Creep is the tendency of a solid material to move slowly or deform permanently under the influence of stresses. It occurs as a result of long-term exposure to high levels of stress that are below the yield strength of the material.

Creep always increases with temperature and is linearly dependent on the stress if the pore water content is constant. If concrete is fully dried, it does not creep, but it is next to impossible to dry concrete fully without severe cracking. Drying shrinkage (es) is the reduction in volume caused principally by the loss of water during the drying process. Creep of concrete sC is the deformation under static compression loading for a prolonged period of time. Lab concrete cylinders are exposed to load of up to 40% of their 28 day compressive strength and readings are taken as per the provisions of ASTM C512-10. The difference (sC-es ) taken as the net creep is measured at 1 month, 3 months, 6 months and finally at one year, with a long term prediction for 20 years. The structural engineer will use all of this information in order to predict shortening values of vertical members. The differences in column shortenings are of particular concern for very tall buildings. In slender structures, creep may cause collapse due to long-time instability. The creep and shrinkage study was required in all our high rise projects.

In the Credit Libanais headquarters project, three test specimens were studied by the RSK - UK lab. The figures below describe the testing results:

As for the Sama Beirut project, 18 specimens of B30, B40 and B70 MPA concrete respective to the compressive strength (f’c) of core walls, shear walls and columns in the tower were sent to the RSK - UK lab with results shown herein: > Drying shrinkage, es at 1 year averages 700 μm/m (0.7mm/m), and 0.85mm/m predicted at 20 years. > Creep values at 1 year averages 1400 μm/m (1.4mm/m) for 70 MPA and 2000 μm/m (2mm/m) for 30 and 40 MPA. As a result, the following measures were implemented on site: - The pouring level of the core walls and slabs were raised each 10 floors by 10mm, accordingly the entire structure was raised by a cumulative of 60mm. - Updated differential shortening values between all vertical members were accounted for in the PT design and applied in every other 10 floors. - The floor, shear walls and columns levels were adjusted during construction above the 2nd floor.