1555Shrinkage

Shrinkage and creep are time-dependent deformations that, along with cracking, provide the greatest concern for designers because of the degree of uncertainty associated with their prediction. Concrete exhibits elastic deformations only under loads of short duration, and due to additional deformation with time, the effective behavior is that of an inelastic and time-dependent material.

Shrinkage of HPC may be expected to differ from normal concrete in three broad areas: plastic shrinkage, drying shrinkage, and autogenous shrinkage:

1. Plastic shrinkage occurs during the first few hours after fresh concrete is placed. During this period, moisture may evaporate faster from the concrete surface than the bleeding water from lower layers of the concrete mass. HPC, which is a paste-rich mix, will be more susceptible to plastic shrinkage than normal concretes.

2. Drying shrinkage occurs after the concrete has already attained its final set and a good portion of the chemical hydration process in the cement gel has been accomplished. Drying shrinkage mainly depends on w/b and total amount of water used. For HPC, its w/b ratio is reduced. However, its total water amount may not be very different from normal strength concrete due to increased cement paste. Although HPC's shrinkage is perhaps potentially larger due to higher paste volumes, it does not, in fact, appear to be appreciably larger than normal concretes. This is probably due to the increase in stiffness of the stronger mixes.

3. Autogenous shrinkage due to self-desiccation is one of the typical characteristics for HPC with very low w/b ratio. Of course, autogenous shrinkage also occurs during the hydration process of normal concrete, while, compared with HPC, the autogenous shrinkage of normal concrete is generally negligible. Shrinkage should not be confused with thermal contraction that occurs as concrete loses the heat of hydration.

The shrinkage properties of HPC with higher compressive strengths are summarized in an ACI state-of-the-art report [43]. The basic conclusions were: shrinkage is not only affected by the w/c ratio but also the percentage of water by volume in concrete; laboratory and field studies have shown that shrinkage of higher-strength concrete is similar to that of lower-strength concrete; shrinkage of high-strength concrete containing high-range water reducers is less than for lower-strength concrete; higher-strength-high-performance concrete exhibits relatively higher initial rate of shrinkage, but after drying for 180 days, there is little difference between the shrinkage of high-strength-high-performance concrete and normal-strength concrete made with dolomite or limestone.

Carette et al. [44] reported a study of HPCs with high volume fly ash from sources in the United States. The concretes had low bleeding, satisfactory slump and setting characteristics, and low autogenous temperature rise. These concretes also had excellent mechanical properties at both early and late ages with compressive strength reaching as high as 50 MPa at 91 days and the drying shrinkage of the concretes was relatively low.

Field measurements of surface shrinkage strains on a mock column, fabricated with HPC, after 2 and 4 years were conducted by Sarkar and A'itcin [45] and the measurements were compared with results on specimens under laboratory conditions. It was shown that the surface shrinkage strains under the field condition were considerably lower than those measured under the laboratory conditions.

In a similar study by Hindy et al. [46], measurements of dry shrinkage were carried out on concrete specimens as well as on instrumented reference columns made with two different ready-mixed HPCs. One had a 91-day compressive strength of 98 MPa and the other had a 91-day compressive strength of 80 MPa. The first contained silica fume but the second did not. The effects of curing time, curing conditions, silica fume content, and w/b ratio were considered. It was found that the longer the curing time the lower the dry shrinkage. The lower the w/b ratio the lower the dry shrinkage. Dry shrinkage of small specimens measured by the conventional laboratory test was found to overestimate shrinkage of the concrete in the real structure. The ACI 209 predictive equation was valid for HPCs only if new values for the parameters were introduced.

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