CANTO PRIMERO
2 ARTE POÉTICA
B.1 General
This annex gives information on the various movements that can occur in masonry. It is extremely difficult, if not impossible, to predict with any degree of certainty the movement that will actually occur. Movement is caused by a complex combination of factors such as temperature and moisture variations (see B.2).
Furthermore, each movement is controlled to some extent by the degree of restraint to which the masonry is subjected. To complicate matters further, the actual effect on movements of the same basic restraint can vary according to the general shape of the building and in many cases it cannot be quantified.
The determination of movement is thus a complex problem which cannot be solved simply by adding or subtracting individual values for thermal movement, moisture movement, creep, deflection, etc. The various individual movements are treated separately in B.4, B.5 and B.6.
Any estimation of movement has to rely to a great extent on engineering judgment, since many factors, such as the temperature and moisture content of the material at the time of construction, weather conditions and degree of restraint, are unpredictable.
B.2 Determination of total movement within a wall B.2.1 General
To determine the movement that is likely to take place within an actual wall, the individual movements described in B.4, B.5 and B.6 should be considered in combination.
An estimate of the total movement may be made by summing all the potential free movements. However, thermal and moisture movements are not directly additive. For example, a wall which expands due to thermal or moisture action alone generally becomes cooler when wetted by rain. The exact effect of such a combination is in practice extremely difficult to determine. All that can be said is that the maximum thermal and moisture movements should not be added together to arrive at the total effective free movement.
B.2.2 Total effective free movement for clay masonry
For clay masonry where the ambient temperature remains reasonably constant, e.g. for internal walls, the long term or time dependent movement described in B.5.2.2 predominates. Since this is an expansive movement, the masonry is unlikely to develop tensile cracks, except in short returns of less than 1 m in length. External masonry of clay masonry units will be subjected to the effects of thermal expansion superimposed on the long term movement.
B.2.3 Total effective free movement for concrete and calcium silicate masonry
Owing to the number of factors involved, it has not been found practicable to recommend coefficients for total effective free movement of concrete and calcium silicate masonry. However, where joints are provided in accordance with 5.4.2.3.5 and 5.4.2.3.6, the total effective free movement will be small, and detailed calculations are unnecessary.
B.3 Determination of spacing and widths of movement joints
There is no convenient mathematical expression for determining the position of movement joints in masonry. However, the basic principle is that the distance between joints should be such that the longitudinal strain induced in the wall is no greater than the strain capacity of the wall. Owing to the difficulty of computing joint spacings on this basis, recommended spacings based on practical experience have been given in 5.4. It is essential that the maximum movement in the masonry should be no greater than the maximum recommended movement in the joint sealant.
The product of the length of the masonry and the effective strain in the wall should be less than the product of the width of the joint and the permitted strain in the sealant.
B.4 Thermal movement
The theoretical free movement due to thermal effects, which is reversible, is equal to the temperature range multiplied by the appropriate coefficient of linear thermal expansion and the length (see Figure B.1).
However, the movement that actually occurs within a wall after construction depends not only on the range of temperature but also on the initial temperature of the masonry units when laid. This will vary according to the time of year and the exact conditions during the construction period and, in some cases, how soon after manufacture the masonry units are used, i.e. when they come straight from the kiln or curing chamber. Thus, in order to determine the potential free movement that could occur in a wall, some estimate of the initial temperature and the likely range of temperature should be made.
This potential free movement then needs to be modified, to allow for the effect of restraints.
Table B.1 indicates typical ranges for coefficients of linear thermal expansion. Some estimate of the actual value for the particular material being used should be made. In many instances, this information can be obtained from the manufacturers.
Figure B.1 ¦ Thermal movement
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Table B.1 ¦ Coefficient of linear thermal expansion of masonry units and mortar
The longitudinal coefficient of linear thermal expansion of masonry may be taken to be the same as that of the constituent masonry units.
Expansion in the vertical direction may be determined by summing the values obtained by multiplying the dimensions of the masonry units and the mortar by the respective coefficients of linear thermal expansion.
It should be borne in mind that the magnitude of movement in the horizontal and vertical directions will differ where:
¦ the coefficients for mortar and masonry units are not the same; and
¦ the height and length of the masonry units are unequal.
B.5 Moisture movement B.5.1 General
The moisture movement of clay masonry units and calcium silicate or concrete masonry units differ in magnitude and in kind. They have therefore been dealt with separately in B.5.2 and B.5.3.
