Key Engineering Materials Vols. 629-630

Abstract: Autogenous shrinkage strains may cause cracking of brittle cement-based systems. The lower the water/cement ratio of a mixture, the higher the autogenous shrinkage and the higher the probability of cracking. Cracking increases the probability of ingress of aggressive substances into the concrete, jeopardizing the durability of the material. Low water/cement ratios, however, result in a dense microstructure, which is one of the desired features of high performance concrete because of the assumed increase in durability. This results in a kind of dilemma: the high density of low water/cement ratio concretes is desired, but the increased risk of (micro) cracking of these mixtures, of course, not. This dilemma is discussed in this contribution. The search for ways to get out of this dilemma starts with understanding the mechanisms of autogenous shrinkage. It is shown that autogenous shrinkage is not only an issue in low water/cement ratio mixtures. In normal strength concretes, with w/c ratios between 0.40 – 0.50, autogenous shrinkage can make out a substantial part of the total strain. Ways to mitigate autogenous shrinkage of mixtures with water/binder ratios ranging from 0.18 to 0.50 are discussed. Finally brief comments are made on autogenous shrinkage values prescribed in current design codes.

Abstract: HPC and UHPC concretes are finding their ways both to new structures and to retrofitting of existing structures. Herein specific material properties as well as structural examples are discussed. New Codes and Recommendations provide description of material properties and design rules for HPC/UHPC structures and structural elements.

Abstract: Multi-properties concrete (MPC) contains the properties as follows: self-compacting, self-curing, heat dissipation at low water content; low shrinkage; high plasticity keeping and high durability. We applied the concept of “multi-properties concrete” (MPC) during the construction of the West Tower and East Tower both at Guangzhou and Jinji Complex at Shenzhen. The multi-properties concrete is more effective in controlling cracks of concrete, while it is a new type concrete with properties of green and low carbon.

Abstract: The Hodder Avenue underpass – recipient of the Precast/Prestressed Concrete Institute (PCI)’s Harry H. Edwards Industry Advancement Award – is a new highway bridge near Thunder Bay, Canada that utilized a modular construction approach facilitated by the extensive use of ultra-high performance fibre-reinforced concrete (UHPFRC) to expedite construction, elevate aesthetic value and enhance quality and durability. Almost all structural components were precast in facilities and assembled on site using UHPFRC joints, which have compact geometries with less complexity, superior durability and strength. The precast elements include a unique UHPFRC pier cap and pier column shells, high performance concrete (HPC) box girders, sidewalks/parapet walls, abutment caps, ballast walls, slope paving panels and approach slabs. Aesthetically, the structure achieved a slender and open form with the use of shallow precast box girders and a unique pier cap visually and structurally integral with the superstructure. The cap beam was prestressed and precast fully with UHPFRC to overcome design challenges such as geometrical limitations and complex loading. The pier cap and girders were made composite using field-cast UHPFRC joints reinforced with stainless steel bars and threaded bolts. The pier columns also utilized a unique design with precast UHPFRC shells serving as an aesthetic stay-in-place form as well as a protective layer for the salt splashes during Thunder Bay’s harsh winter seasons.

Abstract: With the increasing attention towards energy-efficient and zero emission buildings, improvement to concrete properties is becoming more and more significant in construction and building sectors. One such area is to enhance the thermal properties, while maintaining maximum strength of the material. Here, attempts were made to address this challenge by formulating mortar composites with low thermal conductivity while targeting a minimum compressive strength of 20 MPa at 28 days. For this purpose, aerogel was utilized in an ultrahigh performance concrete (UHPC) formulation to create new aerogel-incorporated mortar (AIM). It was found that AIMs possessing 50 vol% aerogel registered a compressive strength of 20 MPa, while displaying a thermal conductivity of ~ 0.55 W/(mK). By adding more aerogel to reach 70 vol%, while the thermal conductivity of the concrete decreased by ~ 20 %, a sharp decrease in strength to 5.8 MPa was observed. This represents only 1/30 of the original strength of the UHPC mortar. Further addition of aerogel till 80 vol% showed negligible compressive strength, attributing to the imbalance of the particle-matrix ratio in the mortar system, causing a decrease in adhesion of the binder-aggregates.

Abstract: High strength concrete for the production of concrete railway sleepers was designed more than 20 years ago. The compressive strength of the concrete was very high from the start, but flexure strengths showed some irregular development - a decrease in time. Later, also a significant decrease of fracture properties was recorded. Microcracking was found to be the reason for this; therefore some modifications were performed to avoid this happening (especially the reduction of the maximum size of aggregates from 22 mm to 16 mm or 11 mm). Some problems concerning frost resistance of the concrete with a slag addition were reduced by applying ternary binders. All of the results are discussed from the point of view of a long-term observation of the strengths and fracture properties ́ development during the time period of 5 years or even more.

Abstract: Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) presents remarkable mechanical and durability properties. Concretes can be considered as UHPFRC when compressive strength and direct tensile strength reach respectively reach 150 MPa and at 7 MPa at 28 days [1]. The UHPFRC matrix is generally composed by a high amount of cement and silica fume as binder, steel fibers, very low Water/Binder (W/B) ratio and high rang water reducer. Special heat treatment is often required in order to achieve compressive strengths over 150-200 MPa. This work focuses on the development of low cost UHPFRC with less energy in production process in order to widespread its use. To reach this objective, some mix criteria have been set up: use of local materials, substitution of silica fume by less expensive and more available flash metakaolin, reduced cement content, without heat treatment. Four UHPFRC mixtures have been fixed. The reference one incorporates silica fume. In the second one, metakaolin substitutes weight by weight silica fume. Two other mixtures have been designed using a wet packing method [2] and a metakaolin water absorption test [3], in order to optimize the metakaolin/cement mass ratio and to reduce cement content. Workability and mechanical tests have been carried out. Results show that the weight by weight substitution of silica fume by flash metakaolin leads to a slight reduction (about 5%) of strengths but a more important reduction of cost (about 10% of UHPFRC material cost per m³). The optimization of metakaolin/cement ratio allows reaching equivalent performances of silica fume reference mix and reducing cement content without significant strengths decrease.

Abstract: Ongoing research and development of ultra-high performance concrete (UHPC) in the Czech Republic has been utilized in design and implementation of light-weight segmental deck of the cable-stayed footbridge over the Labe River in Celakovice with main span of 156 m. Detailing, design issues, construction method and efficiency of using UHPC on this large span lightweight bridge is described in the paper. The superstructure of the Celakovice footbridge was completed in the December 2013 and the bridge was opened for the public in April 2014. Main advantages of this project is not only low maintenance and reasonable life cycle cost but also favourable tender price which was achieved by the contractor Metrostav a.s. due to combination of high-strength modern materials steel and UHPC.

Abstract: In this study, the influence of fiber orientation on the flexural strength of ultra-high-performance fiber-reinforced concrete (UHPFRC) was examined. To this end, a circular UHPFRC panel measuring φ1,200 × 50 mm was cast from its center, and test specimens measuring 50 × 50 × 200 mm with 10 mm notches for three-point bending tests were cut from it with angles of 0, 30, 60 and 90° between the specimen axis and the radial direction of the panel. After the bending test, fiber orientation on the ruptured surfaces of the specimens was observed. The flexural strengths of the specimens cut at angles of 60, 30 and 0° were 80, 40 and 10% of that for the specimen cut at an angle of 90°. It was also found that the flexural strength of specimens cut from a rectangular panel cast from its center point depended on their original positions and orientation within the panel. Similar fiber orientation characteristics were found in the circular and rectangular panels.