4 Building Africa — October 2011

SITE VISIT

Cement creation Part of Morocco Mall’s attraction, apart from its sheer size, is theenormous amounts of concrete needed for its construction. Thisalso posed a major threat to construction timelines and provedchallenging in the initial stages of the project for the contractedengineers, Arup. Much of the cement construction is, however, tobe covered by the building’s façade.

The construction on site was alreadynearing completion when BuildingAfrica visited the site.

Photographs by Dominic Uys

Building Africa — October 2011 5

Acertainty at the moment is that the con -struction activity in Morocco is extensive –evidence of a large investment and focus

by the government. A further certainty, at leastaccording to this writer, is that the constructionindustry will not be slowing down in the foresee-able future. Among the legion of large constructionprojects happening throughout the country, onesite in particular has managed to capture and holdthe attention of the rest of the world: MoroccoMall. This is for two reasons: the sheer size of theproject and the large quantities of cement used inits construction.

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

On the whole, the project seems relativelystraightforward. The building has a more or lesselliptical shape with rounded walls, achieved bymeans of fixing a façade to the building’s outershear walls. On closer inspection though, thenuances to the construction of this massive struc-ture are somewhat intriguing and definitely moredifficult than first imagined.

Miguel Fernandes, one of Design International’sarchitects on site, comments that what makes thisproject complex is the sheer amount of concreteused. The building was, in fact, initially designed tobe constructed almost entirely out of precast con-crete slabs with cast in-situ drop beams. All slabs

were finally constructed out of flat post-tensionedcast in-situ concrete. An intensive endeavour whenevaluating quantities to be used and deadlines tobe met, the project immediately grabbed BuildingAfrica’s attention.

Redesign necessitated There are advantages and disadvantages to beinglocated in Africa. One disadvantage is the apparentunavailability of new technology available on theinternational market.

The original specifications for the building stipu-lated that it be constructed mainly from precastconcrete slabs with cast in-situ drop beams, slabcolumns and shear walls. The slabs would be man-ufactured and stored on site and then moved intoplace. Engineering firm Arup was brought in to con-sult on the structural, mechanical, electrical, fireand façade engineering of the building, and imme-diately spotted a major challenge to using theplanned method.

There were a number of issues related to usingreinforced concrete slabs, according to Arup’s Italygroup leader, Maurizio Teora, who was involved inthe project from the onset. The first of these wasthe fact that the production of the precast concretecomponents would be relatively slow. Given thefact that this project needed to be fast-tracked dueto the client’s deadlines, Teora states that thiswould have made construction timelines extremelyproblematic to adhere to.

The reason for this was the fact that, in order tocreate the concrete slabs for the building, the con-tractor would need to set up a factory for precast-ing. Setting up the site and manufacturing the con-crete slabs would therefore consistently eat into thebuilding’s construction timeline.

Morocco Mall

Post-tensionedconcrete creation

Client

The Aksal Group

Developer

The Aksal Group

Principal agent architect

Design International

Main contractor

SGTM Construction

Post-tensioned concrete design,

supply and installation supervision

CCL

Engineers (civil, electrical, mechanical

and structural)

Arup

Not only is Morocco’s first post-tensioned concrete specialist

constructing the country’s largest shopping mall almost entirely

out of concrete, but the team also looks set to meet this month’s

construction deadline. Dominic Uys investigates.

Project at a glance

Project name Morocco Mall

Project value US$253-million

Size 165 000 m²

650 m-long structure

Location Casablanca, Morocco

Timeline Mid-2008 to October 2011

Challenge Large amounts of concrete were to be

used in the construction, which would

necessitate a large number of expansion

joints, and large numbers of precast

slabs were to be manufactured, at odds

with the tight construction timelines of

the project.

Solution Replace the reinforced concrete floor

slabs and the precast panels with post-

tensioned concrete.

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6 Building Africa — October 2011

Further challenges facedAnother major issue, according to Teora, was thefact that the building would need to contain a largenumber of expansion joints. Not usually a challenge,in this case it proved problematic because the site islocated less than 20 m from the Atlantic Ocean andthe basement level is below sea level. Leaks wouldtherefore not only pose a major inconvenience, butseverely hamper progress.

