Reducing Cracking in Concrete Structures by Using Internal Curing with High Volumes of Fly Ash

T. J. Barrett1, I. De la Varga2 and W.J. Weiss3

1Graduate Research Assistant, Purdue University, School of Civil Engineering, 550 Stadium Mall Drive, West Lafayette, IN 47907; PH (765) 494-7999; email: barrett1@purdue.edu

2Graduate Research Assistant, Purdue University, School of Civil Engineering, 550 Stadium Mall Drive, West Lafayette, IN 47907; PH (765) 494-7999; email: idelavar@purdue.edu

3Professor and Director of Pankow Materials Laboratory, Purdue University, School of Civil Engineering, 550 Stadium Mall Drive, West Lafayette, IN 47907; PH (765) 494-2215; email: wjweiss@purdue.edu

ABSTRACT High volume fly ash (HVFA) concretes have been used in the past; however HVFA has been primarily advocated for use in mass construction applications. Recent interest in developing more sustainable construction materials has led to an increased interest in utilizing these HVFA mixtures in transportation structures such as pavements and bridges. These mixtures have a reduced carbon footprint, in addition to other improvements in the material performance. This paper presents a study utilizing the dual ring test to assess the benefits of the HVFA mortar mixtures with respect to reducing early age cracking. Three mortar mixtures were prepared with a water-to-cement ratio of 0.30. The first mixture is a plain cement mortar, the second mixture is a mortar where 40 % of the cement (by volume) was replaced with C class fly ash, and the third mixture is a mortar where 40 % of the cement (by volume) was replaced with C class fly ash and prewetted lightweight aggregate (LWA) to provide internal curing (IC). The cracking potential due to thermal and autogenous shrinkage was assessed. Results show a lower risk of shrinkage cracking in the HVFA mixture with internal curing. The IC mixture made using HVFA is more robust for construction at early ages.

INTRODUCTION

Fly ash is by-product from coal-powered plants that is broadly used in concrete applications to replace cement (ACAA, 2003; CANMET/ACI, 2007). In practice, a large portion of concrete mixture designs used in the United States contain fly ash. Recently, interest has developed in increasing the volume that fly ash is used to replace cement in concrete (Atis, 2003; Kumar et al., 2007; Mehta, 1999), leading to the production of high volume fly ash (HVFA) concretes.

One of the main challenges of using fly ash in concrete is the slower strength development at early ages. A portion of fly ash can react hydraulically like portland cement (depending on its composition) (Diamond, 1983), however the reaction of the fly ash is slower than portland cement. In addition to reacting on its own, fly ash can participate in a secondary reaction known as the pozzolanic reaction. The pozzolanic reaction takes place at later ages. Due to this delayed reaction, replacing part of the cement with fly ash may compromise the early-age strength. This can be a concern if the strength development is slow enough such that it alters construction operations. One way to counteract this low early-age strength development is by reducing the water-to-cement (w/c) or water-to-cementitious materials (w/cm) ratio in concrete (De la Varga et al., 2012). This reduction in w/c is beneficial since it results in a lower porosity system which can substantially reduce the ingress of fluids containing aggressive ionic species (e.g., salt water or deicing salt). However, this lower w/c comes with other drawbacks including higher self-desiccation (i.e. internal drying) (Bentz et al., 1999; Weiss et al., 1997) and higher temperature rise in the material (cement hydration is an exothermic chemical reaction and in lower w/c systems there is typically a larger volume of cement per cubic yard). This can be problematic since the concrete can set at a relatively high temperature and then as the concrete cools, tensile stresses can be built into the concrete if it is restrained from shrinking. Since fly ash reacts more slowly and there is less cement, it aides in dissipating part of the temperature rise in concrete.

Low w/c (or w/cm) concretes are typically known as high performance concretes (HPC) (Gagne et al., 1989). While HPC is generally considered to be more durable due to the reduction of porosity in the system, the use of the low w/c (or w/cm) may make the concrete more susceptible to early age cracking. It may also be difficult to properly cure the concrete since the low porosity of the system can result in pore depercolation (i.e., disconnection of capillary porosity) and curing water may not be able to penetrate past the top layer of the concrete (in a percolated system the absorption of water may be limited to a depth on the order of centimeters thick). Internal curing (IC) is one alternative to conventional curing which uses water filled inclusions (typically prewetted lightweight aggregate (LWA)) to distribute curing water across the sample cross section (Bentz et al., 2006; Bentz et al., 1999; Bentz et al., 2011). IC not only provides more well distributed curing water in the concrete structure (eliminating the self-desiccation or internal drying mentioned above), but also helps to increase the degree of reaction of the cement and other supplemental cementitious materials since more water is provided into the system (De la Varga et

al., 2012). This effect further tightens the pore microstructure, thereby reducing permeability and further improving durability.

