Two structures, one ancient and one very modern, that used super-strength concrete.
The Pantheon is a building in Rome, Italy, on the site of an earlier building commissioned by Marcus Agrippa during the reign of Augustus (27 BC – 14 AD). The present building was completed by the emperor Hadrian and probably dedicated about 126 AD. He retained Agrippa’s original inscription, which has confused its date of construction.
The building is circular with a portico of large granite Corinthian columns (eight in the first rank and two groups of four behind) under a pediment. A rectangular vestibule links the porch to the rotunda, which is under a coffered concrete dome, with a central opening (oculus) to the sky. Almost two thousand years after it was built, the Pantheon’s dome is still the world’s largest unreinforced concrete dome. The height to the oculus and the diameter of the interior circle are the same, 43.3 metres (142 ft).
The 4,535 metric tons (4,999 short tons) weight of the Roman concrete dome is concentrated on a ring of voussoirs 9.1 metres (30 ft) in diameter that form the oculus, while the downward thrust of the dome is carried by eight barrel vaults in the 6.4 metres (21 ft) thick drum wall into eight piers. The thickness of the dome varies from 6.4 metres (21 ft) at the base of the dome to 1.2 metres (3.9 ft) around the oculus. The materials used in the concrete of the dome also varies. At its thickest point, the aggregate is travertine, then terracotta tiles, then at the very top, tufa and pumice, both porous light stones. At the very top, where the dome would be at its weakest and vulnerable to collapse, the oculus actually lightens the load.
No tensile test results are available on the concrete used in the Pantheon; however, Cowan discussed tests on ancient concrete from Roman ruins in Libya, which gave a compressive strength of 20 MPa (2,900 psi). An empirical relationship gives a tensile strength of 1.47 MPa (213 psi) for this specimen. Finite element analysis of the structure by Mark and Hutchison found a maximum tensile stress of only 128 kPa (18.5 psi) at the point where the dome joins the raised outer wall.
The stresses in the dome were found to be substantially reduced by the use of successively less dense aggregate stones, such as small pots or pieces of pumice, in higher layers of the dome. Mark and Hutchison estimated that, if normal weight concrete had been used throughout, the stresses in the arch would have been some 80% greater. Hidden chambers engineered within the rotunda form a sophisticated structural system. This reduced the weight of the roof, as did the elimination of the apex by means of the oculus.
One World Trade Center (also known as the Freedom Tower, 1 World Trade Center, One WTC and 1 WTC) is the main building of the rebuilt World Trade Center complex in Lower Manhattan, New York City. It is the tallest building in the Western Hemisphere, and the fourth-tallest in the world. The supertall structure has the same name as the North Tower of the original World Trade Center, which was completely destroyed in the terrorist attacks of September 11, 2001. The new skyscraper stands on the northwest corner of the 16-acre (6.5 ha) World Trade Center site, on the site of the original 6 World Trade Center. The building is bounded by West Street to the west, Vesey Street to the north, Fulton Street to the south, and Washington Street to the east.
In early 2010, Eastern Concrete Materials, a U.S. Concrete company, began producing high-strength concrete for One World Trade Center (WTC). Within three years, the New York City-based producer had supplied 150,000 cubic yards of ready-mix for the tower’s superstructure—with a concrete strength that has never been used on such a scale in building construction. Collavino Construction Co. then pumped this mix as high as 103 stories.
Construction began in 2006 and was completed in 2014. Its supporting columns are made of steel and concrete ranging in strength from 8600 psi to 14,000 psi. Columns on the first 40 floors are made from 12,000-14,000-psi concrete and the upper floors with 8,600–10,000-psi mix designs.
The ready-mixed concrete was pumped by Collavino’s crews to the highest elevation to which concrete has ever been pumped in the Americas. Because the mix design was so workable, pumping was accomplished with a single pump that moved the concrete directly from the ground to the top story, instead of to an intermediate station where it would have been remixed before being transferred to a second pump.
The Trump World Tower, the tallest all-residential building in the world when completed in 2001 and the tallest in New York until the 76-story Beekman Tower, engineered by WSP Cantor Seinuk, brought another technological marvel to New York: “super” cement. The super or high-strength concrete is produced by blending fly ash, slag cement and silica fume with concrete. Super high-strength concrete cannot be produced with only concrete.
Typically, the high-strength concrete used in skyscraper cores (in the 1990’s) would have a compressive strength of 8,000 to 10,000 pounds per square inch (psi). Because Trump World Tower is a rather slender high-rise, WSP Cantor Seinuk specified concrete compressive strength of 12,000psi for the first time in New York City.
“On One World Trade Center WSP Cantor Seinuk’s engineers worked on specifying the compressive strength and modulus of elasticity,” says Marcus. “But we needed even higher compressive strength—14,000 psi—for the taller One World Trade building.”
View from the top
Most of the concrete has gone into the tower’s monolithic pedestal. From a footprint of 200-by-200 feet, it rises up for 70 feet. Above ground, this has specially reinforced concrete to defend the building from the blast of a street-level bomb. Below grade, the reinforced-concrete structure is engineered to protect the tower’s structural integrity from a bomb even bigger than the one that exploded in 1993 in the World Trade Center’s parking garage.