Concrete floor slabs sit on a hardcore sub-base. In the past, some hardcore has contained industrial waste products including sulphates. When moisture is present, these sulphates can migrate into the body of the concrete slab, causing an expansive reaction. Common signs of this are uneven floors that have lifted in places, and randomly cracked. This problem can be serious, especially if internal walls have been built off the affected floor slab. Repair works can be disruptive and costly. Our structural engineers can investigate whether a floor has been affected by sulphate attack, carry out intrusive testing and then provide a specification for these works.