HVFA replacement is a strategy that engineers normally use in mass concrete applications, but the push for more sustainable concrete has caused people to question whether HFVA concrete can be used in pavements and the transportation infrastructure. The benefits of using fly ash in terms of reduced heat of hydration (Mehta, 1999) may have a positive impact in reducing early age cracking in bridges or buildings. The importance of evaluating these properties in a proper manner is vital.

In this research, the dual ring test (DRT) was used for evaluating the early age

cracking potential of sealed concrete undergoing a temperature reduction. The DRT is an improvement upon the conventional single ring rest that is used for the determination of the cracking age and the evaluation of the internal tensile stresses induced in restrained concrete structures (ASTM, 2004). The DRT was specifically designed to account for expansion at early ages and to provide appropriate restraint during temperature changes (Schlitter et al., 2010). This allows for the quantification and study of restrained shrinkage behavior due to thermal effects and autogenous deformations. RESEARCH OBJECTIVES

The DRT was used to quantify the thermal and autogenous deformations in three low w/cm (0.30) mixtures: plain, 40 % fly ash (by volume), and 40 % fly ash (by volume) with prewetted LWA. These mortar mixtures have been designated as FA0, FA40, and FA40-LWA100, respectively. With these material variations, it was possible to evaluate the differences that these mixtures have on the risk of cracking. MATERIALS AND TESTING PROCEDURE Materials. An ordinary portland cement (OPC) (ASTM C150-09 Type I/II) was used in this study, with a Blaine fineness of 476 m2/kg, a specific gravity of 3.17, an estimated Bogue composition of 52 % C3S, 18 % C2S, 8 % C3A, 9 % C4AF, and a Na2O equivalent of 0.5. A class C fly ash (ASTM C618-08a) was also used. The fly ash had a specific gravity of 2.63.

The fine aggregate consisted of regular river sand with a fineness modulus of 2.71 and an apparent specific gravity of 2.58. Rotary kiln expanded shale (i.e., a fine lightweight aggregate) was used for providing IC. The specific gravity (oven dry) of the LWA is 1.38. The LWA was measured to have a 24 hour water absorption of 15.9 % by mass. A high-range water-reducing admixture (HRWRA) was added at variable dosage by mass of cement in order to maintain the same slump in all mortars. Mixing Procedure. The mixing procedure was carried out in accordance with ASTM C192 (ASTM, 2007). The fine aggregate, LWA (if used), and a small portion of the

mixing water was first added to a buttered 7.0 liter (0.25 ft3) mixer and mixed until the aggregate was damp. The cementitious materials were then added and blended with the sand. Lastly, the water and HRWRA were added and the mixing began. The mortar was mixed for three minutes, rested for three minutes while the bottom of the bowl was scraped with a spoon, then mixed for an additional two minutes. When LWA was utilized, prior to mixing, the LWA was oven dried, air cooled, then submerged in water for 24 1 hour. The LWA was submerged in water that included the mixing water necessary for cement hydration and the water that would be absorbed by the LWA itself in 24 hours. The excess water (water not absorbed into the LWA during the 24 hours) was then decanted and used as the mixing water. Testing Procedure. The DRT was used to quantify the thermal and autogenous shrinkage deformations of the mortar mixtures (Schlitter et al., 2010). This test is performed by casting an annulus of mortar between two invar restraining rings. The sample is cast in two lifts, being vibrated with a handheld vibrator after each lift then trowel finished upon completion. The sample is placed in an insulated chamber and a copper tubing coil is placed on top of the specimen. The copper tubing coil is connected to an external ethylene-glycol water circulating system to control the temperature throughout the test. Each ring (inner and outer) is instrumented with four equally spaced invar strain gages to measure the strains developed from the sample expanding and contracting. Thermocouples are attached to the rings and the temperature coil to monitor the temperature change of the sample. A data acquisition system was set up to record strain and temperature readings every five minutes (Schlitter et al., 2010). RESULTS AND DISCUSSION Isothermal Comparison. The residual stress that develops when the autogenous shrinkage is restrained over the first four days of age was quantified at an isothermal temperature history of 23 0.2C as shown in Figure 1. To quantify the susceptibility to cracking, the temperature of the sample was decreased at an age of four days at a rate of 2C/hr. Cracking in a sample can be seen by the instantaneous stress release, shown by vertical lines on the plot. The reserve stress of the mixture (stress that had to be added by reducing the temperature) is then computed by subtracting the stress before temperature drop from the peak stress the sample was able to hold. The results of the tests show that the FA0 mixture had a reserve capacity of 3.0 MPa and resulted in cracking after a temperature change of 12C. Similarly, the FA40 mixture had a reserve capacity of 2.9 MPa and also resulted in cracking after a temperature change of 14C, however, an approximate reduction in autogenous shrinkage of 30% was observed in comparison to the FA0 mixture. The FA40-LWA100 mixture showed a reserve capacity of 4.4 MPa and did not result in any autogenous shrinkage or cracking after a temperature change of -32C.

