Chloride Penetration Resistance Research Articles

Applicability of the Formation Factor for Different Alternative Binder Types Investigated on Mortars

The high energy demand and associated CO2 emissions to produce ordinary Portland cement (OPC) create the need for alternative types of binders that can be produced in a more environmentally friendly manner. These alternative binders either have a reduced OPC content or can be produced entirely without the use of OPC clinker. To allow their widespread use also in reinforced concrete components, these binders must meet the same durability requirements as types of cement that have already been standardized. This requires a thorough characterization of the materials and, in addition, a verification of the applicability of the evaluation criteria known from the traditional binders. One of the most important factors influencing the transport processes in the material and therefore the durability is the penetration resistance of the pore structure for aggressive species, such as chloride ions. One key parameter for describing the pore structure is the formation factor (FF), which is used for various applications to evaluate durability. It is determined based on the ratio of the electrical resistivity of the concrete and of the pore solution. A high resistivity also influences the corrosion process of depassivated steel reinforcement in concrete or mortar: by inhibiting ion transport, the corrosion rate may be limited. In this study, mortars produced with different alternative binders are characterized in terms of their FF and pore size distribution. In addition, chloride migration coefficients determined by the rapid chloride migration test are related to the FF. The test program includes nine different alternative binders and two reference cements. In addition to a CEM I and CEM III/B, two calcined clays and a modified steel mill slag were investigated as supplementary cementitious materials, as well as a calcium sulfoaluminate () cement and a calcium silicate hydrate (C-S-H) binder. Four materials belong to the group of alkali-activated mortars: Two geopolymers and two alkali-activated slags were investigated here. It is shown that the AC resistances and the conductivities of the pore solutions differ greatly among the mortars investigated. The FF is a suitable parameter for a simplified estimation of the chloride penetration resistance of the investigated alternative binders, provided that realistic values of the pore solution conductivities are determined and used for the calculations.

The role of internally incorporated nano-silica in recycled aggregate concrete: Modification of transport properties

This paper conducted an experimental investigation on the compressive strength, transport properties behavior and its microstructure of recycled aggregate concrete (RAC) internally modified with nano-SiO2 (NS). The capillary water absorption and chloride penetration resistance of RAC incorporating 0, 30%, 50% and 100% weight replacement of RCAs (namely, R0, R30, R50 and R100) with different NS dosages (0, 1%, 2% and 3% of cementitious materials) were experimentally investigated. The coupled action of NS and RCAs on the resistance to water absorption and chloride penetration into concrete were further discussed. Mercury intrusion porosimetry (MIP) was used to determine the pore structure of mortar in RAC and scanning electronic microscopy-energy dispersive spectrum (SEM-EDS) was selected to characterize microstructural properties of ITZs and mortar. The results indicate that the optimum dosage of NS was determined to be 3%. The 28-day compressive strength of R100 with 3% NS addition was nearly 1.15 time that of R100 without NS addition, and the initial sorptivity and chloride diffusion coefficient of R100 with 3% NS addition was 23.1% and 37.10% lower, respectively, than that of R100 without NS addition. This phenomenon can be attributed to filling effect, nucleation effect and pozzolanic effect of NS. The coupled action of the RCA content and NS dosages has a slightly influence on the sorptivity and chloride diffusion coefficient. The initial sorptivity of R100 with 3% NS addition was about 1.47 time as much as that of R0 without NS addition. The pore structure of specimens is refined by NS according to the obtained MIP results, and a proportion of harmful pores and less harmful pores are transformed into harmless pores. In addition, the microscale analyses suggest that NS promises an optimization of ITZ structures and compacts the mortar matrix, thus improving the mechanical and durability-related properties of RAC.

Mechanical Properties and Durability of Geopolymer Recycled Aggregate Concrete: A Review.

