Aggregate Skeleton Research Articles

Design and construction of a new Mechanised Cement Bound Macadam (MCBM)

The paper describes the development and implementation of a new heavy-duty pavement design and construction technique. The pavement structure consist of a very densely packed and interlocked, mainly single sized large aggregate skeleton, combined with a low cement and fines void filler. The small percentage of cemented finer particles specific to the gradation ensures optimal void fill is achieved without causing separation of the inter-locked large stone particles. This enables direct load transfer to take place between the interlocked large stone particles. The heavy-duty pavement is modelled on the original macadam pavement philosophy with specific reference to cement-bound macadam. However, the advances in modern day construction equipment, and an increased focus on productivity, resulted in the demise of this traditional labour-intensive method. The greatest challenge, therefore, was to mechanise the cement-bound macadam concept, while also ensuring consistent aggregate gradation production and placement. This was achieved through the innovative use of modern recycling equipment and aggregate blending plants. Performance testing of in-situ core and beam cut test samples shows the strength characteristics achieved are similar to lean mix and conventional concrete. The cost of a mechanised cement bound macadam (MCBM) layer is approximately a third to a quarter of an equivalent thickness structural asphalt layer in New Zealand. It is therefore a viable design alternative to structural asphalt, delivering performance more akin to rigid lean-mix pavements, at significantly reduced cost.

Coupled effect of fiber and granular skeleton characteristics on packing density of fiber-aggregate mixtures

The addition of fiber to cementitious materials enhances mechanical performance but can reduce workability of the fiber-reinforced concrete (FRC) mixtures. This can be due to the negative effect of fibers on packing density (PD) of the fiber-coarse aggregate (F-A) combination. The performance of FRC, as a diphasic suspension, is dependent on the characteristics of both F-A (suspended-solid skeleton) and mortar (suspending liquid) phases. PD can reflect the voids within the F-A skeleton to be filled with mortar. An adequate optimization of the characteristics of the F-A skeleton can modify the performance of FRC in fresh and hardened states. The F-A skeleton can be characterized in terms of particle-size distribution, volumetric content, and morphology of the coarse aggregate, as well as size, rigidity, and content of fibers. In this study, a comprehensive investigation was undertaken to identify the coupled effect of the characteristics of fibers and coarse aggregate on the PD of F-A combination used without any cement paste/mortar. The solid components play a key role in the overall performance of the concrete produced. This study was carried out to optimize the F-A combination and enhance the workability design of FRC. Various types of steel, polypropylene, and polyolefin fibers having different sizes and rigidities were investigated. Moreover, four combinations of three different classes of coarse aggregate were used to proportion F-A mixtures. Test results showed that shorter length, smaller diameter, and more flexible fibers can lead to higher PD of F-A systems. Moreover, the coarser aggregate skeleton with larger interparticle voids led to more available length for fibers to be deformed, hence improving the PD of F-A mixtures. New empirical models were proposed to predict the packing density of F-A combinations given the characteristics of coarse aggregate and fibers, as well as the level of compaction. The established models were employed to propose a new proportioning approach for fiber-reinforced self-consolidating concrete mixtures to achieve the targeted workability.

Recycling of waste expanded polystyrene concrete in lightweight sandwich panels and structural applications

The recycling of waste expanded polystyrene (EPS) concrete in the lightweight construction sector is not yet explored. The complexity of recycling resides from concrete crushing that creates two distinct, yet inseparable phases including the hardened mortar fraction and recycled EPS beads. This study investigates the feasibility of using the waste EPS-based concrete in two different lightweight applications; i.e., sandwich panels and structural concrete complying with ACI 213 requirements. Test results showed that the lightweight concrete density and mechanical properties remain unaltered when the virgin EPS beads are replaced by recycled ones at relatively low rates not exceeding 2 kg/m3. The strength degradation becomes evident at higher rates, given the inseparable mortar fraction that weakens the aggregate skeleton due to higher porosity and water absorption. The bond to embedded reinforcement and shear strength of beams without stirrups decreased by about 20% when 3 kg/m3 of virgin EPS are replaced by recycled ones. The incorporation of 0.5% steel fibers, by volume, was efficient to mitigate the detrimental effect of such wastes and restore the lightweight concrete structural properties.