Figure 1. Residual stress development under isothermal conditions up to four days of age

Temperature Profiles. In structures the temperatures are never maintained at isothermal conditions. In reality, a concrete mixture will typically experience a temperature rise at early ages due to the heat of hydration, followed by a gradual reduction in temperature as the mixture equilibrates with its surroundings. In mass concrete, the interior concrete is insulated well and the temperature decrease occurs over long periods of time. This behavior is characterized as being close to that of the adiabatic temperature rise, in which no heat is lost from the material to its surroundings. In many structures, the concrete temperature rises then decreases over the next few days to reach equilibrium with its surroundings. This study assumes that the material is allowed to reach high temperatures at early ages; however, the sample is allowed to cool. Figure 2 shows each of the temperature profiles that the mortar mixtures for this study were exposed to. It should be noted that the temperature drops seen at later ages were done intentionally in attempts to induce cracking in the samples so that the susceptibility for cracking could be observed.

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Figure 2. Temperature history profiles for dual ring test mixtures

Possible Temperature Variations. The results from exposing the FA0 mixture to each of the potential temperature profiles discussed previously can be seen in Figure 3. This was done to elicit the differences in stress development between the three idealized conditions. It can be seen that the performance of the material in the isothermal and adiabatic conditions closely resemble one another, with the adiabatic sample showing earlier expansion due to its temperature rise. The semi-adiabatic sample is exposed to both autogenous shrinkage and thermal shrinkage. It can be seen that the sample undergoing a temperature swing follows the same trend until the 24-36hr period, at which point the temperature of the sample begins to decline (see Figure 2) resulting in thermal shrinkage stresses and hence a deviation from the best-case autogenous shrinkage boundary. It is clear that the time and rate of this deviation is dependent on the time which the temperature is reduced and the rate of temperature reduction. The net effect of the additional thermal shrinkage is an earlier cracking age and an apparent higher ultimate tensile stress capacity (i.e. tensile strength). The higher ultimate strength is likely due to the reduction in damage that results from sustaining restrained shrinkage for longer periods of time as is the case with the isothermal and adiabatic tests. Semi-Adiabatic Comparison. The results of using the semi-adiabatic temperature profiles for the temperature in the dual ring test for all three test mixtures can be seen in Figure 4. Figure 4 shows that the FA0 and FA40 mixtures have similar behavior, with the mixture containing 40% fly ash cracking at a slightly earlier age and lower stress level. When internal curing is included, the stress due to autogenous shrinkage is essentially eliminated. As such, the response for the FA40 LWA100 mixture is mainly that of the thermal component of the shrinkage. This mixture was able to withstand a total temperature drop of -67.5C and resulted in no cracking. It seems that the compressive stresses at early ages (i.e., expansion) were beneficial to the

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mixture since it provided a slight compression in the sample prior to any shrinkage. As the temperature was reduced at later ages this compressive stress was reduced while no significant shrinkage stresses developed. This observed behavior may also be attributed to the more compliant nature of the material due to the lower stiffness obtained with lightweight aggregate. At the end of the test when the temperature was reduced at a faster rate in attempts to induce cracking, the shrinkage stress was also lower in the internally cured mixture.

