Air Void Characteristics Research Articles

Curing performance indicators of exposed aggregate layer in two-lift concrete pavement

The use of exposed aggregate concrete (EAC) in the upper layer of two-lift pavements is spread over many European countries including Poland. However, the EAC technology is considered difficult to apply because of inevitable dependence on operator’s experience to control the whole process properly. The proper timing of technological operations is critical. The investigation on the performance of EAC was performed to gain a better understanding of material and environmental factors involved. EAC slabs were manufactured in the laboratory following the procedure applied at the construction site. Air entrained concrete mix design included a variable water to cement ratio and cement type while the type and the content of aggregate was constant. The effects of curing intensity were studied. The strength properties, air void characteristics of hardened concrete, the freeze-thaw resistance and the saltscaling resistance were tested of specimens cored from the slabs, using the European standard methods. EAC permeability was also evaluated using the methods covered by ASTM C1585 and NT Build 492. Such permeability indicators and frost durability were applied to evaluate the differences in EAC layers performance. The environmental vulnerability of EAC mixes used in the upper layer of two-lift pavements is discussed.

High-temperature behaviour of geopolymer composites containing carbon fibre and nano-silica: Mechanical, microstructure, and air-void characteristics

The fly ash–ground granulated blast-furnace slag-based geopolymer (FSG) represents a groundbreaking material, offering a sustainable and environmentally friendly alternative to conventional construction materials. This paper presents a systematic experimental study on FSG reinforced with carbon fibre (CF, 0 %–1.0 %) and nano-silica (NS, 1 %–3 %) at high temperatures (20, 200, 400, 600, 800, and 900 °C). Various tests concerning appearance, residual strength, uniaxial tension, XRD, FTIR, TGA, SEM, and air-void characteristics were conducted to analyse thermal effects. The findings show that as temperature increases, the colour of the geopolymer transitions from dark grey to yellow grey. Enhanced geopolymerisation during thermal solidification from 20 to 400 °C significantly increases the compressive, flexural, and tensile strengths of the geopolymer. The addition of CF and NS improves the residual strength of geopolymers at high temperatures, with optimal concentrations found to be 0.6 % for CF and 2 % for NS. CF maintains its bridging effect at high temperatures, enhancing fibre–matrix bonding, while NS promotes the formation of additional hydration products on CF surfaces, thereby strengthening the material further. After exposure to 800 °C, despite a weakening of the fibre–matrix interface, the modified geopolymers (FSGC0.6 and FSGC0.6N2) exhibit finer air bubble chord lengths and superior air-void characteristics than the control group. This supports the theory that the synergistic action of CF and NS not only enhances geopolymerisation but also fills pores, thus limiting crack propagation and densifying the pore structure.

Method for prediction of the frost resistance ability of air‐entrained concrete based on the 3D air void characteristics by x‐ray micro‐CT

Method for prediction of the frost resistance ability of air‐entrained concrete based on the <scp>3D</scp> air void characteristics by x‐ray micro‐<scp>CT</scp>

Development of insulating backfill materials produced from the ternary mixtures of red mud, fly ash and preformed foam

Many public utility lines to transport power, water, natural gas, water, sewer, and communication are placed beneath trafficable areas. However, insufficient or inadequate backfill induces sudden subsidence, damage, or pothole on the road. This study aims to develop lightweight controlled low-strength materials (CLSM) with low thermal conductivity using the ternary mixtures of red mud to replace aggregate sand, high carbon fly ash, and preformed foam. Changes in flow consistency, air void characteristics, bulk unit weight, unconfined compressive strength (UCS), and thermal conductivity (k) of the tested materials with varying red mud contents were investigated as a function of the foam volume ratio (FVR). The results demonstrate that the UCS and k of tested materials decreased with increasing FVR due the decrease in unit weight, and a greater UCS but smaller k was observed at a given FVR with increasing red mud content because the inclusion of red mud in the lightweight CLSM mix design helps improve the stability of air bubbles and achieve uniform distribution of air voids. In addition, the red mud can act as a NaOH supplier, leading to the developed material had additional strength gain from the alkali activation. Thus, the developed insulating backfill material showed 43 % decrease in k while maintaining UCS similar to non-foam CLSM without red mud.

