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strength at time of loading [N/mm2] Figure 11.2. Final specific creep as a function of strength at the age of loading [7]

Shrinkage

For normal strength concrete the shrinkage is formulated as

where Scs final shrinkage strain Scd drying shrinkage strain Sca autogenous shrinkage

The component Scd, representing the drying shrinkage strain, is known to be higher for lightweight concrete. In [8] it is reported that the final shrinkage of LWAC is about 1-1.5 times the final shrinkage of NDC of the same strength. Hoffman and Stockl [9] reported for LWAC's with cylinder strengths of 40-50 MPa differences of about 30% with NDC. Theissing [10] reported, on the basis of a literature survey, for concretes with a cylinder strength of 21 Mpa, values which were about 35% higher than for NDC. Probst [11] reported about shrinkage tests on three different LWAC's made with Liapor (fcc = 65 Mpa), Berwilit (fcc = 43 Mpa) and Leca (fcc = 33 Mpa), kept under a RH of 65%, values of 0.55% in axial direction and 0.85 % in transverse direction. This is about the same as found for NDC.

In ENV-1992-1-4 it is stated that final drying shrinkage values for lightweight concrete can be obtained by multiplying the values for normal density concrete with a factor ^3 defined by LC12/15 to LC/20: ^3 = 1.5

Since the information from literature is not fully consistent and the values given in ENV 1992-1-4 will presumably be not be too far from reality they have been maintained in the new draft of 1.1.2000.

A new element in the formulation of shrinkage is the component Sca which represents autogenous shrinkage. The attention to this additional type of shrinkage contribution was drawn during the introduction of high strength normal density concretes, with low water/cement ratio's. Autogenous shrinkage is believed to be caused by "self-dissication", which is a result of a volume reduction of the hydration product compared to the volume of the reacting water and cement, i.e. chemical shrinkage, and goes along with a decrease of the relative humidity in the pore system. This drop in relative humidity is accompanied by a volume reduction of the matrix. This volume reduction is sometimes also denoted with the term "chemical shrinkage". [4].

For normal density concrete the component of autogenous shrinkage is formulated as:

where specific creep [10-5 N/mm2]

specific creep [10-5 N/mm2]

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