Lower Air Void Content Research Articles

The Role of Water Content and Binder to Aggregate Ratio on the Performance of Metakaolin-Based Geopolymer Mortars

Geopolymers are in many applications a perfect alternative to standard cements, especially regarding the sustainable development of green building materials. This experimental study therefore deals with the investigation of different factors, such as the water content and the binder to aggregate ratio, and their influence on the workability of fresh mortar and its mechanical properties and porosity on different size scales. Although increasing the water content improved the workability and flow behaviour of the fresh mortar, at the same time, a reduction in compressive strength in particular and a lesser reduction in flexural strength could be demonstrated. This finding can be attributed to an increase in capillary porosity, as demonstrated by capillary water uptake and mercury intrusion porosimetry measurements. At the same time, the increasing water content led to an improved deaeration effect (low air void content) and to initial segregation (see the µXCT measurements). An alternative approach to enhance the compressive and flexural strengths of the mortar specimens is optimization of the binder to aggregate ratio from 1 to 0.25. This study paves the way for a comprehensive understanding of the underlying chemistry of the geopolymerization reaction and is crucial for the development of sustainable alternatives to cementitious systems.

Research on the durability of nano-SiO2 and sodium silicate co-modified recycled coarse aggregate (RCA) concrete

Compared with natural aggregate (NA), recycled coarse aggregate (RCA) is of lower strength and higher porosity due to the attached mortar, which limits the application of RCA. In this paper, the nano SiO2-sodium silicate mixed solution (NMS) was prepared and used to modify RCA by improving the properties of the attached mortar. Three groups of concrete with NA, RCA, and modified RCA (MRCA), respectively, were prepared. The modification effects of NMS on the concrete were illustrated in regards to the mechanical properties and durability (impermeability, chloride ion resistance, and freezing-thawing resistance). Results indicated that the concrete with the MRCA had higher mechanical properties, better durability properties, and lower air void content. Then, the surface morphology of the MRCA and interfacial transition zone (ITZ) in concrete were both examined by scanning electron microscope (SEM) to understand the modification effect of NMS. It showed that the NMS can fill the pores and repair the cracks on the attached mortar by filling effect (brought by nano SiO2) and film-forming effect (brought by sodium silicate). Moreover, nano SiO2 and sodium silicate could also participate in secondary hydration reaction to form C-S-H gel and strengthen the microstructure of ITZ. The coupling effect both result in the improved mechanical and durable properties of the concrete prepared with MRCA.

Assessment of resistance to permanent deformations of asphalt mixes of low air void content

Abstract This article presents the issue of resistance to permanent deformations of bridge pavements. In Europe, bridge asphalt pavement usually consists of the wearing course and the protective layer, which are paved on the waterproofing. Protective layers of bridge pavement are often constructed using low air void asphalt mixes. Such mixes are characterized by water-tightness, resistance to unfavorable environmental conditions, and enhanced fatigue life. Due to increased binder content, asphalt mixes for bridge pavement may display reduced resistance to permanent deformations. The article presents the test results of resistance to permanent deformations of asphalt mixes for bridge pavement. The study compares the rutting resistance for mixture use with low air void content. This specific kind of mixtures are dedicated to pavements on bridge deck. In this study, the resistance to permanent deformations was tested depending on the content of grit structure and air void content. Laboratory test results were compared for stone mastic asphalt and asphalt concrete, characterized by low air void content. Resistance to permanent deformations was assessed using the cyclic uniaxial compression (dynamic load creep) method. On the basis of analysis of the test results, it was shown that resistance of asphalt mixes to permanent deformations depends most of all on the type of mix designed and the air void content.

Preparation, compression behavior and 3D damage evolution of epoxy syntactic foam with nickel hollow spheres

The nickel thin-walled hollow spheres (NHS) epoxy syntactic foam was fabricated by the infiltration method. The crystal structure and grain size of the NHS were quantitatively analyzed by FIB-TEM. The bulk compressive response of the NHS syntactic foam was determined and compared with glass bubble syntactic foam. The X-ray computed microtomography (XCT) coupling with the interrupted compression tests were conducted on a single NHS and the NHS syntactic foam to demonstrate the 3D deformation history. The bulk behaviors of the NHS syntactic foam were correlated with the XCT examinations to analyze the mechanisms of the failure process. The electrodeposited NHS shows the nanocrystalline and polycrystalline microstructure. The low air void content was achieved in the NHS syntactic foam produced by the infiltration method. Although the NHS syntactic foam's elasticity and yield strength are lower, the compressive resistance and toughness as well as the energy absorption of the NHS syntactic foam are higher than those of glass syntactic foam. The failure process of the NHS syntactic foam is dominated by the NHSs deformation and the resultant strain hardening. The damage initiates from the weak NHSs, then more crushed bands arise with the increased compressive stress. The further deformation of the damaged bands can intermittently develop owing to the strain hardening. During the deformation history, no visible macro cracks can be found in the epoxy matrix from the XCT slices.

