N. H. Thom

The use of additives to enhance properties of pre-formed foamed concrete

This paper describes an experimental study of an enhancement of pre-formed foamed concrete, 1300-1900 kg/m3, by utilising two types of additives, silica fume and fly ash, to partially replace Portland cement and fine sand. It focuses on consistency, mechanical and thermal properties as well as presenting a comparison with normal weight, lightweight and foamed concretes from the literature. In addition to conventional foamed concrete mixes (FC), foamed concrete mixes with high flowability and strength (FCa) were also manufactured in this study. The FC mixes had 28-day compressive strengths from 6 to 23 MPa and corresponding thermal conductivities in the dry state from 0.475 to 0.951 W/mK, whereas for the same density range, the FCa mixes gave 19-47 MPa and 0.498-0.962 W/mK, respectively. Compared to other studies on foamed concrete, the results from the mixes investigated in this study showed higher strengths (for a given density), higher tensile to compressive strength ratios and higher moduli of elasticity.

A study of geogrid-reinforced ballast using laboratory pull-out tests and discrete element modelling

This paper presents an evaluation of the behaviour of geogrid-reinforced railway ballast. Experimental large box pull-out tests were conducted to examine the key parameters influencing the interaction between ballast and the geogrid. The experimental results demonstrated that the triaxial geogrid outperforms the biaxial geogrid and the geogrid aperture size is more influential than rib profile and junction profile. The discrete element method (DEM) has then been used to model the interaction between ballast and geogrid by simulating large box pull-out tests and comparing with experimental results. The DEM simulation results have been shown to provide good predictions of the pull-out resistance and reveal the distribution of contact forces in the geogrid-reinforced ballast system. Therefore, the calibrated geogrid model and the use of clumps to model ballast particles hold much promise for investigating the interaction between geogrids and ballast and therefore optimising performance.

Concrete Pavement Design Guidance Notes

This book provides a guide to the design of concrete pavements which summarises current best practice. Following an overview of the theory involved, the authors detail optimum design techniques with a focus on highway and infrastructure projects. Worked examples and calculations are provided to describe standard design methods, illustrated with numerous case studies. The authors provide guidance on how to use each design method complete with particular projects. Reference is made to UK, European and US standards and codes of practice including US AASHTO methods and guides. The chapters cover: surface slab systems; concrete bases and sub-bases; testing and specifications; concrete slab analysis methods; design inputs and assumptions; concrete pavement design methods; composite pavement design; joints; and detailing.

Experimental Investigation of Bitumen and Mastics under Shear Creep and Creep-Recovery Testing

This paper characterises binder and mastic behaviour under shear creep and creeprecovery tests. Although several researchers have investigated the creep and recovery behaviour of pure and polymer modified bitumen, there is a primary need to investigate the behaviour of mastic in such a way to better correlate with the behaviour of asphalt mixtures. The steady state condition under creep testing and recovery behaviour under creep recovery testing are characterised through testing in the dynamic shear rheometer (DSR) under a wide range of stresses and temperatures. 40/60 pen pure bitumen as well as binder-filler mastics containing limestone blended at three different concentrations [35%, 50%, and 65% by mass of mastic] were utilised for this study. Creep testing results indicated that pure bitumen and mastics behave linearly at low stress levels and nonlinearly at high stress levels. In addition, the stiffening effect of filler is characterised through a new Stiffing Factor. Stiffening Factor was determined for each bitumen-mastic combination obtained under similar testing conditions based on a steady-state curve fitting technique. Results analysis of creeprecovery tests showed that the recovery property of the bitumen is affected by both stress limit and filler concentration but insignificantly influenced by temperature. Moreover, the recovery property of bituminous material reduces with increasing filler content.

Laboratory assessment of the resistance to crack propagation in high-stiffness asphaltic materials

Abstract This paper describes laboratory work undertaken to measure the resistance to crack propagation of bituminous materials comprising a range of binder grades from different sources. The compact tension (CT) test has been adapted for use with bituminous materials, and an image analysis technique, based on digital photographs of the specimen taken during testing, has been used to quantify crack length. Linear elastic fracture mechanics (LEFM) principles have been used to calculate the mode I stress intensity factor (SIF), and the Paris law has been used to quantify crack growth. Results show that the general trend is for the Paris law exponent n to decrease and for the multiplier A to increase with increasing temperature for each of the materials, indicating that, the harder the bitumen, the greater is the sensitivity to overloading. A good correlation was found between the values of A and n, and also between the values of A in the Paris law and the stiffness modulus of the asphaltic mixture. It was found that mixtures containing the nominal 15 penetration grade binders were least sensitive to variations in stiffness of the mixture, and mixtures containing the nominal 35 and 50 penetration grade binders were most sensitive to variations in stiffness of the mixture. The number of standard wheel loads required to cause a crack to propagate through the asphalt layer of a typical minor and major flexible pavement structure have been estimated using data from the CT testing. Results show that there is a good correlation between the stiffness modulus of the asphalt mixture and the predicted number of load applications to failure. The predicted number of load applications to failure increases as the stiffness modulus of the asphalt increases.

