Architectural Precast

The Emir of Qatar has placed emphasis on the creation of a solid infrastructure in the country; one in which medium-sized enterprises of non-oil-related industries can prosper. Adjacent to the international airport of Doha, a complex of four office buildings was planned as a new headquarters for Manateq, a developer within Qatar. Laceco Architects and Engineers from Lebanon won the international design competition.

Bernhard Sagmeister, Durcrete, Limburg, Germany

UHPC shading elements create an iconic building in Qatar

(Ph

oto

s: DC

S, Aym

an B

adr.)

The design of the Manateq HQ was conceived as a composition of four inverted cubes sheltering an internal oasis. The outside of the iconic building should reflect the brand of Manateq and reinterpret traditional Qatari architecture using a modern language. The headquarters’ parametric façade was inspired by Manateq’s logo and optimised daylighting in offices. To achieve a long-lasting, high-quality and low-maintenance performance, an ultra-high-performance concrete (UHPC) was selected as material for the permanent shading construction.

RequirementsThe façade elements were produced within Doha; the material is highly adapted to local climatic conditions (corrosion, dust, etc).

The single panels have a length of approximately 9m and a width of approximately 3.3m. All panels are visible from the outside as well as from the inside (offices), so that all sides should be as-struck concrete. As there is a groove on both sides of the curved panels, the production was done in vertical glass-fibre-reinforced plastic (GFRP) moulds using self-compacting concrete.

The contractor for the shading elements was Doha Cladding Solution (DCS). In 2015, the owner of the prefabrication plant decided to move into the business of UHPC façade elements. The management refused to buy ready-dry mixes of mortar or concrete, without knowing the ingredients and details of the mixture, as they preferred solutions where the formulation can be adjusted during the production process. With the high-performance binder Nanodur, DCS was able to batch a UHPC mix within short notice and to adapt this recipe to the varying requirements of the mould, mixer configuration, ambient temperature, change of raw materials, strength, flowability and ductility. The subcontractor carried out a tough prequalification and the client had to be convinced by the testing of specimens, that all the requirements of the tender could be fulfilled.

Structural designThe structural analyses for the UHPC elements, as well as for the fixing to the steel construction, were the responsibility of the subcontractor. It was performed by a local engineering office in Doha, following British Standards.

Architectural Precast

Each panel is connected as simply supported directly on the steel back frame. The fixing is strain free to avoid stresses due to temperature, elongation and warping in the concrete element, as well as strain due to different deformations between steel frame and elements. A maximum permanent deflection a value of span/35000 was calculated; similarly, a maximum wind-load deflection a value of span/15000 was calculated, which means nearly no deformation occurs. The elements are calculated with a linear elastic calculation in an uncracked state. The maximum tensile stresses due to dead weight and wind are compared with an admissible strength of 5MPa.

As the chosen Nanodur UHPC has a minimum flexural tensile strength of the uncracked concrete of 15–20MPa, there is a safety factor of 3.0. Experience over the past ten years with this material, suggests this should be enough to avoid uncertainties of the load, material, decrease of tensile strength due to temperature and humidity, fatigue and other influences. Fibres and reinforcement within the UHPC may increase ductility, tensile and compressive strength. However, all these positive effects were not considered; the design was carried out with the pure flexural strength of the UHPC. Doing so, the concrete remains uncracked and the visual characteristics remain perfect. The given wind load onto the UHPC elements was accounted for in X and Y directions, and dimensioning was done with unsymmetrical bending.

The connection between the UHPC elements and the steel support should allow movements up to 5mm in order to avoid thermal strain. Due to the tolerance compensation during the erection, it was decided to avoid embedded threaded sleeves. The stainless steel M12-size bolts are undercut anchors with a length of 120mm. The anchors are drilled and installed at the site. Intensive testing was required to get the ultimate loads of the fixing in various directions. The panel thickness of the in-plane shear connection was only 130mm.

Mix designThe tender documents required an UHPC with a strength of 150-200MPa in accordance with ASTM and a flexural

tensile strength of the uncracked concrete (modulus of rupture) of a minimum of 12MPa. The Young’s modulus had to exceed 55,000MPa. Deviations, bowing and warpage due to production and shrinkage were restricted.

The subcontractor used Nanodur Compound 5941 as a UHPC-binder premix of Portland and slag cements, admixtures and quartz flour. Instead of silica fume or metakaolin, very fine cement particles as well as synthetic oxides were used to fill the gaps between the cement particles and to influence the hardening process. This results in very smooth, flowable and self-compacting UHPC mixes without the need for further post-processing. Common UHPC formulations with silica fume often need thermal curing between 60° and 80°C over a one- to two-day period, or even a treatment in an autoclave to reach the required strength. The binder and admixtures were imported from Germany, aggregates from India, steel fibres from China. The final mix design is shown in Table 1.

ProductionAll particles finer than 0.63mm were combined within the cement plant using a high-intensity mixer. To batch the concrete, a simple pan mixer in the prefabrication plant was sufficient and no special equipment was necessary. The mixing time depended on the polycarboxylate superplasticiser (PCE) used and was normally between 8 and 10 minutes per charge. At temperatures of more than 30°C, most PCEs lose their effectiveness, so the material was stored inside and not in the sunlight. The use of high-intensive shear mixers was found not to shorten the required mixing time and did not improve the result;

Above: Pouring of panel.

Table 1 – UHPC mix design and test resultsNanodur Compound 5941 1320g/m³Special aggregates size 3–5mm 312kg/m³Special aggregates size 1–3mm 864kg/m³Quartz sand from India 160kg/m³Superplasticiser – PCE 26kg/m³Shrinkage reducer 16kg/m³Steel fibres 70kg/m³Water 160kg/m³Mean 100mm cube compressive strength at 56 days >180MPa

Architectural Precast

very often they cause problems because they bring in too much energy and increase the temperature of the mix. The Nanodur binder was delivered in large bags and dosage of all ingredients was done by hand. Pouring of concrete was possible with buckets or with a wheelbarrow; no special equipment was required. The greater the flow of concrete, the better the loss of entrapped air and the surface quality of the element.

The challenge in production was not the recipe or the mixer but the crucial task of constructing the mould. The self-compacting concrete with a lot of fine particles gives an exact mirrored impression of the mould. Every small error and uncleanliness marked on the mould surface is reflected in the element. The air bubbles rise to the top and get trapped at corners or edges within the mould. The shrinkage of the concrete of millimetres over the whole length leads to deviations in dimensions, which should be defined within given tolerances. All inner formwork and preformed voids are restrained within the concrete due to shrinkage. These should be constructed in a ‘soft and weak’ material construction in order to facilitate the demoulding and to avoid cracking of the concrete. However, the formwork should be not too weak, so that the given tolerances are not exceeded.

DCS used GFRP for the construction of the mould. This material also has the advantage of being lightweight, so that moulding and demoulding procedure are possible to do by hand, without using a crane.

Above: The front of the building.

Below: Building site at night.

Corners and edges.

August 2020Volume 24 Number 2

Sustainability

CONCRETEENGINEERINGInternational

Formwork and Falsework

Historic Concrete

Concrete and the circular economy