Which type of concrete is characterized by having a slump of 1 inch or less?

No-slump concrete is characterized by having a slump of 1 inch or less, which indicates that it is very stiff and has a low water-to-cement ratio. This type of concrete is often used for applications where high strength and durability are required, such as in pavements, precast products, and certain structural components. The low slump signifies that the mixture has been designed to retain its shape and stability when placed, making it suitable for applications where flowability is not necessary.

In contrast, other types of concrete listed, such as lightweight insulating concrete, heavyweight concrete, and refractory concrete, have different characteristics and applications that involve varying amounts of water and aggregates, leading to different slump values. For example, lightweight insulating concrete typically has a much higher slump due to the inclusion of lightweight aggregates and higher water content for workability, while refractory concrete is designed to withstand high temperatures and may also have a different consistency. Hence, the defining characteristic of no-slump concrete directly aligns with the specified slump measurement in the question.