Lightweight Concrete for High Strength - Expanded Shale & Clay

B.3 Creep and Shrinkage Models
Among the variety of methods proposed for creep and shrinkage in concrete, seven of
them are as follows: American Concrete Institute committee 209 (ACI-209, 1997), American
Association of State Highway and Transportation Officials (AASHTO-LRFD, 1998), Comite
Euro-Internacional du Beton and Federation Internationale de la Precontrainte (CEB-FIP, 1990),
Bažant and Panula’s (BP, 1978), Bažant and Baweja’s (B3, 1995), Gardner and Lockman’s (GL,
2001), and Sakata’s model (SAK, 1993). Five methods aimed to be used for high strength
concrete are as follows: CEB-FIP as modified by Yue and Taerwe (1993), BP as modified by
Bažant and Panula (1984), SAK as modified by Sakata et al. (2001), Association Française de
Recherches et d'Essais sur les Matériaux de Construction (AFREM, 1996), and AASHTO-LRFD
as modified by Shams and Kahn (2000). Most of the expressions presented here are empirical,
so they have different versions depending on the unit system. Only the three models found most
applicable are present.
B.3.1. ACI-209 Method (Normal Strength Concrete)
American Concrete Institute through its committee 209 “Prediction of Creep, Shrinkage and
Temperature Effects in Concrete Structures” proposes an empirical model for predicting creep
and shrinkage strain as a function of time. The two models have the same principle: a
hyperbolic curve that tends to an asymptotic value called the ultimate value. The shape of the
curve and ultimate value depend on several factors such as curing conditions, age at application
of load, mix design, ambient temperature and humidity.
Creep Model. Creep model proposed by ACI-209 has three constants that determine the
asymptotic value, creep rate and change in creep rate. The predicted parameter is not creep
strain, but creep coefficient (creep strain-to-initial strain ratio). The latter allows for the
calculation of a creep value independent from the applied load. Equation B.1 presents the
general model.
ψ
( t − t'
)
φt
=
⋅φ
ψ u
d + ( t − t'
)
where
ø t : creep coefficient at age “t” loaded at t′
t: age of concrete (days)
(B.1)
B-4