block & beam fl oor systems and hollow core slabs

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characteristics and propertiesFloors and fl oor plates in post-war housing were commonly constructed with wood, concrete and/or ceramics. Leaving aside the timber fl oor joists and the solid, cast in situ concrete fl oor plates, two popular systems with prefabricated elements were block & beam systems (in concrete and ceramics) and hollow core slabs (in reinforced or prestressed concrete).

The block & beam systems consist of small beams, often with an inverted T-shape, in between which small, hollow and light elements are placed as fi ller blocks. The profi le of the beams and blocks is designed in such a way that they hook together, while creating a fl oor plate with a fl at bottom side. After the blocks are placed, a top layer of concrete is cast on top to create a monolithic fl oor. Each part of the fl oor fulfi lls a specifi c function. The beams, which are often made in prestressed concrete or in ceramics with a prestressed concrete core, create the span (up to approx. 6 m) and take the tensile forces. The cast in situ top layer, which is usually slightly reinforced, takes up the compression forces. Structurally, the infi ll blocks have no loadbearing function: they literally fi ll in the space between the beams in

block & beam fl oor systems and hollow core slabs

764

the easiest and lightest way possible to create two horizontal planes, one of which is the ceiling (which needs no further fi nishing in some cases) and the other is the permanent formwork for the compressive layer of concrete on top. One of the most important advantages of the block & beam fl oor system is that it consists of small, prefabricated elements: the elements can be carried without heavy machinery or mechanical equipment. There is no need for temporary supports or formwork, and the fl oors can be constructed relatively quickly (except for the top layer of concrete which has to set). Once the blocks are in place, an instant fl oor is ready to be used. The blocks and beams exist in various sizes, to suit different spans and loads. To simplify the number of construction materials and elements that were used on the construction site, it was not uncommon to use the beams also as lintels above door and window openings.

Hollow core slabs are prefabricated elements in reinforced or prestressed concrete, or exceptionally in gypsum (e.g. the French system Gypsolith). The rectangular cross section shows adjacent, longitudinal cavities: other than the top and bottom of the slab, which have a structural and practical function, the inner mass can be hollowed without any loss of function, yet realizing a very important dead weight reduction. The hollow cores can be created in different ways, for example by using tubes in waterproof cardboard (e.g. brandname ‘Monotub’, which could also be used on site for customized hollow core slabs) or by using metals moulds which were retracted after hardening. The most common way to create hollow core slabs was by extrusion: if the concrete mixture was very dry, no formwork at all was needed to create the cores. As the technological equipment of the precast factories evolved, the production of hollow core slabs changed as well. In a fully automatic process, a mobile casting machine is moved along a long work bench or long production line, casting one, very long hollow core slab (often in prestressed concrete) which can be cut or sawn in pieces of the desired length afterwards. Like the block & beam systems, hollow core slabs exist in different sizes, from 45 to 160 cm wide and commonly up to 6 or even 8 m in length. The long edges are often chamfered: a reinforcement bar is inserted when the joint is cast with concrete (or when a complete top layer of concrete is cast, as desired).

brands and productsBlock & beam fl oor systems and hollow core slabs were not invented during the post-WWII period, but the development of the prefabrication industry enabled a

765

widespread and large scale application of both concepts during those years. An early system of blocks & beams is the Herbst system, which was developed by the German engineer W. Herbst in ca. 1903 and commercialized in Belgium by the Société Belge des Bétons. The system existed of small beams in reinforced concrete and hollow tubes in ash concrete, on which a layer of rich concrete was poured. Early examples of the hollow core slabs in Belgium are the systems Siegwart (invented by architect Hans Siegwart from Luzern), Moyse (invented by the Liège engineer/architect/contractor Simon Moyse), Mihrtadiantz (invented by the Brussels engineer Maksoud Mihrtadiantz) and Monobloc (by the fi rm Knapen). Most of these early hollow core slabs have only one hollow core, in contrast to the ‘genuine’ post-WWII hollow core slabs, which are broader and have a series of hollow cores within the same element. Next to these block & beam systems and hollow core slabs, numerous other fl oor systems in prefabricated concrete were developed before or after WWI, each claiming to be quick, easy and cheaper than the rest. Yet hardly any of these systems lasted for more than a few years, or until after WWII.

