Long-term Durability Of Concrete Structures Research Articles

Characteristics of non-steady-state chloride migration of self-compacting concrete containing recycled concrete aggregate made of fly ash and silica fume

Chloride migration poses a significant challenge to the long-term durability of concrete structures due to its potential to corrode reinforcement and deteriorate the structure. This study focuses on chloride migration in self-compacting concrete (SCC) made with pozzolans, namely silica fume (SF) and fly ash (FA), along with recycled concrete aggregate (RCA). Particularly, the effects of external electrical potential (power-on) duration and RCA content on the features of SCC were examined, alongside the influence of pozzolanic materials, focusing on the chloride migration coefficient and mechanical properties. Six groups of SCC mixtures were created with varying proportions of SF and FA, and different levels of RCA replaced the natural coarse aggregate. The findings reveal a multifaceted impact on SCC behaviour. The chloride migration coefficient exhibits a distinct pattern as the power-on duration increases from 12 to 36 h. Initially, the coefficient increases and then decreases, demonstrating a healing and sealing mechanism within the concrete matrix, which enhances resistance to chloride penetration. The chloride migration coefficient significantly decreased with adding SF and FA. With 15% SF and 30% FA, the coefficient decreased to 59.9% and 49.5%, respectively. Moreover, incorporating RCA in SCC mixtures significantly influences coefficient levels. RCA inclusion at 20% remarkably decreased the coefficient after 12 h by 16.17%. Subsequently, the levels decreased by 39.49% and 46.87% after 24 and 36 h, respectively, compared to the control mixture. This behaviour highlights the importance of RCA in enhancing the resistance of SCC to chloride ingress, a crucial factor for long-term durability. Additionally, mechanical properties such as splitting tensile strength, ultrasonic pulse velocity and surface hardness consistently improved with longer power-on durations and higher RCA and SF content. SCC with RCA and pozzolanic materials exhibits enhanced chloride resistance and superior mechanical performance, making it ideal for sustainable concrete production.

Precise control mode for concrete vibration time based on attention-enhanced machine vision

In concrete construction, vibration is an essential process used to ensure the structural soundness and long-term durability of concrete structures. Automating concrete vibration monitoring enables far more precise control compared to subjective human judgments. In this paper, a control mode for concrete vibration time used on the vibrating robot is proposed. The control mode utilizes YOLOv8 model to recognize the best-vibrating position and eliminate the rebar part of concrete surface image and employs attention-enhanced squeeze-and-excitation neural network to regress the vibrating completion degree. After deployment to the embedded system of vibrating robot, experimental results demonstrate superior performance of the proposed method over state-of-the-art algorithms with the highest recognition rate and maximum R2 value. The control mode enhances vibration quality while liberating workers from manual tasks. A commercial mall construction project validates the practicality of the presented control model.

Performance recovery of high-temperature damaged ultra-high-performance concrete under different curing environments

The self-healing capability and susceptibility to salt solution erosion in ultra-high-performance concrete (UHPC) significantly influence the mechanical properties and long-term durability of concrete structures within coastal or underground environments. Acknowledging the intricacies of real-world environments, this study focuses on UHPC that has endured high-temperature damage at 800 °C. The research aims to analyze the property variations within 28 days following exposure to diverse dry and wet conditions, as well as salt solution environments. The findings demonstrate a substantial recovery in both mechanical and transport properties of the high-temperature-damaged UHPC, attributed to its inherent self-healing capability. Among the dry and wet conditions, the optimal performance recovery of UHPC specimens is observed when they are subjected to submerged and immersed environmental conditions. Furthermore, when immersed in a salt solution environment, the beneficial impacts attributed to the self-healing property tend to outweigh the detrimental effects of salt solution erosion. All scenarios exhibit a trend of enhanced properties, with the presence of chloride salts being particularly conducive to the recovery process. However, exposure to a sulfate-rich environment induces deterioration in transport properties and pore structure, resulting in a partial incongruity with the mechanical property results. Microscopic analysis techniques reveal that the primary contributors to enhanced properties encompass a range of hydration products formed due to the interaction between water, salt solutions, and the compromised UHPC. Nevertheless, owing to the intricate nature of the environment, disparities arise in both the type and quantity of hydration products, contributing to variations in the extent of property alterations.

