International Concrete Abstracts Portal

A special concrete was used to erect the MAXXI building in Rome designed by Zaha Hadid and her team with long, inclined, curvilinear walls. Due to the very congested reinforcements, the original concrete issued by Zaha Hadid and her team was self-compacting concrete (SCC). However, irregular cracks -caused by the restrained drying shrinkage- appeared on the surface of this concrete a few days after removing the formworks. On the other hand, due to aesthetic reasons, neither saw cuts in the hardened concrete to produce regular contraction joints -carried out to avoid the irregular cracks caused by a restrained drying shrinkage- were accepted by the Architects. Therefore, a special 3-SC mixture was developed and used; it is characterized to be: - a self-compacting concrete based on the use of an acrylic superplasticizer, a viscosity modifier to avoid the bleeding risk, and a special particle size distribution of the aggregates; - a self-compressive concrete due to the use of a CaO-based expansive agent; - a self-curing concrete based on the use of a shrinkage-reducing admixture (SRA). This concrete called 3-SC, because it is 3 times “self”, was very successful in producing a crack-free concrete surface even in the very long, curvilinear, and inclined walls: after 18 years of building the long, inclined, curvilinear walls of the MAXXI museum have been carefully examined and during the last inspection their surface resulted to be still sound and crack-free. However, just before the building’s inauguration in 2009, in very few areas some micro-cracks were observed on the concrete surface and considered to be dangerous for the future of the building. Therefore, the concrete surface was treated with a transparent varnish in order to avoid the ingress of the aggressive humid air to protect the steel reinforcements from the corrosion promoted by the carbonation process.

The present paper preliminarily illustrates the mechanism of damages caused by the alkali-silica reaction (ASR) between the high alkali content of the dry shake-hardener due to the high cement content on the top of the concrete industrial floors and the alkali-reactive coarse aggregate in the concrete substrate. To mitigate or prevent these damages a special dry shake-hardener, based on the partial replacement of the Portland cement by siliceous fly ash, is used. The beneficial influence of the fly ash, as well as that of other fine pozzolanic materials, is due to the distribution of a very large number of amorphous silica-based fine particles which can potentially react with the alkali in the same way as the amorphous or badly crystallized silica of the alkali-reactive coarse aggregates. The introduction of a very high number of pozzolanic particles significantly reduces the alkali availability for the reaction with the few alkali-reactive coarse aggregates. In other words, the alkalis instead of concentrating their aggression on a few grains of the alkali-reactive coarse aggregates, usually 5 to 15 mm (2 to 6 in.) in size, spread their action on a large number of very fine pozzolanic particles so that their expansive and destructive power is lost. However, another problem can arise when the Portland cement is partially replaced by fly ash due to the longer setting time, particularly in cold weather, of the dry shake-hardener, so that the workers must wait a very long time before the mechanical troweling and the opening of the finished surface to the pedestrian traffic. To avoid this drawback a combined use of the siliceous fly ash and a setting accelerator, based on tetra-hydrate calcium nitrate in powder form [4H₂O∙Ca(NO₃)₂ > 4H₂O∙CaO∙N₂O₅ > H₄CN₂] has been studied at three different temperatures: 35°C (95°F), 20°C (68°F) and 5°C (41°F). In warm weather, at temperatures as high as 35°C (95°F), there is no need for H₄CN₂ since the Portland cement hydration occurs at a very great rate and only the dry shake-hardener containing fly ash without H₄CN₂ can be applied within few hours and incorporated into the concrete substrate. At 20°C (68°F) the delay in the setting times caused by the partial replacement of Portland cement by fly ash can be compensated by the use of H₄CN₂ at 1% by weight of the cementitious materials. In cold weather, such as that caused by a temperature as low as 5°C (41°F), a much higher percentage of H₄CN₂, up to 5% by weight of the cementitious materials, must be used to reduce the setting times at approximately the same values as those recorded at 20°C (68°F) when the dry shake-hardener without fly ash is used.

Ye’elimite-rich cements or calcium sulfoaluminate cements (CSA) are commercialized to prepare shrinkage compensation and self-stressing concretes. Moreover, CSA cements show environmentally friendly characteristics associated to their production, which include reduced CO₂ footprint. The expansive behavior of CSA cements is mainly controlled by ettringite amount, produced upon hydration of the key-phase, ye’elimite [Ca₄(Al₆O₁₂)SO₄]. This paper presents, on one hand, the optimal conditions for the synthesis of highly pure ye’elimite by solid state reactions, and on the other hand, it shows a fundamental description of ye’elimite formation mechanisms. Another aspect of the study encompasses the influence of fineness and citric acid addition on ye’elimite phase dissolution, then on hydrates composition of lab made ye’elimite-rich cement. For the fineness effect study, a highly fine and pure ye’elimite was originally synthetized by sol-gel methods. Various experimental techniques were performed to conduct the different aspects of the present study, namely XRD-Quantitative Rietveld analysis, Thermal analysis (TGA, DTA and Dilatometry), SEM (BSE imaging and EDS mapping), BET analysis, PSD by laser diffraction, and Image analysis (2D porosity and 2D PSD).

The Joint ACI-ASCE Committee 445 published a document titled Report on Torsion in Structural Concrete that contained an in-depth review of historical theory development, design models, and simplified design procedures for the effect of torsion in concrete structures. That document contained three design examples that were relatively simple. An important goal of this ACI Special Publication is to provide more realistic design examples that are usable by design professionals. This paper satisfies that goal by showing a detailed solution to a realistic example that has been encountered on several occasions by one of the authors. Another goal of the ACI Special Publication is to show applications where torsion is combined with flexure and shear. In this example, the torsional effects are combined with biaxial flexure and biaxial shear forces. This example includes a check of the new provisions in ACI 318-19 for bi-axial shear effects.

This paper shows a detailed solution for the design of a reinforced concrete grade beam subjected to torsional effects combined with biaxial shear and biaxial flexure. The grade beam is a portion of a structural screen wall system. A 25 psf (1.20 kPa) strength level wind pressure acts on a 20 ft (6.10 m) tall CMU wall supported by a continuous grade beam. The 21 in (533 mm) wide by 18 in (457 mm) deep grade beam is isolated from an expansive soil and is supported by drilled shafts 21 ft (6.40 m) on center. The wind load and gravity loads induce torsion, biaxial bending moments, and biaxial shear forces in the grade beam. This example shows how to calculate the internal forces in the grade beam at the critical section and design the required longitudinal and shear reinforcement according to the ACI 318-19 code.

The design of the grade beam includes closed stirrups of #4 (Ø 12) bars spaced at 5.5 in (140 mm), five #8 (Ø 25) bars used near the top and bottom faces and one #6 (Ø 16) bar used at mid-height near the side faces.

Bridge embankments serve a vital role in raising the roadway profile to the bridge deck elevation for passage of vehicles. It is common practice to construct embankments utilizing compacted lifts of soil obtained from nearby borrow pits. Soil borrowed from regions of predominantly expansive clay soils can be problematic for bridge embankment construction. High plasticity soils swell in contact with moisture, inducing vertical and lateral pressure on embankments. Mechanically Stabilized Earth (MSE) walls are particularly susceptible to soil expansion as they try to confine high soil expansion pressures through soil reinforcement and mobilization of a stabilized volume behind the face of the wall. This paper provides insight into the investigation of MSE wall movement, abutment movement and corresponding bridge beam distress, and reinforced concrete failures resulting from high plasticity soil backfill in existing bridge embankments. Remediation strategies are discussed which are directed at the expansive soil behavior within the embankment.