Geopolymer recycled aggregate concrete (GPRAC) is a new type of green material with broad application prospects by replacing ordinary Portland cement with geopolymer and natural aggregates with recycled aggregates. This paper summarizes the research about the mechanical properties, durability, and microscopic aspects of GPRAC. The reviewed contents include compressive strength, elastic modulus, flexural strength, splitting tensile strength, freeze-thaw resistance, abrasion resistance, sulfate corrosion resistance, and chloride penetration resistance. It is found that GPRAC can be made to work better by changing the curing temperature, using different precursor materials, adding fibers and nanoparticles, and setting optimal mix ratios. Among them, using multiple precursor materials in synergy tended to show better performance compared to a single precursor material. In addition, using modified recycled aggregates, the porosity and water absorption decreased by 18.97% and 25.33%, respectively, and the apparent density was similar to that of natural aggregates. The current results show that the performance of GPRAC can meet engineering requirements. In addition, compared with traditional concrete, the use of GPRAC can effectively reduce carbon emissions, energy loss, and environmental pollution, which is in line with the concept of green and low-carbon development in modern society. In general, GPRAC has good prospects and development space. This paper reviews the effects of factors such as recycled aggregate admixture and curing temperature on the performance of GPRAC, which helps to optimize the ratio design and curing conditions, as well as provide guidance for the application of recycled aggregate in geopolymer concrete, and also supply theoretical support for the subsequent application of GPRAC in practical engineering.

A time-variant model of chloride diffusion in prestressed concrete cylinder pipe (PCCP) considering the effects of curing age

Chloride penetration is considered as the most responsibility to steel corrosion for PCCP during service. Compared with ordinary concrete structures, the curing method of PCCP is generally steam-curing. Despite numerous contributions have shown that curing age has significant effect on the resistance to chloride penetration of concrete. There is still a lack of critical understanding of the curing age effect on the chloride migration in steam-cured concrete. For this reason, this paper aims to establish a chloride diffusion model suitable for PCCP. In this study, the non-steady-state chloride diffusion coefficient and chloride concentration of steam-cured concrete are measured through RCM test and chloride diffusion simulated test, respectively. The apparent chloride diffusion coefficient and surface chloride concentration are calculated based on Fick’s second law. Moreover, the instantaneous chloride diffusion coefficient was estimated. It is found that the steam-cured concrete cannot ignore the influence of curing age on the chloride penetration resistance, according to RCM test results, the non-steady-state chloride diffusion coefficient of 28 days decreased by 33 % compared with that of 3 days, while the chloride diffusion coefficient of 90 days decreased by 15 % compared with 28 days under steam curing. The larger the curing age, the smaller the chloride concentration at each depth, as well as the chloride diffusion coefficient and surface chloride concentration. In addition, it can be concluded that with the increase of exposure time, the chloride diffusion coefficient decreases gradually, while the surface chloride concentration increases gradually. Thus, the two kinds of test results and theoretical analysis were combined to establish the time-variant models of instantaneous chloride diffusion coefficient and surface chloride concentration of steam-cured concrete considering the effects of curing age and exposure time. On this basis, a new chloride diffusion model considering the effects of curing age, exposure time, the time-variant chloride diffusion coefficient and time dependence surface chloride concentration was established, which plays an important role in durability evaluation of PCCP.

Durability properties of metakaolin based geopolymer polypropylene fibre reinforced concrete

As the modernization is taking place these days, adaption to the new innovations are going on, which is also true for the civil construction sector. Conventional concrete i.e. Oridinary Portland Cement (OPC) has a very high carbon dioxide (CO2) emission nearly from 5%−7% in its production, leading to the major by-product pollution as greenhouse gases. Due to these back falls, need of eco-friendly and sustainable material like Geopolymer (GP) comes into place, however, even geopolymer exhibits low tensile strength and quasi-brittle behaviour which needs to be improved by adding or replacing certain components such as Meta Kaolin (MK) and Polypropylene Fibres (PPF). Also it consumes less energy and is cost effective and also helps in low emission of greenhouse gases. The durability properties of the Meta kaolin-based geopolymer (MKG) with the inclusion of polypropylene Fibres (PP) are elaborately discussed in this research paper. This Geopolymer Fibre Reinforced Concrete (GPFR) offers myriad of advantages. Sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) are used as the alkali activator solution in this for geopolymerization process. With the inclusion of polypropylene fibres, drying shrinkage decreases and various other factors like chloride penetration and temperature resistance increases. Nevertheless, in all these years only limited study has been done in this particular area. Various tests are to be investigated like Ultrasonic Pulse Velocity test (UPV), water absorption test etc for finding its different aspects in different environmental conditions.