Influence of ground granulated blast furnace slag on recycled concrete powder-based geopolymer cured at ambient temperature: Rheology, mechanical properties, reaction kinetics and air-void characteristics

In this paper, ground granulated blast-furnace slag was used to enhance the properties of recycled concrete powder based geopolymer cured at ambient conditions. The rheology, workability, strength, hydration reaction kinetics, microstructure, and air-void characteristics of the geopolymer were investigated. The results showed that adding the slag accelerated the setting process and reduced the water absorption and porosity of the geopolymer mixtures. Excellent mechanical properties were achieved when the slag content exceeds 30 %. The 28-d compressive and flexural strength of the geopolymer with 50 % slag addition reached 61.2 and 5.17 MPa, respectively. The yield stress and plastic viscosity of the geopolymer increased as the slag content increased. A quantitative model between the slag content, resting time, and rheological parameters was established, with high accuracy, realistically reflecting the rheological properties of the geopolymer. The addition of the slag provided nucleation sites for the C-S-H gel, which promoted the hydration reaction, increased the hydration products and the total heat release, and formed a denser and more uniform microstructure, thereby increasing the strength of the geopolymer. The slag addition was also found to reduce the air content, spacing factor, and average chord length of the geopolymer matrix. As the slag content increased, the air bubbles became smaller and more stable, and exhibited a more uniform distribution.

Study on the influence of fractal dimension and size effect of coarse aggregate on the frost resistance of hydraulic concrete

In cold regions, hydraulic concrete structures are exposed to harsh environmental conditions that greatly impact their durability and safety. The role of aggregates in influencing concrete performance is paramount. This study aims to investigate how the fractal distribution and size of aggregates affect the frost resistance of hydraulic concrete, with air void characteristics, mass loss rate, Elastic Modulus of dynamic, and strength serving as primary references. Two experiment series, fractal distribution influence and aggregate size influence of concrete frost resistance, were conducted to analyze the frost resistance of hydraulic concrete with different aggregates. Based on the fractal theory, four grading fractal distributions were utilized to fractalize aggregate gradation, aiming to investigate the influence of aggregate fractal dimension on the frost resistance of hydraulic concrete. Simultaneously, concrete specimens of fully graded aggregate and wet-screened aggregate were also constructed to simultaneous freeze-thaw cycle tests, the impact of aggregate size on concrete's frost resistance performance was also studied by analyzing the air void structure in the frost-resistant concrete specimens. Results indicate that the specimens exhibit varying frost resistance performance under different aggregate gradation fractals, ranking from strongest to weakest as follows: D=2.5, D=2.3, D=2.1, D=2.7, which is attributed to the differences of air void spacing factors. Increasing aggregate size leads to greater internal structural heterogeneity in concrete, resulting in an increase in defects and weak points. This causes microcracks to develop more rapidly in the transition zone of fully graded concrete during freeze-thaw cycles, leading to more severe damage. The findings of this study can serve as a fundamental scientific basis for the designing and constructing of hydraulic concrete structures in cold areas.

A novel approach for modifying air-voids in concrete by short-term low-air pressure intervention

If it is desired to experimentally verify the regularity in the effects of air-voids on the properties of concrete, methods of altering voids must be relied on. Consequently, this research explored a method that reduced the curing air pressure after mixing concrete so as to modify air-void characteristics. In addition, in order to avoid introducing variables besides air-voids, a limit was placed on the duration of low-pressure maintenance, which was restoring the air pressure to atmospheric pressure after the formation of the concrete's air pressure transient resistant strength. The results showed that if the reduction in air pressure was insufficient, the differential pressure between the inside and outside of the original bubbles was not enough to cause them to break through energy barriers resulting in expansion. Only bubble regeneration due to the decrease in gas solubility was observed. When the differential pressure exceeded the energy barriers, bubble expansion could be achieved. In addition, it was possible to achieve the air pressure transient resistant strength in short time, which was achieved in only 7 h for the concrete used in this paper. Water loss calculation, X-ray diffraction, thermogravimetry-derivate thermogravimetry, scanning electron microscope, and mercury intrusion porosimetry were used to evaluate the hydration of the concrete confirming that the low-air pressure curing of 7 h had almost no effect on hydration. The method proposed can provide an effective means of specimen preparation for scientific research involving the effects of air-voids on the properties of concrete.