A further design issue identified prior to com-mencing construction was the fact that the parkinggarage at the base of the building contained toomany supporting columns which severely reducedthe number of parking bays. The building there-fore needed to be redesigned to accommodate anew solution.

Savvy solutionA solution was eventually identified. It was not obvi-ous at first, according to Teora, because the tech-nology was not readily available in Morocco at thetime of the building’s initial design. After some backand forth communication with the relevant role-play-ers, however, Arup was able to replace the rein-forced concrete floor slabs with post-tensioned con-crete. This not only allowed for the building to havefewer expansion joints, but also for construction tobe sped up significantly.

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

around 20%,” Teora says. The post-tensioned slabsalso offer the same amount of strength that is offeredby the thicker reinforced slabs while still being lighter,thus reducing the number of columns required.

Architectural insight The building’s architect, Davide Padoa from DesignInternational, points out that the post-tensionedconcrete had another advantage: “The great advan-tage that the post-tensioned system had for us wasspace. Firstly, we had severe height restrictionsimposed by the Casablanca authorities, so wewanted to maximise the clear height of each leveland this could be achieved if we eliminated the extraconcrete beams left over from the initial design as apost-tensioned slab system can remove the needfor beams.”

He adds that the decline of the constructionindustry in the Middle East in 2009 gave the teamthe opportunity – at no extra cost – to switch to theomega precast slab system, with a post-tension sys-tem, that they had first intended on using. “It allowedus to give the shops some extra 500 mm in clearheight. This also benefited the rents revenue assome shops could introduce mezzanines and extrastorage space,” says Padoa.

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

SITE VISIT

Putting post-tensioning into play at Morocco Mall

The advantages of using the post-ten-

sioning method have been widely publi-

cised. They include:

• savings in materials (steel and

concrete) used in comparison with

reinforced concrete;

• smaller deflections than with steel

and concrete, deflection can be

controlled by adjusting prestress;

• reduced slab thickness;

• longer spans which results in

fewer columns;

• resistance to cracking;

• high fatigue strength; and

• considerable reduction in

construction time.

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The bigger picture

Anchor assembly Stressing the strand

Advantages of post-tensioning

Building Africa — October 2011 7

strengthening the slab. The cable duct is thengrouted to protect the strands from corrosion andensure full adherence between the strands and theconcrete, making the post-tensioned slabs behavelike traditional reinforced concrete slabs.

The technology most certainly is not new, havingbeen implemented in many locations around theworld, but it is rare in this part of Africa. Post-ten-sioned concrete technology is, however, specialisedand Arup was unaware of any contractors operatingin Morocco at the time.

Opportune timingThis proved opportune for CCL, a specialist com-pany expert in pre-tensioned and post-tensionedconcrete technology, which approached the build-ing’s contractor and offered to assist in the imple-mentation of the post-tensioned concrete technol-ogy on site. This involved the design of a post-ten-sioned alternative, supply of the post-tensioningmaterial and specialised equipment, and the super-vision of the installation on site. Morocco branchmanager for CCL, Antoine Zein, tells Building Africathat the initial proposal took some convincing.

“We approached the contractor, which agreedthat this technology was a good solution to the con-struction team’s predicament. The project developerwas also immediately convinced by the benefits ofthe alternative. This removed the drop beams toleave flat soffits, thereby facilitating the distributionof electromechanical, air-conditioning, fire protectionand other networks. At first, however, the idea was

resisted by the consulting engineers which pointedto a number of areas where they felt that our systemcould fall short. However, we were able to provethat in the areas where the current design was lack-ing, namely space and time, our system would beup to the task,” Zein says.

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

Skills transferred On commencement of construction, CCL set towork helping the on-site SGTM team become spe-cialised in post-tensioned concrete construction.“An important point to note here is that the systemwe used really does not need specialised skills. Thefirst thing we had to do was to gather the requiredskills which were already available in Morocco. Weneeded steel workers, carpenters and concreteworkers with their team leaders,” says Zein.

The CCL on-site team of workers is relativelysmall, according to Zein, and comprises around 20workers to take charge of the supervision of slabconstruction, compared to around 1 000 workersfor the main contractor. “From our side, weappointed two experienced technicians from CCL tosupervise. We trained a further 10 local workers toassist and a team to carry out duct manufacture onsite. Skills issue addressed,” says Zein.