Internal deformation monitoring of granular material using intelligent aggregate

Particle motion is the critical factor leading to the internal deformation of granular materials. However, relying on conventional techniques to monitor particle motion in internal deformation of granular materials has limitations. This study presents a novel method to monitor the internal movement of granular materials using intelligent aggregate. The intelligent aggregate packaging structure is fabricated through industrial CT technology and 3D printing. The six position method and fast fourier transform method are used to eliminate the measurement error caused by the packaging structure and the rutting tester vibration noise respectively. The results show that intelligent aggregate could maintain good linkage with surrounding aggregates under load, and the fine aggregates have a greater impact on the stability of the coarse aggregate skeleton. This method based on intelligent aggregates provides a basic technology for further real-time monitoring of granular material properties.

A Triaxial Viscoplastic Model with Temperature Dependence for Asphalt Concrete Permanent Deformation

Asphalt concrete is a composite material comprised of aggregate skeleton and asphalt cement. Its permanent or viscoplastic (VP) deformation is characterized by time and temperature dependence, memory effect, and pressure sensitivity. To the best knowledge of the authors, there have been no VP models explicitly considering the nonlinear temperature effects in permanent deformation of asphalt concrete. In previous work, the authors developed a triaxial VP model that featured a VP relaxation spectrum and a modulus parameter [Formula: see text],[Formula: see text]. The spectrum was postulated as a material function representing the time dependence and memory effect, while [Formula: see text],[Formula: see text] was only a function of the stress condition to capture the pressure dependence. The objective of this study was to incorporate temperature dependence into the model. It was achieved by introducing the concept of VP shift factor solely to translate the relaxation spectrum. This strategy was inspired by the time–temperature superposition principle in the viscoelastic (VE) theory. The shift factor was identified experimentally at three temperatures different from the model calibration temperature. The VP shift factor function was thus established, and comparison with its VE counterpart showed that the VP response exhibited greater temperature sensitivity. Numerical simulation was conducted at several temperatures to illustrate the nonlinear temperature effects on the VP deformation and relaxation of the internal stress that served as the hardening/softening variable.

Evaluation of coarse aggregate movement and contact unbalanced force during asphalt mixture compaction process based on discrete element method

To evaluate the stability of asphalt mixture aggregate network and the mechanism of coarse aggregate movement of the asphalt mixture during the compaction process, the Marshall impact compaction (MIC) process of asphalt mixture was simulated through Discrete Element Method (DEM) and the new index of the coarse aggregate particles contact unbalance force was proposed. Firstly, the mechanical parameters of the asphalt mortar at the compaction temperature were calculated based on the time–temperature superposition principle and the Dynamic Mechanical Analysis (DMA) tests. Secondly, the aggregate morphology properties were collected with a 3D blue-ray scanner, and the virtual aggregate database and the DEM model of asphalt mixture during MIC process with aggregate morphology properties were constructed. Thirdly, the volumetrics evolution information of the asphalt mixture specimen was obtained, and the coarse aggregates movement and contact unbalanced force were evaluated. The results indicated that the vertical displacement of coarse aggregate particles of asphalt mixture was larger than horizontal displacement in the MIC process. The vertical displacement increased with the increment of the aggregate particles size, the reason was that the contact number and the vertical unbalance force increased with the increment of the aggregate particles size. The horizontal displacement and rotation angle of coarse aggregate shown similar trend during MIC process, both of them increased greatly during the initial compaction process and tended to remain stable after that, and the horizontal displacement and rotation angle increased with the decrement of the aggregate particles size. It also found that the particles sizes have an impact on the unbalance force distribution of coarse aggregate, and the unbalance force of coarse aggregate decreased with the increment of blows which indicated that the asphalt mixture tends to become stable with the effects of compaction. And the result also indicated that the aggregate location has an impact on the aggregate movement displacement and aggregate skeleton stability.

