long walk down to safety in case of an emergency or fire. Special mixes of concrete were made to withstand the extreme pressures of the massive building weight; as is typical with reinforced concrete construction, each batch of concrete was tested to ensure it could withstand certain pressures. CTLGroup, working for Skidmore, Owings and Merrill, conducted the creep and shrinkage testing critical for the structural analysis of the building. The consistency of the concrete used in the project was essential. It was difficult to create a concrete that could withstand both the thousands of tonnes bearing down on it and Persian Gulf temperatures that can reach . To combat this problem, the concrete was not poured during the day. Instead, during the summer months, ice was added to the mixture and it was poured at night when the air was cooler and the humidity was higher. Cooler concrete cures more evenly and is, therefore, less likely to set too quickly and crack. Any significant cracks could have put the entire project in jeopardy. Milestones January 2004: Excavation commences. February 2004: Piling starts. 21 September 2004: Emaar contractors begin construction. March 2005: Structure of Burj Khalifa starts rising. June 2006: Level 50 is reached. February 2007: Surpasses the Sears Tower as the building with the most floors. 13 May 2007: Sets record for vertical concrete pumping on any building at , surpassing the to which concrete was pumped during the construction of Taipei 101, while Burj Khalifa reached the 130th floor. 21 July 2007: Surpasses Taipei 101, whose height of made it the world's tallest building, and level 141 reached. 12 August 2007: Surpasses the Sears Tower antenna, which stands . 12 September 2007: At , becomes the world's tallest freestanding structure, surpassing the CN Tower in Toronto, and level 150 reached. 7 April 2008: At , surpasses the KVLY-TV Mast to become the tallest human-made structure, level 160 reached. 17 June 2008: Emaar announces that Burj Khalifa's height is over and that its final height will not be given until it is completed in September 2009. 1 September 2008: Height tops , making it the tallest human-made structure ever built, surpassing the previous record-holder, the Warsaw Radio Mast in Konstantynów, Poland. 17 January 2009: Topped out at . 1 October 2009: Emaar announces that the exterior of the building is completed. 4 January 2010: Burj Khalifa's official launch ceremony is