Figure 3. Isothermal, Adiabatic, and Semi-Adiabatic residual stress development of the FA0 mixture

Figure 4. Semi-Adiabatic residual stress

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development comparison of test mixtures

CONCLUSIONS

This investigation focused on combining internal curing with high volume fly ash to create more sustainable mixtures with improved early age cracking resistance. The results of this study confirm that the dual ring test is a way of quantifying the cracking potential of concrete materials. Results of the semi-adiabatic tests show that while plain concrete may be susceptible to cracking, the use of fly ash as a cement replacement combined with internal curing provides sufficient resistance to shrinkage cracking, thereby improving the sustainability and durability of structures. While promising, future research is needed to correlate the small geometry of the dual ring test to large scale structures. ACKNOWLEDGEMENTS The authors of this work acknowledge the Federal Highway Administration for partially supporting this project together with Purdue University. The contents of this paper reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein, and do not necessarily reflect the official views or policies of the Federal Highway Administration, nor do the contents constitute a standard, specification, or regulation. The experiments reported in this paper were conducted in the Pankow Materials Laboratories at Purdue University. The authors acknowledge the support that has made this laboratory and its operation possible. REFERENCES ACAA. (2003). Fly Ash Facts for Highway Engineers (No. FHWA-IF-03-019):

Federal Highway Administration (FHWA). ASTM, Standard C 192/C 192M 07 (2007). Standard Practice for Making and

Curing Concrete Test Specimens in the Laboratory. (2007). ASTM International, West Conshohocken, PA Vol 4(2).

ASTM, Standard C 1581-04 (2004). Standard Test Method for Determining Age at Cracking and Induced Tensile Stress Characteristics of Mortar and Concrete under Restrained Shrinkage. (pp. 787-792). ASTM International, West Conshohocken, PA Vol 4(2).

Atis, C. D. (2003). High-Volume Fly Ash Concrete with High Strength and Low Drying Shrinkage. Journal of Materials in Civil Engineering, 15, 153-156.

Bentz, D. P., & Jensen, O. M. (2006). Mitigation strategies for autogenous shrinkage cracking. Cement and Concrete Composites, 26(6), 677-685.

Bentz, D. P., & Snyder, K. A. (1999). Protected paste volume in concrete: Extension to internal curing using saturated lightweight fine aggregate. Cement and Concrete Research, 29(Compendex), 1863-1867.

Bentz, D. P., & Weiss, W. J. (2011). Internal Curing: A 2010 state of the art review. Gaithersburg, MD: National Institute of Standars and Technology, U.S. Department of Commerce.

CANMET/ACI. (2007). 9th CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag, and Natural Pozzolans, Warsaw, Poland.

DelaVarga, I., Castro, J., Bentz, D., & Weiss, J. (2012). Application of Internal Curing for Mixtures Containing High Volumes of Fly Ash. to be submitted to Cement and Cocnrete Composites.

Diamond, S. (1983). On the glass present in low-calcium and in high-calcium fly ashes. Cement and Concrete Research, 13, 459-464.

Gagne, R., Pigeon, M., & Aitcin, P. C. (1989). Deicer salt scaling resistance of high performance concrete. Paper presented at the Symposium on Performance of Concrete.

Jensen, O. M., & Hansen, P. F. (2001). Water-entrained cement-based materials: I. Principles and theoretical background. Cement and Concrete Research, 31(4), 647-654.

Kumar, B., Tike, G. K., & Nanda, P. K. (2007). Evaluation of Properties of High-Volume Fly Ash Concrete for Pavements. Journal of Materials in Civil Engineering, 19, 906-911.

Mehta, P. K. (1999). Concrete Technology for Sustainable Development. Concrete International, 21(11), 47-52.

Philleo, R. (1991). Concrete Science and Reality. In J. P. Skalny & S. Mindess (Eds.), Materials Science of Concrete II. Westerville, OH: American Ceramic Society.

Schlitter, J., Senter, A., Bentz, D., & Weiss, J. (2010). A Dual Concentric Ring Test for Evaluating Residual Stress Development due to Restrained Volume Change (submitted).

Weiss, W. J., Yang, W., & Shah, S. P. (1997). Shrinkage Cracking in High Performance Concrete. Paper presented at the PCI/FHWA International Symposium on High Performance Concrete New Orleans, Louisiana.