Morocco Mall: post-tensioned concrete creation

Description: I was on site at the construction of the Morocco Mall in Casablanca, the country's largest shopping mall. The photographs I took of the site can be viewed at the URL.

Published: Building Africa Magazine October 2011

URL: https://www.scribd.com/document/363743826/201205705-Post-Tensioned-Concrete-Case-Study-pdf

Not only is Morocco’s first post-tensioned concrete specialistconstructing the country’s largest shopping mall almost entirelyout of concrete, but the team also looks set to meet this month’sconstruction deadline. Dominic Uys investigates.

A certainty at the moment is that the construction activity in Morocco is extensive –evidence of a large investment and focus by the government. A further certainty, at least according to this writer, is that the construction industry will not be slowing down in the foresee-able future. Among the legion of large construction projects happening throughout the country, one site in particular has managed to capture and hold the attention of the rest of the world: Morocco Mall.This is for two reasons: the sheer size of the project and the large quantities of cement used in its construction.

Mall manifesto

The Morocco Mall in the city of Casablanca is hailed as one the largest shopping complexes to be constructed in Africa. Constructed at a cost of around US$253-million by one of Morocco’s largest firms, SGTM Construction, work on this 165 000 m², 650 m-long structure was started in the second half of 2008 and is set to be completed this month.

On the whole, the project seems relatively straightforward. The building has a more or less elliptical shape with rounded walls, achieved by means of fixing a façade to the building’s outer shear walls. On closer inspection though, the nuances to the construction of this massive structure are somewhat intriguing and definitely more difficult than first imagined.

Miguel Fernandes, one of Design International’s architects on site, comments that what makes this project complex is the sheer amount of concrete used. The building was, in fact, initially designed to be constructed almost entirely out of precast concrete slabs with cast in-situ drop beams. All slabs were finally constructed out of flat post-tensioned cast in-situ concrete.

An intensive endeavour when evaluating quantities to be used and deadlines to be met, the project immediately grabbed Building Africa’s attention.

Redesign necessitated

There are advantages and disadvantages to being located in Africa. One disadvantage is the apparent unavailability of new technology available on the international market.

The original specifications for the building stipulated that it be constructed mainly from precast concrete slabs with cast in-situ drop beams, slab columns and shear walls. The slabs would be manufactured and stored on site and then moved into place.

Engineering firm Arup was brought in to consult on the structural, mechanical, electrical, fire and facade engineering of the building, and immediately spotted a major challenge to using the planned method.

There were a number of issues related to using reinforced concrete slabs, according to Arup’s Italy group leader, Maurizio Teora, who was involved in the project from the onset. The first of these was the fact that the production of the precast concrete components would be relatively slow.

Given the fact that this project needed to be fast-tracked due to the client’s deadlines, Teora states that this would have made construction timelines extremely problematic to adhere to.

The reason for this was the fact that, in order to create the concrete slabs for the building, the contractor would need to set up a factory for precasting. Setting up the site and manufacturing the concrete slabs would therefore consistently eat into the building’s construction timeline.

Further challenges faced

Another major issue, according to Teora, was the fact that the building would need to contain a large number of expansion joints. Not usually a challenge, in this case it proved problematic because the site is located less than 20 m from the Atlantic Ocean and the basement level is below sea level. Leaks would therefore not only pose a major inconvenience, but severely hamper progress.

A further design issue identified prior to commencing construction was the fact that the parking garage at the base of the building contained too many supporting columns which severely reduced the number of parking bays. The building therefore needed to be redesigned to accommodate a new solution.

Savvy solution

A solution was eventually identified. It was not obvious at first, according to Teora, because the technology was not readily available in Morocco at the time of the building’s initial design.

After some back and forth communication with the relevant role players, however, Arup was able to replace the reinforced concrete floor slabs with post-tensioned concrete. This not only allowed for the building to have fewer expansion joints, but also for construction to be sped up significantly.

“All in all, we are looking at a 20% reduction in everything. The slabs are around 20% thinner and lighter, and use around 20% less concrete and steel. Curing and construction times are also reduced by around 20%,” Teora says.

The post-tensioned slabs also offer the same amount of strength that is offered by the thicker reinforced slabs while still being lighter, thus reducing the number of columns required.

Architectural insight

The building’s architect, Davide Padoa from Design International, points out that the post-tensioned concrete had another advantage.

“The great advantage that the post-tensioned system had for us was space. Firstly, we had severe height restrictions imposed by the Casablanca authorities, so we wanted to maximise the clear height of each level and this could be achieved if we eliminated the extra concrete beams left over from the initial design as a post-tensioned slab system can remove the need for beams.”

He adds that the decline of the construction industry in the Middle East in 2009 gave the team the opportunity – at no extra cost – to switch to the omega precast slab system, with a post-tension system, that they had first intended on using.

“It allowed us to give the shops some extra 500 mm in clear height. This also benefited the rents revenue as some shops could introduce mezzanines and extra storage space,” says Padoa.

Post-tensioned priority

Post-tensioned concrete slabs, as Teora explains, are slabs that are embedded with cables (tendons) that are tensioned after the slabs have cured. The cables compress the concrete slab, thereby strengthening the slab.

