Speed-up of hardening by heating or steam curing
1 General
This chapter is related to production of precast concrete elements. The adding rules for precast concrete element and structure are described in EN 1992-1-1 chapter 10 and annex B. This heating influences maturity of concrete and relaxation loss of prestressing steel.
2 Speed-up of hardening by heating or steam curing
The user can select sped-up hardening in the Library of Stressing beds. If the user marks check box Speed-up of hardening by heating or steam curing, two new check boxes appear in this dialogue. There he can choose if heating influences Maturity of concrete and Relaxation acceleration.
Fig. 1 Dialogue of stressing beds
2.1 Maturity of concrete
2.1.1 Theory
The effect of elevated or reduced temperatures within the range 0 – 80 °C on the maturity of concrete may be taken into account by adjusting the concrete age according to formula B.10 from EN 1992-1-1.
Where
| t_T | is the temperature adjusted concrete age which replaces t in the corresponding equations |
| T(delta_ti) | is the temperature in °C during the time period delta_ti |
| delta_ti | is the number of days when a temperature T prevails |
2.1.2 Implementation in SCIA Engineer
New check box Maturity of concrete was implemented in Stressing beds dialogue. If user turns ON this check box, he has an opportunity to define the beginning of time interval (ti) where temperature (Tc(dti)) is acting by a three-dot button. The lengths of the intervals (dti) are calculated from the difference of the user defined times (ti). The temperature should be defined as an average value Tc(dti) in the defined time interval dti according to formula B.10.
Fig. 2 Time-temperature cycle
Fig. 3 New time-temperature interval
Fig. 4 Maturity of concrete table
There are concrete characteristics in the bottom part of the dialogue. These characteristics are informative for the user only and the values depend on the selected class of concrete.
The program recalculates from these input values new adjusted time ti(adj) according to formula B.10. This adjusted time is adapted to the time dependent construction stages analysis. The user has possibility to see it in the Beam settings in Local time axis. The maturity of concrete of the beam for which Beam settings are defined will be adapted by adjusted time of casting.
Fig. 5 Beam settings dialogue
3 Relaxation acceleration
3.1.1 Theory
For pre-tensioned members, the effect of increasing the temperature while curing the concrete on the relaxation losses is considered. An equivalent time teq is added to the time after tensioning t in the relaxation time functions, given in 3.3.2(7), to take into account the effects of the heat treatment on the prestress loss due to the relaxation of the prestressing steel. The equivalent time can be estimated according to formula 10.2 from EN 1992-1-1.
Where
| t_eq | is the equivalent time (in hours) |
| T(delta_ti) | is the temperature (in °C) during the time interval delta_ti |
| T_max | is the maximum temperature (in °C) during the heat treatment |
3.1.2 Implementation in SCIA Engineer
New check box Relaxation acceleration was implemented in Stressing beds dialog. If user turns ON this check box, he has an opportunity to define the beginning of time interval (ti in hours) where temperature (Tp(dti)) is acting by the three-dot button. The lengths of the intervals (dti) are calculated from the difference of the user defined times (ti). If the user turns on check box Relaxation acceleration; he has an opportunity to influence relaxation acceleration in the new table.
Fig. 6 New time-temperature interval
Fig. 7 Maturity of concrete table
The values in the Relaxation acceleration table can be loaded from Maturity concrete table by button Load.
The time teq is used for calculation of duration of short-term relaxation of pretensioned strands and it is possible to view it in Beam strand pattern dialogue using button Losses.
Fig. 8 Short-term losses of pretensioned strand.