Corrosion protection of steel bar enabled by monofluorophosphate-bearing composites with sustained-release capability

This paper selected the polyethylene (PE) and polystyrene (PS) as polymer carriers of the sodium monofluorophosphate (MFP) to prepare PE/MFP and PS/MFP composites using melt-extrusion technique with a sustained-release capacity for smart corrosion protection. The releasing behavior of two MFP-containing composites in the simulated concrete pore solution was evaluated in terms of residual content, releasing rate and the side/fractured surface micro-morphologies. Besides, related releasing mechanisms of MFP-bearing polymer composites are explained, which are different from the mechanisms of most microcapsules and tablets. The experimental analyses showed that two composites all displayed a uniform elemental distribution on their side surface and intrinsic hydrophobicity. The PE/MFP composites obtained higher initial encapsulation efficiency and a lower but relatively controllable releasing rate compared to the PS/MFP composites due probably to the higher hydrophobicity and crystallinity of the PE carrier. In addition, the physical appearance of the corroded carbon steel bar after immersion in sodium chloride-contaminated simulated concrete pore solutions and corresponding electrochemical outcomes revealed that PE/MFP composites generally led to better corrosion protection performance than the PS/MFP counterpart, which could be due to their better sustained-release capacity. The obtained composites provide another promising option as concrete inhibitors, making inroads into improving the long-term durability of concrete structures.

In-situ shrinkage measurement of alkali-activated materials using focused ion beam combined with environmental scanning electron microscopy

Drying shrinkage of alkali-activated slag (AAS) has gained significant attention since the volumetric instability of this material can generate premature cracking and degrade the long-term durability of concrete structures. The unique shrinkage behavior of AAS originates mainly from the particular characteristics of its main hydrated products. However, few studies have even at-tempted to investigate the shrinkage behavior of hydrated products in AAS materials. This paper presented a new method of investigating drying shrinkage behavior of AAS using focused ion beam (FIB) combined with environmental scanning electron microscopy (ESEM). This innovative experimental method allowed the in-situ measurement of phase-specific shrinkage. The results showed FIB/ESEM technique can be successfully implemented to cementitious material to prepare phase-specific samples. Furthermore, object-oriented finite element method (OOF2) has been utilized to compute the shrinkage behavior of AAS concrete using a proposed multi-scale scheme. The computations can be made at multi-scales which highlight the effect of hydration products on shrinkage behaviors. OOF2 calculation at the micro-length scale predicted an increase in the shrinkage due to the well-dispersed hydration products inside the matrix. At larger scale, the decrease in the overall shrinkage is related to the extremely low shrinkage value by aggregates. The findings reveal OOF2 offers the capability to accurately measure local stress and strain distribution within heterogeneous materials, a feature notably absent in conventional numerical homogenization approaches. The importance of this benefit is highlighted particularly in the context of free-restrained shrinkage prediction, as conventional homogenization methods fail to account for stress.

Calcium Leaching of 3D-Printed Cement Paste Exposed to Ammonium Chloride Solutions

Understanding the degradation of 3D-printed cement paste (3DPC) is essential to evaluate changes in the long-term durability of concrete structures subjected to aggressive water. However, the degradation mechanism of 3DPC has yet to be reported, as the microstructure and pore characteristics of 3DPC are very different from those of its cast counterparts. This work studies the performance anisotropy of 3DPC due to calcium leaching to investigate the degradation mechanism. Samples with aggregate micro fines (AMF) and fly ash (FA) were prepared. A 6 mol/L NH4Cl solution was used in the accelerated experiment. At specific leaching durations, performances were tested on the samples in different leaching directions. The performance anisotropy of 3DPC exposed to aggressive water was investigated by comparing the changes in bulk density, water absorption, leaching depths, and compressive strength in different leaching directions. X-ray diffraction (XRD), differential thermal gravity-thermogravimetric (DTG−TG), and mercury intrusion porosimetry (MIP) were used to characterize the changes in hydration products and pore structure in different leaching directions. The results show that the performances of 3DPC in aggressive water have a significant anisotropic behavior. The evolution of pore defects and hydration products mainly governs the performance anisotropy of degraded 3DPC. The remaining hydration products of the surface of 3DPC leached in the Z direction are fewer than the other two directions because calcium ions leached in the Z direction are difficult to compensate through weak interfaces between layers. The test results clearly show that the calcium leaching mechanism of 3DPC in aggressive water is directly influenced by the hydration products, such as portlandite (CH) and C-S-H, and the pores. The current study may help us understand the degradation mechanism of 3DPC to assess its durability performance anisotropy.