Effects of re-vibration on mechanical properties and air-void characteristics of cement-stabilised macadam materials

ABSTRACT Layered continuous paving (LCP) technology is an effective choice for improving the integrity and construction efficiency of the thick cement-stabilised macadam (CSM) base course. However, the effects of re-vibration generated by construction equipment during the paving process are not well understood. In this investigation, the effects of re-vibration on mechanical properties and air-void characteristics were investigated by mechanical performance tests, chloride migration experiments and industrial computerised tomography tests. The results demonstrate that compared with the control group, the void characteristic parameters were reduced by applying re-vibration within 12 h, resulting in a more compact structure. Thus, the mechanical properties were improved. However, the improvement effect gradually weakened over the delay time. CSM interlayer bonding strength were related to the time point of applying re-vibration and the cement paste dosage sprinkled between the layers. It is suggested that12 h be used as the critical time point for the LCP.

Paste, aggregate, or air? That is the question.

The Ambassador Bridge between Detroit, Michigan, and Windsor, Ontario, has served for almost 100 years as North America's busiest international border crossing. But in 2025, the Ambassador will be replaced by the new Gordie Howe International Bridge. The Gordie Howe is a cable-stayed bridge, with two massive 220 m tall concrete piers on opposite banks of the St. Claire River, a single clear span of 853 m, and 42 m of clearance over this busy waterway. To ensure durability in this harsh freeze-thaw environment, air-entrained concrete is specified throughout. And, to ensure the quality of air entrainment, the ASTM C 457 Procedure C, Contrast Enhanced Method is employed. While a similar automated microscopic approach has been in use for well over a decade according to EN 480-11 Determination of air void characteristics in hardened concrete, this is the first large-scale application of automated air void assessment in North American infrastructure. According to the ASTM Procedure C, the air void characteristics are determined through digital image processing, while the paste content may be determined by either mix design parameters, manual point count, or 'other means'. Of these three options, point counting is used for Gordie Howe; but in parallel, during each point count, the digital image coordinates and phase identifications for each evaluated stop are recorded. This allows for training of a neural network, for automated determination of paste content, as demonstrated here.

Study on the influence mechanism of recycled concrete aggregate on strength of asphalt mixtures

Recycled concrete aggregate (RCA) has great potential use in pavement. However, unclear characteristics of RCA and its relationship with the properties of asphalt mixture hinder severely its promotion. The objective of this study is to characterize the microscopic morphology and structure of RCA, further investigate its impact on the strength of asphalt mixture and associated mechanisms, by means of various test method. Results showed that cement mortar was the vital factor to affect the properties of RCA. Hence, the asphalt mixtures with RCA had characteristics of higher air voids (VV) and voids in the mineral aggregate (VMA), lower asphalt saturation (VFA), compared with conventional asphalt mixtures (CAM). In addition, according to the results of Micro-Scratch (MS) test, the compression depths of aggregate, interfacial transition zone (ITZ) and asphalt mortar in RAM are larger than that in CAM. Therefore, the influence of RCA on the performance of asphalt mixture is mainly in the following two aspects: reducing the strength of asphalt mortar; decreasing the skeleton strength of asphalt mixtures.

