EVAPORATION, SETTLEMENT, TEMPERATURE EVOLUTION, AND DEVELOPMENT OF PLASTIC SHRINKAGE CRACKS IN MORTARS WITH SHRINKAGE-REDUCING ADMIXTURES Pietro Lura (1), Guy B. Mazzotta (2), Farshad Rajabipour (2), and Jason Weiss (2) (1) Technical University of Denmark, Lyngby, Denmark (2) Purdue University, West Lafayette, IN, US Abstract This paper deals with the development of plastic shrinkage cracks in mortars containing a commercially available shrinkage-reducing admixture (SRA). Mortars containing SRA show fewer and narrower plastic shrinkage cracks than plain mortars when exposed to the same environmental conditions. It is proposed that the lower surface tension of the pore fluid in the SRA mixtures results in less evaporation, reduced settlement, reduced capillary pressure, and lower crack-inducing stresses at the topmost layer of the mortar. It is hoped that by improving the understanding about how SRA work, mixtures containing SRA can be more appropriately designed and utilized in cases where early-age plastic cracking is a potential problem. 1. INTRODUCTION One of the age-old problems in concrete is cracking. In addition to being unsightly, cracks have the potential to act as weak planes for further distress or as conduits for the accelerated ingress of aggressive agents that may lead to reduced durability. In particular, fresh concrete may be susceptible to plastic shrinkage cracking, which occurs between the time of placement and the time of initial set [1]. In the literature, plastic cracks are generally attributed to four driving forces: rapid evaporation of water that creates menisci and high tensile stresses in the capillary water near the surface [2,3]; differential settlement, since plastic shrinkage cracks are frequently observed above reinforcing steel or at locations where there is a sudden change in cross-sectional thickness [1,4,5]; differential thermal dilation due to temperature gradients and temperature development within the fresh concrete; and autogenous shrinkage that occurs during the plastic phase [6,7]. While these factors are often addressed independently, in reality they will act simultaneously to increase the potential for cracking. Over the last two decades, significant research has examined the use of SRA’s in concrete. A recent review article [8] reported that SRA mixtures generally have lower shrinkage, lower or equal chloride penetration indexes, reduced sorptivity, and reduced cracking potential, despite having similar or slightly lower strength, modulus of elasticity, and fracture toughness to plain concrete. However, substantially fewer studies have been conducted on the role of Page 1 SRA in fresh (i.e., plastic) concrete systems. Bentz et al. [9] examined the role of SRA in pastes and mortars at early ages using x-ray absorption. Mora et al. [10] presented an experimental study where a reduction of evaporation and of plastic shrinkage cracking was observed in concrete with three different types of SRA. A previous work by the current authors [11] used a theory originally developed for drying of gels [12] to explain how SRA, by reducing the surface tension of the pore fluid, influences the rate of evaporation, temperature evolution, capillary stresses, and cracking of mortars. In this paper, further experimental evidence is presented to support this hypothesis. It is hoped that by improving this understanding, SRA mixtures can be more appropriately designed and utilized in cases where early-age plastic cracking is a potential problem. 2. Experimental Investigation 2.1 Materials The SRA used in this research is a commercially available product (Tetraguard® AS20). This product belongs to a class of admixtures that are specifically designed to lower the surface tension of the pore water. Standard mortars were studied with a water-to-cement mass ratio of 0.50, 55% aggregate volume, and 0%, 1%, 2% and 5% SRA (the SRA replaced an equal amount of the mixing water to maintain a constant liquid-to-cement ratio). The cement used in this investigation was a Type I ordinary Portland cement with a Blaine fineness of 360 m2/kg and a Bogue phase composition of 60% C3S, 12% C2S, 12% C3A, 7% C4AF and a Na2O equivalent of 0.72%. River sand was used with a fineness modulus of 2.60 and grading consistent with the requirements of ASTM C 33 [13]. Prior to mixing, the constituent materials were conditioned in a sealed container in an environmental chamber (30±1°C) for 12 hours. After preconditioning, the materials were mixed in a 0.05-m3 rotary pan mixer in accordance with ASTM C 305 [14]. During mortar placement, external vibration was applied using a standard vibration table. The specimens were screeded perpendicular to the stress riser before a smooth steel trowel finish was applied. Fifteen minutes after water was added