International Concrete Abstracts Portal

Casting of concrete specimens in the field is governed by ASTM C31. Its origin can be traced to 1920, with adjustments before 1970 to include internal vibration. Concrete properties can be strongly affected by vibration, with insufficient vibration leading to increased air content and excessive vibration possibly resulting in phase separation. This paper investigates the density and compressive strength of concrete cylinders for different internal vibration procedures. Concrete mixtures with different paste volume, w/cm, aggregate grain size distributions, fly ash, and plasticizer quantities were produced. For stiff concrete mixtures, insufficient vibration led to significant decreases in concrete density and compressive strength. For all other mixtures and for stiff mixtures with high vibration energy, significant decreases in strength were observed despite increases in density. A hypothesis for this behavior is phase separation, reflected by a strong increase in type 5 or 6 breaks compared to the reference rodding procedure from ASTM C31. As such, consolidating concrete in 100 × 200 mm cylinders by means of vibration is not recommended, except for very stiff mixtures.

Crack densities obtained from on-site surveys of 74 bridge deck placements containing concrete mixtures with paste contents between 22.8 and 29.4% are evaluated. Twenty of the placements were constructed with a crack-reducing technology (shrinkage- reducing admixtures, internal curing, or fiber reinforcement) and 54 without; three of the decks with fiber reinforcement and nine of the decks without crack-reducing technologies involved poor construction practices. The results indicate that using a concrete mixture with a low paste content is the most effective way to reduce bridge deck cracking. Bridge decks with paste contents exceeding 27.3% had a significantly higher crack density than decks with lower paste contents. Crack-reducing technologies can play a role in reducing cracking in bridge decks, but they must be used in conjunction with a low-paste-content concrete and good construction practices to achieve minimal cracking in a deck. Failure to follow proper procedures to consolidate, finish, or cure concrete will result in bridge decks that exhibit increased cracking, even when low paste contents are used.

ASTM C31/C31M describes the procedure of making concrete specimens in the field. Its origin can be traced to 1920, proposing rodding or stroking each 100 mm thick layer 25 to 30 times. Concrete technology has evolved tremendously over the last century, but specimens are still prepared following this 100-year-old methodology. This paper investigates the density and compressive strength of concrete cylinders for different consolidation procedures. Mixture design variations include paste volume, water-cement ratio (w/c), aggregate grain size distribution, fly ash, and water-reducing agent. An increase in compressive strength of approximately 5 MPa can be obtained if 100 x 200 mm cylinders are rodded in four layers, 25 rods each, if the slump is not over 100 mm. For all other mixtures, the current rodding procedure of two layers, 25 rods each, is recommended. For mixtures with higher slump, two layers with less rodding per layer deliver similar strength values, but the variability is high.

Using vibration to consolidate concrete is a standard task when placing normal concrete, and dates back to the early 1900s. Since then, concrete vibrators have been optimized to provide efficient consolidation, which includes an increase in their vibration frequency up to 200 Hz. On the other hand, compared to the concrete of the early 1900s, modern concrete has also been improved by reducing the proportion of water content. Both have been changed, but the vibration energy transfer has not been quantitatively evaluated lately for the updated vibrators and modern concrete. Herein, the attenuation of concrete, assuming a cylindrical wavefront and exponential decay for P-wave propagation, is measured and quantified. As a result, it can be concluded that the attenuation coefficient of modern concrete is distributed from 1 to 3 m–1. The notional power density, the maximum vibration energy imposed by a conventional vibrator, is 100 to 300 W/m3, excluding the instability of near-field liquefaction.

Ultra-high-performance concrete (UHPC) is a new class of cementitious material with unique characteristics, including self-consolidation, and excellent mechanical and durability properties. To achieve the desired properties, a very dense internal structure and a very low water-binder ratio (w/b) are necessary. Due to the very different mixture design compared to conventional concrete, it is critical to incorporate different types of chemical admixtures to achieve appropriate fresh concrete behavior of UHPC. To ensure the successful placement of UHPC, it is important to have a good understanding of the workability and rheological characteristics of UHPC with different types and dosages of chemical admixtures. This paper presents a detailed study of the impact of high-range water reducer, workability-retaining admixture, and anti-foaming admixture on the workability and rheological characteristics over different mixture elapsed times. Besides the flowability, both Bingham and modified Bingham models were used to obtain key rheological parameters, including yield stress, viscosity, and thixotropy. Furthermore, the authors developed stability indexes to assess the fiber stability of UHPC in both fresh and hardened states. Based on the experimental results, the paper presented suggested criteria to ensure appropriate flowability and fiber stability for UHPC placement.