During the post-WWII period, important changes occurred in the production methods, material compositions, the production volume and the equipment, both in the factory and on the building yard. The two concepts of blocks & beam systems and hollow core slabs responded differently to the changes in the building climate. The block & beam fl oor systems remained ‘systems’, i.e. a fi xed set of elements with specifi c characteristics and dimensions, designed and produced by a specifi c manufacturer, whose elements were often not interchangeable or to be used with other brands or products. A few international brands succeeded to market their products (and brand name) on an international scale. An example of such an internationally successful brand is the Swiss system Stalton (Stahlton) for blocks & beams. The Belgian company Novobric also designed some popular block & beam systems in ceramics and concrete. The production and application of hollow core slabs evolved in a quite different way after WWII. Instead of specifi c, ‘closed’ and often patented systems, hollow core slabs became ‘generic’ construction elements. Hollow core slabs became one of the most widespread construction elements in the post-WII period, easy to combine with other construction elements and easily integrated in each design and typology.

Hollow core slabs became one of the most important, yet also most undervalued elements of the post-war building practice. Almost every prefabrication company

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and concrete contractor produced one or more types of hollow core slabs, all referring to the same advantages and sales arguments such as the minimum fl oor thickness, economical, light yet robust, easy to handle and quick to construct, a specifi c type of slab for each span and load, good thermal and acoustic insulation, fi re-resistant, etc. Although based on the same concept, there was a great diversity between the slabs of different companies and manufacturers, yet not in such a way that it became a ‘closed’ construction element that could not be combined with other manufacturers’ products. Differences are found in the material (reinforced concrete, prestressed concrete, vibrated concrete, lightweight concrete, ceramics) but even more in dimensions, weight and loadbearing capacity. Hollow core slabs existed in almost every possible dimension, so it would be close to impossible and pointless to give a complete list of all sizes and formats. Just to give a few examples: the hollow core slabs of the Etablissements Bekaert were 10, 14 or 17 cm thick and up to 6 m long; the hollow core slabs by Usines Regnier & Cie were up to 8,50 m long. Also the shape and cross section of the slabs varied: most slabs were rectangular with circular openings, but rectangular openings were also common. More peculiar were the hollow core slabs ‘Record’ manufactured by the Tubize company and the ‘Zig-Zag slabs S.V.K.’ manufactured by the Usines Réunies Scheerders van Kerchove, which were both made with hollow elements with a very specifi c cross section. Depending on the size of the company, and whether or not the slabs were put on the market to be used by other construction companies as well, catalogues were published giving an overview of the production gamut, with varying dimensions, cross sections and load schemes. In the 1970s the Federation of the Precast Concrete Industry in Belgium edited a brochure on the Standardisation of prefabricated concrete elements for buildings, with the help of CBR, Structo, Ronveaux and other precasting companies. One of the things put forward in the brochure was a standard width of 60 cm or 1,20m for hollow core slabs; the thickness was preferably between 15 and 40 cm, with regular intervals of 5 cm (including the top layer in concrete).

Steenbakkerijen van PloegsteertThe company ‘Steenbakkerijen van Ploegsteert’ (‘Brickworks of Ploegsteert’), founded in the 1920s and located in the town with the same name close to the French border, produced the Swiss block & beam system Stahlton (Stahlton has no ‘h’ in Dutch) from the early 1950s onwards. Next to these ‘hollow fl oors in prestressed ceramics’, Ploegsteert also produced façade bricks, bricks with

767

insulating properties (Iso Briques/Iso sponsensteen), lightweight ceramic blocks (Bloc treillis/Tralie bouwblokken) and a beam&block system (Ridec) in prestressed concrete for larger spans (over 9 m) and greater loads.