Influence of Low- to Medium-Kaolinite Clay on the Durability of Limestone Calcined Clay Cement (LC3) Concrete.

The kaolinite content is principally responsible for the durability performance of Limestone Calcined Clay Cement (LC3), which calls into question its global applicability. The clay supply has a significant impact on the LC3 system's reduced carbon footprint advantage. The influence of kaolinite concentration from two separate clays (collected in East South-East Europe) on the durability performance of concrete was investigated in this study. The low-kaolinitic clay had 18% kaolinite, while the medium-kaolinitic clay contained around 41% kaolinite. The compressive strength, chloride intrusion, electrical conductivity, surface resistivity, and sorptivity index were measured on concrete after 28 days. Furthermore, the pore structure development of these mixtures was investigated in relation to the kaolinite content of the mixtures. The reactivity test was performed on clays to measure their reactivity levels within the cementitious system. The results show that kaolinite content has a moderate effect on compressive strength, but it has a considerable effect on other durability indices. When compared to the Portland cement mixture, the chloride migration and diffusion coefficients were reduced by 50% and 36%, respectively, in the combination with a medium kaolinite content (more than 40%). The low-kaolinitic clay, on the other hand, achieved 60% of the chloride penetration resistance of the medium-kaolinitic clay. Furthermore, low-kaolinitic clay has been demonstrated to be suitable for low-carbon concrete in moderate exposure conditions.

Enhancing the corrosion resistance of recycled aggregate concrete by incorporating waste glass powder

Conventional supplementary cementitious materials would decrease the alkalinity of pore solution, which is not beneficial for the stabilization of passive film on the surface of steel bars in the marine environment. By leverage of the alkali-rich and pozzolanic characteristics, this study first valorized waste glass powder (GP) to enhance the corrosion resistance of recycled aggregate concrete (RAC). The results showed that partially replacing cement with GP could improve the steel corrosion resistance of RAC to a level better than conventional concrete prepared with natural aggregates. Moreover, the presence of 20% GP significantly enhanced the chloride penetration resistance at a later age without sacrificing the compressive strength. The enhanced corrosion resistance of GP-RAC was attributed to two main reasons: i) the refined pore structure by the formation of secondary C-(N)-S-H gels with a lower Ca/Si ratio; ii) the increased alkalinity of pore solution due to the depolymerization of GP and the released Na+ from GP. As a result, the use of GP in low-carbon cement was able to compensate for the inferior durability of RAC. The proposed corrosion-resistant concrete with low ecological and economic costs has a promising application potential to promote the wider use of GP and RAC in the construction sector.

Study on mechanical properties, freeze–thaw and chlorides penetration resistance of alkali activated granulated blast furnace slag-coal gangue concrete and its mechanism

In this paper, the durability and strength of coal gangue (CG) based geopolymer concrete by adding granulated blast furnace slag (GBFS) were studied systematically. The effects of the GBFS content and alkali activator modulus on the fluidity, strength, freeze–thaw resistance, chlorides penetration resistance, and microstructure of alkali activated GBFS-CG were analyzed. The results showed that the freeze–thaw resistance of geopolymer concrete can be improved by more than two times with proper GBFS content and alkali activator modulus. The deterioration of pores caused by freeze–thaw attack was the main reason for the reduction of strength and chloride penetration resistance.