Performance criteria, environmental impact and cost assessment for 3D printable concrete mixtures

Extrusion-based concrete 3D printing is a promising technology and has the potential for being a sustainable construction solution by utilizing structural optimization and reduced material usage. However, studies focusing on the sustainability assessment of 3D printable concrete mixtures are not many. In the current study, a quantitative assessment of the environmental and economic impact of multiple 3D printable concrete mixtures for the production of one cubic meter volume is performed. 3D printable concrete mixtures made with (i) different binder systems and (ii) increasing aggregate content and modified gradation of the aggregate skeleton by adding natural and recycled coarse aggregates, satisfying a set of performance criteria, were evaluated. It was observed that the use of calcium sulfoaluminate-limestone binder systems has significantly lower global warming potential; however, the depletion of fossil resources indicator is much higher than compared to the Portland cement-based mixtures. Increasing the aggregate content decreases environmental impact; however, incorporating recycled and coarse aggregates do not significantly decrease the environmental impact at a lower replacement level. Also, it was found that the contribution of the chemical admixtures to the total material cost is significantly higher in the case of 3D printbale concrete mixtures in comparison to the conventional mould cast mixtures. The study provides insights into the environmental and economic impact of extrusion-based concrete 3D printing materials satisfying the same functional requirements.

Triaxial testing of new and degraded ballast under dry and wet conditions

While many studies focused on both new and fouled ballast behavior due to contamination by external fouling agents, far fewer studies have focused on the mechanical breakdown of crushed ballast stones and the behavior of used ballast due to gradual aggregate degradation. This paper presents triaxial test results conducted on both new and degraded limestone ballast samples to investigate deformation trends and evaluate effects of varying moisture contents. Based on the limited experimental results, degraded ballast samples in dry conditions and typically at a denser state compared to the new and clean ballast did not result in significant strength loss. In most cases, the degraded ballast yielded even higher strengths, which is different than reported in previous studies focused on testing contaminated ballast. However, when subjected to repeated load cycles, degraded ballast specimens exhibited much higher permanent deformation accumulations. When moisture was introduced, the degraded ballast showed significantly larger permanent deformations and strength losses compared to the results observed from dry testing conditions. Negative impacts of moisture on degraded ballast were generally similar as on the contaminated ballast, but the mechanical property changes were more abrupt mainly due to size reductions in coarse aggregate particles and the aggregate skeleton itself.

Investigating the Effect of Gradation, Temperature and Loading Duration on the Resilient Modulus of Asphalt Concrete

This research was carried out to assess the effect of aggregate skeleton, temperature variation, and loading duration on the resilient modulus of asphalt concrete mixtures. Two different gradation methods, i.e., the conventional method of gradation and the Bailey method of gradation, were adopted to design the aggregate skeleton. The effect of these gradation methods, with temperature and loading duration, on the resilient modulus of asphalt concrete has not been previously investigated. The Modified Marshall Test was used to determine optimum binder content against 4% air voids, and then volumetric and strength parameters were calculated against optimum binder content. For performance tests, specimens were prepared at optimum binder content using a Superpave gyratory compactor. An indirect tensile strength test on both types of mixtures was conducted, and a 20% value of indirect tensile strength was kept for peak load, whereas 10% was kept for seating load for conducting resilient modulus tests. The tests were conducted at 100 and 300 ms duration loads under two different temperatures, i.e., 25 oC and 40 oC. The results declared that aggregate skeleton, temperature, and loading duration have a prominent effect on the resilient modulus of asphalt concrete mixtures. Bailey gradation mixtures disclosed higher resilient modulus values than conventional gradation mixtures. Higher values of resilient modulus were observed for both gradation mixtures at low temperatures and under small duration loads than at high temperatures and large duration loads. The results of the two-way factorial design also confirmed the above findings. Doi: 10.28991/CEJ-2022-08-02-07 Full Text: PDF

The Mechanical Resistance of Asphalt Mixture with Steel Slag to Deformation and Skid Degradation Based on Laboratory Accelerated Heavy Loading Test.

