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Enhanced Entrained Air Void System Characterization for Durable Highway Concrete

Status: Complete

Introduction

The impact of the air void system depends on the total volume, size, and dispersion of the air voids in the concrete system as well as their compatibility with various material properties. Properties that are affected by the air void system include workability, cohesion, density (fresh and hardened), strength, finishability, and freeze-thaw resistance. The most important of these in terms of long-term performance of highway concretes, is the resistance to freeze-thaw durability. To effectively provide freeze-thaw resistance, the air void system must have a total volume of empty air voids that equals or exceeds the volume of water or ice not accommodated by empty space in the capillary pore system. Additionally, important is that the air voids must be dispersed throughout the cement paste so that nearly all of the paste is within an air-void system zone of influence. The current state of the art test method to study the entrained air void system in concrete is ASTM C457 – “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete” [originally proposed by Brown and Pierson]. The test method consists of polishing a hardened concrete sawn sample and placing the sample under a microscope. The operator systematically makes measurements, by counting the air voids that come into view and statistical estimates are produced from the measurements that define the air content, paste content, air-void size distribution, and spatial dispersion. There are two alternative methods also described in ASTM C457: i) the Rosiwal linear traverse technique (Procedure A) and ii) the modified point-count method (Procedure B) resulting in equivalent results. Although ASTM C457 is the standard test method for characterizing the entrained air void system in concrete, there still exists some major sources of variability and uncertainty of the test results. The major sources of variability include precision and bias, inherent statistical uncertainty, level of magnification of the microscope, and operator subjectivity. Additionally, the parameters associated with this characterization and the results of these laboratory tests do not always reflect the observed field performance nor do they consider the possible effects on other concrete properties. Therefore, there is a need to identify the characteristics of the air void system that relate to field performance of the concrete system. This information will help highway agencies prepare specifications for concrete procurement that will provide the air void characteristics for freeze-thaw resistance and de-icer scaling needed for enhanced durability of highway structures and pavements.

This study will be implemented through the following objectives:

  1. Develop two alternative entrained air void system characterization and measuring techniques;
  2. Compare alternative techniques to that of the current practice (ASTM C457) and Super Air Meter; and
  3. Correlate entrained air void system data to the fresh and hardened properties of highway concrete.

Tasks

Task 1: Initial Memorandum on Expected Research Benefits and Potential Implementation Steps

During the project phase and the development of the work plan, key benefits were selected to clearly define the benefits for MnDOT; receive results and conclusions of this research. This task will provide an initial assessment of research benefits, a proposed methodology, and potential implementation steps.

  • Deliverable: A memorandum providing initial estimates of expected research benefits, documentation of the methodology, and potential implementation steps. Task 1 memo (doc), 4/26/2021
  • Date due: May 31, 2021

Task 2: Concrete Mixing, Fresh Air Content, Super Air Meter, and Compressive Strength Testing

The University will focus on producing a minimum of four straight cement air entrained highway concrete pavement mixtures. Each mixture will be produced using a standardized ASTM C150 Type I/II cement as well as siliceous river gravel and river sand. The air entraining dosages will be added at various ranges sufficient enough to provide a low (3-4%), moderate (5-7%), and high (> 8%) air contents in the produced mixtures. A non-air entrained mixture will also be included as a reference in the study. This will be completed in order to produce hardened samples to characterize via ASTM C457 and the proposed alternative method. Preliminary trial mixtures will be done in order to achieve a consistent workability range (target slump 3- 5”) between each mixture and repeatability in target air contents based on air entraining admixture dosage. This task will also determine the fresh air content (ASTM C231) value as well as the Super Air Meter number to draw comparative results between fresh and hardened air content properties. All samples will be simultaneously cast and evaluated during this task. Additionally, 7-day and 28-day compressive strength testing will be performed in accordance to ASTM C39 to determine the impact of the air void system on the compressive strength.