Transient Nonlinear Heat Conduction in Concrete Structures: A Semi-Analytical Approach.

Thermal loading, especially in fire scenarios, challenges the safety and long-term durability of concrete structures. The resulting heat propagation within the structure is governed by the heat conduction equation, which can be difficult to solve analytically because of the nonlinearity related to the thermophysical properties of concrete. A semi-analytical approach for the transient nonlinear heat conduction problem in concrete structures was established in the present work. The nonlinearity related to the temperature-dependent thermal conductivity, mass density, and specific heat capacity of heated concrete was taken into consideration. A Taylor series approximate solution was first established within a small neighborhood, employing the Boltzmann transformation in combination with the mean value theorem. Thereafter, it was extended to the whole domain by utilizing the Bernstein polynomial. The semi-analytical approach was validated by comparing it with the numerical results of two independent Finite Element simulations of nonlinear heat conduction along concrete plates, subjected to either moderate or fierce thermal loading. Absolute values of the relative errors are smaller than 5%. The validated semi-analytical approach was further applied to prediction of the temporal evolution of the temperature field of a scaled model of a subway station, subjected to fire disaster. The nonlinearities, related to the time-dependent surface temperature and the temperature-dependent thermophysical properties of concrete, were taken into consideration. The predictions agree well with the experimental measurements. The established semi-analytical approach exhibits good accuracy and stability, providing insight into the interaction between the thermophysical properties of concrete in the heat conduction process.

Prediction the Chloride Ion Permeation Coefficient of Concrete Based on A Hybrid Intelligent Algorithm

Chloride ion penetration resistance (CIPR) is a critical concern in engineering to ensure the long-term durability of concrete structures, accurately predicting concrete CIPR is essential for designing the appropriate mix ratio. The rapid chloride migration (RCM) test is the most commonly used experimental method, typically employed to measure CIPC. To efficiently and accurately predict the CIPR of concrete, a Bayesian Optimization (BO)-Light Gradient Boosting Machine (LGBM) model is developed. Through this research, it can be concluded that (1) BO can effectively search and optimize the hyperparameters in LGBM. Within 100 iterations, BO optimization can search the hyperparameters effectively and find the optimal solution quickly.(2) BO-LGBM has a strong predictive ability, and its prediction accuracy is superior than the other three prediction models. The outcomes indicate that the application of this model has important practical significance for predicting the CIPC of concrete, optimizing the design of the concrete mix ratio and improving the durability of concrete.

A Critical Review of Existing Test-Methods for External Sulfate Attack.

External sulfate attack (ESA) of cementitious materials has been studied worldwide for a very long time. This physical/chemical interaction between sulfate ions and the cement hardened elements affects the long-term durability of concrete structures: cracking, spalling or strength loss of concrete structures. To study these damaging phenomena, some standardized and non-standardized accelerated aging tests are used to evaluate the performance of cements in sulfate-rich environments. However, these existing methods do not adequately predict field performance and some shortcomings or deficiencies still exist: change of degradation mechanisms when using high concentrations of sulfate, variable boundary conditions and small specimens compared to the real concrete structures. In this work, a critical review of some existing test methods and foreign national standard methods for ESA are presented, analyzed, and discussed. This results in some proposed recommendations for improving these methods to meet the needs of structure managers.

