Navigating Challenges in the Concrete Industry – Simulating Pumping Processes and Assessing Aggregate Reactivity Potential

Theme of the Peer-to-Peer Webinar: Navigating Challenges in the Concrete Industry – Simulating Pumping Processes and Assessing Aggregate Reactivity Potential

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Speaker 1: Tooran Tavangar, Post Doctoral Fellowship, Université de Sherbrooke (Canada)

Title: Numerical Simulation of Self-Consolidating Concrete Flow as Homogeneous Fluid(s) and Heterogeneous Material During Pumping Process

Pumping is one of the most commonly used methods to transport self-consolidating concrete (SCC). The prediction of pumping pressure drop is of particular interest to properly design the pumping circuits for successful pumping process given the application on hand. This is necessary to fulfil the targeted flow rate and ensure homogeneity of concrete during and after pumping, respectively. During the pumping process, a thin layer of highly fluid fine mortar enriched with high volume of cement paste and fine particles, namely lubrication layer (LL), is formed in vicinity of the pipe walls. It is revealed in literature that concrete pumpability is significantly controlled by the LL characteristics, in terms of thickness and flow properties, as well as the rheological properties of the bulk concrete (BC). This study firstly aims to predict the pressure required to pump SCC and to characterize the LL in large-scale pipelines using computational fluid dynamics (CFD). Although the concrete pumping process seems conceptually simple, the challenge underlying material physics is complex and covers a broad range of time and length scales. Another challenge comes from the interaction between suspending fluid phase (mortar) and diversity of particle sizes (aggregate) in concrete matrix, causing heterogeneous rheological behavior across the pipe section, and formation of the LL. The main mechanisms of LL formation, including the shear-induced particle migration (SIPM) and wall effects, were also simulated using multiphasic simulation techniques, including discrete element method (DEM) and coupled CFD-DEM.

First, a comprehensive literature review on numerical methodologies used to model concrete pipe flow as well as their advantages and disadvantages was presented. In addition, potential solutions were introduced to predict concrete pipe flow given an application on hand in reasonable time and precision. This study was conducted in two complementary phases. The Phase 1 included developing computational models to predict the pressure loss in a full-scale concrete pumping circuit, as a function of the pipes’ geometry, rheological properties of BC, and the LL characteristics. Two novel tri-viscous models were developed to simulate the presence of the LL and BC as a homogeneous fluid with variable rheological properties across the pipe, corresponding to the BC and LL, using OpenForam software.

In the second phase, the main mechanisms of LL formation, including the wall effect and SIPM, were simulated using DEM and coupled CFD-DEM approaches. The pipe flow of concrete was simulated as a biphasic suspension to evaluate the coupled effect of characteristics of the suspended particles (i.e., granular skeleton) and the suspending cement paste/mortar matrix. The interaction of the suspended particles and suspending fine mortar was simulated by considering the collision of particles with different particle-size distributions (PSD) and concentrations (and pipe wall), as well as the suspending fluid’s drag force. A multiscale soft-sphere discrete element method (DEM) and its four-way CFD-DEM coupling were employed in Phase 2 to simulate the rheological heterogeneity across the pipe and formation of the LL. Accordingly, the coupled effect of PSD, concentration, and mean diameter of particles, as well as the rheological properties of the suspending fluid and flow Reynolds number on wall effect, SIPM, and rheological heterogeneities across the pipe was evaluated. The LL of the investigated mixtures were eventually characterized in terms of the thickness and rheological properties.

Speaker 2: Rodolfo Castillo Araiza, PhD Candidate, Université Laval (Canada)

Title: A new accelerated methodology to assess the potential oxidation reaction of sulfide-bearing aggregates on concrete specimens.

After 10-20 years of construction, several deteriorations on concrete foundations and other structural elements of both residential and commercial buildings have been reported worldwide. Manifestations of this damage include cracks, yellowish exudations/stains, and surface spalling. In many instances, the extent of the damage necessitates the demolition and replacement of the affected elements, resulting in repair costs that sometimes exceed the original construction value. The damage is attributed to the oxidation of iron sulfides, particularly pyrrhotite, present in the coarse aggregate of concrete. That reaction generates secondary “rust” products and releases sulfur (S) that converts into sulfuric acid to cause an internal sulfate attack with the formation of secondary gypsum (CaSO₄·2H₂O), ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O) and thaumasite (CaSiO₃·CaCO₃·CaSO₄·15H₂O) through reactions with the cement paste's solid phases (portlandite, monosulfoaluminates, C-S-H).

This presentation outlines the research development of an accelerated laboratory test method for assessing the potential reactivity of concrete aggregates containing iron sulfides in a shorter period of time. The aim is to establish a new criterion for assessing the quality of aggregates in concrete, with the goal of helping concrete producers prevent future structural damage associated with undesirable chemical reactions.

To achieve this objective, a proposed electrochemical treatment that has been applied over various testing periods to a series of laboratory-made concrete specimens containing reactive sulfide-bearing aggregates as well as non-reactive aggregates. Subsequently, these concrete specimens underwent testing using non-destructive methods such as Ultrasonic Pulse Velocity, semi-destructive methods including expansion, modulus of elasticity, and stiffness damage tests, and destructive methods like concrete polished section examination and compression testing. These tests were conducted to evaluate the impact of the deleterious reaction caused by the sulfide-bearing aggregates.

The results shows that a 35-day duration of electrochemical treatment is appropriate for revealing the damage caused by the harmful reactivity of sulfide-bearing aggregates. Concrete containing these reactive aggregates experiences a significant reduction in mechanical performance, primarily characterized by a substantial decrease in the modulus of elasticity (approximately 30% reduction after 35 days of electrochemical treatment).

This webinar is brought to you by the RILEM Youth Council (RYC) and hosted by José Vidal González Aviña (Latin America RYC representative) in collaboration with the American Concrete Institute student chapters of Sherbrooke University and Laval University.