Steel slag is a main form of solid waste. Using this component to replace part of the aggregate in an asphalt mixture is an effectively way of treating solid waste. To study the performance degradation of asphalt mixture with various content of steel slag under heavy loading, some large-sized basalt hot mixed asphalt mixture (BHMA) and steel slag hot mixed asphalt mixture (SHMA) in a form of specimens were prepared and a heavy loading wheel tracking test was conducted. The aggregate characteristics of basalt and steel slag were measured. The deformation and skid resistance of the asphalt mixture with different content of steel slag was tested and analyzed. Due to the particle characteristics of steel slag aggregate, it was found that a low content of steel slag can effectively improve the resistance to deformation and skid resistance of the asphalt mixture under heavy loading. The response of SHMA can still meet the pavement technical requirement after long-term heavy loading service. The main change in the mixture under heavy loading is the crushing of the 9.5–16 mm aggregate, and the content of this part also significantly affects the deformation. This study explains the mechanism of degradation of SHMA under heavy loading: the large aggregate is crushed and forms a new aggregate skeleton structure.

Evaluation on the influence of dynamic water pressure environment on viscoelastic mechanical performance of asphalt mixture using the bending beam rheometer method

With the degraded service performance induced by the erosion of dynamic water pressure environment in summer, asphalt pavement has to subsequently overcome the challenge of low-temperature environment in winter. Considering the unique phenomenon of the gradual disintegration of asphalt mixtures’ components, it is still a lack of systematic research on moisture damage induced by dynamic water pressure environment from the perspective of viscoelastic properties. This study adopted the bending beam rheometer (BBR) method to evaluate the influence of dynamic water pressure environment on viscoelastic mechanical performance for asphalt mixture. The results indicated that both creep stiffness modulus and relaxation modulus of asphalt mixture showed similar downward trends with the increase in severities of different water environments, while values of relaxation modulus were slightly smaller than that of creep stiffness modulus. Master curves of creep compliance gradually moved upward with the increase in severities of dynamic water pressure environments, while master curves of relaxation modulus inversely moved downward. Specially, master curves of both creep compliance and relaxation modulus exhibited irregular trends for the dynamic water pressure environment of 60 ℃ − 0.414 MPa, which implied that significant erosion had been brought into asphalt mixture. A relative larger size of coarse aggregate skeleton could improve the resistance of asphalt mixture to moisture damage. The positive effect on improving resistance of asphalt mixture to moisture damage induced by SBS modified asphalt binder was stronger than the negative effect induced by the gradation with a relatively small nominal maximum aggregate size (NMAS). Differences existed in the reaction mechanisms of different types of asphalt binder subjected to the erosion of different water environments.

Effect of slag coal ash and foamed glass on the mechanical properties of two-stage concrete