Methods for characterising the steel–concrete interface to enhance understanding of reinforcement corrosion: a critical review by RILEM TC 262-SCI

The steel–concrete interface (SCI) is a complex, multi-phase and multi-scale system. It is widely known to influence the performance and long-term durability of concrete structures. However, a fundamental understanding of its properties and effects on corrosion initiation of embedded reinforcing steel remains elusive. This is attributed to its complicated heterogeneity and time-dependent nature, exacerbated by the lack of suitable techniques for systematic and detailed characterisation. This paper, prepared by members of the RILEM Technical Committee 262-SCI, critically reviews available information regarding current methods (laboratory or field-based) for characterising local properties of the SCI that have been identified as governing factors affecting corrosion initiation. These properties include characteristics of the steel such as mill scale and rust layers, and characteristics of the concrete such as interfacial voids, microstructure and moisture content. We evaluated over twenty methods and summarised their advantages, applications and limitations. The findings show a severe lack of well established, non-destructive techniques that are suitable for direct monitoring of the SCI at a representative scale with sufficiently high resolution (spatial, temporal), particularly for moisture related aspects. Several promising novel techniques with significant potential for further development and application were identified and discussed. Finally, we provide several recommendations for future research needs that are required to advance this critically important topic.

The evolution of early-age cracking of cement paste cured in low air pressure environment

In recent years, infrastructure constructions have rapidly developed in plateau areas. The research on the surface cracks of cement-based materials in low air pressure environment in plateau areas is critical in improving the long-term durability of concrete structures as well as the harmony between the building and environment. In this work, the evolution of early-age cracking in cement pastes cured in low air pressure environment was examined by drying measurement, thermogravimetric analysis (TG), nitrogen adsorption-desorption, and micromorphology. The cumulative moisture loss and evaporation rate, total crack area, degree of hydration, and microstructure analyses were evaluated to analyze the evolution mechanism of early-age cracking. The results indicate that the development of early-age cracking on cement paste surfaces is related to air pressure. The moisture evaporation of cement paste accelerates in low air pressure environment, making early-age cracking and pores more noticeably exposed to the cement paste surface. Further, the cement hydration decreases and the volume of micropores and mesopores of cement paste increases in low air pressure environment, forming a loose and porous microstructure and increasing the risk of early-age cracking on the cement paste surface.

Long-term durability of concrete structures

The issue of long-term strength of concrete in construction structures is considered, taking into account the features of concrete structure and the impact on the durability of concrete and force resistance to deformation and destruction. An approach to assessing the long-term strength of concrete as a thermodynamic system has been proposed. The effect of the growth of concrete strength at an early age when firming it in different temperature and humid conditions is shown to change the initial module of concrete deformations in time due to the physical and chemical reaction of hardening. The article proposes an approach to building the functions of aging concrete based on the cynetics of the physical and chemical reaction of cement hydration in a concrete mixture using its stehometry. The form of aging functions is presented using the analysis of the chemical reactions of cement hydration in concrete.

Correlation between concrete cracks and corrosion characteristics of steel reinforcement in pre-cracked plain and fibre-reinforced concrete beams

This paper presents results on corrosion characteristics of 66 rebars extracted from un- and pre-cracked plain concrete and fibre-reinforced concrete (FRC) beams suffering from corrosion for more than 3 years. The influences of fibre reinforcement, flexural cracks, corrosion-induced cracks and loading condition on the maximum local corrosion level (defined as the maximum cross-sectional area loss percentage) and pit morphology were examined. With 3D-scanning, the corrosion characteristics were analysed, and pit types were classified based on the maximum local corrosion level and geometric parameters of pits. Corrosion pits were observed near some flexural cracks, while the bars at other cracks were free from corrosion. Most rebars in FRC had less maximum local corrosion level than those in plain concrete under the same loading condition and maximum flexural crack width. However, the maximum local corrosion level was not dependent on the maximum flexural crack width (0.1 and 0.4 mm). Longitudinal cracks (corrosion-induced cracks) aggravated the total steel loss and changed the pit morphology by promoting the pit length development. However, longitudinal cracks did not always form, even with severe pitting corrosion. A hypothesis about the time-dependent interplay between transverse and longitudinal cracks and corrosion development was proposed. Further studies on predicting the pitting corrosion evolution and experimental work on specimens exposed for longer periods are needed to understand and quantify the long-term durability of concrete structures reinforced with both conventional reinforcing bars and fibres.

