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that often reach 106°F (41°C) and sometimes levels. Cement, which is produced locally by the exceeding 112°F (44.5°C). For the workers’ government, and chloride-contaminated potable protection, all work was restricted if the ambient water also contribute to chlorides in mixtures and temperature exceeded 113°F (45°C). must be closely monitored. For both RPS projects, the QCS 2010 required Inconsistent Shotcrete Mixture the use of QCS 2010 C40 for the shotcrete mix- The concrete mixture used for the wet-mix ture. C40 refers to a concrete cube 6 x 6 x 6 in. shotcrete presented the greatest challenge for two (150 x 150 x 150 mm) sample that must break reasons. First, although shotcrete mixtures are higher than 40 N/mm2 typically cement rich and, consequently, sticky, adding silica fume caused the mixture to become 40 N/mm2 (cube) = 32 MPa (cylinder) = 4600 extremely sticky. The second challenge was that psi (cylinder)* the concrete delivered to the site was rarely delivered Low breaks were never a concern because the with a consistent slump. With over 40 concrete mixture design consistently outperformed the C40 suppliers covering the construction boom in Doha requirements with an average 28-day strength and most concrete placement occurring at night, exceeding 6700 psi (46.2 MPa). many people consider the concrete truck Doha’s summer nocturnal creature. QCS 2010 C40 requirements for cylinder Throughout the nation, older concrete structures strength and durability: are crumbling due to corrosion of internal rein- forcement caused by the high internal chloride Requirements content in the original concrete mixtures. As a Strength (28-day) 4600 psi (32 MPa) result, Qatar has established strict requirements Chloride content in the national building code called the Qatar ASTM C1281 0.06% Construction Standards (referenced locally as Durability (56-day) QCS 2010). Local limestone aggregates are only to be used on a limited basis due to their substantial Water penetration 15 mm (0.6 in.) nonuniform qualities, uncontrolled water absorption, BS 12390-8 and abrasion quality. Imported limestone and Water absorption 2 Gabbro aggregates (igneous rock) from the United BS 1881 Arab Emirates and Saudi Arabia are widely Chloride migration 5 x 10-12 employed for nearly all construction activities. RCP ASTM C1202 2000 coulombs Although most aggregates are imported with low chloride levels, mixtures are continually checked *refer to Shotcrete Corner on pg. 22 for a more detailed to ensure the chlorides are kept below allowable explanation. Shotcrete • Spring 2014 9


2014SprShotcreteEMag
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