Watertightness in anti-flotation slabs. MIS-RJ Case. Estanqueidade de lajes de subpressão. Caso MIS-RJ - PDF

Volume 7, Number 6 (December 2014) p ISSN Watertightness in anti-flotation slabs. MIS-RJ Case Estanqueidade de lajes de subpressão. Caso MIS-RJ C. BRITEZ a

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Volume 7, Number 6 (December 2014) p ISSN Watertightness in anti-flotation slabs. MIS-RJ Case Estanqueidade de lajes de subpressão. Caso MIS-RJ C. BRITEZ a P. HELENE b S. BUENO c J. PACHECO d Abstract It is common in coastal cities as Rio de Janeiro, that buildings located close to the shoreline have their basements below water table level. In most cases, the engineering solution for these buildings is to design a massive anti-flotation slab to satisfy, principally, the issues related to structural dimensioning and calculation hypothesis. On the other hand, the execution of this solution imply in significant construction problems related to reinforced concrete watertightness and durability. This paper presents a case study about challenges and solutions devised to execute an antiflotation, 1m thick, 1200m³ reinforced concrete slab for the new Museu de Imagem e Som (MIS) Sound and Image Museum, located at 50m from the seashore, at Copacabana in Rio de Janeiro, RJ. The results show that concrete proportions, concreting plan and pouring method adopted were decisive in obtaining a watertight structure, avoiding thus the employment of traditional waterproofing alternatives. Keywords: anti-flotation slab, watertight concrete, watertight concrete structures, concrete at seashore. Resumo Tem sido comum em cidades litorâneas, como a do Rio de Janeiro, observar a construção de subsolos em edificações localizadas nas proximidades de orlas marítimas. Na maioria dos casos, a solução de engenharia envolvida nesses projetos é o uso de lajes de subpressão com o objetivo de garantir, principalmente, os aspectos relacionados com o dimensionamento estrutural e hipóteses de cálculo. No entanto, há complexidades significativas quanto à execução desse tipo de solução, no que tange aos aspectos de estanqueidade e durabilidade do concreto armado. Este artigo apresenta um estudo de caso sobre os desafios e as engenhosidades envolvidas para concretagem da laje de subpressão em concreto armado da nova sede do Museu de Imagem e do Som (MIS), com 1m de espessura e volume de 1200m³, situada a 50m da orla marítima, na região de Copacabana, Rio de Janeiro, RJ. Os resultados demonstraram que a composição do concreto, o plano de concretagem e os procedimentos executivos empregados foram decisivos para promover uma estrutura íntegra e com propriedades estanques, dispensando, nesse caso, alternativas tradicionais e convencionais de impermeabilização. Palavras-chave: laje de subpressão, concreto estanque, estanqueidade estruturas de concreto, concreto em orla marítima. a b c d Civil Engineering Construction Department at Escola Politécnica da Universidade de São Paulo. PhD Engenharia; Full Professor at Universidade de São Paulo. PhD Engenharia; Escritório Técnico Julio Kassoy e Mario Franco Eng. Civis Ltda. (JKMF); PhD Engenharia. Received: 13 Mar 2014 Accepted: 19 Sep 2014 Available Online: 01 Dec IBRACON Watertightness in anti-flotation slabs. MIS-RJ Case 1. Introduction To make the new building for the Museu de Imagem e do Som (MIS) one more international architectural landmark for Rio de Janeiro city, known by the creativity of its artistic expressions and by the richness of its musical rhythms, the Secretaria de Estado e Cultura do Rio de Janeiro, together with Fundação Roberto Marinho (FRM), promoted recently an important international architectural contest, to choose a futurist design. The American architectural bureau Diller Scofidio + Renfro won the contest among other seven architectural bureaus selected for the final decision. The design was developed in Brazil by the renowned bureau Índio da Costa Arquitetura, Urbanismo, Design e Transporte (ICA). Located at Atlântica avenue, at Copacabana, just at 50m from the beach, the new MIS headquarters will substitute the old museum premises. The MIS, opened in 1965, is installed simultaneously in two places, at Praça XV, located in downtown Rio, and at Lapa. The new building will also host the Carmen Miranda Museum, which is now located at Flamengo neighborhood (CORBIOLI, 2011)[1]. It should be noted that the American architects proposed a museum as a vertical boulevard, with seven stories, a continuous external promenade and a display of sequential ramps and floors. The building will have a total area of seven thousand square meters approximately, including foyers, box office, wardrobe, exposition rooms, auditorium, didactical activities room, shop, cafeteria, panoramic restaurant, bar, terrace, piano bar, belvedere, administration areas, storage spaces for collections, parking and load/unload facilities. The new MIS premises, shown in Figure 1, are being built by the construction company Rio Verde. Construction works are being managed by the company