Stalton

The Swiss company Stahlton S.A. was founded in 1945 to commercialize the block&beam system invented and patented by the three Swiss engineers M.R. Roš, A. Brandestini and M. Birkenmaier. The system was soon exported and applied in other countries all over the world, including Belgium.

The Stahlton system is composed of ceramics, concrete and prestressing steel. The base of the beams is a hollow ‘tray’ in ceramics, with internal ribs or stiffeners in the same material, which are placed one after the other. Prestressing cables are placed in these ceramic elements before they are fi lled with concrete to create one monolithic element. These beams can be used as such, e.g. as lintels or beams, or together with light infi ll elements in ceramics (topped off with a compression layer of concrete) in the block & beam system.

NovobricThe Belgian company Novobric - Damman S.A. designed several fl oor systems (as well as building blocks for walls and prefabricated panels), all based on hollow ceramic elements. The elements were very light, easy to handle, economical (both in regards to the labour and the weight that had to be supported by the foundations), loadbearing, insulating (thermal and acoustic), insusceptible to cracks or rust. Because of the enclosed air in the hollow elements, the λ-value of the ceramic elements was equal to 0,145 W/mK - yet this value needs to be nuanced for Novobric fl oors, because the fl oor elements are fi lled or covered with concrete, reducing the insulating capacities.

Atlas

The Atlas fl oor system is an alternative to the ‘regular’ hollow core slabs: the hollow elements in ceramics are placed next to each other, reinforcement bars are inserted between them to form ribs in reinforced concrete (with the ceramic elements as permanent moulds, so that the bottom side of the fl oor is completely in ceramics) and the whole is covered with concrete. The main characteristic, distinguishing it from other hollow core slabs, is the cross-section of the ceramic element, which is

768

rather complicated compared to regular (concrete) hollow core slabs. The ceramic elements were fabricated in several sizes: the elements were between 8,5 and 20,5 cm wide and 25 or 33 cm high, plus an extra 3 cm for a concrete screed fl oor if desired. The maximum length was limited to 8 m for a load of 250kg/m²; if the load bearing capacity was to be increased to 1000 kg/m², the length was limited to 4,95 m. Novobric also produced prefabricated panels with the Atlas blocks: instead of casting the beams in situ, these were cast in the factory, so that entire panels could be delivered on the construction site, saving time and on site labour. The panels were between 1,65 m and 6,85 m long.

Excelsior

For small spans, Novobric produced uniform hollow fl oor plates: the hollow panels Excelsior, which didn’t need to be covered with concrete, were between 1 and 1,6 m long, 25 cm wide and 4 or 6 cm high. Given their limited length, they could only be used for relatively small spans, and seem more suited for walls or roofs than for fl oors.

Listex

Another Novobric-product was Listex, a block & beam fl oor system in reinforced ceramics: hollow ceramics beams, fi lled with reinforced concrete, supported hollow elements in ceramics (with a complex cross-section, similar to the Atlas elements), after which a layer of concrete connected everything together. These fl oors were characterized by the general Novobric properties (lightweight, easy and quick to put in place, economical, good insulating capacity, no risk of cracks), plus a high rigidity and an ideal surface for fi nishing. The Listex fl oor also existed in different sizes: the ceramic blocks could be either 10,5 cm, 12,5 cm or 16,5 cm high. For bigger heights, two blocks of 10,5 cm could be stacked on top of each other. The width of the blocks was always the same (37 cm at the broadest point, or 32 cm at the base between the beams). The beams were always 10,5 cm high and 13 cm wide. The maximum span varied between 1,75 m (blocks of 10,5 cm, without a screed fl oor, under a load of 1000 kg/m²) and 8,55 m (two blocks of 10,5 cm plus a screed fl oor of 3 cm, under a load of 250 kg/m²).