Study of High-Temperature Properties of Asphalt Mixtures Used for Bridge Pavement with Concrete Deck.

The paper presents the issue of resistance to permanent deformations of bridge pavements placed upon concrete bridge decks. In Europe, bridge asphalt pavement usually consists of a wearing course and a protective layer, which are placed over the insulation (waterproofing). Protective layers of bridge pavement are commonly constructed using low air void content asphalt mixes as this provides the suitable tightness of such layers. Due to increased binder content, asphalt mixes for bridge pavement may have reduced resistance to permanent deformations. The article presents test results of resistance to permanent deformations of asphalt mixes for the protective layers. In order to determine the composition of mixtures with low air void content and resistance to permanent deformation, an experimental design was applied using a new concept of asphalt mix composition. Twenty-seven different asphalt mixture compositions were analyzed. The mixtures varied in terms of binder content, sand content and grit ratio. Resistance to permanent deformation was tested using the laboratory uniaxial cyclic compression method (dynamic load creep). On the basis of experimental results and statistical analysis, the functions of asphalt mixture permanent deformation resistance were established. This enabled a determination of suitable mixture compositions for protective layers for concrete bridge decks.

The development of a novel, microwave assisted, half-warm mixed asphalt

Global warming is an imminent threat that the world and its inhabitants have to confront. As such, it is the duty of the pavement industry as a contributor of greenhouse gas emissions, to pave the way by lowering its carbon footprint. It is vital that new measures are adopted in order to do so such as employing the use of emulsion-based mixtures (EBM). The problem with this is that the performance properties of such mixtures are inferior to that of traditional hot mix asphalts (HMAs). The air void content of EBM is very high and considered unacceptable by road engineers for application as a surface layer. That said, these mixtures are not only environmentally friendly but also boast ecological and economic advantages. An innovate approach was applied in this research by using a pre-compaction microwave processing technique to develop a novel, half-warm mix asphalt mixture (H-WM). This new mix was shown to have improved mechanical properties and lower air void content. EBM mixtures comprised of cementitious binary blended filler, were prepared using microwave heating applied over different lengths of time. Stiffness modulus, air voids content and temperature were used to establish the optimum microwave radiation time. The results indicated that 1.5 min of microwave processing decreased air void content from 8.92% to 7.12%. A 7% improvement in stiffness modulus was also found and the temperature was within lower limits (43°C). Hydration was accelerated by the microwave radiation, and the demulsification of bitumen emulsion was promoted. Microwave processing was found to have a positive impact on permanent deformation at elevated temperatures in comparison to the two reference HMAs used. It also proved to be an adequate technique to produce a fast-curing H-WM with lower air voids content. Water damage resistance for the microwaved H-WM (99%) is better than the reference HMA mixes. The findings of this study show that the novel half-warm asphalt mixture has superior properties in comparison to EBM.

Laboratory and field performance comparison of dense graded and stone mastic asphalt as a runway surface

ABSTRACT Ungrooved stone mastic asphalt (SMA) provides an alternate to grooved dense graded asphalt (DGA) as a runway surface. To be verified as suitable in this application, SMA must provide similar or better asphalt surface performance, compared to DGA, and must achieve regulated aircraft skid resistance requirements. A pilot runway resurfacing project provided the basis for this laboratory and field comparison of ungrooved SMA to grooved DGA as a runway surface. Laboratory performance testing demonstrated adequate and comparable deformation, fatigue and moisture resistance of SMA, while the quality assurance records indicated acceptable asphalt production. The construction records indicated that SMA had a lower air void content in the compacted layer, which can only provide a more durable surface. Most importantly, the aircraft skid resistance of SMA exceeded that of grooved DGA, based on surface macro-texture and wet friction measurements. It was concluded that SMA performed equal to or better than DGA, particularly with regard to surface texture and wet friction and it is recommended that airports consider ungrooved SMA as a runway surface in the future. The demonstration project should also be monitored in the future, to verify the long-term performance of SMA, compared to the DGA.

