Self-consolidating concrete mixture successfully used for thick slabs

       Upon its completion, the North Jeddah Main Wastewater Pumping Station in Saudi Arabia became one of the deepest and largest such facility in the world. The project involved the construction of three cast-in-place reinforced concrete(RC) bottom slab foundations 70m (230ft) below ground level(Fig.1). Slabs designated as B1 and B2 are located in pump shafts. Each has a diameter of 42.6 m(140 ft), a horizontal area of 1423m2 (15,350 ft2), and a thickness of 5.5 m(18ft), with an estimated volume of ready mixed concrete of about 7845m3 (10,260 yd3). Slab B3 is the foundation for the screen shaft for the facility. It has a diameter of 44 m(144 ft), an area of 1521m2 (8440 ft2),and a thickness of 4.5 m(15ft). The volume of concrete needed for B3 was about 6846m3 (8950 yd3).

      Considering that the three slabs must be classified as mass concrete and that thermal cracking would be a serious risk, the original plans were to construct each in three layers, allowing time between subsequent layers for dissipation of heat of hydration before the placement of a new layer. This approach would have been quite problematic, however, because the slabs were at the bottom of very deep shafts and had very dense mats of top reinforcement-- it would have been extremely difficult to consolidate the concrete in the bottom layers. Thus, it was decided to construct the massive slabs using a continuous placing process and self-consolidating concrete(SCC). The expected placement time for each of the three foundation slabs was 24 to 30 hours, at placement rates of 262 to 327m3/h(340 to 430 yd3/h) for Slabs B1 and B2, and at placement rates of 229 to 286 m3/h(300 to 375 yd3/h) for Slab B3. No cold joints were allowed in this continuous casting process.

       The 28-day design strength of the concrete was 37 MPa (5370 psi), based on cube tests. In addition to the need to address the potential for cracking due to volume change and thermal differentials, it was necessary to design the concrete mixtures for resistance to penetration of seawater and diffusion of harmful substances such as chloride and sulfate ions to ensure durability and mitigate corrosion of reinforcing steel. The mixture also needed to be capable of consolidating under self-weight without vibration and resistant to segregation and bleeding.