Two-stage concrete (TSC) is known by various names such as colcrete, Polcrete, preplaced aggregate concrete and prepacked concrete. It is different from traditional concrete in two fundamental ways, namely method of construction and mix proportion. Two-stage concrete (TSC) is defined as firstly, coarse aggregates are placed into the formwork and grout is applied to fill in the between coarse aggregate particles voids. Secondly, the percentage of coarse aggregates in the mix proportion of TSC is higher than that in normal concrete. The typical value is about 60% as compared with 40% in traditional concrete. As coarse aggregates are preplaced first, they can occupy up to 60–70% of the total volume. As coarse aggregates are not involved in the mixing process, TSC is environmentally friendly with lesser consumption of energy. With a higher content of aggregates, TSC reduces the use of cement by 20–30% and may minimize the temperature rise. Engineering properties of TSC, including its stress–strain relationship, is mainly governed by the properties of coarse aggregates as stress is transferred from the skeleton of aggregates to hardened grout. Main advantages of TSC include a higher volume of coarse aggregates and the ability to use larger size coarse aggregates. The latter also reduces the cost of crushing. TSC has beneficial properties such as low drying shrinkage, high bonding strength, high modulus of elasticity, and excellent durability. The method of TSC has proved particularly useful in a number of applications like underwater construction, and masonry repair, where placement by conventional methods is extremely difficult. The method is also applicable in case of massive concrete where low heat of hydration is required. It is studied the feasibility of casting two stage concrete with 100% steel slag as coarse aggregate. In term of formulation, to adopt two stage concreting method we could minimize the risk of concrete bleeding and segregation due to high water absorption and quite high density of slag aggregate. The effect of slag coal ash and foamed glass on the mechanical properties of two-stage concrete has rarely been reported. Thus, the development of an eco-efficient alkali-activated grout for two-stage concrete is a new research topic that has no robust results to draw solid conclusions and it should blaze the track towards a cleaner production of building materials with outstanding sustainability.

Effects of Mixture and Aggregate Type on Over-Compaction in Hot Mix Asphalt in Tennessee

Correct compaction is vital for asphalt mixture service life. An adequately compacted mixture with inferior properties can achieve better performance than a mixture with excellent properties but poorly compacted. This study investigated resistance to damage caused by over-compaction by utilizing the locking point concept. Over-compaction might cause damage to the aggregate structure and decrease service life. The locking point is defined as the moment during mixture compaction at which an aggregate skeleton is developed and becomes stable. Beyond the locking point, more compaction energy does not significantly increase mixture density and can damage aggregate particles. A total of 15 mixtures was utilized and evaluated using the gyratory compactor. Among them, five dense-graded plant mixtures contained different aggregates and binders, and 10 laboratory mixtures (three types: the surface, the base, and stone mastic asphalt [SMA]) were designed with the most popular coarse aggregates in Tennessee: hard limestone, soft limestone, gravel, and granite. The results of this study show that the highest locking point was reached by the mixtures containing gravel. The SMA mixtures have, on average, lower locking points than the dense-graded mixtures. Most of the dense-graded mixtures made with crushed stones failed in the range of +20 to +30 gyrations, whereas the samples made with gravels failed in the range of +30 to +40 gyrations, indicating that gravel seems to be the most resistant to damage.

Performance Characterization of Stone Mastic Asphalt using Steel Fiber

Stone Mastic Asphalt (SMA) is a gap-graded hot mixture designed to provide higher resistance towards permanent deformation and rutting potential by 30% to 40% more than dense-graded asphalt, due to its stable aggregate skeleton structure. However, compared to other types of hot mix asphalt, SMA unfortunately has some shortcomings in term of its susceptibility towards moisture-induced damage due to its structure and excessive bitumen content in the composition. This research aims to assess the performance of a SMA mixture with steel fiber by enhancing overall stability, abrasion resistance, and, most importantly, moisture susceptibility. This study involved the incorporation of various steel fiber proportions of 0%, 0.3%, 0.5% and 0.7% by the total weight of mixture. The steel fiber modified SMA was made up of 6.0% PEN 60/70 bitumen content. The performance of SMA were evaluated through Marshall stability and flow test, Cantabro loss test and indirect tensile strength test. The results obtained from the testing showed that the incorporation of steel fiber is significantly effective to enhance the resistance towards moisture damage, while increasing the stability and reducing the abrasion loss of SMA mixture, compared to conventional mixture. Overall, it can be concluded that the addition of steel fiber in asphalt mixture specifically SMA, has improved the mechanical performance in the application of asphalt pavement with the optimum steel fiber proportion of 0.3% by the weight of mixture. The developed models between the independent variables and responses demonstrated high levels of correlation. The study found that Response Surface Methodology (RSM) is an effective statistical method for providing an appropriate empirical model for relating parameters and predicting the optimum performance of an asphaltic mixture to reduce flexible pavement failure.