Engineering. The building has 2 basement floors and a technical mezzanine also located below grade, besides the superior floors. As the building is being built very close to the sea shore (50m), very special procedures and engineering techniques were required to design a watertight anti-flotation slab, with considerable volume (1200m³) and thickness (1m). Other problems are, besides ensuring technical parameters related to watertightness and high concrete strength (characteristic compressive strength was specified at f ck 50MPa at 28 days), weather factors (hot weather, concreting commonly occur at temperatures about 35ºC) and logistic (the concrete batch plant is approximately 30km afar from the working site, mixing trucks drive through a tourist route, with heavy traffic. At working days and hours, mixing trucks take at last 1 hour to transport the concrete). The anti-flotation slab was executed during November 2012 and January 2013, at an elevation about 10m under sea level. Concrete mix proportions, as well as some execution details, mainly those related to watertightness will be described below. A substantial part of procedures employed is included in Brazilian current standards (ABNT NBR 6118:2007 [2]; ABNT NBR 12655:2006 [3]; ABNT NBR 14931:2004 [4]) and in international literature (KOS- MATKA; WILSON, 2001 [5]; KENNEDY, 2005 [6]; LAMOND; PIEL- ERT, 2006 [7]). However, most part of procedures was based on directives supported by the reference paper published by Helene, Terzian e Sardinha (1980) [8]. 2. Watertightness concept It is important to explain that concrete, as a material, is capable of promoting sufficient conditions for a very low permeability which means that it can be considered watertight. As it can promote an efficient barrier against water penetration, concrete is largely employed to build large reservoirs, pools, dams, water and effluent treatment plants, etc. Nevertheless, most problems of employing this potentially watertight material, arose from the difficulty of obtaining the watertightness of structures, which depends not only on the material, but mainly on proceedings. Thus, besides a material of adequate quality, the procedures related to good engineering practices are needed to avoid honeycombs, defective compaction, unpredicted fissures, construction joints or defective joints through which water may percolate. The watertightness of a concrete structure may be understood as the capacity of this structure to avoid percolation of liquids through Figure 1 Future MIS headquarters, Copacabana, Rio de Janeiro, RJ, Images by Diller Scofidio + Renfro for the international contest. (Courtesy of ICA) 914 C. BRITEZ P. HELENE S. BUENO J. PACHECO walls, joints or slabs that contain those liquids. It mainly involves issues related to good construction practices and requiring special care during execution. Thus, it can be understood that to build a successful, watertight anti-flotation slab, at least two aspects shall be carefully and deeply considered: n The first one is related to the material (concrete) that shall be resistant, sound, of very low permeability and durable; n The second one is related to special cares and procedures that constitute the set of good construction practices in order to get a final watertight structure. However, experience has proven that bigger and most frequent failures in liquid retaining structures are related more to execution techniques and procedures (good construction practices) than to material quality. 3. Design data, materials and procedures adopted for MIS 3.1 Historical and basic design data Initially, the solution proposed by the structural design bureau, Escritório Técnico Julio Kassoy e Mario Franco Eng. Civis Ltda. (JKMF), for MIS foundation, consisted in diaphragm walls for lateral retention, cast-in-place piles with steel profiles and structure construction downwards from grade up to reach foundation level, when direct foundations, columns and a 30cm thick anti-flotation slab with tiebacks anchors into earth. Notwithstanding, it was verified that this procedure would be rather slow. Them it was studied a second alternative, drilling provisional tiebacks anchored to the soil in three lateral faces of the site, where no buildings existed at neighbor land plots. In this way it would be possible to reach the foundation level and build the structure upwards at a much higher speed. Main columns would have direct foundations. This method was adopted and at one of the lateral sides of the plot (where neighbor buildings existed), provisional slabs were built working as horizontal beams, which would enable proceed with excavation as those slabs attained required design strength. As the site is very close to the sea, and the foundation design was given a prize (Prêmio Milton Vargas 2012), during excavation procedures the well points for water table depressing had several clogging problems and also soil breakouts with the consequent flooding of the excavation. Under these conditions it became more and more unfeasible the execution