Compressive Strength and Chloride Resistance of Slag/Metakaolin-Based Ultra-High-Performance Geopolymer Concrete

Ultra-high performance geopolymer concrete (UHPGC) has been favored due to its excellent sustainability and outstanding mechanical properties. This study was conducted to explore the mechanical and durability properties of slag/metakaolin-based UHPGC with steel fibers reinforcement. The uniaxial compression test and rapid chloride migration test were conducted to measure the compressive strength and chloride penetration resistance of UHPGC. A total of nine groups of mixture proportions were designed and tested to investigate the influences of steel fiber dosage and sodium hydroxide (NaOH) solution concentration. The results showed that an increased steel fiber dosage and alkali concentration can improve compressive strength, and the maximum compressive strength can reach more than 140 MPa. In addition, the rapid chloride migration test showed that the chloride penetration resistance of the slag/metakaolin-based concrete was moderate, with a non-steady chloride migration coefficient ranging from 6.5 × 10-12 m2/s to 14.1 × 10-12 m2/s. The increase in steel fiber volume content slightly enlarged chloride penetration depth, while the higher concentration of sodium hydroxide solution was beneficial as it improved chloride penetration resistance. The results suggest that although ultra-high compressive strength can be achieved, the durability issues of steel fiber reinforced slag/metakaolin-based geopolymer concrete still need considerable attention.

Synergetic effect of fly ash and ground-granulated blast slag on improving the chloride permeability and freeze–thaw resistance of recycled aggregate concrete

In this study, the synergetic effects of fly ash (FA) and ground-granulated blast slag (GGBS) on the compressive behavior and durability of recycled aggregate concrete (RAC) were comprehensively investigated. FA and GGBS were incorporated separately or simultaneously into the RAC containing 0%, 35%, and 50% recycled coarse aggregate (RCA) to replace partial cement. Moreover, the frost resistance and chloride penetration resistance of RAC, respectively, were experimentally determined by the rapid freeze–thaw cycle test and rapid chloride permeability test (RCPT). Results showed that an optimum replacement ratio could exist for improving the properties of RAC by partially replacing cement with FA and GGBS. The optimum ratio of the simultaneous incorporation of FA and GGBS to replace partly cement for improving the 90-day-compressive strength and the chloride penetration resistance of RAC were 10% and 20%, respectively, but for enhancing the frost resistance of RAC were 15% and 15%. In addition, the synergetic effects of simultaneously incorporating FA and GGBS were more effective than the effects of separately incorporating FA or GGBS on enhancing the performance of RAC. When adopting the simultaneous incorporation of 15% FA and 15% GGBS, the 90-day-compressive strength, chloride penetration resistance, and frost resistance of RAC incorporating 50% RCA were better than those of ordinary concrete.

Effect of pretreated biomass fly ash on the mechanical properties and durability of cement mortar

As the reduction of CO2 emissions is set as an international goal, the use of biomass is increasing. However, the large amount of ash generated after using biomass is an important issue to deal with. The chemical composition of some biomass fly ash is similar to that of C-class fly ash, so it is suitable for use as supplementary cementitious materials, and many analyses have been performed in existing studies. However, due to its high porosity and unburned particles, it is insufficient as a complete alternative material for cement. Therefore, this paper conducted a study on the mechanical properties and durability of mortar including BFA by improving the irregular particle structure of BFA and removing unburned particles by applying two processes. Specimens were manufactured by mass substitution 10%, 20%, and 30% of BFA for cement, and a water reducer was added for improving the workability. As a result, mortar containing pretreated BFA (PBFA) had significantly improved strength and chloride penetration resistance compared to mortar containing untreated BFA (UBFA). The average pore size of the specimen including PBFA was greatly reduced, and the untreated ash showed an irregular average pore size depending on the replacement rate. Thus, this study showed that pretreatment on BFA could have a positive effect on mechanical properties and durability.