PL 46

The block & beam fl oor system ‘PL 46’ was very similar to the Listex system, except for using prefabricated prestressed concrete beams instead of ceramic beams fi lled

769

with reinforced concrete, enhancing even more the loadbearing capacities. The blocks were identical to the Listex-blocks, and the beams had the same dimensions, but they could be combined differently: not only the two small blocks but also the two biggest blocks could be stacked on top of each other, with not one but two prestressed beams between them, so to create a fl oor of 36 cm high (16,5 cm + 16, 5 cm + 3 cm screed) and a maximum span of 10,25 m.

CBR-ErgonErgon, one of the major prefabrication companies in Belgium, was one of the companies with a standardized, modular gamut of hollow core slabs. Ergon was founded in 1963 by one of Belgium’s largest cement companies Cimenteries et Briqueteries Réunies CBR to produce prefabricated cement and concrete products. Within a few years, Ergon was one of the major precast companies in Belgium, next to the Ets. Ronveaux in Ciney and the NV Structo in Bruges. Ergon provided a full service, from the preliminary design to production and assembly, with technical study and inspection services linked to an extensive production department. Ergon focused on the mass production of beams, columns, panels, TT-fl oor slabs and hollow core slabs.

SP-fl oor plate

The Ergon SP-fl oor plate was a fl oor plate in prestressed concrete, made by the extrusion process. To reduce the weight of the slabs, they could be made with lightweight aggregates ‘Agral’ instead of regular concrete. Both the top and the bottom surface of the fl oor plate were fl at. The cross section was rectangular (1,20 m wide and 20, 27 or 32 cm thick), with six circular cavities running over the entire length. The fl oor plates were made by extrusion, on four lines of 80 m in length: a concrete tank or reservoir is moved along these lines, squeezing out the very dry concrete through a mould in the desired form with cavities. The number of prestressing cables, which are positioned on the extrusion lines, varies with the desired loadbearing capacity. The concrete reservoir moves along rails with a speed of one meter per minute. After the concrete has set, the plate with a length of 80 m, is cut into pieces with a diamond blade saw. The spans could vary between 6 and 14,5 m, inversely proportional with a service load varying between 2000 and 250 kg/m². The SP-fl oor plates were mainly used for spans between 6 and 9 m, whereas hollow core slabs of 60 cm in width and up to 6 m in length were more common in house building.

770

applications in house building in brussels

771

bibliography

characteristics and properties

block & beam fl oor systems and hollow core slabs

brands and products

block & beam fl oor systems and hollow core slabs

776

PLOEGSTEERT-STALTON

777

STEENBAKKERIJEN VAN PLOEGSTEERT

Produkten in gebakken aarde, Iso, Tralie, StaltonBouwen en Wonen1956, vol. 3, no. 7.

778

BRIQUETERIES MÉCANIQUES DE PLOEGSTEERT

Iso, Treillis, StaltonLa Maison1958, vol. 14, no. 6.

779

BRIQUETERIES MÉCANIQUES DE PLOEGSTEERT

Stalton, RidecLa Maison1962, vol. 18, no. 9, p. CCXIX.

780

BRIQUETERIES DE PLOEGSTEERT

Treillis, briques ordinaires, Stalton, Spanfloor, RidecLa Maison1967, vol. 23, no. 3, p. LX.

781

782

NOVOBRIC

783

NOVOBRIC-DAMMAN

NovobricLa Maison1959, vol. 15, no. 9, p. CXC.

784

NOVOBRIC-DAMMAN

NovobricLa Maison1960, vol. 16, no. 6, p. CXXXVI.

785

NOVOBRIC-DAMMAN

Atlas NovobricLa Maison1961, vol. 17, no. 1, p. IX.

786

NOVOBRIC

ListexLa Maison1962, vol. 18, no. 1, p. XVII.

787

NOVOBRIC-DAMMAN

Atlas NovobricLa Maison1962, vol. 18, no. 8, p. CCXI.

