Heat Transfer Characteristics of Crushed Rock and Lightweight Aggregate Materials

Over the last decades there has been a major transition to using crushed rock materials for road and railway construction instead of natural materials such as sand or gravel. In addition, it is common practice to use very coarse and sometimes open graded materials for structural layers. Knowledge of the thermal properties of construction materials is crucial for proper thermal design for road and railway structures. Thermal conductivity is the governing heat transfer mechanism in most such materials. However, with coarse open graded materials, natural air convection can make a significant contribution to the overall heat transfer. This study investigates the heat-transfer characteristics of granular materials with a focus on natural air convection. It consists of three parts: i) small-scale experiments on particle thermal conductivity and crushed rock thermal conductivity, and the validation and adaptation of an existing thermal conductivity model; ii) large-scale experiments on natural convection, the establishment of intrinsic permeability and subsequent modeling of large scale experiments; iii) investigations of possible air convection under field conditions in a full-scale road structure. The small-scale experiments showed to have good results on thermal conductivity measurements of particle thermal conductivity and thermal conductivity of crushed rock materials. For the validation and adaptation of the thermal conductivity model, 42 samples were prepared and 328 tests conducted in unfrozen and frozen states with different degrees of moisture content. A large-scale heat transfer cell with an inner volume of 1m3 was used to measure the increase in heat transfer due to natural air convection. A method based on the analytical Nusselt (Nu)-Rayleigh (Ra) number relationship was used to establish the intrinsic permeability (K) of different construction materials. These included three crushed rock materials (20/120, 40/120 and 20/250 mm) and two lightweight aggregates: expanded clay and foam glass. The experimental results showed that natural air convection could be initiated in all three crushed rock materials at a fairly low temperature gradient (∇T) of 4.5 to 6.5° C/m. The established K for crushed rock materials ranged from 1.1 to 2.2x10-6 m2. Air convection for foam glass and expanded clay (10/20 mm) material was initiated at critical ∇T of 11.0 and 22.5°C/m respectively. The corresponding K values based on the experimental measurements were 0.86 and 0.17x10-6 m2. The overall results show that air convection can be induced at a relatively low ∇T. To observe material characteristics under field conditions, a full-scale test site was constructed incorporating ten different sections. This study, however, focuses on only two sections, which had layers of open graded materials. Observations of the temperature profiles during the cooling period found that ∇T exceeded the critical value in all open graded layers. However, given that the air convection also depends on layer thickness, apparent Rayleigh number (Ra*) was calculated. The results show that Ra* exceeded the critical value only in a 1.0 m thick frost protection layer (40/120 mm crushed rock) and a 0.6 m thick foam glass layer (10/60 mm). To further investigate possible air convection under field conditions, a numerical model was developed. The results from the model clearly show the presence and effect of air convection. However, it could be that the effect of convection was overestimated, given that the K values were established under laboratory conditions with materials with no compaction. To sum up, this study has shown that a simple thermal conductivity model could be adapted, with slight calibration, for the estimation of thermal conductivity in crushed rock materials. The study has also shown that natural air convection could easily be initiated in coarse materials used in road construction under relatively low temperature gradients. The study at the field test site found that layers with coarse, open graded materials are exposed to very high temperature gradients. The field observations and analysis together with the numerical model showed the possible presence of natural air convection. Air convection can have an adverse effect on seasonal freezing environments by increasing the depth of frost penetration, resulting in possible differential heaving

Experimental study on the effect of soil moisture content on critical temperature rise for typical cable backfill materials

The increased utilization of power grid requires operating power cables close to their thermal limit. Buried power cables in such condition experience thermal instability where thermal resistance increases as the moisture migrates away from the proximity of the cable forming a dry-out zone. While it is common to use a two-zone model to account for the dry-out zone in ampacity calculations, there is a limited number of studies on characterizing backfill materials for their critical temperature rise ( Δθx ), especially for crushed rock sands. In addition, dependency of the Δθx on moisture content is sometimes not acknowledged. This study measures the Δθx for three typical backfill materials and investigates the relationship between Δθx and moisture content. The results show that crushed rock sand has higher Δθx compared to natural sand. Overall, large range of Δθx was measured depending on moisture content and type of soil. Ampacity calculations with the established Δθx show that at low moisture content, the thermal resistivity of the soil has a higher influence on ampacity than Δθx . At relatively high moisture content, the Δθx becomes the predominant factor governing the overall ampacity

Laboratory investigations into convective heat transfer in road construction materials

This paper presents a laboratory investigation into natural air convection and the establishment of intrinsic permeability of road and railway construction materials. The laboratory investigations were performed using a heat transfer cell with an inner volume of 1 m3. The study shows the importance of natural air convection and a practical method for establishing the intrinsic permeability of coarse granular materials. Three different open-graded crushed rock materials and two lightweight aggregates were tested. All materials were tested for downward (conduction only) and upward (convection and conduction) heat flow conditions. The experimental results revealed that all three crushed rock materials are prone to developing natural air convection in thermal gradients of 4.5 to 11 °C/m, depending on the particle size distribution. Foam glass aggregates showed a convective heat transfer flow at the fairly low temperature gradient of 6.5 °C/m. No natural air convection was achieved in expanded clay aggregates within the temperature gradients imposed. Intrinsic permeability values were established based on the experimental results. The intrinsic permeability of crushed rock materials ranged from 1.1 to 2.2 × 10−6 m2 while that of foam glass materials was 0.9 × 10−6 m2.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author