Effect of Varying w/c Ratios on the ITZ Characteristics of Internally Cured RCC Pavements

Abstract: While low w/c ratio concretes are desirable for paving application, it often experiences self-desiccation due to reduced internal moisture availability relative to the cement content. This in turn can lead to incomplete hydration of the cement particles which affects the long-term performance of paving concretes. Internal curing (IC) is an effective method that can provide a supplementary source of water within the concrete matrix, often in the form of lightweight aggregates (LWA) that are pre-saturated with water. By providing a supplementary source of water, IC can facilitate higher degrees of hydration with improved water distribution which can significantly improve the cracking resistance of concretes such as drying shrinkage and early-age cracking. In conventional portland cement concrete (PCC) pavements, where the surface area-to-volume is typically high, chances of excessive drying and subsequent mechanical strength reduction becomes prominent, especially in the ITZ region. This weakened ITZ region between the aggregate and surrounding cement matrix often becomes a primary pathway for cracking and increased permeability, thereby negatively affecting the durability of the concrete. In this study, expanded clay aggregates (ECA) was used as the IC agent to produce paving concretes at varying w/c ratios viz, 0.30, 0.35, and 0.40. The study investigates the mechanisms through which varying w/c ratios impact the ITZ and the microstructural evolution of internally cured mixtures after 7 days of curing. The investigation also aims to achieve performance comparable to 0.42 w/c ratio concrete, which theoretically provides sufficient water for complete hydration. The required internal curing water was designed based on the chemical shrinkage of each w/c ratio to ensure optimal hydration. Advanced characterization methods, including Thermogravimetric analysis (TGA), X-ray Diffraction (XRD), Nanoindentation, and Backscattered electron images were conducted to study the hydration level and microstructural developments due to internal curing.
This investigation provides a comprehensive understanding of the mechanisms of internal curing at different w/c ratios. The results indicate that internal curing significantly enhances hydration even at lower w/c ratio, comparable to or exceeding those of 0.42 w/c ratio. Pore structure analysis showed a refined and less connected capillary network in internally cured samples, particularly in the ITZ of lower w/c ratios, resulting in improved curing affected zone as studied from nanoindentation test. The study evidence that internal curing effectively mitigates self-desiccation even at lower w/c ratios which reduce shrinkage stress and enhance ITZ density. The enhanced hydration, ITZ and microhardness at lower w/c ratios through internal curing facilitates lesser chance of early age cracking and drying stresses which reduce the need for external curing.