of direct foundations, as the soil did not remain stable when excavated to execute the foundations. The designed method became a painful procedure. It was then decided to study a radier type foundation, excavating the whole construction site to a single level, with a 1,0m thick slab that would work as a foundation slab for the columns as well as an anti-flotation slab. It was verified too that 1,0m thickness will be insufficient for all embedded footings inside this anti-flotation slab. Even with a concrete strength of 50MPa, stresses inside the slab would be too high for that thickness. To solve this problem, the inferior radier elevation was lowered and the columns with higher loads had their footing height increased up to 47cm. The cross section of some columns was also increased in order to reduce the slab punching shear stresses. As this was the adopted solution, the major challenge consisted in finding a proceeding that could assure concrete integrity, avoiding pathological manifestations that may jeopardize the stress distribution and the structural performance. In short, the MIS anti-flotation slab was conceived as a trapeze in plan view, measuring approximately 51m long (parallel to Atlântica Avenue), with sides 25m and 20m, respectively. The four sides are embedded into diaphragm walls with variable thickness depending on direction, neighbor constructions and soil conditions. The characteristic design compressive concrete strength is f ck 50MPa, the slab volume is approximately 1200m³ and the mean thickness is 1m. The slab was reinforced as a cage and at intermediate thirds in height (at 33cm each), a complementing reinforcing mesh was added, having 1 ø 12,5mm each 30cm of spacing in both directions. The steel/concrete ratio of the anti-flotation slab excepting footing and complementary reinforcing is 72kg/m³. Considering footing and already mentioned complementing reinforcing, this ratio increases to 105kg/m³. 3.2 Concrete mix and employed ingredients The concrete mix for the anti-flotation slab was developed by PhD Engenharia (Concrete technology consultancy company for Fundação Roberto Marinho) from an extensive laboratory program, jointly with Votorantim Cimentos / Engemix S. A. (concrete supplier). Prototypes were tested at construction site in secondary structures, in order to verify fresh concrete conditions and hardened concrete strengths. The standard ABNT NBR 12655:2006 Concreto de cimento Portland preparo, controle e recebimento Procedimento prescriptions were followed, specially item Condições especiais de exposição, as well as minimal design specifications: n f ck 50MPa (at 28 days), as specified in concrete structural design; n water/cement ratio 0,4 (linked to aggressiveness class); n mortar ratio from 52 to 55% (related to pouring height and workability); n slump from 16 to 25cm (fluid concrete: related to segregation and bleeding issues); n silica fume addition (at least 5%) (high strength concrete and AAR prevention); n waterproofing admixture, acting by integral crystallization, promoting self-healing of fissures (dosing ratio depending on concentration/supplier). It should be observed that the waterproofing admixture, acting by integral crystallization that promotes self-healing of fissures was dosed per manufacturer s technical instructions, including the recommendation of obligatory visits of the their technical representative, who went several times to the concrete batch plant to verify procedures. In this case, it was employed 1,0% by cement weight of XYPEX NF 500 (concentrated), supplied by MC-Bauchemie Brasil. It is also important to stress that the chosen mix proportions were developed having in mind the ingredients that are available at Rio de Janeiro (which are not the most appropriate for a fluid, high strength concrete, with consequently high elasticity modulus, as per design specifications). The worst problem for the ingredients was the high fineness modulus of sands. Grain grade curves submitted by Engemix, normally employed at Caju batching plant, showed lack of fines in both sands (natural and artificial) which corresponds to higher fineness modulus (between 2,3 and 2,9 respectively). 915 Watertightness in anti-flotation slabs. MIS-RJ Case The mix proportions as well as the suppliers of the ingredients for the anti-flotation slab concrete are described in Table 1. It can be observed that no water was employed, besides that existing in sands (to be subtracted from the whole amount of ice). All mixing water was replaced by ice cubes with initial specified temperature of -10ºC (delivered by refrigerator trucks with a Thermo King type refrigeration unit). The technical specification to accept concrete at working site asked for a 20ºC temperature. In reality, that temperature went up to 25ºC in very hot days. As concreting batches had maximum volumes of about 150m³ and the slab had a high reinforcement ratio with well distributed rebars through the 1m thickness, the temperature difference in hot days did not imply in fissures of thermal origin. The mean time of arrival, acceptance and discharge of mixing trucks was 12 minutes, which is rather quick. In order to not slowing down the concrete production before concreting events, based on a large data obtained from the concrete batch plant about sand humidity, it was admitted the weighted average of 5% as a constant for both sands (natural and artificial) and, consequently, the amount of ice was also constant for each mixing truck. This amount was calculated as 130kg per cubic meter, with the explicit exception written in the technical specification of a corrective procedure in case of rainy days or if very high humidity batches of sand were detected by the control technicians at the concrete mix plant. 