Autogenous shrinkage is caused by self-dessication in young concrete as water is consumed during hydration reaction. This occurs majorly within the early days of casting the concrete, while drying shrinkage is the reduction in volume caused mainly by the loss of water during the drying process and this continues perhaps for years after the concrete is placed. This means that drying shrinkage commences after curing. Shrinkage in concrete is usually the sum of autogenous shrinkage and drying shrinkage.
Mechanism of Autogenous Shrinkage
(1) During the hardening process of concrete, water from the mix accumulates into the pores, voids, and capillaries of the mixture.
(2) Water binds itself into the grains of the binder, as well as moving into the sorroundings whose relative humidity is usually less than that of the concrete.
(3) In order to move water from the tiny voids, consdierable forces are required to overcome the surface tension, as well the forces of sdhesion which binds the water to the voids.
(4) These forces act on the load bearing structure of the concrete which at that time has low stiffness, can cause deformations in the concrete which can lead to cracking.
Autogenous shrinkage cracks can be largely avoided by keeping the surface of the concrete continuously wet; conventional curing by sealing the surface to prevent evaporation is not enough and water curing is essential. With wet curing, water is drawn into the capillaries and the shrinkage does not occur.
Practically, autogenous shrinkage happens in the interior of a concrete mass. According to BS EN 1992-1-1, autogenous shrinkage occurs in all concretes and has a linear relationship to the concrete strength.
According to EN 1992-1-1, autogenous shrinkage is given by Equation (1)
εca(t) = βas(t)εca(∞) ————————— (1)
εca(∞) = 2.5(fck – 10) x 10-6
βas(t) = 1 – e(√0.2t)
fck = characteristic strength of concrete (MPa)
Where t is given in days.
As concrete matures, moisture is gradually lost from the concrete mass to the atmosphere. This moisture loss is accompanied by volume change which is referred to as drying shrinkage. Drying shrinkage therefore depends on the relative humidity (for instance, indoor concrete will shrink more readily when compared with external concrete), the quantity and class of cement (rich concrete mixes will shrink more than leaner mixes), and the member size (thinner sections will shrink more quickly than thicker sections). According to EN 1992-1-1, drying shrinkage strain develops slowly, since it is a function of the migration of the water through the hardened concrete.
Expression 3.9 in EN 1992-1-1 gives the development of drying shrinkage strain with time as given in Equation (2);
εcd(t) = βds(t, ts).kh.εcd,0 ———————- (2)
Where kh is a coefficient depending on the notional size h0 (see Table 1).
βds(t, ts) = (t – ts)/[(t – ts) + 0.04√(h03)]
t is the age of the concrete at the moment considered (in days)
ts is the age of the concrete (in days) at the beginning of drying shrinkage (normally this is at the end of curing)
h0 is the notional size (in mm) of the cross-section = 2Ac/u
Ac is the concrete cross sectional area (for slabs, Ac = thickness x 1000 mm)
U is the perimeter of that part of the cross section exposed to drying (for slabs, take u = 2000 mm)
εcd,0 is the basic drying shrinkage strain. This is given in Annex B of EN 1992-1-1 as Equation (3);
εcd,0 = 0.85[(220 + 110.αds1).e(-αds2.0.1fcm)] x 10-6 x βRH ——— (3)
βRH = 1.55[1- (RH/RH0)3] ————- (4)
fcm is the mean compressive strength (Mpa)
αds1 is a coefficient which depends on the type of cement
= 3 for cement class S
= 4 for cement class N
= 6 for cement class R
αds2 is a coefficient which depends on the type of cement
= 0.13 for cement class S
= 0.12 for cement class N
= 0.11 for cement class R
RH is the ambient relative humidity (%)
RH0 = 100%