MATERIALS ENGINEERING CONCEPTS - Materials for Civil and Construction Engineers - page 14

Wood elements loaded for a few days can carry higher stresses than ele-
ments designed to carry “permanent” loads. On the other hand, creep of con-
crete is associated with microcracking at the interface of the cement paste
and the aggregate particles (Mehta and Monteiro 1993).
The viscous flow models are similar in nature to Hooke's law. In lin-
early viscous materials, the rate of deformation is proportional to the stress
level. These materials are not compressible and do not recover when the
load is removed. Materials with these characteristics are Newtonian fluids .
Figure 1.9(a) shows a typical creep test in which a constant compres-
sive stress is applied to an asphalt concrete specimen. In this case, an elas-
tic strain will develop, followed by time-dependent strain or creep. If the
specimen is unloaded, a part of the strain will recover instantaneously,
while the remaining strain will recover, either completely or partially, over
a period of time. Another phenomenon typical of time-dependent materi-
als is relaxation, or dissipation of stresses with time. For example, if an as-
phalt concrete specimen is placed in a loading machine and subjected to a
constant strain, the stress within the specimen will initially be high, then
gradually dissipate due to relaxation as shown in Figure 1.9(b). Relaxation
is an important concern in the selection of steel for a prestressed concrete
design.
Materials exhibiting both viscous and elastic responses are known as
viscoelastic . In viscoelasticity, there are two approaches used to describe
how stresses, strains, and time are interrelated. One approach is to postulate
mathematical relations between these parameters based on material func-
tions obtained from laboratory tests. The other approach is based on com-
bining a number of discrete rheological elements to form rheological models ,
which describe the material response.

Time Time

Creep

Elastic
rebound

Relaxation

Elastic
strain

Recovery

Time Time

(a) (b)

FIGURE 1.9 Behavior of time-dependent materials: (a) creep and

(b) relaxation.