100% recycled high-modulus asphalt concrete mixture design and validation using vehicle simulator

High modulus asphalt concrete (HMAC) mixtures are designed for high rutting resistance, high modulus and excellent fatigue performance. This is achieved through the use of high content of hard bitumen, low air void content, and use of performance-based testing for mixture design. Such approach is presumably well-matched for application of Reclaimed Asphalt Pavement (RAP): the RAP binder is hard because of aging, RAP mixtures inherently have low air voids and performance-based mix design is recommended to increase the degree of reliability when using high RAP mixtures. Here we present a study focused on designing and validating performance of HMAC from 100% RAP. Through multiple design iterations we found that it was not possible to fully fulfill the fatigue, modulus and rutting requirements for either of the recycled HMAC mixture types (C1 or C2) although the performance of one of the 100% RAP mixtures came close to the HMAC design requirements. Nevertheless, validating of the best-performing 100% RAP HMAC slabs using vehicle load simulator demonstrated that the recycled HMAC is significantly less resistant towards crack propagation compared to a conventional HMAC. This highlights the importance of testing cracking resistance for high RAP mixtures.

Optimising the moisture durability SATS conditioning parameters for universal asphalt mixture application

The saturation ageing tensile stiffness (SATS) procedure is the first laboratory asphalt mixture durability protocol to combine moisture damage and oxidative ageing during the conditioning of asphalt mixtures prior to mechanical testing. However, the application of the SATS procedure has tended to be predominantly targeted at base materials with relatively ‘hard’ binders and overall high stiffness modulus requirements and is considered potentially too severe for more standard asphalt mixtures. This research study focused on understanding the effect of changes to the SATS conditioning parameters on the relative moisture damage assessment performance of asphalt mixtures, particularly ‘softer’ binder, high binder content and, finally, low air void content mixtures. Using four different aggregate types, together with ‘hard’ 10/20 pen and ‘softer’ 40/60 pen bitumen, the parameters of pressure, temperature and duration were altered, either one at a time or in combination, to arrive at a suitable combination for testing more commonly used 40/60 pen material. The optimum combination of parameters to allow the SATS procedure to be used for a wide range of typically used asphalt mixtures was found to be 0.5 MPa pressure, 85°C temperature and 24 h duration. The revised SATS durability conditioning protocol was still able to successfully discriminate between ‘poor’ moisture susceptible aggregate and ‘good’ material.

Characterizing Permanent Deformation and Fracture of Asphalt Mixtures by Using Compressive Dynamic Modulus Tests

Permanent deformation and fracture may develop simultaneously when an asphalt mixture is subjected to a compressive load. The objective of this research is to separate viscoplasticity and viscofracture from viscoelasticity so that the permanent deformation and fracture of the asphalt mixtures can be individually and accurately characterized without the influence of viscoelasticity. The undamaged properties of 16 asphalt mixtures that have two binder types, two air void contents, and two aging conditions are first obtained by conducting nondestructive creep tests and nondestructive dynamic modulus tests. Testing results are analyzed by using the linear viscoelastic theory in which the creep compliance and the relaxation modulus are modeled by the Prony model. The dynamic modulus and phase angle of the undamaged asphalt mixtures remained constant with the load cycles. The undamaged asphalt mixtures are then used to perform the destructive dynamic modulus tests in which the dynamic modulus and phase angle of the damaged asphalt mixtures vary with load cycles. This indicates plastic evolution and crack propagation. The growth of cracks is signaled principally by the increase of the phase angle, which occurs only in the tertiary stage. The measured total strain is successfully decomposed into elastic strain, viscoelastic strain, plastic strain, viscoplastic strain, and viscofracture strain by employing the pseudostrain concept and the extended elastic-viscoelastic correspondence principle. The separated viscoplastic strain uses a predictive model to characterize the permanent deformation. The separated viscofracture strain uses a fracture strain model to characterize the fracture of the asphalt mixtures in which the flow number is determined and a crack speed index is proposed. Comparisons of the 16 samples show that aged asphalt mixtures with a low air void content have a better performance, resisting permanent deformation and fracture.

