👉 Underfill, also known as a "melted in place" fill, is a type of construction material that is used to fill spaces between two layers or structures. It is typically filled with a mixture of sand and water, which are mixed together to create a cohesive substance. Underfill is often used to fill gaps or cracks in concrete, masonry, or other building materials. Underfill can be a relatively inexpensive option compared to traditional filling methods such as concrete or asphalt, but it requires