The amount of thermal expansion and contraction of concrete varies with factors such as type and amount of aggregate, richness of mixture, water-cement ratio, temperature range, concrete age, and degree of saturation of concrete. A typical value of thermal expansion and contraction of concrete is on the order of 10 millionths per degree Celsius, that is, very near the same as that for steel. If an unrestrained slab or wall 30 meters long has a temperature variation from summer to winter of 38 degrees Celsius, the total thermal movement might be about 1.5 to 1.8 centimetres. Movements occur at the exposed surface of the concrete, which cools off more quickly, before they occur in the interior of the section, leading frequently to additional warping or curling effects. Observations of buildings in service indicate the total movement is usually less than half of that which might be anticipated by combining the contraction due to temperature drop with the shrinkage. This is due to restraining effects of the reinforcing steel and restraining effects of columns, walls, and foundations.