3.3 Execution recommendations The whole technical specification was elaborated based on reference paper Considerações sobre estanqueidade de estruturas de concreto published by Helene, Terzian and Sardinha (1980) [8]. Besides basic guidelines exposed in the paper about lean concrete/structural concrete interface treatment, assurance of rebar recover and placing, other procedures were essential to assure the watertightness of the structure, the main one was related to concreting joints and the other with proceedings. Besides general recommendations, the Contractor Rio Verde team, which was responsible for project execution, was given specific technical guidelines about proceedings for antiflotation slab by PhD consultants Concreting joints Principally, due to time limits for mixing trucks circulation at Copacabana coastal avenue, as well as existing legislation related to noise produced by construction sites, it was not possible to program slab concreting in a single, continuous event, what would be ideal (and feasible) by the use of special additives and procedures. Concreting, thus, was divided into 10 different phases (determined by the structural designer). When casting a concrete watertight structure, construction joints or concreting joints are one of the most sensible points which ask for more attention. Execution shall be planned in a way to minimize nonconformities at these points. At all concreting joints, it was recommended to use poultry wire mesh as an incorporated formwork, structured by the steel reinforcement existing at the joint, together with hydro swelling tapes. Reinforcement design included an C clamp shaped bar each 25cm, designed to work as a support to tie the mesh, all the way up every joint (at one of the sides). Thus, a vertical joint was obtained, allowing concrete to be duly vibrated through the whole slab and principally next to concreting joints, which are the most critical places. The typical detail of the poultry wire mesh used as an incorporated formwork and of the hydro swelling tape at the slab, can be seen in the perspective of Figure 2. It should be noted that the ideal location for the hydro swelling tape should be as close to the wire mesh bottom as possible. However, when the reinforcement for the next concreting phase is already completed, this place is almost inaccessible. The tape was then placed at a higher region, approximately 30cm below the slab up- Table 1 Concrete mix proportions for the anti-flotation slab by weight, dry materials for 1m³ of concrete. Designed for a characteristic compressive strength f ck of 50MPa at 28 days. Mix proportions Designed for f ck =50MPa Cement per m 3 (CP III-40 RS Votoran Moagem Santa Cruz) 448kg Sílica fume addition (Silmix) 30kg Water/cement+additions (cementing materials) ratio 0,35 Water (from sand humidity only, mean value fixed at 5%) + ice 168kg Medium sand, natural (Areal D. Lucia) 650kg Artificial sand, crushed sand type II (A 21 Mineração) 73kg Crushed stone 0 (A 21 Mineração) 162kg Crushed stone 1 (A 21 Mineração) 921kg Waterproofing admixture, acting by integral crystallization (XYPEX NF 500 concentrado, MC-Bauchemie) 4,5kg Polyfunctional plasticizer admixture (MIRA RT 75, Grace) 4,0kg Superplasticizer admixture (Tecflow 9040, Rheoset/Grace) 2,9kg 100% of ice replacing free mixing water (humid aggregates, sand humidity weighted average is 5%) 130kg (ice) 916 C. BRITEZ P. HELENE S. BUENO J. PACHECO per face, as can be seen in Figure 2. The same procedure (tapes) was implemented at reinforcing slabs intersection with diaphragm walls. Figure 3 exemplifies actual situation at worksite before, during and after concreting a generic phase. The hydro swelling tape was put in place only a few minutes before concreting the phase juxtaposed to the already concreted phase. As these phases were not concreted in sequence with previous phases, the time interval was at least 7 days. A wooden barrier was also placed at the bottom of the poultry wire mesh to prevent cement slurry leakage. Obviously, there was a normal controlled leakage of cement slurry through the wire mesh (a very small amount), because the employed concrete was of the fluid type, but nothing that could jeopardize the objective of having a rough surface with the incorpor
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