to the mixer, all of the prepared specimens were transferred into the environmental chamber where testing took place. Setting time was measured on the mortars at 23±1°C according to ASTM C 403/C 403M [15]. The addition of SRA delayed both initial and final set of the mortars. Initial and final setting times varied from 3.7 and 5.4 hours in the plain mortar to 4.7 and 6.8 hours in the mortar with 5% SRA. 2.2 Experimental methods The du Noüy Ring Method was used to measure the surface tension of solutions of SRA and deionized water at a temperature of 23±1°C. The method is based on determining the force required to detach a ring from the surface of a solution. Measurements were taken using a tensiometer with a platinum-iridium alloy ring, in accordance with ASTM D 971 [16]. The tensiometer was calibrated using deionized water at 23°C, which has a surface tension of 0.072 N/m. All mortar samples were tested in a chamber, where environmental conditions were maintained constant throughout the investigation: temperature 30±1°C, relative humidity Page 2 90 mm 280 mm 560 mm 12.7 mm 32 mm 25 mm 68º 68º 63.5 mm 18 gage steel 355 mm 100 mm 160 mm 50±2%, and wind velocity 24±2 km/h. Immediately after mixing, all mortar specimens were placed in the chamber and exposed to the environment to accelerate evaporation. An electronic balance, equipped with data acquisition at 30-second intervals, measured the weight loss of cylindrical samples of mortar (75 mm depth and 100 mm diameter). Three specimens were tested for each mixture. In addition, evaporation of SRA-deionized water solutions was also measured using identical cylindrical molds. Copper-Constantan thermocouples were used to measure the temperature profile in identical cylindrical specimens during the evaporation process. Thermocouples were placed into the mortar at depths of 2, 4, 6, 9, 12 and 20 mm and automated temperature readings were taken at 30-second intervals until 6 hours. Settlement of mortars was measured with a non-contact laser system [17, 18]. The vertical displacement of the surface of mortars was measured every 30 seconds from the time of placement (15 minutes after water was added during mixing) until an age of six hours. Mortar samples were cast in the same molds used in the evaporation and temperature measurements. The non-contact laser measurement devices consisted of a laser beam projected at a small angle from the vertical direction toward the specimen surface. The distance from the laser head to the surface is calculated by measuring the horizontal displacement of the reflected beam. It was observed that a film of bleed water present on the surface of the samples did not substantially influence these measurements. To study the plastic shrinkage cracking, a setup similar to the one proposed by Berke and Dalliare [19] was adopted. However, the dimensions of the slab were slightly varied, as described in Qi et al. [20, 21] (Figure 1). In particular, the cover above the stress riser was reduced. This geometry provided a base restraint, while a stress riser placed in the centre of the slab significantly reduced the slab depth and provided a preferential location for cracking. Stress Riser Geometry Side View Top View Figure 1: Geometry of the plastic shrinkage cracking form After 6 hours, the environmental chamber was turned off and the specimens were stored there for an additional 18 hours at 23±1°C and 50±2% RH. At an age of 24 hours, the slab was ready for image acquisition. The crack width was measured using an automated image analysis approach that takes approximately 300 measurements for each specimen, thereby providing statistically valid information about crack width and its variability [21]. The automated image processing and analysis procedures used in this study are described in reference [20]. Page 3 3. EXPERIMENTAL RESULTS 3.1 Surface tension of solutions The addition of SRA significantly decreases the surface tension of the deionized waterSRA solutions. The surface tension decreases rapidly until a plateau is reached at approximately 15% SRA replacement by mass. A semi-empirical expression describing the surface tension of dilute aqueous solutions of organic substances [22] has been used to fit the surface tension of SRA solutions measured in the experiments [23]: ⎧ ⎡ ⎛ C ⎞⎤ if 0.1% ≤ CSRA < 15% ⎪γ SOL = γ Water ⎢1 - 0.0795 ln⎜ SRA ⎟⎥ (1) ⎨ ⎝ 0.0164 ⎠⎦ ⎣ ⎪ γ SOL = 0.033 N/m if CSRA ≥ 15% ⎩ where CSRA is the SRA content by mass of solution expressed as percentage (i.e., for 1% SRA, CSRA = 1). These measurements compare reasonably well to others previously reported [9,24]. Specific Change in Mass (kg/m2) 3.2 Evaporation of solutions and mortars Figure 2 shows the mass change due to evaporation for samples of deionized water and SRA-deionized water solutions (where SRA content is defined by mass of water). The mass change is normalized to the surface area of the specimen. It is noticed that the addition of SRA to water appears to have little influence on the evaporation rate of deionized water in the first few hours. 