The relatively high presence of silica found in the rocks in the quarries of the State plays a pivotal role in the damage of asphalt roads and severe pavement distress immediately after a heavy spell of rain in Kerala, says a joint study conducted by Kerala Highway Research Institute (KHRI), and department of Geology, University of Kerala, Thiruvananthapuram.
The research team headed by Sreejith G.H., Deputy Director, KHRI, and Anilkumar Y., Assistant Professor, University of Kerala, found that the silica content (SiO2) in rocks sourced from the quarries in the State—collected mainly from Thiruvananthapuram, Kollam, and Pathanamthitta districts—vary from 52.46% to 73.09%, posing a serious challenge to the engineering properties of the bituminous mixes used for road construction. Acidic rocks having an excess content of silica have a tendency to detach from bitumen in the presence of water.
The SiO2 content, found based on X-ray Fluorescence (XRF) analysis of 15 aggregate samples sourced from southern Kerala, varied from 52.46% to 73.09%. Out of the 15 samples, nine samples were highly acidic (i.e. SiO2 Content > 63%) and six were intermediately acidic in nature (i.e SiO2 ranged between 52% and 63%), says the study.
According to Mr. Sreejith, aggregates are usually classified into two categories—hydrophilic (water-loving) and hydrophobic (water-repelling). Hydrophilic aggregates are characterised by their acidic chemical nature and typically possess a high silica content. Since the mineralogical composition of the aggregate appears to be a primary factor in effecting better adhesion with bitumen, the excess presence of silica in the rocks would lead to its early detachment from bitumen during the rainy season, he says.
”We announced the result soon after the first phase of the research in which samples collected from major quarries were subjected to petrological studies and moisture susceptibility tests, as majority of the rocks prevalent in Kerala has a documented silica content exceeding 52%, according to some previous studies, leaving no doubt that the rocks in Kerala lean predominantly towards an acidic nature. This aspect was further corroborated through this study,” he says.
The study, however, also suggest that rocks treated with anti-stripping additives like lime or cement can address the issue to some extent. Up to 50% increase was observed in Tensile Strength Ratio (TSR) value—an indication of the relative strength of asphalt mixes under wet and dry conditions—across aggregates sourced from all six quarries with the addition of 2% Hydrated lime (by weight of dry aggregates).
Also, 42% increase was observed in TSR value across aggregates sourced from all six quarries with the addition of 3% cement. Considering the weather of Kerala, which receives rains for about six months a year, the KHRI recommended that TSR testing shall be strictly enforced for all HMA road construction projects in Kerala.
