Page 133 - Design of Reinforced Masonry Structures

MATERIALS OF MASONRY CONSTRUCTION 3.27

3.10 CREEP OF MASONRY

Deformation under loads is a common characteristic of structures. Deflection of beams
and slabs (due to flexural loads), and shortening of columns (due to compressive loads) are
examples of these deformations. Strain is simply a measurement of change per unit length
of the material.
Materials such as masonry and concrete experience two types of deformations under
loads: (1) instantaneous deformation, which is elastic and (2) creep. However, there is a
difference in the manner in which the two occur. Instantaneous deformation occurs as soon
as the load is applied, and is recoverable; deformation would disappear upon the removal
of loads as along as the deformation was within the elastic limit. Creep, on the other hand,
is a deformation that continues to occur slowly under sustained loading at stresses within
the elastic range over a period of time. Furthermore, this deformation is inelastic (i.e., unre-
coverable). Most deformation due to creep occurs during the first year and increases at a
decreasing rate during the time of loading, and its magnitude can be 2 to 3 times the instan-
taneous (elastic) deformation. Thus, creep is a time- and load-dependent (i.e., depends on
the magnitude of sustained loading) phenomenon [3.39, 3.40]; the greater the sustained
load, the greater the creep. Creep is often associated with shrinkage because both continue
to occur simultaneously over time, with the net effect of increases deformation with time.
Information is available for predicting creep [3.46, 3.47]. The internal mechanism of
creep, or “plastic flow,” as it is sometimes called, may be due to any one or a combination
of the following factors:
1. Crystalline flow in the aggregate and hardened cement paste.
2. Plastic flow of the aggregate and hardened cement paste surrounding the aggregate.
3. Closing of internal voids.
4. Flow of water out of the cement gel due to external loading and drying.
A number of factors affect the magnitude of creep of concrete [3.44]. These include:

1. Constituents of concrete, such as size, grading and mineral content of aggregates,
composition of cement, and the admixtures. Magnitude of creep is largest in con-
cretes containing high cement-paste content; it is less in concretes containing a
large percentage of aggregates because only the paste creeps, and aggregates act to
restrain creep.
2. Mix proportion such as water content and water-cement ratio.
3. Curing temperature and humidity.
4. Relative humidity during period of use.
5. Age of loading.
6. Duration of loading.
7. Magnitude of sustained stress (i.e., stress due to sustained loads).
8. Surface-volume ratio of member.
9. Slump of concrete (which is affected by water-cement ratio).
Creep characteristics of a material are defined by a coefficient called coefficient of creep
(k ), which is the ratio of creep strain (after a very long time) to initial elastic strain. The
c
coefficient of creep, k , for concrete masonry is much greater than that for clay masonry
c
(which is negligible) [3.4]. This is attributed, in part, to the fact that concrete blocks are
products of portland cements (the cement paste, not the aggregates, is the problem) [3.44].