Measuring the Complex Modulus of Asphalt Concrete Using Ultrasonic Testing

Dynamic modulus mastercurves are essential for the design and modeling of asphalt concrete (AC). One way of improving the accuracy of the upper asymptote of the mastercurve is to test at extremely high frequencies or extremely low temperatures. Ultrasound is used extensively in the nondestructive testing of materials and the work completed here demonstrates the potential for the application of this technology to AC. Since testing at extremely low temperatures is not practical, a new ultrasonic technique is developed for measuring the complex moduli of AC. A theoretical explanation of the measurement process is provided. Two AC specimens were tested using the ultrasonic method and the dynamic modulus method in the indirect tensile test (IDT) mode. Both test techniques were performed at four different temperatures. The mastercurves were constructed using time-temperature superposition on the IDT test data and the upper asymptotes were extrapolated. The ultrasonic data was shifted to the desired reference temperature and the predicted moduli were compared to those of the IDT test. It was found that the moduli predicted using the ultrasound measurement agreed well for the specimen with a lower air-void content and differed more for the specimen with a higher air-void content. The phase angles predicted by the ultrasonic method were higher than those obtained from the IDT test. It is believed that this was a result of wave scattering from air-voids and aggregates. Suggestions are made to further increase the accuracy of the technique.

Influence of water and temperature on mechanical properties of selected asphalt pavements

One of the main causes of distress in asphalt pavements is water damage. The purpose of this paper is to compare different test methods to evaluate moisture susceptibility. This is of special importance because of the insufficient effectiveness of the test procedures currently used. In this research, experiments were conducted to investigate the effects of water and temperature on mechanical properties of mixtures with different, air void content. The evaluation of such properties concentrates on the following three approaches: innovative (Coaxial Shear Test), traditional, (Indirect Tensile Test) and empirical (Cantabro Test). Specimens were prepared by means of Superpave Gyratory Compactor (SGC) and divided in two different subsets for controlled dry and wet conditioned testing. Test results indicated that the Indirect Tensile Test (IDT) is not able to discriminate between wet and dry condition as the Coaxial Shear Test (CAST) does. The CAST method reproduces closest the real field conditions and indicates clearly the risk of water damage for open graded mixtures (high air void content). Dense graded mixtures (low air void content) showed less influence probably due to the reduced amount of penetrating water. Cantabro Tests (CAT) provided also significant results in good correlation with air void content and material properties of asphalt mixes.

Improving silica fume cement by using silane

Silica fume cement was improved by using silane, which was introduced by either coating silica fume particles with silane or using silane as an admixture. Both methods of silane introduction similarly enhanced the workability, tensile strength and compressive strength, but the latter method resulted in lower compressive ductility, lower damping capacity, more drying shrinkage, lower air void content, higher density, higher specific heat and greater thermal conductivity, mainly due to the network of covalent coupling among the silica fume particles.

Effects of Asphalt Content and Air Void Content on Mix Fatigue and Stiffness

The primary objective of most procedures for asphalt concrete mix design is to find an asphalt content that minimizes the possibility of stability failure while providing adequate fatigue and other durability characteristics. To date, the consequences of asphalt content selection and construction compaction on fatigue performance and flexural stiffness have not been thoroughly investigated and documented with experimental data. The results of laboratory-controlled strain flexural beam testing, (i.e., fatigue life and flexural stiffness) for one aggregate and asphalt cement combination, five asphalt contents, and three air void contents are presented. The results clearly indicate the benefits of a lower air void content on both fatigue life and initial stiffness. Increased asphalt content was found to increase fatigue life and reduce stiffness. Alternative models for predicting fatigue life and initial stiffness using asphalt content, air void content, voids filled with bitumen, and the volume concentrations of asphalt and aggregate were evaluated. Elastic-layer theory was used to simulate the effects of air void content and asphalt content on the fatigue life of several example overlays using the models for stiffness and fatigue life from the laboratory testing. The simulations indicated an increase in predicted pavement fatigue life for lower air void contents and higher asphalt contents. Example simulations of the effects of increased asphalt content and decreased air void content at the bottom of thick overlays indicated a marked increase in predicted fatigue life. It was also concluded that stiffness should not be included in regression for fatigue life models for mix design unless there is a clear understanding of the effects of other variables in the model that correlate with both fatigue life and stiffness.