Logistics of Concrete Pours:

In relation to any concrete pour, after the trucks that have delivered the concrete leave, the construction crews must finish the freshly poured slab.  This process involves the concrete finishers waiting until the concrete hardens enough (1-3 hours sometimes more, depending on ambient temperature) to allow the finishing machines on top of the slab to provide a smooth level concrete surface.  If this were not done the concrete would harden creating a deficient structural component, jeopardizing the integrity of tower construction.

Timing of Concrete Pours:

Prior to any concrete pour the required forming and rebar work must be completed. This frequently takes several hours. As well, the timing of concrete pours are dependant on the concrete suppliers availability to deliver concrete to the site.
In a large pour, for example for a floor slab, even if it is possible to begin in the morning, the work may still stretch into the evening, given the amount of concrete and the time it takes to pour and finish it.
Every effort is made to have work stay within the prescribed times in the City of Toronto’s construction noise by-law.
Please note that the City of Toronto’s construction noise by-law permits continuous concrete pours to continue past 7:00 pm when construction logistics require this.

TORONTO MUNICIPAL CODE  NOISE § 591-2.1 [Excerpt]
Construction. [Added 2007-12-13 by By-law No. 1400-2007]

(1) No person shall emit or cause or permit the emission of sound resulting from any operation of construction equipment or any construction, if it is clearly audible at a point of reception:
(a) In a quiet zone or residential area within the prohibited period of 7:00 p.m. one day to 7:00 a.m. the next day, 9:00 a.m. on Saturdays, and all day Sunday and statutory holidays; or
(b) In any other area within the prohibited period of all day Sunday and statutory holidays.
(2) Subsection B(1) does not apply to the continuous pouring of concrete, large crane work, necessary municipal work and emergency work that cannot be performed during regular business hours.

To read the full by-law please click on this link: http://www.toronto.ca/legdocs/municode/1184_591.pdf