Engineering properties and air void characteristics of cold recycled mixtures with different compaction methods

Cold recycled mixtures (CRM) offer advantages in terms of resource conservation and reduced CO2 emissions. This paper utilizes the vertical vibration testing method (VVTM), Marshall compaction (MC), Superpave Gyratory Compactor (SGC), and field core samples to analyze the effects of different compaction methods on the engineering properties of CRM. The study begins by discussing the volumetric and mechanical parameters of CRM with different compaction methods, followed by an analysis of fatigue performance. Additionally, CT scanning technology is used to study the effect of different compaction methods on the void characteristics of CRM. The results demonstrate that VVTM specimens offer better mechanical properties compared to MC and SGC specimens. Furthermore, the RT (splitting tensile strength), MS (dry-wet tensile strength ratio), and TSR (freeze-thaw splitting strength ratio) of VVTM specimens exhibit the highest correlation with field core samples, reaching 97%, 97%, and 95%, respectively. The VVTM specimens also exhibit the biggest fatigue, which is similar to that of the field core sample. The Weibull distribution model is utilized to predict the fatigue life of CRM with different compaction methods, with the fatigue equation indicating that VVTM specimens have better fatigue performance. In addition, the number of voids, diameter of voids, and fractal dimension of VVTM specimens along the longitudinal direction are closer to those of field cores. The fractal dimension of CRM specimens with different compaction methods is between 1.2 and 1.4, indicating that the complex shape of voids may alter stress concentration and impact the performance of CRM.

Mechanical responses of buoyant bio-inspired foamed concrete structures

This work proposes a new type of lightweight cellular foamed concrete (FC) inspired by natural cellular structures with controllable mechanical properties for different prefabricated engineering applications. The 3D printed formworks are inspired by the lattice structure and triply periodic minimal surface (TPMS) architectures, including gyroid and primitive prototypes. These 3D printed sacrificial thermoplastic Polylactic Acid (PLA) formworks were then infiltrated with two types of foamed concrete with different densities of 600 kg/m3 and 800 kg/m3 and subjected to uniaxial compressive loadings. Furthermore, the X-ray computed tomography (CT) scan was conducted on the prefabricated structures to further observe the PLA-foamed concrete contact region and to investigate the air void characteristics. Numerical simulation was employed for further investigation on the structural performance and failure mechanism of three bio-inspired structures (TPMS-Primitive, -Gyroid, and strut-based lattice). By both numerical and experimental investigations, the gyroid cellular structure yields the highest compressive capacity, followed by the primitive and lattice structures, respectively. Furthermore, based on the X-ray micro-CT scan, the bio-inspired architecture of the moulds showed a significant effect on the size and distribution of air voids, which highly influenced the compressive strength of the infiltrated foamed concrete.

Mutual compatibility of superplasticizers (PC, SNF), grinding aids (TEA, glycol) and C3A in Portland cement systems – Hydration, rheology, physical properties and air void characteristics

The compatibility of grinding aids (TEA/glycol) and superplasticizers (PC/SNF) in low- and high-C3A cements, as well their mutual effect on the properties of pastes and fresh/hardened mortars were evaluated. The influence of various surfactant combinations on the hydration rate, flowability, compressive strength, rheology, surface tension and air void characteristics was studied. For this purpose, 6 cements based on industrial clinkers and 18 mortars were prepared under laboratory conditions. PC is more effective than SNF in low-C3A cements. In high-C3A cements, both superplasticizers have a comparable impact on consistency. Grinding aids, especially TEA, have a strong influence on the effectiveness of PC and SNF, leading to changes in the induction period, plastic viscosity and yield stress. A synergy effect of grinding aids and superplasticizers increases the air content of mortars, especially for TEA-SNF combination. Surfactants decrease surface tension, particularly PC, thus increasing the amount of air voids >300 µm.

How nano-bubble water and nano-silica affect the air-voids characteristics and freeze-thaw resistance of air-entrained cementitious materials at low atmospheric pressure?

With more infrastructures being built in plateau regions, low atmospheric pressure is put forward as a new factor affecting the mechanical properties and durability of cementitious materials, but its influence is inconclusive, and the failure of freeze-thaw has not been effectively solved. This research explored the influences of both nano-bubble water (NBW) and nano-silica (NS) on air-voids characteristics and freeze-thaw resistance of cementitious materials formed and cured at different air pressure (0.4P, 0.7P, P; P=1atm); and investigated the effects and its mechanisms of low air pressure on performances of cementitious materials. The air-voids structure analyser, SEM, XRD and TG-DSC were employed to evaluate the microstructure and cement hydration of hardened cement paste samples. Results show that low-pressure samples have abnormal air-void parameters and poor bubble stability, which lead to a decrease in freeze-thaw resistance. It is proved that both NBW and NS can improve the freeze-thaw resistance and mechanical strength of samples at low atmospheric pressure by accelerating cement hydration, enhancing bubble stability, and optimizing air-voids parameters. The mechanisms of low pressure, NBW and NS on properties of cementitious materials are expounded. This research proposes new perspectives for improving the durability of air-entrained cementitious materials in plateau and enables us to further comprehend the effects and principles of low air pressure on performances of cementitious materials.