An Accelerated Test Method to Evaluate Cementitious Mixtures Subjected to Chemical Sulfate Attack

Abstract External sulfate attack continues to be a major threat to the long-term durability of concrete structures. Soil or water containing sufficient levels of sulfates can penetrate and chemically react with the hydrated cement compounds, leading to volumetric expansion, and in severe cases, softening of the hardened cementitious paste. While ASTM C1012, Standard Test Method for Length Change of Hydraulic-Cement Mortars Exposed to a Sulfate Solution, is the most widely accepted performance test method used to access the sulfate resistance of cementitious binders, the test requires at least six months and often up to twelve months to perform. This article presents a new accelerated method that places mortar bar specimens under high-vacuum while immersed in sulfate solution to accelerate the penetration and diffusion of sulfates in the specimens and therefore, the degradation and measured linear expansion of the specimens. Expansion, mass change, and visual degradation were assessed over time on mortar specimens immersed in a 5 % and 0.89 % sodium sulfate solution (Na2SO4) and subjected to the vacuum saturation procedure. Expansion results are compared with companion mortar bars that followed the ASTM C1012 method without vacuum saturation. When compared with the current ASTM method, the new test method showed a significant acceleration in the rate of expansion with severe visual deterioration observed two to three times quicker. Visual degradation and measured expansion occurred more rapidly for specimens subjected to the accelerated method and immersed in 5 % Na2SO4; however, mixtures subjected to the accelerated method and immersed in 0.89 % Na2SO4 still showed much faster expansion than companion specimens tested according to ASTM C1012 in 5 % Na2SO4.

High-efficient Construction and Long-term Durability of Concrete Structures with Low Dust Emission Type Dry Spray System

High-efficient Construction and Long-term Durability of Concrete Structures with Low Dust Emission Type Dry Spray System

Multiscale modeling of ion diffusion in cement paste: electrical double layer effects

Understanding the mechanism of ion diffusion in hardened cement paste is of great importance for predicting long-term durability of concrete structures. Gel pores in calcium silicate hydrate (CSH) phase forms dominant pathway for transport in cement paste with low w/c ratios where the electrical double layer effects play an important role. Experimental results suggest that the effective diffusivity of chloride ions is similar as that of tritiated water (HTO) and higher than the sodium ions. This difference can be attributed to the electrical double layer near the charged CSH surfaces. In order to understand species transport processes in CSH and to quantify its effective diffusivity, a multiscale modeling technique has been proposed to combine atomic-scale and pore-scale modeling. At the pore scale, the lattice Boltzmann method is used to solve a modified Nernst Planck equation to model transport of ions in gel pores. The modified Nernst Planck equation accounts for steric and ion-ion correlation effects by using correction term for excess chemical potential computed through the results from the grand canonical Monte Carlo scheme at atomic scale and in turn bridges atomic scale model with pore scale model. Quantitative analysis of pore size influence on effective diffusivity carried out by this multiscale model shows that the contribution of the Stern layer to ion transport is not negligible for pores with diameter less than 10 nm. The developed model is able to reproduce qualitatively the trends of the diffusivity of different ions reported in literature.

Quantitative measurement of the influence of degree of saturation on ion penetration in cement paste by using X-ray imaging

