This study investigates the effects of spatial distribution of lightweight aggregates (LWAs) on internal curing of concrete. As replacements for normal aggregates, different sizes and amounts of natural pumice LWAs were used as water reservoirs to provide internal curing in mitigating autogenous deformation. Water in the pre-soaked LWAs flows into cement paste during hydration and provides internal curing to counteract the RH loss due to self-desiccation of binding paste. The results show that variations in the autogenous strain of concrete can be evaluated in terms of LWA-LWA proximity. The protected paste volume approach, previously used for air-entrained concrete, is applied to calculate the internally-cured volume of paste. The results show that the experimental rate of mitigation of autogenous strain for different series of concrete specimens, with respect to the reference concrete, gave the best-fitted values at water flow distance of 1 mm. The results indicate that the protected paste volume in internal curing can be determined by calculating the water-entrained volume using image analysis. (c) 2010 Elsevier Ltd. All rights reserved.