Carbonation and curing of cement stabilized gravels for use in subbases.
Egou, Stephen Isaac
Chesuro, Benerd Boris
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Naturally occurring granular materials are an important source for base course and subbase materials in the construction of flexible pavements in many countries (Emery, Cooks, & Keeley, 2007). In Uganda, locally available materials are given priority over imported materials during road construction projects. This is partly due to the costs involved in sourcing the gravels and the associated haulage distances. In order to enhance environmental sustainability and cost effectiveness of road construction and rehabilitation, there is need to make use of marginal road construction materials and to recycle existing pavement materials. This is frequently accompanied by the use of chemical stabilization to ensure that the appropriate material quality and structural capacity is achieved (Paige-Green, 2008). The most commonly used chemical stabilizing agents for gravels are cement and lime. Cement treatment of gravels to create cement-stabilized gravel has been widely used to improve the properties of gravel (Kenichiro & Tomomi, 2015). Chemical stabilization is increasingly becoming popular in improving the quality of sub-standard materials and recycling old pavement materials (Paige-Green, 2008). Paige-Green (2011) notes that despite many years of experience with lime and cement stabilization of road construction gravels and the comprehensive stabilization specifications, a noticeable increase in problems related to chemically stabilized materials has been observed. One of the frequently reported challenges in the use of cement in stabilization is the occurrence of carbonation during the stabilization process. In various investigations carried out on stabilized layers in roads mostly in South Africa as reported by Paige-Green (2009), it was shown that the problems related to the loss of stabilization and disintegration of stabilized layers was related to carbonation of the stabilized materials during curing and after construction. Carbonation of stabilized materials is a particular problem that needs to be assessed according to Paige-Green (2011). In Uganda, many roads have been chemically stabilized using cement (Uganda National Roads Authority [UNRA] reports on Tirinyi – Pallisa – Kumi road, 2018 and Kasese – Katunguru – Ishaka road, 2004). These roads have exhibited different levels of performances. Some of the recently upgraded UNRA roads whose subbases have been stabilized with cement include the 2 Musita – Lumino / Busia – Majanji, the Ishaka – Katunguru and the Tirinyi – Pallisa – Kumi roads. Failures have been reported on these roads and the UNRA technical teams have cited carbonation in the cement stabilized subbase as a major cause of these failures. According to UNRA, the Tirinyi – Pallisa – Kumi road subbase was stabilized using cement and challenges on curing were encountered due to the temperature changes which causes evaporation of water sprinkled on the road surface hence increasing the number of days required for minimum strength gain. Carbonation is an inevitable time dependent environmental process. It is the reaction between carbon dioxide (CO2) and the products of Portland cement hydration, the main participant being Calcium Hydroxide to produce calcium carbonate (CaCO3) precipitate. As a result of carbonation, a number of cementitious compounds in the stabilized gravels such as Calcium Hydroxide and Magnesium Hydroxide decompose and get depleted and destroyed (Beardmore, 2018). Carbonation causes premature decrease in stiffness (durability issues) and apparent recurrence in its pavement life. The consequences of carbonation could be detrimental due to the loss of strength and fatigue life of a pavement layer. Netterberg & Paige-Green (1987) articulated that the problem of deterioration of cement stabilized pavement layers during curing, before sealing and in service is partly ascribed to carbonation of the stabilizer during curing and subsequent exposure to the atmosphere before sealing. Research findings on the impact of carbonation have shown that there is progressive loss of strength of stabilized materials. The factors that contribute to carbonation are either extrinsic or intrinsic. However, taking cognizance of the extrinsic factors (which can only be controlled in limited ways in construction), the role of intrinsic factors is important and an understanding of these variables leads to improved understanding of the contributing factors (Beardmore, 2018). It is against this background that an investigation on the extent of carbonation during curing of cement stabilized gravels for use in subbases after standard time periods, was undertaken in this study.