Link to Latest Report : Coming Soon.
Background :
Among the various deterioration mechanisms that affect the lifespan of concrete, transport properties have been identified as one of the most severe issues. The penetration of water and harmful ions (e.g., chlorides) significantly impacts the durability of concrete. Concrete resistivity measurement has undergone significant advancements over the past few decades and is now widely regarded as a reliable indicator of concrete durability. Electrical resistivity is a critical property used to evaluate the ionic conductivity of concrete, which directly correlates with its permeability and vulnerability to deterioration caused by chloride ion penetration and the corrosion of embedded steel reinforcement. Early laboratory applications of resistivity measurements included the rapid chloride penetration test (RCPT) and ultrasonic pulse velocity measurements during rapid freezing-thawing tests. These techniques, introduced as early as the 1980s, marked a turning point in assessing concrete durability, offering a scientific basis for predicting its performance under harsh conditions. Over time, technological improvements have refined these methods. Resonant frequency testing emerged as a more accurate alternative to ultrasonic pulse velocity measurements, offering superior precision in evaluating the relative dynamic modulus of elasticity. Similarly, bulk resistivity and surface resistivity measurements began to replace the RCPT test due to their non-destructive nature and expedited testing procedures. These methods offered a more practical approach for evaluating concrete’s transport properties, particularly its resistance to chloride penetration and its overall durability. Despite these advancements, the widespread adoption of resistivity-based methods is hindered by their reliance on controlled laboratory conditions. Most require meticulous sample preparation, extended durations to ensure full saturation, and ideal environmental conditions for accurate results. Among the existing methods, surface resistivity is the only technique suitable for field applications. However, its reliability is often compromised by external factors such as varying levels of concrete saturation and temperature, and it is limited to the assessment of surface layers, such as those on bridge decks. This highlights an urgent need for a novel, rapid, and accurate testing method that can be deployed directly in the field with minimal environmental interference, addressing the limitations of current technologies while maintaining robust accuracy.
Objectives :
The first objective is to determine the threshold pressure required for water penetration in various concrete types without compromising the integrity of their pore structures. This step is crucial to ensure the method’s suitability across diverse mix designs and environmental conditions. The second objective is to establish a clear relationship between the pressure drop and concrete pore properties, such as pore size distribution, connectivity, and tortuosity. This will be investigated experimentally using advanced characterization techniques, such as micro-X-ray computed tomography (µCT) and BET surface area analysis. These experimental insights will be further validated with theoretical models developed, such as by using Darcy’s law and the Kozeny-Carman equation, to provide a comprehensive understanding of water permeability and pressure drop behavior. Building on these findings, the next objective is to correlate WPDT results with standardized test outcomes, such as RCPT, rapid freeze-thaw, and bulk resistivity tests.
Scope :
Task 1 – Determining the threshold pressure applied to the test.
As claimed in the previous report, the pressure applied during the WPDT must be sufficient to
ensure that water penetrates more than 2 inches into the concrete, allowing for a thorough evaluation of its permeability. This is particularly important for concrete structures such as bridge decks, which typically have a thickness of 7 to 8 inches. However, the required pressure varies depending on the concrete mix design, as each formulation results in different microstructures and pore distributions. While the pressure needs to be high enough to achieve effective water penetration, excessive pressure could damage the concrete’s pore structure, leading to inaccurate results. Thus, determining a universal threshold pressure applicable to all mix designs is critical for obtaining reliable and consistent data. To determine this threshold, 8 to 12 mix designs with water-to-cement (w/c) ratios ranging from 0.2 to 0.6 will be selected in collaboration with local stakeholders such as FDOT and Ready Mixed Concrete Plants. For each mix design, 20 identical specimens, each measuring 4 inches in diameter and 8 inches in height, will be cast. These specimens will be cured under ambient temperature for at least 56 days to avoid the potential impact of ongoing hydration reactions. During casting, moisture sensors will be installed at 1-inch intervals along the depth of the specimens to monitor water movement during testing.
Task 2 – Correlating the pressure drop to concrete pore structures.
