Materiales: obtención y pre-procesamiento

5.8.2.1 General

When calculating the forces in tendons at the various stages considered in design, make allowance for the appropriate losses of prestress resulting from

a) relaxation of steel of the tendons,

b) the elastic deformation and subsequent shrinkage and creep of the concrete, c) slip or movement of tendons at anchorages during anchoring, and

d) other causes in special circumstances, for example when steam curing is used with pre-tensioning.

If experimental evidence on performance is not available, take the properties of the steel and of the concrete into account when calculating the losses of prestress from these causes. The provisions given in the following subclauses are applicable to a wide range of structures, especially buildings. It must be recognized, however, that these recommendations are necessarily general and approximate.

5.8.2.2 Loss of prestress due to relaxation of steel

5.8.2.2.1 Ensure that the loss of force in the tendon allowed for in the design is double the maximum relaxation after 1 000 h duration, for a jacking force equal to that imposed at transfer.

5.8.2.2.2 When there is no experimental evidence available, the relaxation loss for normal stress-relieved wire or strand may be assumed to decrease linearly from 10 % for an initial prestress of 80 %, to 3 % for an initial prestress of 50 %. This would apply when the estimated total creep and shrinkage strain of the concrete is less than 500 x 10^-6. When the creep plus shrinkage strain exceeds 500 x 10^-6, the loss for an initial stress of 80 % should be reduced to 8,5 %. Losses for low-relaxation tendons may be assumed to be half the above values.

5.8.2.2.3 Make no reduction in the value of the relaxation loss for a tendon when a load equal to or exceeding the relevant jacking force has been applied for a short time prior to the anchoring of the tendon.

5.8.2.2.4 In special cases, such as tendons exposed to high temperatures or subjected to large lateral loads, greater relaxation losses will occur. Consult specialist literature in these cases.

5.8.2.3 Loss of prestress due to elastic deformation of the concrete

5.8.2.3.1 Calculation of the immediate loss of force in the tendons due to elastic deformation of the concrete at transfer may be based on the values for the modulus of elasticity of the concrete given in table 1 when the actual experimental values are not available (see annex C). The modulus of elasticity of the tendons may be obtained from 3.4.2.3.

5.8.2.3.2 For pre-tensioning, calculate the loss of prestress in the tendons at transfer on a modular ratio basis, using the stress in the adjacent concrete.

5.8.2.3.3 For elements with post-tensioning tendons that are not stressed simultaneously, there is a progressive loss of prestress during transfer, due to the gradual application of the prestressing force.

Calculate the resulting loss of prestress in the tendons on the basis of half the product of the modular ratio and the stress in the concrete adjacent to the tendons averaged along their length; alternatively, the loss of prestress may be accurately calculated by basing it on the sequence of tensioning.

5.8.2.3.4 In making these calculations, it may usually be assumed that the tendons are located at their centroid.

5.8.2.4 Loss of prestress due to shrinkage of the concrete

5.8.2.4.1 The shrinkage strain to be considered depends upon the following:

a) the aggregate used;

b) the original water content;

c) the effective age of transfer;

d) the effective section thickness; and e) the ambient relative humidity.

5.8.2.4.2 The loss of prestress in the tendons due to shrinkage of the concrete may be calculated as the product of the shrinkage per unit length of the concrete (see table 35) and the modulus of elasticity of the tendons (as in 3.4.2.3).

Table 35 - Shrinkage of concrete

5.8.2.4.3 Some adjustment to the figures in table 35 will be necessary for other ages of concrete at transfer, for other conditions of exposure, or for massive structures, in which cases specialist literature should be consulted.

5.8.2.4.4 When it is necessary to determine the loss of prestress and the deformation of the concrete at some stage before the total shrinkage is reached, it may be assumed that half the total shrinkage takes place during the first month after transfer and that three-quarters of the total shrinkage takes place within the first 6 months after transfer.

5.8.2.4.5 In certain regions of South Africa, the aggregate may exhibit abnormally high shrinkage characteristics. The fine-grained shales and sandstones of the Beaufort group of the Karoo sequence are those most likely to lead to high dimensional changes in concrete. Seek advice when these aggregates or others of a similar type are to be used.

5.8.2.5 Loss of prestress due to creep of the concrete

5.8.2.5.1 The loss of prestress in the tendons may be calculated on the assumption that creep is proportional to the stress in the concrete (see 5.8.2.5.4). The loss of prestress is obtained as the product of the creep per unit length of the concrete adjacent to the tendons and the modulus of elasticity of the tendons (see 3.4.2.3). When calculating this loss, it is usually sufficient to assume that the tendons are located at their centroid.

5.8.2.5.2 For pre-tensioning at between 3 d and 5 d after concreting and for humid or dry conditions of exposure where the required cube strength at transfer exceeds 40,0 MPa, take the creep of the concrete per unit length as 48 x 10^-6 per megapascal. For lower values of cube strength at transfer, assume the creep per unit length to be 48 x 10^-6 x 40,0/f_ci per megapascal, where f_ci is the concrete strength at transfer.

5.8.2.5.3 For post-tensioning at between 7 d and 14 d after concreting and for humid or dry conditions of exposure where the required cube strength at transfer exceeds 40,0 MPa, take the creep of the concrete per unit length as 36 x 10^-6 per megapascal. For lower values of cube strength at transfer, take the creep per unit length as 36 x 10^-6 x 40,0/f_ci per megapascal.

5.8.2.5.4 The values as in 5.8.2.5.2 and 5.8.2.5.3 are applicable when the maximum stress anywhere

in the section at transfer is less than one-third of the cube strength of concrete. Where the maximum stress anywhere in the section at transfer exceeds one-third of the cube strength of the concrete, the value for the creep per unit length should be increased up to the maximum value equal to 1,25 times the values given in 5.8.2.5.2 and 5.8.2.5.3, as relevant. This maximum value is applicable when the maximum stress at transfer is half the cube strength. For intermediate stresses, the values for the creep per unit length should be interpolated linearly.

5.8.2.5.5 The values in the preceding subclauses relate to the ultimate creep after a period of years.

When it is necessary to determine the deformation of the concrete due to creep at some earlier stage, it may be assumed that half the total creep takes place in the first month after transfer and that three-quarters of the total creep takes place in the first 6 months after transfer.

5.8.2.5.6 When applying the provisions given above, which are necessarily general, consult specialist literature for more detailed information on the factors affecting creep, particularly those such as aggregates used, original water content, effective age at transfer, effective section thickness, ambient relative humidity and ambient temperature. Care should be taken when using Reef quartzite, aggregates of the Beaufort group of the Karoo sequence and the Lesotho basalts, since the values may be three times bigger. (See also figure C.1.)

5.8.2.6 Draw-in during anchorage

In post-tensioning systems, make allowance for any movement of the tendon at the anchorage when the prestressing force is transferred from the tensioning equipment to the anchorage. The loss due to this movement is particularly important in short elements and for such elements check, on site, the allowance made by the designer.

5.8.2.7 Loss of prestress due to steam curing

Where steam curing is used in the manufacture of prestressed concrete elements, consider changes in the behaviour of the material at temperatures higher than normal.