Laboratory Mix Design Procedure for Foamed Bitumen Mixtures

The primary objective of this study was to propose a practical and consistent mix design procedure for foamed bitumen mixture (FBM). The main focus of the proposed methodology was the use of the gyratory compaction method. The study maximized the mix design parameters, such as mixing water content (MWC) and compaction effort. The study was initially carried out on FBMs with virgin limestone aggregate without reclaimed asphalt pavement (RAP) material, and a mix design procedure was proposed. The proposed methodology was validated on FBMs with 50% RAP and 75% RAP. Efforts were also made to optimize the foamed bitumen (FB) content in FBMs with and without RAP. Optimum MWC was achieved by optimizing mechanical properties such as indirect tensile stiffness modulus and indirect tensile strength (ITS-dry and ITS-wet). A rational range of 75% to 85% optimum water content was obtained by the modified Proctor test and found to be the optimum range of MWC that gave optimum mechanical properties for FBMs. The study used a gyratory compactor for FBM compaction and found that a unique, mixture-specific number of gyrations that was independent of the FB content could be established. The study also found that the presence of RAP influenced the designs FB content: treating RAP as black rock in FBM mix design was not appropriate.

Mix design considerations of foamed bitumen mixtures with reclaimed asphalt pavement material

Abstract In the present work, a mix design parametric study was carried out with the aim of proposing a practical and consistent mix design procedure for foamed bitumen mixtures (FBMs). The mix design parameters that were adopted in the study are mixing and compaction water content (MWC), compaction effort using a gyratory compactor and aggregate temperature. This parametric study was initially carried out on FBMs with virgin limestone aggregate without reclaimed asphalt pavement (RAP) material and a mix design procedure was proposed. This proposed methodology was also found to apply to FBMs with RAP. A detailed consideration was also given to characterising the RAP material so as to understand its contribution to the mechanical properties of FBMs. Optimum MWC was achieved by optimising mechanical properties such as indirect tensile stiffness modulus and indirect tensile strength (ITS-dry and ITS-wet). A rational range of 75–85% of optimum water content obtained by the modified Proctor test was found to be the optimum range of MWC that gives optimum mechanical properties for FBMs. It was also found that the presence of RAP influenced the design foamed bitumen content, which means that treating RAP as black rock in FBM mix design is not appropriate. To study the influence of bitumen and water during compaction, modified Proctor compaction and gyratory compaction were employed on mixes with varying amounts of water and bitumen. By this, the work also evaluated the validity of the total fluid (water + bitumen) concept that is widely used in bitumen–emulsion-treated mixes, and found it not to be applicable.

Micromechanical Structure-Property Relationships for the Damage Analysis of Impact-Loaded Sustainable Concrete

In this study, quantitative microstructure-property relationships are mainly used to characterize the damage due to high-strain-rate impact loading and the mechanical behavior of concretes prepared by substituting natural aggregate (gravel) with recycled aggregates having different rigidities (blue brick and rubber). Based on the results obtained, a possible mechanism for microstructural damage in concrete is proposed. It is concluded that the aggregate causes a change in the initial interfacial transition zone (ITZ) condition, and it is this altered ITZ condition that has a major effect on overall mix behavior. The analysis also indicates that there is almost a linear correlation between the roughness values (Ra) of the region near the paste–aggregate interface and the dissipated surface fracture energy values of the specimens. Moreover, three-dimensional topographic images of the specimens constructed using a vertical nanotech scanning interferometer show that the paste region of the gravel specimen has the smoothest profile due to the relatively strong hydrated paste.

Testing of Unbound Materials in the Nottingham Asphalt Tester Springbox

The current trend in mechanistic (analytical) pavement design is to use the mechanistic properties of pavement materials to optimize design. This is compatible with the move toward performance-based specifications and away from traditional empirically based design methods and recipe specifications. Other drivers-such as the Europeanwide adoption of aggregate mixture standards, which no longer differentiate on source, but moves toward sustainable construction-mean that a wide range of recycled, secondary, and primary aggregate sources can potentially be used in highway construction. The requirement for accelerated, performance-based testing is therefore coming to the fore. The U.K. Highways Agency has funded an accelerated testing program across a range of unbound capping and subbase materials. The performance parameters assessed, over a range of moisture and soaking conditions, are resistance to permanent deformation and resilient stiffness. The apparatus used during the unbound mixture assessments is the newly developed Springbox, which uses the standard Nottingham Asphalt Tester loading frame and software. This follows a simplification of the K-mold test and facilitates repeated loading of 170-mm cubic specimens under variable confinement. Samples are compacted into stainless steel liners. If required, samples can then be soaked before placement within the Springbox apparatus for testing. The following are discussed: the aggregate mixture performance parameters being measured, sample preparation procedures, the Springbox equipment and test procedures, a testing program of unbound capping and subbase materials, and results. The performance of the Springbox apparatus is discussed, and conclusions on the relative performance of the aggregates are presented.

Characterisation of high-performance cold bitumen emulsion mixtures for surface courses

Abstract Cold bitumen emulsion mixture (CBEM) is not yet widely used as a surface course around the world. In this study, 0/14-mm-size dense-graded surface course CBEMs have been investigated. The mechanical performance was evaluated in terms of stiffness modulus over 3 months and resistance to permanent deformation under three different stress levels (100, 200, 300 kPa), whilst durability evaluation was carried out in terms of resistance to moisture and frost damage. The study has also investigated the incorporation of low cement content (1%) with relatively sustainable by-product fillers, namely ground-granulated blast furnace slag (GGBS) and fly ash (FA) type 450-S on both mechanical and durability performance. A comparison has been carried out between the low and high cement content CBEM, as well as with respect to corresponding hot mix asphalt (HMA). The results revealed that the incorporation of GGBS and FA in CBEMs leads to superior performance, similar to CBEMs treated with high cement content and comparable to an equivalent HMA. Furthermore, GGBS replacement exhibited better performance than that of FA replacement. The findings suggest that the new sustainable types of CBEM can be developed for using as a surface layer for medium- to heavy-trafficked roads.