Performance of Fly Ash Concrete with Nickel Slag Fine Aggregate in the Marine Environment

This research aims to assess the feasibility of the mechanical strength and the durability of the concrete containing 50% nickel slag and a combination of 15% and 30% fly ash with a water-cement ratio of 0.25 and 0.45 in a marine environment. Four types of concrete, namely OPC-sand (C) as control concrete, OPC-50GNS (S), 15FA-50GNS (F1), and 30FA-50GNS (F2) as comparison concrete, were tested with a 100×200 mm cylindrical specimen. The results showed an increase in the mechanical strength and potential resistance of the comparison concrete at the age of 28 days. While at the age of 180 days, fluctuating changes were found. The compressive strength of S concrete increased by 36.9 and 9.3% respectively, F1 concrete by 37.7% and 1.7%, F2 by 33.7% and 5.9% at ratio 0.45 and 0.25. Likewise, the value of the split tensile strength and modulus of elasticity of concrete. This result was followed by reduced porosity, sorptivity, and chloride penetration resistance as an indication of better concrete durability. Fly ash appears to have a greater positive impact on potential durability than mechanical strength at a water cement ratio of 0.25 versus 0.45. Although the chloride penetration resistance is decent, the compressive strength of concrete with a water-cement ratio of 0.45 does not qualify for application in the marine environment. In contrast, concrete with a water-cement ratio of 0.25 containing 50% nickel slag and the addition of class C fly ash up to 30% was declared suitable for application to concrete in the marine environment zone C2 according to ACI 318-19. Doi: 10.28991/CEJ-2022-08-12-010 Full Text: PDF

Influence of supplementary cementitious materials on ITZ characteristics of recycled concrete

The poor performance of recycled aggregate concrete (RAC) limits its application on wide filed. This paper attempted to study the influence of fly ash (FA), slag powder (SP) and silica fume (SF) on the performance of RAC from a microscopic point of view. The compressive strength and chloride resistance were evaluated when FA, SP and SF were used. Then the evolution of ITZ performance with different admixtures was observed by backscattering scanning electron (BSE) microscopy image, Mercury intrusion porosimetry (MIP), microhardness and so forth.Results indicate that, (1) the supplementary cementitious materials (SCMs) can improve the compressive strength and chloride penetration resistance of RAC, with a certain dosage range for different SCMs; (2) the SCMs can refine thepore structure of ITZs for RAC, increase the microhardness and modify the phases around ITZs; (3) the three SCMs both fill the cement paste voids and promote the compactness of ITZ structure by reacting with calciumhydroxide to generate CSH gels, which in turn enhance the macroscopic performance of RAC. The results of this research can provide a meaningful reference for further study of SCMs to improve the ITZs structure of RAC.

Transfer Parameter Analysis of Chloride Ingress into Concrete Based on Long-Term Exposure Tests in China’s Coastal Region

Chloride penetration resistance is one of the most important performance measures for the evaluation of the durability of concrete under a chloride environment. Due to differences in theory and experimental conditions, the durability index (chloride diffusion coefficient) obtained from laboratory accelerated migration tests cannot reflect the real process of chloride ingress into concrete in the natural environment. The difference in test methods must be considered and the transfer parameter kt should be introduced into the service life prediction model when the test results of accelerated methods are used. According to the test data of coastal exposure in South China, the attenuation rule of the chloride diffusion coefficient of different cement-based materials changed with time and was analyzed in this paper. Based on the diffusion coefficient-time curve, the theoretical natural diffusion coefficients of 28 d and 56 d were deduced, which were compared with the chloride diffusion coefficients obtained from the non-steady-state rapid migration method (RCM) at the same age. Therefore, the transfer parameter kt that expounds the relationship between concrete resistance to chloride permeability under a non-stationary electrical accelerated state and natural diffusion in the marine environment can be calculated; thus, the RCM testing index can be used to evaluate the long-term performance of the concrete structure in the marine environment. The results show that the value of kt was related to environmental conditions, test methods, and binder systems.