The cable duct is then grouted to protect the strands from corrosion and ensure full adherence between the strands and the concrete, making the post-tensioned slabs behave like traditional reinforced concrete slabs.

The technology most certainly is not new, having been implemented in many locations around the world, but it is rare in this part of Africa.

Post-tensioned concrete technology is, however, specialised and Arup was unaware of any contractors operating in Morocco at the time.

Opportune timing

This proved opportune for CCL, a specialist company expert in pre-tensioned and post-tensioned concrete technology, which approached the building’s contractor and offered to assist in the implementation of the post-tensioned concrete technology on site.

This involved the design of a post-tensioned alternative, supply of the post-tensioning material and specialised equipment, and the supervision of the installation on site.

Morocco branch manager for CCL, Antoine Zein, tells Building Africa that the initial proposal took some convincing.

“We approached the contractor, which agreed that this technology was a good solution to the construction team’s predicament. The project developer was also immediately convinced by the benefits of the alternative. This removed the drop beams to leave flat soffits, thereby facilitating the distribution of electromechanical, air-conditioning, fire protection and other networks.

At first, however, the idea was resisted by the consulting engineers which pointed to a number of areas where they felt that our system could fall short. However, we were able to prove that in the areas where the current design was lacking, namely space and time, our system would be up to the task,” Zein says.

“This was our first project in the Moroccan construction sector, but we have since been pulled into a number of other projects being undertaken by SGTM, as well as by other construction companies, especially where high-rise and large-scale construction is concerned,” he says.

Skills transferred

On commencement of construction, CCL set to work helping the on-site SGTM team become specialised in post-tensioned concrete construction.

“An important point to note here is that the system we used really does not need specialised skills. The first thing we had to do was to gather the required skills which were already available in Morocco. We needed steel workers, carpenters and concrete workers with their team leaders,” says Zein.

The CCL on-site team of workers is relatively small, according to Zein, and comprises around 20 workers to take charge of the supervision of slab construction, compared to around 1 000 workers for the main contractor.

“From our side, we appointed two experienced technicians from CCL to supervise. We trained a further 10 local workers to assist and a team to carry out duct manufacture on site. Skills issue addressed,” says Zein.

Pouring to placement

The slab construction process requires formwork to be installed and tendons to be placed at specified locations in the slab. A small amount of rebar is also added, in large part to comply with Moroccan building regulations.

The concrete needs to be certified to 35 MPa according to Zein.

After the concrete has been left to cure for about two days, jacks are used to stress the tendons, which are anchored, and compress the concrete. The tendon ducts are then grouted to protect the tendons.

“As the tendons are anchored, the slab is lifted slightly from the formwork which can then be removed quite easily. The formwork is then moved for preparation of the next slab,” says Zein.

Consistent integrity

Fernandes describes the ongoing construction of this building as “organic”, meaning that while construction is taking place, the architects are constantly involved in redesigning aspects of it here and there.

This is due to a number of reasons, one of them being that the new facilities management team has requested a few changes to the building in order to make it more manageable when operational.

A potential obstacle to the constant redesign is the fact that a number of holes and openings, 15 000 to be exact, have had to be made in the concrete slabs at the end of slab construction to accommodate electrical wiring, plumbing and the like.

This could certainly have negatively impacted the strength of the overall build by compromising the integrity of the slabs.

Zein explains that because the slabs’ tendons have been grouted, the slabs behave like normal reinforced concrete or better and can accommodate the new openings in the slabs without compromising their integrity.

Vertically limited

The system is, however, limited in its application. Zein adds that the one limitation of the post-tensioned slab alternative is the need for vertical shear walls for lateral stability with post-tensioned thin slabs acting as diaphragms, compared to the “columns/dropped beams” frame effect.

“We are researching remedies to this and it might be quite interesting if we succeed,” says Zein.

Building bottom line

Zein might make a strong case for CCL’s system but, as with most good ideas, costing usually spoils the party. However, with regards to the construction of the Morocco Mall specifically, Zein makes an interesting point in this regard.

“When compared to reinforced concrete construction, I would agree that our method uses 20% less in terms of resources. This does not necessarily mean that the method is cheaper at dry cost, however.

While we use fewer materials, the scope of supplies that we require with regard to specific materials, like the tendons, has to be added bringing the cost of construction on a project like this back to what it would have been, had it been constructed with ordinary reinforced concrete.

“The main economy comes by dramatically reducing execution time, several months of fixed costs for example, which creates a substantial margin on the execution planning schedule and avoids delay penalties,” says Zein.

“One thing that also plays a role in using a post-tensioned slab alternative is the length of the spans between the columns supporting the slabs; post-tensioning will be more cost-effective with large spans and large cantilevers.

In this case, the fact that 95% of the building’s slabs were 8 m x 8 m made it cost-effective to build in this manner. Had the slabs been 5 m x 5 m for example, we might have advised the client to look for an alternative solution,” says Zein.

Although ideal for this Morocco-based construction project, it is not necessarily a universal solution for all concrete-intensive building sites. In Zein’s opinion – and this writer’s as well – the use of the post-tensioned slabs was worth it when offset by the fact that the construction target date could be reached much sooner than initially speculated; that is to say, on time.