and the aluminates in the hardened cement phase when a compressive load is sustained for a longer period, as already suggested in this publication [16].…”

“…This observation, although in a slightly different context, was also observed in the previous investigation by Gu et al, where a densification of the porous matrix was suggested [25,30]. The reduction in the pore connectivity has also been suggested by Wang et al who studied the chloride diffusivity under a small compressive force [16]. Then, two opposite factors must be considered when the mortar specimens are under the influence of a compressive force for some time: New conduction paths are created, and some paths are then obstructed.…”

“…A recent publication has revealed the opposite result, concluding a lower diffusivity under longer-term (up to eight weeks) compressive loads below 40%F u [16]. This apparent contradiction is likely related to an enhanced chemical interaction between the chloride ions and the aluminates in the hardened cement phase when a compressive load is sustained for a longer period, as already suggested in this publication [16]. The resulting product of this reaction blocks the capillary pores [24] and finally contributes to the referred diffusivity reduction.…”

“…A reduction in the time required for the corrosion initiation was proved but the arduous methodology used makes almost impossible the transference to the real structure monitoring. On the contrary, other authors pointed to a reduced chloride penetration depth obtained under a long-term compressive force [16]. A clear correlation between the mechanical action and the chloride diffusion has not been established so far and, thus, research is still required to clarify the combined effect of both variables.…”

Effect of the Mechanical Load on the Carbonation of Concrete: A Review of the Underlying Mechanisms, Test Methods, and Results

“…The threshold for carbonation under sustained compressive load seems to be higher than the threshold for gas and water permeability. When comparing with unloaded specimens, the carbonation rate under sustained compressive load is still lower when 0.4 [ 46 ], 0.5 [ 47 , 48 ], and even 0.75 [ 49 ] of the failure stress is reached. On the one hand, the pressure during a gas permeability test is much higher than the atmospheric pressure during the carbonation test, which will absolutely cause a discrepancy in the results.…”