A Review of the Durability-Related Features of Waste Tyre Rubber as a Partial Substitute for Natural Aggregate in Concrete

As the number of discarded tyres continues to increase, causing serious environmental problems, the need of recycling the waste tyre rubber become extremely urgent in worldwide. Today, there is an increasing focus on recyclable materials. The reuse of waste tyre rubber in concrete contributes to sustainable development. In the past 10 years, numerous experiments on the recovery of rubber from waste tyres to produce concrete products have been conducted. In this review, we conclude the major achievement of rubberized concrete (RC) durability, discuss and analyse the influence of rubber replacement rates, replacement patterns, particle size and treatment methods. Results show that an increase in rubber content can improve the chloride penetration resistance, acid and sulphate attack resistance, freeze–thaw resistance, and alkali–silica reaction damage resistance of concrete, and the content of 5–20% has a significant improvement effect. Rubber replacing fine aggregate is the best scheme for durability, followed by cement and coarse aggregate. In addition, the recommended rubber particle size is 0–3 mm. However, the rubber particle has adverse effects on abrasion resistance, impermeability, water absorption resistance and carbonation resistance. The pre-treatment of rubber or the addition of supplementary cementitious materials are effective and viable ways of improving the durability of RC. Further research is needed on the long-term durability of RC, as well as on ductility, energy absorption, and thermal and corrosion resistance.

Sustainable utilization of hybrid recycled powder and recycled polyethylene terephthalate fiber in mortar: Strength, durability and microstructure

The effects of hybrid recycled powder (HRP) from composite concrete and brick waste as well as recycled polyethylene terephthalate fiber (RPETF) on the performance of mortar were investigated in this study. Recycled brick powder (RBP) was used as comparison to evaluate the influence of HRP. The mechanical properties, including compressive strength, flexural strength and elastic modulus as well as durability, including capillary water absorption, chloride penetration resistance, carbonation resistance and frost resistance were tested. The hydration products and micro pore structure of mortar were studied to reveal the influence mechanism of HRP and RPETF. The results show that the 28 d compressive and flexural strength of mortar with 30% HRP reached 78.60% and 80.36% of that of cement mortar, respectively, which was similar to that of RBP. The addition of HRP reduced the capillary water absorption, chloride ion permeability coefficient of mortar. This is related to the refined pore structure of mortar. The proportion of pores with the diameter lower than 100 nm increased due to the filling effect, nucleation effect and secondary hydration of HRP and RBP. When the volume fraction of RPETF was in 2%, the mechanical properties of mortar increased with the increase of RPETF content. The 28 d compressive strength and flexural strength of mortar with 2% RPETF were 14.25% and 22.50% higher than that without RPETF, respectively. The utilization of RPETF improved the durability of mortar with HRP. A greater reduction of mass loss and increase of dynamic elastic modulus of mortar with RPETF compared to that without RPETF after freeze-thaw cycles were obtained.

Effects of CEA on the microstructure evolution and chloride ion migration of marine concrete

In this paper, autogenous shrinkage, compressive strength, microstructure evolution and also chloride migration coefficient of marine concrete with CaO-based expansive agent (CEA) were investigated. The results showed that the autogenous shrinkage of marine concrete was effectively compensated by adding CEA, especially in the early stage. However, as the CEA dosage increased to 12%, re-expansion of concrete in the later stage was observed to cause the problem of poor volume stability of marine concrete. The re-expansion of 12% CEA was due to the rehydration of unreacted CEA. While, Ca(OH)2 produced by CEA hydration accelerated the hydraulic reaction and pozzolanic reaction the of slag in the later stage. As a result, more C-S-H gel was generated to fill the pore and micro-cracks and refine the pore structure, which can partially repair the inner damage of marine concrete in the later stage. Although the chloride migration coefficient gradually increased and the compressive strength decreased with the increase of CEA dosage, the self-healing of partial damage for marine concrete made the chloride ion more difficult transport into the marine concrete. Thus, the impact of CEA dosage on chloride migration coefficient of marine concrete was significantly weakened in the later stage. When the dosage of CEA was no more than 6%, the marine concrete behaved a good volume stability and chloride penetration resistance in the later stage.

Prediction of Rapid Chloride Penetration Resistance to Assess the Influence of Affecting Variables on Metakaolin-Based Concrete Using Gene Expression Programming.

