NOVEL PERFORMANCE TESTS FOR  EVALUATION OF ALKALI-SILICA REACTION

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NOVEL PERFORMANCE TESTS FOR  EVALUATION OF ALKALI-SILICA REACTION

ABSTRACT

Alkali-silica reaction (ASR) is a leading cause of premature concrete deterioration, resulting in cracks that develop in concrete structures. ASR is a slow chemical reaction, taking years to manifest. Therefore, ASTM currently has several widely used laboratory tests to more rapidly assess the ASR risk of aggregates and durability of concrete mixtures. ASTM C1260 uses an extremely accelerated testing environment (specimens are stored in 1N NaOH at 80OC) to reduce the testing time to 16 days, but this dramatically reduces the test reliability. ASTM C1293 uses less harsh conditions (100% relative humidity at 38oC) to accelerate the reaction to identify reactive aggregates in 1 year, but an experimental artifact called alkali leaching can ultimately stop ASR, leading to possible false aggregate identification.

The motivation, objectives, and outline of this thesis are described in Chapter 1. Chapter 2 details the development and evaluation of two new tests to identify ASR. These new tests address the flaws in existing ASTM testing methods. In the sealed concrete prism test (S-CPT), a membrane is applied to the surface of concrete prisms to prevent alkali leaching. In the water entrained concrete prism tests (WE-CPT) the same moisture barrier prevents alkali leaching, but pre-saturated light weight aggregates are added to the mixture to provide excess moisture for expanding ASR gel. Four varying levels of reactive aggregates were tested at both 38oC and 60oC, to determine the effectiveness of these new test methods and the possibility of accelerating the tests. Expansion, mass change, relative humidity, and pore solution chemistry were experimentally measured to evaluate S-CPT and WE-CPT. The results suggest that alkali leaching can be lessened by a moisture barrier and important relationships between expansion, mass gain, and relative humidity were developed during this research.

In Appendix A, various moisture barriers were tested for their effectiveness at creating the closed system necessary for S-CPT and WE-CPT. Based on the results, a vapor permeable membrane was selected and used in S-CPT and WE-CPT because it was able to lessen alkali and OH leaching, while also maintaining ASR promoting levels of relative humidity inside concrete. Subsequent Appendices present experimental results for ASR tests conducted during this research.

TABLE OF CONTENTS

LIST OF FIGURES ………………………………………………………………………………………………….. v

LIST OF TABLES ……………………………………………………………………………………………………. viii

ACKNOWLEDGEMENTS ……………………………………………………………………………………….. x

Chapter 1  Introduction and Thesis Outline ………………………………………………………………….. 1

Motivation …………………………………………………………………………………………………. 1

Objectives ………………………………………………………………………………………………….. 2

Thesis Outline ……………………………………………………………………………………………. 3

References …………………………………………………………………………………………………. 3

Chapter 2  Two New Performance Tests to Identify Alkali-Silica Reaction ……………………… 4

Abstract …………………………………………………………………………………………………….. 4

Background ……………………………………………………………………………………………….. 4

Current ASTM ASR test methods …………………………………………………………………. 6

Other attempts at Novel ASR test methods …………………………………………………….. 11

New ASR tests developed in this research ……………………………………………………… 12

Materials and methods ………………………………………………………………………………… 15

Experimental Measurements ………………………………………………………………………… 20

Results and Discussion ………………………………………………………………………………… 23

Conclusions ……………………………………………………………………………………………….. 35

References …………………………………………………………………………………………………. 37

Appendix A  Finding a suitable moisture barrier …………………………………………………………… 40

Vacuum Seal Bags – low humidity environment ……………………………………………… 40

Vacuum Seal Bags – high humidity environment…………………………………………….. 41

Commercial Tapes ………………………………………………………………………………………. 43

Pore solution analysis of barrier systems ……………………………………………………….. 45

Conclusions from moisture barrier experiments ……………………………………………… 48

References …………………………………………………………………………………………………. 48

Appendix B  Mixture proportions and results for other aggregate and concrete mixtures tested 50

Appendix C  Compiled data for expansion, mass gain, and RH for concrete undergoing ASR

testing ………………………………………………………………………………………………………………. 61

Chapter 1

 

Introduction and Thesis Outline

Motivation

Alkali-silica reaction (ASR) is a leading cause of premature concrete deterioration, leading to increased maintenance costs and a shortened service life for affected structures (including highway pavements and bridges, walls, dams, and roadway barriers). ASR occurs when metastable forms of silica in aggregates dissolve in the highly alkaline pore solution of concrete, and then form an expansive silicate gel that swells in the presence of moisture (Rajabipour et al. 2015). ASR results in cracks that create pathways for other forms of deterioration (freeze thaw damage, rebar corrosion, and chemical attack) to rapidly reduce the serviceability of the structure.