In Task 2, we will focus on establishing a relationship between the pressure drop (or decay factor K) and the pore structures of concrete. This process will involve both theoretical analysis and experimental validation to develop a deeper understanding of how pore characteristics influence the results of the WPDT. This involves applying the Kozeny-Carman equation, a model that correlates key pore properties such as pore size distribution, pore connectivity, and pore tortuosity with the permeability of concrete. By using this equation, we will quantify the relationship between the concrete’s microstructure and its ability to allow fluid movement, thus providing insights into how different pore configurations influence the material’s overall permeability. Then, the developed model will be further linked to Darcy’s Law, which describes the flow of fluid through porous media. By connecting the permeability values from the Kozeny
Carman equation to Darcy’s Law, we will create a practical framework for predicting pressure drops in concrete during the WPDT. This connection will establish a theoretical relationship between the observed pressure drops and the underlying pore properties of the concrete. To achieve this, the study will focus on the pore structures of concrete cylinders from six different mix designs with varying w/c ratios. The pore structure of these cylinders will be evaluated using µCT, or other similar techniques.
Task 3 – Linking pressure drop to standardized resistivity tests vis pore structures.
Building on the insights gained from Task 1 and Task 2, the next objective is to establish a reliable correlation between the WDPT and other widely used standardized resistivity tests for concrete. To achieve this, 8-12 mix designs from Task 1 will be reproduced under fully saturated conditions, ensuring consistency across samples. These specimens will undergo a series of established resistivity tests, which are commonly used in concrete durability assessments:
- Longitudinal resonant frequency measurements will be taken after conducting a rapid freeze-thaw test using an EModumeter. This test evaluates the concrete’s ability to resist freeze-thaw cycles, which is critical for determining its long-term durability in varying temperature conditions.
- Bulk resistivity tests will be performed to assess the overall resistance of the concrete to electrical current, a key indicator of its permeability. Higher resistivity typically correlates with lower permeability, meaning the concrete is more resistant to the ingress of water and harmful chemicals.
- Capacitance measurements will be obtained using Electrochemical Impedance Spectroscopy (EIS), a technique that evaluates the capacitive properties of the concrete. This measurement provides insights into the concrete’s ability to store charge, which is closely related to its porosity and permeability.
- Electromigration charges will be measured using the Rapid Chloride Permeability Test (RCPT), which helps determine how well chloride ions can migrate through the concrete. This test is especially useful for assessing concrete susceptibility to chloride-induced corrosion.
Task 4 – Validating the effectiveness of the developed test on UHPC.
The addition of silica fume in UHPC plays a critical role in refining its pore structure, leading to the formation of a denser microstructure with significantly smaller and more tightly packed pores. This modification results in a concrete matrix that is not only stronger but also more resistant to the penetration of water and other harmful substances. The impact of this dense microstructure on the WPDT is expected to be significant. Specifically, the WPDT will require a higher initial pressure to achieve water penetration to a depth of 2 inches or more in the concrete, as determined in Task 1. The tighter, more impermeable pore structure of UHPC means that the water faces more resistance and must be applied with greater force to penetrate the material effectively. This will be a crucial factor in assessing the applicability and reliability of the WPDT for evaluating the permeability of UHPC, which has a vastly different microstructure than conventional concrete. To assess this, six cylindrical samples will be prepared from two distinct UHPC mix designs. These mix designs will vary in their silica fume content and other key
parameters, allowing for a comparison of how different formulations influence the concrete’s permeability and WPDT performance. The pressure drop trend during the WPDT will be recorded for each sample, tracking how pressure varies with the increasing depth of water penetration. This data will be used to establish correlations between the pressure drop and the pore properties of UHPC, providing valuable insight into how silica fume impacts the material’s resistance to water ingress. To evaluate the effectiveness and reliability of the developed WPDT method for UHPC, the tests outlined in Tasks 2 and 3 will be repeated with the UHPC samples. These tasks, which involve correlating pore structure with pressure drop and linking these properties to standardized resistivity tests, will be essential in confirming the WPDT’s accuracy and applicability to UHPC. By comparing the results from the WPDT with other established tests, such as resistivity and chloride permeability tests, the ability of the WPDT to assess the
durability and impermeability of UHPC will be evaluated.
Task 5 – Field – testing preparation.
To facilitate the field application of WPDT, we will initiate discussions with local stakeholders, such as FDOT, to explore potential field-testing opportunities. During this task, we will document the standardized testing procedure and share it with FDOT for review and evaluation. We anticipate a series of back-and-forth discussions regarding technical details and field challenges, which will inform necessary adjustments to the operating procedures to align with FDOT requirements. Additionally, we will begin conversations at the national level with the Federal Highway Administration to explore further opportunities for demonstrating the WPDT in other states, under varying environmental conditions. This collaboration may help extend the application of the test and identify its potential for broader use in different regions.
Research Team :
Principal Investigator : Linfei Li, Ph.D.