The useful life of a concrete structure is highly dependent upon its durability, which enables it to withstand the harsh environmental conditions. Resistance of a concrete specimen to rapid chloride ion penetration (RCP) is one of the tests to indirectly measure its durability. The central aim of this study was to investigate the influence of different variables, such as, age, amount of binder, fine aggregate, coarse aggregate, water to binder ratio, metakaolin content and the compressive strength of concrete on the RCP resistance using a genetic programming approach. The number of chromosomes (Nc), genes (Ng) and, the head size (Hs) of the gene expression programming (GEP) model were varied to study their influence on the predicted RCP values. The performance of all the GEP models was assessed using a variety of performance indices, i.e., R2, RMSE and comparison of regression slopes. The optimal GEP model (Model T3) was obtained when the Nc = 100, Hs = 8 and Ng = 3. This model exhibits an R2 of 0.89 and 0.92 in the training and testing phases, respectively. The regression slope analysis revealed that the predicted values are in good agreement with the experimental values, as evident from their higher R2 values. Similarly, parametric analysis was also conducted for the best performing Model T3. The analysis showed that the amount of binder, compressive strength and age of the sample enhanced the RCP resistance of the concrete specimens. Among the different input variables, the RCP resistance sharply increased during initial stages of curing (28-d), thus validating the model results.

A two-step approach for calculating chloride diffusion coefficient in concrete with both natural and recycled concrete aggregates

This paper presents an analytical approach to calculate the effective diffusion coefficient of chlorides in concrete with both natural and recycled concrete aggregates. In the approach the concrete is treated as a composite consisting of three phases, namely mortar, natural aggregate plus interfacial transition zone, and recycled concrete aggregate plus interfacial transition zone. The effective diffusion coefficient of chlorides in the composite is calculated through two steps. The first step is to calculate the effective diffusion coefficients of chlorides in the natural aggregate plus interfacial transition zone and in the recycled concrete aggregate plus interfacial transition zone by using multilayer spherical approximation, the results of which provide the information about the quality of recycled concrete aggregate in terms of chloride penetration resistance. The second step is to calculate the effective diffusion coefficient of chlorides in the three-phase concrete composite by using effective medium approximation, the results of which provide the information about the influence of recycled concrete aggregate on the diffusivity of recycled aggregate concrete. The analytical expression of the effective diffusion coefficient is derived and carefully compared with the results obtained from both the experiments and numerical simulations, which demonstrates that the present analytical model is rational and reliable. The analytical expression presented can be used to predict the service life of recycled aggregate concrete exposed to chloride environment.

Prediction of the durability of high-performance concrete using an integrated RF-LSSVM model

In extremely cold and complex marine environments, strong concrete chloride ion penetration resistance is the key to ensuring the durability of buildings (structures). Quickly and accurately predicting the resistance of concrete to chloride penetration is critical to optimizing the concrete mix proportions. A hybrid intelligent prediction model that integrates random forest (RF) and least squares support vector machine (LSSVM) algorithms is proposed to rapidly and accurately predict the resistance of high-performance concrete (HPC) to chloride penetration. The initial index system of HPC chloride penetration resistance was established from substantial research and practical projects. An RF is employed to screen the initial index parameters and to provide an optimal index set for predicting concrete resistance to chloride penetration. Based on a sample data set, the LSSVM is implemented to establish high-precision predictions regarding the resistance of concrete to chloride penetration. The results indicate that (1) the RF method effectively screens important indicators and provides the optimal data set for the prediction of concrete resistance to chloride penetration and (2) the developed RF-LSSVM hybrid intelligent approach can effectively and accurately predict the resistance of concrete to chloride penetration. For the test set, the RMSE and R2 of the prediction model reach 0.0491 and 0.941, respectively, representing accuracy better than that typical for machine learning algorithms. The proposed RF-LSSVM hybrid intelligence model can provide a basis for optimizing the concrete mix proportion and can be applied in practical projects to help solve similar problems.