The effect of size and shape of pores on the prediction model of compressive strength of foamed concrete

The Balshin’s model is the most well-known of the models used to predict the compressive strength of aerated concrete. Among the pore characteristics such as the amount of porosity, the size of the pores, and their shape, Only the amount of porosity is placed as a specific parameter in the Balshin’s model. The impact of the other two characteristics, on the other hand, is included in the power value of the Balshin’s equation. Compressive strength tests and identifying the characteristics of air voids using microscopic imaging were performed on the mixtures produced in this study, which had weight ratios of sand to the cement of 1, 1.5, and 2, and the percentage of foam volumes of 0, 15, 25 and 35. In this study, a review of some of the factors influencing the change in the pore characteristics was performed, including the porosity of the foamed mixture and the size of sand used, in addition to the effect of the pore characteristics such as size and shape on the compressive strength of the foamed mixture. This study aims to conduct a comparative analysis of the models introduced in previous research to determine the effect of the pore characteristics of semi-structural and structural foamed mixtures (with sand) caused by mixture porosity and the sand size on the Balshin’s model. Among the types of relationships presented for the Balshin’s model, the relationship based on the paste fraction porosity was used. According to the findings of this study, foam porosity is the most important factor influencing pore size change, followed by sand size. The increase in the size of the pores and their non-sphericality decreased the relative compressive strength and, as a consequence, increased the power of the Balshin’s model because the maximum level of stress concentration was created adjacent to the middle diameter of the pores at the period of concrete rupture.

Effect of air void structure and mechanical properties of high strength fiber reinforced mortar composed with different construction sand species based on X-CT technology

The significant effect of desert sand on cement-based composites was observed in the previous investigation. However, how the composition of fine aggregate affects the formation of air void structure remains to be investigated. In this paper, mortars composed of five different sand mixtures were prepared, and the specific surface area (SSA) of the sand mixture with different content of fine aggregate was calculated. It was observed that the air void content decreased as the SSA value of the sand mixture increased. A negative-slope linear relation was observed between the SSA values and air void content in the composite construction sand fiber reinforced mortar system. With the increase of SSA, the air void number of every high-strength fiber reinforced mortar decreased from 12,446 to 6652. With the gradual increase of desert sand content, the count of air void number decreased. When the fine aggregate was completely desert sand, the air void content of the fiber-reinforced mortar dropped from 1.21% to 0.98%. Through the analysis of the 3D air void system, it can be concluded that the mixture of fine aggregate with different particle size distributions plays a positive role in optimizing the air void characteristics. Reasonable utilization of desert sand can significantly improve the pore structure characteristics and compressive strength of cement-based composites, which provides an effective theoretical basis for the development and utilization of desert sand resources.

Characterization of Rutting Damage by the Change of Air-Void Characteristics in the Asphalt Mixture Based on Two-Dimensional Image Analysis.

In the process of the rutting test, the air-void characteristics in asphalt mixture specimens are a dynamic change process. It is of great significance to systematically study the correlation between the change of air-void characteristics and the depth of the rutting slab and establish a relationship with damage. In this paper, the air-void information of rutting specimen sections with different loading cycles (500, 1000, 1500, 2000, 2500, and 3000 times) is obtained by two-dimensional image technology. The dynamic change process of the micro characteristics of internal air voids of two graded asphalt mixtures (AC-13 and AC-16) under cyclic wheel load is analyzed, and it is used as an index to characterize the microstructure damage of the asphalt mixture. The results show that the variation of air-void distribution, air-void shape characteristics, and air-void fractal dimension with the loading process can well characterize the permanent deformation law of the rutting slab. The fractal dimension of the air void increases with the increase in load. It is a dynamic process in which the air-void content changes with crack initiation and propagation. After rutting deformation, the total air-void area and average air-void size of the sample increase, and the total air-void number decreases. Because microcracks are formed in the specimen after rutting damage, the aspect ratio of the air void increases, and the roundness value decreases.