Internal structure change of porous asphalt concrete under coupled conditions of load, moisture and temperature

In the actual working environment, the pore water pressure generated by wheel loading may accelerate the damage accumulation to the internal structure of porous asphalt concrete (PAC), resulting in the performance deterioration. In this study, a modified immersion wheel tracking test was developed by incorporating the image processing and analysis method to capture the internal structure change of the PAC at the different loading time. The varying temperatures and moisture conditions were also considered to evaluate the effect of the coupled condition. According to average rutting depth rate, the loading process was divided into three stages of condensation, transition and stabilization. The change of mesoscopic indices corresponding to each stage were analyzed. Based on void area change under wheel loading, it has been found that the rising temperature may worsen failure development and moisture damage may promote the condensation of the specimen. In comparison to a single-layer specimen, a strong constraint of the bottom layer inside a two-layer specimen on the top layer may limit void expansion. The change of aggregate skeleton was evaluated based on contact length. The loss of contact length could be due to filling failure, rheological failure and over-compaction failure. Because of the negative effect of increased temperature and moisture on asphalt mortar, wheel loading may cause more extensive skeleton degradation, leading in more contact length loss. The findings of this study reveal important information about the rutting resistance deterioration of PAC from a mesoscopic viewpoint.

Deformation superposition effect of asphalt mixture based on experiments and micromechanical modeling

Under multi-wheel heavy load, the asphalt mixture is prone to exhibit the deformation superposition effect, which exacerbates the damage of pavement structure. Multi-point penetration tests and numerical simulations by discrete element method (DEM) are performed to investigate the deformation superposition effect and micromechanical characteristics of asphalt mixture. The effect of wheel spacing, wheel group, and the evolution of micromechanical deformation superposition behavior are analyzed. Results indicate that the deformation superposition resistance of the asphalt mixture under the multi-wheel load decreases dramatically with the decrease in wheel spacing and the increase in the number of wheels, specifically the wheel spacing is 54 mm and the number of wheels is 4. The DEM simulations reflect the micromechanical property of asphalt mixture in the multi-point penetration test. The reduction of tensile chains is the internal reason for asphalt mixture deformation superposition, indicating the decrease of the adhesive strength of the material. A remarkably positive correlation is found between the reduction of the tensile chain and the deformation effect coefficient. In the process of superposition, the aggregate skeleton force chains are gradually destroyed and decrease to zero until cracking. The numerical simulation outcome is consistent with the laboratory penetration test outcome.

Effect of Coarse Recycled Aggregate on Failure Strength for Asphalt Mixture Using Experimental and DEM Method

Coarse aggregate is the major part of asphalt mixture, and plays an essential role in mechanical performance of pavement structure. However, the use of poor-quality coarse recycled aggregate (CRA) reduces the strength and stability of the aggregate skeleton. It is a challenge to predict accurately the influence of CRA on the performance of asphalt mixture. In this study, both a uniaxial compression test and a direct tensile test were carried out to evaluate the failure strength of asphalt concrete with four CRA content. The discrete element method (DEM) was applied to simulate the specimen of asphalt concrete considering the distribution and properties of CRA. The results showed that temperature and loading rate have a significant influence on failure strength, especially when the CRA content was more than 20%. With the increase of CRA content, both cohesion force and internal friction angle were gradually weakened. The proposed model can be used to predict the failure strength of asphalt mixture, since both experimental and simulated results had a high consistency and repeatability. With the decrease of CRA strength, the nominal cohesion force of the specimen decreased, while the internal friction angle increased.

Study on the Road Performance of Foamed Warm-Mixed Reclaimed Semi-Flexible Asphalt Pavement Material.

