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Showing 1-5 of 647 Abstracts search results

Document: 

SP-355_37

Date: 

July 1, 2022

Author(s):

Mario Collepardi, Silvia Collepardi, Roberto Troli

Publication:

Symposium Papers

Volume:

355

Abstract:

The present paper shows the study of a mixture design of the concrete used in the reinforced foundations of the bridge on the Straits of Messina in Italy. A cube compressive characteristic strength of 35 MPa (5,075 psi) is required for the foundation concrete. Due to the peculiar shape of the concrete foundations (completely embedded in the excavated ground), the damages caused by the thermal stress, the steel corrosion, and the alkali-silica reaction cannot be monitored and repaired. Therefore, a concrete structure must be designed without any damage for at least 200 years due to the very important role of this structure from a social point of view. In order to guarantee this long-term durability, there are two problems to be faced and solved: 1) the heat of cement hydration could cause cracks inside the foundation due to thermal gradients between the hotter nucleus of the massive structure and the colder peripheral areas; 2) the corrosion of the metallic reinforcements caused by the reaction between the metallic iron and the oxygen (O2) present in the air to an extent of about 20%; 3) the alkali-silica reaction causing a local disruption in the concrete. All these problems can be solved using a blast-furnace slag cement such as CEM III B 32.5 R characterized by a very small heat of hydration and adopting a ground coarse aggregate with a maximum size as large as 32 mm (1.28 in): the choice of this aggregate produces a reduction in the amount of mixing water and then of the cement content and reduces the volume of the entrapped air at about 1.3% by concrete volume. This amount of O2 would cause the corrosion of a negligible amount of iron corresponding to only 10 to 13 g (0.4 to 0.5 oz) of steel in 1 m3 (1.31 yd3) of concrete of each foundation. In order to prevent any ingress of air from the environment, the top of the foundation should be protected by self-compacting, self-compressing, and self-curing concrete.

DOI:

10.14359/51736049


Document: 

SP-354_15

Date: 

July 1, 2022

Author(s):

Yuya Akao

Publication:

Symposium Papers

Volume:

354

Abstract:

The application of a novel superabsorbent polymer (SAP) as a multifunctional chemical admixture for concrete properties is expected to contribute to the overall durability and sustainability of concrete structures. SAPs are well known to quickly absorb and retain a significant amount of water within the concrete matrix as a means of providing internal curing. However, the rate of water uptake can significantly affect the rheology of fresh concrete such as reduced flowability. This paper introduces a novel SAP that features slow water absorption and swelling behavior, and its resulting impact on both fresh and hardened concrete properties. The novel SAP has been shown to delay swelling for several hours in cement filtrate, followed by a predictable absorption of water over a 24-hour period comparable to conventional SAP. The delayed swelling effect observed with the novel SAP eliminates the need for additional water to obtain a similar flowability, but with a very slight increase in viscosity, compared to a concrete mixture without SAP. Moreover, the internal curing capability of the novel SAP can result in an increase in both early age and long-term compressive strengths, improved freeze-thaw resistance, and a reduction in autogenous shrinkage under sealed and air curing conditions.

DOI:

10.14359/51736071


Document: 

SP-354_29

Date: 

July 1, 2022

Author(s):

Mario Collepardi, Silvia Collepardi, Giuseppe Marchese and Roberto Troli

Publication:

Symposium Papers

Volume:

354

Abstract:

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.

DOI:

10.14359/51736085


Document: 

SP-355_14

Date: 

July 1, 2022

Author(s):

Harald Justnes, Tobias Danner

Publication:

Symposium Papers

Volume:

355

Abstract:

The long-term stability of aluminum metal in binders intended for concrete has been studied. Aluminum was cast in paste where 55% cement was replaced by calcined smectitic clay. After 7 days of curing several samples were submersed in either distilled water or in a 6% NaCl solution. After 1 year of submersion, the microstructure of samples submitted to both exposure conditions was analyzed. Chloride corresponding to 1.5-2% Cl- of cement mass had reached the Al-bar. The surface of the Ø 10 mm aluminum bar had corroded to a depth of less than 10μm, irrespectively if it was submerged in water or in 6% NaCl. There was no sign of intergranular corrosion. The hydrated binder consisted of amorphous calcium silicate hydrate gel and crystalline layered double hydroxides of hydrocalumite-type. When cured in water, the crystals were a mix of hemi- and mono-carboaluminate, but when stored in 6% NaCl, the carbonate/hydroxide was partly replaced by chloride. There was a dense binder zone formed around the ≈ 15 μm oxide layer of the aluminum bar richer than the overall binder in aluminum. Some of the alumina formed on the metal surface may have been partly dissolved by alkalis and precipitated/reacted with the nearest binder region and densified it and thus preventing further reaction.

DOI:

10.14359/51736020


Document: 

SP-355_06

Date: 

July 1, 2022

Author(s):

Víctor Revilla-Cuesta, Marta Skaf, José A. Chica, Roberto Serrano-López, Vanesa Ortega-López

Publication:

Symposium Papers

Volume:

355

Abstract:

Increasing sustainability is currently one of the main objectives of the construction sector. One of the most widespread sustainable practices to reach this goal is the recycling of industrial residues within concrete mixes. What began as a practice to produce low-quality concrete has now spread, for example, to self-compacting concrete (SCC), which has a high flowability in the fresh state. Recycled concrete aggregate (RCA) is a waste that can be used to develop SCC of good mechanical performance. RCA produced from the crushing of rejected precast components for the prefabricated concrete industry is assessed in this study. It was demonstrated that the addition of 100 % coarse RCA and 50 % fine RCA contributed to the production of an SCC with adequate flowability and optimal mechanical and durability behavior. To do so, the design of the mixture, its flowability, its strength behavior at different curing ages, and its performance in freeze/thaw, moist/dry, and sulfate-attack tests are discussed. The study is complemented by analyzing the interaction of RCA with different natural fine aggregates, such as siliceous or limestone sand. It is concluded that the SCC developed with large amounts of RCA was of sufficient strength and durability for general usage.

DOI:

10.14359/51736012


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