HPC is characterized by a lack of bleeding due to the low w/c ratio and a high heat of hydration, a result of the high cementitious factors used. The lack of bleeding results in a quick drying at the surface, if it is left unprotected, and bleed water does not readily move to the surface [16]. The surface dries out relative to the interior which leads to plastic shrinkage cracking. These cracks are either parallel or in a mesh form and they appear in the first 10 hours after placement [16], sometimes as early as 30 minutes after casting.
To prevent this type of cracking it is important to follow proper curing practices that are correctly timed. The three methods adopted are wet burlap or similar fabric, curing compounds, and fog sprays. In extreme cases, more than one method is used simultaneously. The timing of application is critical to avoid plastic shrinkage cracking. Rostam [9] recommends that all free surfaces must be protected before 0.5 kg/m 2 (0.1 lb/ft2 ) of water has evaporated. He suggested that the rate of evaporation should not exceed 0.3 kg/m2/hr (0.06 lb/ft2/hr). ACI recommends that protective measures be used when the evaporation rate exceeds 0.5 kg/m2/hr, which might not be sufficient precaution for some HPC concrete mixtures, especially with those mixtures incorporating silica fume.
In the survey, 62 percent of the contractors used a water mist or wet burlap, while 16 percent used a curing compound. Only less than 1 percent reported that nothing was used to protect the concrete during the first 24 hours. The duration of the curing also varied from 24 hours (16 percent) to 7 days (20 percent) while 1 percent protected the structure to about 14 days. HPC typically uses high dosages of HRWRA that retard the setting time and the development of early age strength. Therefore, often a longer curing time is necessary to obtain the high strength specified.
The other type of cracks that are likely to appear are thermal cracks, due to the high heat of hydration associated with HPC mixtures. Thermal cracking is due to the thermal gradient within the concrete mass or between a previously cast portion and the newly placed concrete. To prevent thermal cracks, the solution depends on the case. In the first case, a temperature gradient between the interior of the concrete and the ambient atmosphere should be avoided. Many authors [17, 18, 19, 20, 21] advise that the temperature should be closely controlled and that concrete surface be insulated with a blanket or foam during setting and hardening period, as it can take up to 5 days for the concrete to equilibrate with ambient temperature [16]. In the second case, the remedy is to lower the temperature rise of the newly cast concrete as much as possible, e.g., by lowering the as-placed concrete temperature, by replacing the cement by supplementary cementitious materials or by cooling the structure by passing water through embedded pipes.
It might be worth using finite element calculations for a better control of the temperature related cracking [8]. Tanigawa [20] showed that compressive strength of concrete in the center of a structure decreases with higher temperature. He recommends the use of low heat cements and emphasizes proper curing of the structure. On the other hand it seems that the final strength of HPC is less sensitive to high temperature, relative to conventional concrete. In our survey, only one contractor for one job indicated that the temperature was maintained below maximum of 50 C (122 F).
From the survey, only 17 percent reported problems with plastic shrinkage or drying shrinkage cracking. Most of the reported cracks were due to plastic shrinkage. This finding is quite surprising because the literature reports cracking to be a real problem, where it seems that no large job is totally immune from cracks.
In addition to plastic shrinkage and thermal shrinkage, another type of cracking at very early age has been reported [22], which is due to the autogenous shrinkage of concrete. HPC displays a rather high autogenous shrinkage due to its low water/binder ratio [23]. Unlike conventional concrete, HPC autogenous shrinkage is high enough to provoke cracking when fully restrained.
Finally some cracking phenomenon has been reported in HPC bridge decks, although the concrete was properly cured and still in the dormant period [24]. It is believed that the chemical shrinkage could be the main cause.