Use of X-ray computed tomography to evaluate the gradual behaviour of air voids in asphalt mixtures during permanent deformation

ABSTRACT X-ray computed tomography (CT) as a non-destructive testing method has been used to evaluate the air voids and void-related distresses in asphalt mixtures. To evaluate the air void development in asphalt mixtures under loading, this study proposed a volume-equivalent maximal ball model based on CT to characterise the changes of void connectivity. The void distribution and shape features were described using the variation coefficient of modified annular-sector segmentation, positional eccentricity ratio, and sphericity. The results indicate that voids are relatively heterogeneous and discrete in stone mastic asphalt (SMA-13) mixture compared with dense-graded asphalt concrete (AC-13), particularly for those with volume smaller than 0.01 mm3. The void connectivity and distribution in SMA-13 seem to be more susceptible to load as its coarse aggregates tend to shift their positions during deformation. Rutting reduces the vertical inhomogeneity of AC-13 but increases that of SMA-13. Moreover, loading complicates the void geometry of AC-13 whereas the opposite is true for SMA-13, foreboding that SMA mixtures maintain the potential to resist rutting failure after 1 h of loading compared to AC types. In general, the topological characteristics of air voids within asphalt mixtures and their dynamic response during permanent deformation are principally dependent on mixture gradations.

Investigation on frost heaving stress (FHS) of porous cement concrete exposed to freeze-thaw cycles

Frost Heaving Stress (FHS) is one of the main causes of freeze-thaw (FT) damage in porous cement concrete. This study customized a device for the measurement of FHS in the laboratory. Firstly, rodding, vibration, and static compaction methods were compared for the preparation of porous cement concrete in terms of the air void characteristics and air void distribution. Based on the proposed measurement device, the influence of curing time, air void, saturation degree, and freeze-thaw cycles on the FHS evolution were discussed, respectively. Besides, the release characteristics of FHS in the thawing process were also characterized. The results indicated that the evolution of FHS in the freezing process can be divided into three stages that accounted for the thermal contraction, phase transformation of water, and the end of phase transformation. The FHS of porous cement concrete can be reduced through the extension of curing or reduction of air void content. In general, the increase of saturation degree induced the growth of FHS. With respect to the release characteristics of FHS, the FHS would not completely dissipate and the remaining FHS would accumulate as the F-T cycle increased.

Properties and Microstructural Characteristics of Manganese Tailing Sand Concrete.

In this work, manganese tailing sand concrete (MTSC) was prepared using manganese tailing sand (MTS) in replacement of river sand (RS) to alleviate the shortage of RS resources and achieve clean treatment and high-value resource utilization of manganese tailing stone. The effects of MTS content on the slump, mechanical strength, air void characteristics, hydration products and micromorphology of MTSC were studied experimentally. The leaching risk of harmful substances in MTSC was also explored by testing the concentration of Mn2+. The results show that the utilization of MTS reduces the slump of MTSC to a certain extent. When the MTS content is lower than 40%, the gypsum introduced by MTS and C3A in cement undergoes a hydration reaction to form ettringite, which decreases the number of pores with a diameter less than 0.1 mm and promotes strength development in MTSC. Additionally, when the MTS content exceeds 40%, the large amount of gypsum reacts to form more ettringite. The expansive stress generated by the ettringite severely damages the pore structure, which is not conducive to the mechanical properties of MTSC. In addition, the leaching of hazardous substances in MTSC is insignificant, and the incorporation of cement can effectively reduce the risk of leaching hazardous substances in MTSC. In summary, it is completely feasible to use MTS to replace RS for concrete preparation when the substitution rate of MTS is less than 40%, with no risk of environmental pollution. The results and adaptation in the concrete industry can reduce the carbon footprint, which is in line with the current trend in civil and materials engineering.