Task 3: Hardened Sample Preparation

The University will focus on minimizing or removing the sample preparation requirements of ASTM C457 Procedure C, which requires saw cutting, lapping, polishing, and dying of the sawn surface. These steps are required by ASTM C457 and are necessary for the proposed alternative technique however, the researchers will attempt to reduce the time required for sample preparation and evaluation. According to ASTM C457 the surface preparation requires the surface to be lapped with successively finer abrasives until it is suitable for microscopical observation. The standard recommends beginning with a nominal 150 μm grit size followed by 75, 35, 17.5, and 12.5 μm grit sizes, and perhaps 5 μm (No. 2500 grit) aluminum oxide. In between each successive lapping the sample is investigated under a microscope to determine if the air voids are clearly distinguishable. ASTM C457 describes a distinguishable air void as having well-defined edges, which will be sharp and not eroded or crumbled. This includes air-void sections as small as 10 μm in diameter. In this task, samples will be polished to meet the specifications in ASTM C457, and additional samples will be polished at varying degrees to investigate the possibility of reducing the preparation step. Also required in ASTM C457 Procedure C, is placing the sample surface into a contrast enhanced (black/white) color scheme. This is accomplished by applying black ink to the entire surface followed by a white powder/paste to fill the air voids. This will create a surface that specifically highlights only the air voids. This process will allow the microscope software to analyze the sample surface quickly and accurately. The investigators will also investigate the likelihood of reducing (or removing) this step as well.

  • Deliverable: A word document summarizing the information learned and results obtained for each procedural step used in the investigation.
    Hardened sample preparation (DOC), 10/12/2021
  • Date due: October 31, 2021

Task 4: ASTM C457 Sample Characterization

The University will characterize all sample surfaces following strict ASTM C457 procedures. Procedure A – Linear Traverse Method, Procedure B – Modified Point and Count Method, and Procedure C – Contrast Enhanced Method will be completed. This task is necessary in order to create a baseline for comparison.

Task 5: Alternative Air Void Characterization Method Testing and Development

The University will focus on characterizing the entrained air void system using the digital optical microscope (Keyence). In addition to obtaining key entrained air void system data, the investigators will focus on quantifying the time required, the ease of use, the repeatability, and the quality of the results.

Task 6: Results Analysis and Comparison/Correlation of Alternative Technique Procedures

The University will compare and correlate the results obtained from the three ASTM C457 procedures to that of the alternative characterization technique as well as the fresh air content properties obtained in Task 2. In addition to the results comparison, a procedure and time requirement (feasibility) comparison will be completed.

  • Deliverable: A word document describing the results, comparison, and an analysis. Also included will be a summary of the feasibility (time required, equipment required, materials required, etc.). Lastly, the final procedures to complete the alternative technique will be provided.
  • Date due: June 30, 2022

Task 7: Final Memorandum on Research Benefits and Implementation Steps

A final technical memorandum at the end of the project that provide details of the methodology, steps and approach for evaluating benefits, benefits quantification results, and discussion of next steps for implementation.

  • Deliverable: A final technical memorandum at the end of the project that provide details of the methodology, steps and approach for evaluating benefits, benefits quantification results, and discussion of next steps for implementation.
  • Date due: July 31, 2022

Task 8: Draft Final Report

Compile Report, Technical Advisory Panel Review, and Revisions. The Principal Investigator (PI) will prepare a draft final report, following MnDOT publication guidelines, to document project activities, findings and recommendations. This report will be reviewed by the Technical Advisory Panel (TAP), updated by the PI to incorporate technical comments, and then approved by the Technical Liaison (TL) before this task is considered complete. If possible, a TAP meeting will be scheduled to facilitate the discussion of the draft report.

  • Deliverable: A draft final report for TAP review, and a revised report that is technically complete and approved by the TL for publication.
  • Date due: August 31, 2022

Task 9: Editorial Review and Publication of Final Deliverables

During this task, the PI will work directly with MnDOT to address editorial comments and finalize all final deliverables in a timely manner. MnDOT will publish the report and ensure it meets publication standards. Final deliverables include the Final Report and recommendations on acquiring equipment necessary to complete the alternative air void characterization technique.

  • Deliverable: A final publishable report that meets MnDOT’s Editorial Guidelines and standards.
  • Date due: September 30, 2022

Project team

Email the Project Team
Principal Investigator(s): Anthony Torres, Ph.D., Associate Professor in the Concrete Industry Management (CIM) Program housed in the Department of Engineering Technology at Texas State University, anthony.torres@txstate.edu; Federico Aguayo, Ph.D., Assistant Professor in the Concrete Industry Management (CIM) Program housed in the Department of Engineering Technology at Texas State University, fred.aguayo@txstate.edu
Technical Liaison: Tom Burnham, MnDOT, tom.burnham@state.mn.us
Project Technical Advisory Panel (TAP):

  • Bernard Izevbekhai, MnDOT
  • David Lim, Caltrans
  • Maria Masten, MnDOT
  • Angel Mateos, UC-Berkley
  • Peter Taylor, Iowa State U. CPTech Center
  • Brett Trautman, MoDOT

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