B.5.2 Clay masonry units B.5.2.1 Wetting movement
It has been shown that clay masonry units can exhibit small reversible dimensional changes due to changes in moisture content. The effective movement that occurs within a wall is similar to that of thermal movement, but controlled by the minimum, initial and maximum moisture content. The actual movement is modified by the effect of any restraints. The typical range of movement to be expected is generally less than 0.02 %, which is comparatively insignificant.
B.5.2.2 Long term expansion
Although it is found that the general wetting movement is extremely small in clay masonry units, there is an irreversible expansion which occurs as a result of adsorbing5) moisture from the atmosphere. This occurs in both internal and external walls but can take place slightly more quickly in the latter. The rate of expansion is at its greatest just after the masonry units have cooled and decreases thereafter. The amount of expansion depends on the type of clay and the degree of firing. The actual movement within a wall depends on the degree of restraint and the amount of creep that has taken place in the mortar.
Material Coefficient of linear thermal expansion
10-6K-1
Clay masonry unitsa 4 to 8
Concrete masonry unitsb 7 to 14
Calcium silicate masonry units 11 to 15
Mortars 11 to 13
Natural limestone masonry unitsc 3 to 10 Natural sandstone masonry unitsc 5 to 12 Natural granite masonry unitsc 5 to 10
a Thermal movement of clay masonry units depends on the type of clay.
b Thermal movement of concrete masonry units depends on the type of material and the mix proportions.
c Thermal movement of natural stone masonry units depends on the type of stone.
5) Adsorption is the term used to describe the bonding of water molecules to the surface of the masonry material. It should not be confused with absorption, which refers to the entry of water molecules into the pores of the masonry.
B.5.3 Concrete and calcium silicate masonry units
The potential free movement that can occur in concrete and calcium silicate masonry units depends on the minimum, initial and maximum moisture contents (see Figure B.2).
In considering Figure B.2, it can be seen that the potential free movement within a wall is related to the moisture content at the time of laying. Since concrete and calcium silicate masonry units have a general tendency to shrink as they dry out, it is clear that keeping these masonry units as dry as practicable before and during construction reduces any subsequent movement. Also, the expected movement can be less for walls built under cover than external walls, subject to the relative humidity.
The potential free movement may be modified by restraints. It should be noted that such restraints, particularly at the end of a wall, are likely to increase the tensile stresses in the wall.
B.5.4 Natural stone masonry units
Movement with changes in moisture content in natural stone masonry units depends on the type of stone.
Some sandstone can exhibit noticeable movements with changes in moisture content, but limestones and igneous rocks only respond an insignificant amount.
Figure B.2 ¦ Moisture movement of concrete and calcium silicate masonry units
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Moisture movement can contribute to the opening of coping joints, and cracking at the ends of sandstone sills and lintels built into brick masonry. This has been known to occur in circumstances that suggest shrinkage of the component on drying may be the cause.
B.5.5 Mortar
The free moisture movement of mortar is similar to that of aggregate concrete masonry units, although the potential free movement is likely to be greater, since initial moisture loss does not take place before construction (see Figure B.3). The effect of mortar on longitudinal movement may be neglected.
Typical shrinkage values for mortars are given in Table B.2. The actual values will depend on the
constituents of the mortar, the proportions of the mix and the ambient relative humidity. For convenience, however, the lower values in the table may be taken to apply to mortars in external walls and the higher values to mortars in internal walls. The resulting movement of internal walls may generally be neglected, since they are unlikely to become wet after drying out initially.
Table B.2 ¦ Shrinkage of mortars due to change in moisture content
It should be noted that the values given in Table B.2 relate to unrestrained mortar. In practice, movement in a horizontal direction will largely be controlled by the surrounding masonry. However, for clay masonry, the effect of the mortar shrinkage can counteract the long term expansion described in B.5.2.2. Movement in the vertical direction will usually be unrestrained and will thus contribute to the total movement of the masonry in that direction.
B.6 Movement due to carbonation
An additional shrinkage of concrete masonry units and mortar can occur as a result of carbonation of the cement by atmospheric carbon dioxide. The extent of carbonation and the subsequent movement depends on the permeability of the concrete and on the ambient relative humidity. In dense masonry units and in autoclaved masonry units, the magnitude of this movement is extremely small and may be
neglected. In unprotected open textured masonry units and mortar, the shrinkage due to carbonation can be between 20 % and 30 % of the initial free moisture movement.
Figure B.3 ¦ Moisture movement of mortars
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%
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