788

NOVOBRIC

Blocs de maçonnerie et hourdisLa Maison1963, vol. 19, no. 1, p. XXI.

789

NOVOBRIC

NovobricLa Maison1963, vol. 19, no. 2, p. XLV.

790

NOVOBRIC

Hourdis précontraint type pl. 46La Maison1964, vol. 20, no. 1, p. XXI.

791

NOVOBRIC

Hourdis précontraint type pl. 46La Maison1964, vol. 20, no. 2, p. XLII.

792

NOVOBRIC

ListexLa Maison1964, vol. 20, no. 5, p. CXLI.

793

NOVOBRIC

AtlasLa Maison1964, vol. 20, no. 6, p. CLXXVI.

794

NOVOBRIC

PL 46, trirex, atlasLa Maison1966, vol. 22, no. 2, p. XLVIII.

795

NOVOBRIC

Briques de façade, hourdis préfabriqués et précontraints, blocs pour maçonnerie TRE-I. S.B.B. et briques creusesLa Maison1967, vol. 23, no. 10, p. CCLIII.

796

OTHER BRANDS AND PRODUCTS

797

ETABLISSEMENTS BEKAERT

Hourdis préfabriqués en béton armé vibréLa Maison1946, vol. 2, no. 1, p. III.

798

ETABLISSEMENTS BEKAERT, ETABLISSEMENTS DUYCK FRÈRES

Planchers en béton armé vibréLa Maison1946, vol. 2, no. 2, p. VIII.

799

ETABLISSEMENTS BEKAERT

Plancher préfabriqué en béton armé vibréLa Maison1946, vol. 2, no. 5.

800

ETABLISSEMENTS BEKAERT

Hourdis en béton armé vibréLa Maison1946, vol. 2, no. 6, p. I.

801

BASCLa Maison1951, vol. 7, no. 2, p. XXXII.

802

BRIQUETERIES DU BRABANT

Hourdis et brique “Record”La Maison1952, vol. 8, no. 4, p. LIV.

803

USINES REGNIER

Blocacier, Blockisol, BlocmixteLa Maison1952, vol. 8, no. 8, p. CXXXI.

804

SCHEERDERS VAN KERCHOVE

Hourdis Zig-Zag S.V.K.La Maison1953, vol. 9, no. 6, p. LXXXVIII.

805

GELDERBETON

HourdisLa Maison1954, vol. 10, no. 10, p. CLXXXI.

806

GELDERBETON

Tous éléments en béton et agglomérésLa Maison1955, vol. 11, no. 6, p. CVI.

807

RICHARD DE CLERCQ

HourdisLa Maison1956, vol. 12, no. 3, p. LII.

808

USINES DAUCHOT

Hourdis préfabriquésLa Maison1957, vol. 13, no. 8, p. CLXVIII.

809

USINES FIXOLITE

Plaques isolantes, hourdis coffrage perdu, blocs creux isolants, dalles armées isolantesLa Maison1957, vol. 13, no. 10, p. CCXI.

810

STRUCTO

Éléments de hourdis pour toitures et planchersLa Maison1957, vol. 13, no. 11, p. CCXXVIII.

811

STRUCTO

Béton précontraintLa Maison1962, vol. 18, no. 8, p. CCVI.

812

ARGEX

Chapes, hourdis, dalles armées, planchers composés de poutres, claveaux et dalle de compression, coffrage perduLa Maison1963, vol. 19, no. 8, p. CCI.

813

ETABLISSEMENTS SCHMIDT

Hourdis F.S.A.La Maison1966, vol. 22, no. 7, p. CLXXXIII.

814

FIXOLITE

Dalles armées, éléments préfabriqués décoratifs pour façades, plaques isolantes, coffrages perdusLa Maison1966, vol. 22, no. 8, p. CCVI.

815

ETERNIT

Panneaux en asbeste-ciment extrudéLa Maison1968, vol. 24, no. 2.