Browsing by Author "Engelbrecht, Francois"

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    Age-related performance of typical seal binders in South Africa
    (Stellenbosch : Stellenbosch University, 2018-03) Engelbrecht, Francois; Jenkins, Kim; Bredenhann, Steph; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The age-related performance of typical seal binders in South Africa was studied to understand the behaviour of the binders as it ages. The three main failure mechanisms for surfacing seals include surface cracking, ravelling and texture loss. These mechanisms shorten the life of the pavement exponentially. Various factors may be the cause of these failures. This study includes the analyses of the rate at which oxidation influences the performance of surfacing seals. Rheology was used to analyse the age-related performance of the selected seal binders. Low and high temperature testing was conducted in the laboratory at various ages such as RTFO and PAV (20hr, 40hr and 80hr) which includes short-term ageing and long-term ageing of a binder. Several ageing parameters were used to analyse the results of each seal binder. The South African Performance Grade Specification and the AASHTO M320 specification are established to set the required performance of asphalt binders. These specifications were used to compare the performance of surfacing seal binders to asphalt binders. Results showed that ageing influences the modification within surfacing seal binders. It appears that the performance of the elastic component in the modifier decreases as the binders age. The low temperature results show that the difference between the unmodified and modified binders are minimal. The high temperature results indicate that the highly modified binders perform well considering the influence of ageing.
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    Large uncertainties in future biome changes in Africa call for flexible climate adaptation strategies
    (John Wiley & Sons Ltd, 2020) Martens, Carola; Hickler, Thomas; Davis-Reddy, Claire; Engelbrecht, Francois; Higgins, Steven I.; Von Maltitz, Graham P.; Midgley, Guy F.; Pfeiffer, Mirjam; Scheiter, Simon
    Anthropogenic climate change is expected to impact ecosystem structure, biodiversity and ecosystem services in Africa profoundly. We used the adaptive Dynamic Global Vegetation Model (aDGVM), which was originally developed and tested for Africa, to quantify sources of uncertainties in simulated African potential natural vegetation towards the end of the 21st century. We forced the aDGVM with regionally downscaled high-resolution climate scenarios based on an ensemble of six general circulation models (GCMs) under two representative concentration pathways (RCPs 4.5 and 8.5). Our study assessed the direct effects of climate change and elevated CO2 on vegetation change and its plant-physiological drivers. Total increase in carbon in aboveground biomass in Africa until the end of the century was between 18% to 43% (RCP4.5) and 37% to 61% (RCP8.5) and was associated with woody encroachment into grasslands and increased woody cover in savannas. When direct effects of CO2 on plants were omitted, woody encroachment was muted and carbon in aboveground vegetation changed between –8 to 11% (RCP 4.5) and –22 to –6% (RCP8.5). Simulated biome changes lacked consistent large-scale geographical patterns of change across scenarios. In Ethiopia and the Sahara/Sahel transition zone, the biome changes forecast by the aDGVM were consistent across GCMs and RCPs. Direct effects from elevated CO2 were associated with substantial increases in water use efficiency, primarily driven by photosynthesis enhancement, which may relieve soil moisture limitations to plant productivity. At the ecosystem level, interactions between fire and woody plant demography further promoted woody encroachment. We conclude that substantial future biome changes due to climate and CO2 changes are likely across Africa. Because of the large uncertainties in future projections, adaptation strategies must be highly flexible. Focused research on CO2 effects, and improved model representations of these effects will be necessary to reduce these uncertainties.