The external penetration of ions into concrete plays an important role in long-term durability of concrete structures. In addition, the degree of saturation (DoS) of the paste has a large impact on rate and mechanisms of fluid penetration into concrete. There is little knowledge of the dynamic process of fluid penetration into these materials because few experimental techniques are able to give quantitative spatial measurements of the fluid movement without damaging the sample. This paper uses laboratory transmission X-ray microscopy (TXM) to investigate the in-situ penetration of an iodide based tracer into cement paste with water-to-cement ratio of 0.40 conditioned at four different relative humidities (RH). This technique can non-destructively image the distribution of the dissolved ions at 8.8μm resolution in minutes. In addition, micro X-ray fluorescence (µXRF) imaging and conventional gravimetric measurements were used to compare I and Cl penetration rate and validate the results from TXM. Mathematical models have been proposed to account for ion penetration in partially-saturated samples. The results show that samples that were ponded for two days after being conditioned in 35% and 70% RHs (DoS of 40.8% (±0.1%) and 68.8% (±0.5%) respectively) show a 40× and 10× increase respectively in the penetration coefficient over the saturated sample. Furthermore, important insights about combined ion transport mechanisms have been observed from concentration profiles by TXM.

Using X-ray imaging to investigate in-situ ion diffusion in cementitious materials

The external penetration of ions into cementitious materials plays an important role in long-term durability of concrete structures. Current test methods to study bulk diffusion of ions in cement-based materials are typically destructive, time consuming and labor intensive. This research work uses laboratory transmission X-ray microscopy (TXM) and X-ray computed microtomography (μCT) to image iodide diffusion in cement paste. These techniques can non-destructively image the distribution of the dissolved ions at 8.8μm resolution in minutes with a 2D analysis and a few hours for 3D. This method does not aim to measure the size of the pores, instead it aims to image the movement of the fluid through the pores by imaging an electron dense tracer with similar behavior as chloride ions. The tests were completed on cement paste mixtures with water-to-cement ratios (w/c) of 0.35, 0.40, and 0.45. The findings were validated with micro X-ray fluorescence (μXRF) imaging. Iodide and chloride diffusivity is also compared for paste samples with w/c of 0.40 and shown to provide comparable results. Results are also presented of 3D ion diffusion and void filling at high degrees of saturation.

Modelling the Swelling due to Delayed Ettringite Formation - Application to a Real Bridge

Delayed ettringite formation (DEF) can affect the long-term durability of concrete structures by causing cracking and expansion of the material. Consequently, mechanical properties decrease which may cause large structural disorders due to unexpected deformations and additional stresses in concrete and reinforcement. This reaction consists in ettringite crystallization within concrete after hardening is substantially complete, and in which no sulphates come from outside the cement paste. It may occur in materials that have been subjected to temperature above about 65 °C at early age and to high humidity. At this high temperature, the ettringite turns unstable while the concrete is still plastic and forms again after cooling in the hardened material, thus generating swelling due to crystallisation pressure.This article aims to present a new model for the calculation of structures affected by DEF and to study the effect of the prefabrication temperature on the development of this reaction. In this context, the elaborated model was applied to the 3D simulations of a real bridge affected by this phenomenon. The results highlight that the temperature reached in the precast beams of the studied bridge during prefabrication has a significant effect on the displacements and stresses. Therefore, more precise control of the prefabrication temperature has to be applied in order to prevent the swelling and damage to structures.

Performance Based Specifications for Durability of Mass Concrete Blocks Subjected to Harsh Arabian Gulf Marine Environment

Concrete structures in the Eastern region of Saudi Arabia are exposed to the harsh Arabian Gulf environment, and they suffer from deterioration due to accelerated corrosion of the reinforcing steel resulting in reduced service life. The service life of reinforced concrete structures in this environment is strongly affected by the quality of concrete. The resistances of concrete to chloride penetration, the chloride threshold for corrosion initiation and corrosion rate of the reinforcing steel are critical in ensuring long-term durability of concrete structures. In order to ensure that the concrete functions in the harsh environment for which it is expected to meet the service life requirement, the concrete must be designed for durability performance. A prescriptive concrete specification emphasizing on limits on mix parameters is generally ineffective for durability issues. A performance based specification based on concrete quality represented by durability indices and compliance criteria measured on cast-in-place concrete provides a framework for the contractor and ready-mix concrete supplier to produce a structure meeting the stipulated service life by the client. This paper presents the results of experimental investigations conducted for mass-concrete blocks used in the quay walls off the coastline on a reclaimed land. For long-term durability, a concrete mix in which 50% and 70% of cement was replaced by ground blast furnace slag was used to cater for the durability performance requirements.