Warm-mixed reclaimed asphalt pavement (RAP) technology has been widely studied worldwide as a recycled environmental method to reuse waste materials. However, the aggregate skeleton structure of the warm-mixed reclaimed asphalt mixture is not stable because of the existence of the recycled materials. Warm-mixed recycled semi-flexible pavement material can solve the defects of the above materials. In this study, five different types of open-graded asphalt mixtures containing different contents of RAP were designed, and relevant laboratory tests were conducted to assess the road performance of the warm-mixed recycled semi-flexible pavement material. The test results indicated that the road performance of warm-mixed reclaimed semi-flexible pavement materials has good resistance to rut deformation ability. Furthermore, the materials also had good water stability and fatigue performance. The grey correlation analysis shows that the asphalt binder content has the most significant correlation with the high-temperature stability, and the correlation between RAP content and the fatigue performance was the greatest. Furthermore, the curing age has the most remarkable with the low-temperature crack resistance of the warm-mixed reclaimed semi-flexible material.

Synthesis and characterization of sustainable eco-friendly unburned bricks from slate tailings

The volume of tailings in the world has increased rapidly in recent years, which poses a substantial ecological threat. With the objective of reducing negative impacts on environment and reuse the abandoned slate tailings, the possibility of making eco-friendly unburned bricks by using slate tailings from Shaanxi Province of China was investigated. Raw materials included slate tailings, fly ash and cement, and bricks were produced through the process of mixing, molding, and curing. The optimal compressive strength of the product was found at the slate tailings: fly ash: cement = 5:2:3 (wt.%) with 7-day curing at room temperature and 1-day curing in oven, and the forming water content is 15 wt.%. Under these conditions, the compressive strength and water absorption rate were 26 MPa and 14.32% respectively. Other physical properties and durability conform to the Chinese GB/T4111-2013 standard for test methods for concrete blocks and bricks and the Chinese JC/T422-2007 standard for unfired bricks made of tailings. In addition, a variety of methods were combined to explore the formation mechanism. The phase composition, chemical structure and microstructure of the brick are analyzed by XRD, SEM and AFM. Finally, molecular dynamics (MD) was used to simulate main cementation component, the hydrated calcium silicate (C–S–H) gel, to advance the understanding of mechanical properties of brick at the nanoscale. The results show that the C–S–H gel formed in the curing process acted as the main adhesive component, and the slate tailings are mainly worked as aggregate skeleton to provide support strength for unburned bricks.

Mix proportion design method of recycled brick aggregate concrete based on aggregate skeleton theory

Due to the specific gravity of recycled brick aggregate (RBA) is less than that of natural aggregate (NA), the traditional mix proportion design of concrete can not be used. The purpose of this study is to propose a new mix proportion design method, which is suitable for recycled brick aggregate concrete (RBAC) or lightweight aggregate concrete. In this test, RBA volume fraction, sand volume fraction and water/cement ratio were considered; calculation formula of paste thickness on coated aggregate was established; calculation model of RBAC compressive strength was established, in which RBA volume fraction, the compressive strength of cement mortar and water/cement ratio were considered; mix proportion design method of RBAC based on aggregate skeleton theory was established. The results show that: when the volume fraction of RBA is between 44% and 47%, and the total volume fraction of RBA and sand is about 70%, the compressive strength of RBAC reaches the maximum; when the volume fraction of RBA is between 40% and 50%, the skeleton contribution strength of RBA is greater than that of cement mortar. When sand/cement ratio is about between 0.35 and 2.1, the sand skeleton contribution strength is higher. When the volume fraction of RBA is between 40% and 50%, the actual average paste thickness on coated coarse aggregate (APTCCA) is between 10.04 mm and 5.86 mm and the APTCCA simulated by PFC3D software is between 9.21 mm and 5.75 mm. In the compressive strength calculation model of RBAC, the relative errors of the predicted values are concentrated within 10%, the average relative error is 0.53%, and the correlation coefficient between the predicted values and the test values is 0.96; Compared with ACI 211.2 and ACI 211.6, the mix proportion design method proposed in this paper is to mix more RBA and less sand and cement in concrete mix proportion under the condition of ensuring concrete strength and slump, which is beneficial to protect the environment.