Many natural aggregates used in concrete mixtures are ASR prone, however the reaction typically takes 10 to 20 years to show symptoms. Therefore, accurate and quick laboratory test methods are needed to identify the potential for ASR. When aggregate reactivity is accurately determined, preventative measures such as chemical or mineral admixtures, can be implemented to prevent ASR from occurring. Currently, the concrete prism test (CPT, ASTM C 1293-08) and the accelerated mortar bar test (AMBT, ASTM 1260-14) are the common standard laboratory tests used to identify the potential for ASR in aggregates, but these tests have noted flaws. CPT takes 1 to 2 years, and an experimental artifact influences the results. AMBT exposes mortars to unrealistically harsh conditions resulting in poor test reliability (Ideker et al. 2012).

Researchers have identified these flaws in the current standards and have developed new ways to identify ASR prone aggregates. Researchers have tried developing adaptations of AMBT that allow the incorporation of coarse aggregates, and have shown good correlation between the new test and CPT results (Latifee and Rangaraju 2015). Other groups have increased the size of the specimens tested to limit alkali leaching, while others have wrapped the specimens in high pH cloths (Yamada et al. 2014). While these tests have identified ASR, they provide external sources of alkalis and expose concrete to unrealistic conditions. A paradigm shift is needed that removes external sources of alkalis, and instead creates a closed system that will better replicate field conditions.

Objectives

In this research two new ASR test methods were developed and evaluated; a sealed concrete prism test (S-CPT) and a water entrained concrete prism test (WE-CPT). Both tests attempt to create a closed system by sealing specimens using a breathable membrane to reduce alkali and OH leaching, while also maintaining high internal relative humidity necessary for ASR. In addition to sealing, WE-CPT entrains excess water into the concrete prisms through prewetted lightweight aggregates, which over time desorb water to the expanding ASR gel. These modifications allow field conditions to be more closely replicated (thereby improving test reliability), and also allow testing at higher temperatures, which increases the rate of ASR and decreases the necessary time of testing.

The main objectives of this research are:

  • To limit alkali leaching from concrete prisms undergoing ASR testing, to improve reliability of the test results;
  • To allow for the reduction of testing time by increasing temperature;
  • To evaluate the applicability of these two novel tests using aggregates of various reactivities (from moderately to very highly reactive) in mixtures with or without ASR mitigation.
  • To provide a better understanding of the relationships among ASR expansion, concrete moisture gain, internal relative humidity (RH), pore solution chemistry, and temperature.

Thesis Outline

The following chapter in this thesis is composed of a journal paper detailing the methodology, results, and conclusions of evaluating S-CPT and WE-CPT as new ASR performance tests. The Appendices include supporting information including the methodology and experimentation used to select the moisture barrier and the results from the ASR tests used during this study.

 References

Rajabipour, F., Giannini, E., Dunant, C., Ideker, J. H., & Thomas, M. D. (2015). Alkali–silica reaction: Current understanding of the reaction mechanisms and the knowledge gaps. Cement and Concrete Research, 76, 130-146.

Ideker, J. H., Bentivegna, A. F., Folliard, K. J., & Juenger, M. C. (2012). Do current laboratory test methods accurately predict alkali-silica reactivity?. ACI Materials Journal, 109(4).

 

Latifee, E. and Rangaraju, P. (2015). ”Miniature Concrete Prism Test: Rapid Test Method for Evaluating Alkali-Silica Reactivity of Aggregates.” J. Mater. Civ. Eng., 27(7), 04014215.

 

Yamada, K., Karasuda, S., Ogawa, S., Sagawa, Y., Osako, M., Hamada, H., & Isneini, M. (2014). CPT as an evaluation method of concrete mixture for ASR expansion. Construction and Building Materials, 64, 184-191.

NOVEL PERFORMANCE TESTS FOR  EVALUATION OF ALKALI-SILICA REACTION

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