4 Grouting

The bigger pictureThe process being applied to thesecond floor of the MoroccoMall in the earlier stages ofconstruction (the roof of thebasement level).

Anchor assemblyThe anchor assemblygraphically depicted. During theprocess, a tendon will haveanchors on each end to transmitthe force into the structure.Longer tendons, as used in themall, also have intermediateanchors along their length.

Stressing the strand Tension is applied by using ahydraulic stressing jack whichbears against anchorsembedded in the concrete,pulling the steel to apredetermined force or stress.

Grouting Grouting involves the insertionof grout into the duct – wherethe strand is located – to createa bond between the tendon andthe concrete.

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8 Building Africa — October 2011

SITE VISIT

Pouring to placementThe slab construction process requires formwork to be installed and ten-dons to be placed at specified locations in the slab. A small amount ofrebar is also added, in large part to comply with Moroccan building regu-lations. 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 areused to stress the tendons, which are anchored, and compress the con-crete. The tendon ducts are then grouted to protect the tendons. “As thetendons are anchored, the slab is lifted slightly from the formwork whichcan then be removed quite easily. The formwork is then moved for prepa-ration of the next slab,” says Zein.

Consistent integrityFernandes describes the ongoing construction of this building as“organic”, meaning that while construction is taking place, the architectsare constantly involved in redesigning aspects of it here and there. This isdue to a number of reasons, one of them being that the new facilitiesmanagement team has requested a few changes to the building in orderto make it more manageable when operational.

A potential obstacle to the constant redesign is the fact that a numberof holes and openings, 15 000 to be exact, have had to be made in theconcrete slabs at the end of slab construction to accommodate electricalwiring, plumbing and the like. This could certainly have negativelyimpacted the strength of the overall build by compromising the integrityof the slabs. Zein explains that because the slabs’ tendons have beengrouted, the slabs behave like normal reinforced concrete or better and

Encompassingconstruction Closer to completion,the building structureresembled a skeletonwith the metalstructures to which thefaçade was being fixedclearly visible.

Sea sighted The location of the build site – less than 20 m from the ocean – posed achallenge as the basement level is located below sea level. Damp and leakstherefore became a certainty that needed to be addressed.

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Building Africa — October 2011 9

can accommodate the new openings in the slabswithout compromising their integrity.

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

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

Building bottom line Zein might make a strong case for CCL’s systembut, as with most good ideas, costing usually spoilsthe party. However, with regards to the constructionof the Morocco Mall specifically, Zein makes an inter-esting point in this regard.

“When compared to reinforced concrete con-struction, I would agree that our method uses 20%less in terms of resources. This does not necessarilymean that the method is cheaper at dry cost, how-ever. While we use fewer materials, the scope ofsupplies that we require with regard to specific mate-rials, like the tendons, has to be added bringing the

cost of construction on a project like this back towhat it would have been, had it been constructedwith ordinary reinforced concrete.

“The main economy comes by dramaticallyreducing execution time, several months of fixedcosts for example, which creates a substantial mar-gin on the execution planning schedule and avoidsdelay penalties,” says Zein.

“One thing that also plays a role in using a post-tensioned slab alternative is the length of the spansbetween the columns supporting the slabs; post-tensioning will be more cost-effective with largespans and large cantilevers. In this case, the factthat 95% of the building’s slabs were 8 m x 8 mmade it cost-effective to build in this manner. Hadthe slabs been 5 m x 5 m for example, we mighthave advised the client to look for an alternative solu-tion,” says Zein.

Although ideal for this Morocco-based construc-tion project, it is not necessarily a universal solutionfor all concrete-intensive building sites. However, inZein’s opinion – and this writer’s as well – the use ofthe post-tensioned slabs was worth it when offset bythe fact that the construction target date could bereached much sooner than initially speculated; that isto say, on time. ■

Constant support During initial design stages therewere too many columns in thebasement, reducing parkingcapacity. Utilising the post-tensioning technique allowed thenumber of columns to bedecreased, while maintainingtheir supportive strength.

The finaleWhen Building Africa visited theMorocco Mall site, constructionwas nearing completion, withcontractors applying finishingtouches to the mall’s interior.

Constant support

The finale

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“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.”