News & Media

18 September 2009

SMOOTH CONCRETE FINISHES AT PPC HERCULES PROJECT

The deployment of skilled personnel and the appropriate resources is testimony to the excellent concrete finish on the clinker and cement storage silos being constructed by Concor Civils for PPC Hercules.

Designated the Ntšhafatso Project at PPC Hercules, the scope of the project is to construct a 33 750 ton clinker storage silo and a 10 000 ton duo-cell cement storage silo both with their own elevator shafts.

The Ntšhafatso expansion project also includes the construction of foundations for a new one-million-ton-a-year vertical roller cement mill as well as construction of related materials handling and storage systems.

Cement Storage Silo
The cement storage silo has a finished height of 51.2 metres with an internal diameter of 24.2 metres while the elevator shaft that will service this silo is 58.8 metres high.

“The silo is constructed with an inner and an outer cell as it will be used to store two different cement products,” Tristan Cowley, site manager at Concor Civils, says. “Work started on the silo construction in September 2007 and was delayed due to unexpected ground conditions,” Cowley adds.

The first 13 metres of the cement silo was slid using slip formwork to produce walls of 800 mm in thickness thereafter the shutter was stepped in to reduce the outside diameter and the remaining portion was then slid to produce the upper walls with a 300 mm thickness.

Once the silo sides were constructed the support work for the inverted cone construction began. “The inverted cone supports the inner cell and precast panels were used to form the soffit of the cone. These panels, which were precast on site, have tapered sections with a length of 15 metres,” Cowley says.

The precast panels were lifted into position using a tower crane and once in place the ring beam was cast followed by the cone section.
 
The elevator shaft is also equipped with a structural steel staircase installed by Concor Civils and a lift will be installed by a sub contractor.  “Both the elevator shaft and the silo will be finished off with a concrete roof supported on steel beams. The beams will be lifted into place using a tower crane,” Cowley adds.

Clinker storage silo
“The clinker storage silo was designed as a post tensioned structure because of the volume of product which will be stored, and included four buttresses to accommodate the stressing anchors. The clinker silo diameter is 30 metres and it has a height of 44.2 metres with the elevator shaft adjacent to this silo being 52.06 metres high,” Cowley explains. “Post tensioning makes this construction that much more complex, as stressing-ducts and anchors have to be put in place.”

The clinker storage silo has two tunnels that run underneath it and construction of this lower section was done using Peri Wiehahn formwork. “The lower walls, which are 9 metres high, were cast in one lift over a two month period as schedule constraints necessitated this,” Cowley says.

Cowley says that the sheer size of the silo is also a challenge. “It is 100 metres in circumference and as the slide progresses material has to be hoisted to higher levels to facilitate ongoing construction. This necessitates careful planning of materials and equipment plus other resources.
 
One of the most important factors to achieve the necessary concrete outputs is that Concor Civils has established its own batch plant on site. “The volume of concrete being used dictated this was the way to go and allows greater control over concrete production as well as concrete quality,” Cowley says.

“Testimony to this is the finish on the silo which can be clearly seen even with the naked eye. The concrete design mix was formulated in accordance with the specific project requirements and in total some 16 000 m3 of concrete and 2 000 tons of reinforcing will be used on this project,” he adds.

Apart from the two silos and elevator shafts, Concor Civils has also been responsible for ancillary civils works, including the construction of foundations for the new finishing mill being installed, the filter bag house and the MCC building. The civils works started in September 2007 and scheduled completion is September 2009.

“At peak the total manpower on the project was 280 and we have two full time safety officers on site. We place a huge emphasis on the safety of both employees and the public, and each member of the team is totally committed to the Concor Group and PPC safety policies,” Cowley says.

“The entire project achieved 1 million injury free hours with an LTIF (Lost Time Injury Frequency) rate of zero in May 2009. Concor Civils alone contributed over 800 000 hours to this impressive achievement,” Cowley says.

The volume of the product which will be stored in the clinker silo called for a post tensioned construction.	The cement silo with an elevator shaft which will service the silo.