0.00 -1.00 -2.00 -3.00 Deionized water -4.00 1% SRA Solution 5% SRA Solution -5.00 0 1 2 3 4 5 6 Time of Drying (hours) Figure 2: Mass change due to evaporation of solutions at 30°C, 50% RH, and 24 km/h While these environmental conditions correspond directly to the environmental conditions of the plastic shrinkage test, it became clear through the course of this investigation that the need existed to understand how the evaporation rate influenced the behaviour of the specimens during the first few hours. As a result a testing apparatus has been recently developed to measure the evaporation rate of solutions and mortars, shown in Figure 3. The measurement device consists of two scales on base vibration pads that are connected to a computer for continuous data acquisition. Above the scales, a frame supports two acrylic chambers (one over each specimen) that are designed to enable a fan to pass a very constant Page 4 wind across the surface of the specimens. The entire apparatus is placed inside a chamber with the ability to control the relative humidity and temperature. Support Frame Fan Acrylic Tunnel Solution Container Scales Vibration Isolation Figure 3: Illustration of the chambers designed to measure evaporation rates Evaporation Rate of Solution Evaporation Rate of Water Figure 4 shows the rate of mass change due to evaporation over a 20 hour period for two 75 mm thick samples of deionized water and SRA-deionized water solutions at 30°C and 30% RH. To more clearly illustrate if a difference exists between the evaporation rate of a SRA solution and water, the evaporation rate of each solution was normalized by the evaporation rate of pure deionized water. The solutions are preconditioned to the same temperature of the chamber before being stirred and placed in the evaporation dishes. The results shown in Figure 4 confirm that indeed there is little or no influence of the SRA content (other than possibly at the beginning of the test) on the rate of evaporation of the solution, up to a SRA content of 7.5% by mass of water. Measurements on pure SRA show no evaporation. 1.50 2% SRA 5% SRA 7.5% SRA 1.25 1.00 0.75 0 5 10 15 20 Time of Drying (hours) Figure 4: Rate of mass change due to evaporation of solutions at 30°C and 30% RH Page 5 Specific Change in Mass (kg/m2) It should be noted, however, that for the specimens shown in Figure 4 the size of the water reservoir is relatively large and as a result the concentration of the solution is not significantly altered at the early stages of drying. Additional measurements are being performed on solutions to further investigate the influence of the concentration of solutions containing SRA and their rate of evaporation. This is intended to better correspond with the conditions that may be expected in a mortar where the overall volume of the solution is more limited. 0.00 -1.00 -2.00 Mortar 5% SRA -3.00 Mortar 1% SRA -4.00 Plain Mortar Deionized water -5.00 0 1 2 3 4 5 6 Time of Drying (hours) Figure 5: Mass change due to evaporation of mortars at 30°C, 50% RH, and 24 km/h In addition to measuring the evaporation rate in solutions, the mass change due to evaporation was measured for mortars, as shown in Figure 5. The rate of evaporation in the first minutes appears similar to that of pure water for all mortars. After about 1 hour, the rate of evaporation began decreasing in all mortars. The rate of evaporation was first reduced in the 5% SRA mortar, followed by the 1% SRA mortar and by the plain mortar in the following hours. The evaporation rate decreased steadily over time. Temperature (oC) 35 0% SRA 1% SRA Chamber 32 Chamber 29 Depth (mm) 0 2 4 6 9 12 20 Depth (mm) 0 2 4 6 9 12 20 26 23 20 0 1 2 2% SRA Chamber 3 4 5 6 Time of Drying (hours) 0 1 2 3 4 5 Depth (mm) 0 2 4 6 9 12 20 6 Time of Drying (hours) 0 1 2 3 4 5 6 Time of Drying (hours) Figure 6: Temperature measurements during evaporation in mortar specimens Page 6 3.3 Temperature measurements on mortars Figure 6 shows results of temperature measurements on mortars with 0%, 1%, and 2% SRA. It should be noted here that the temperature readings were taken with uncalibrated thermocouples and this explains some scatter in results. Immediately after placing the samples in the environmental chamber, a drop in temperature occurred due to evaporative cooling. In all mortars, the temperature dropped to approximately 23°C, or the wet-bulb temperature. In the plain mortar, the temperature started increasing at 2 hours and equilibrated with the external temperature at about 6 hours. In the mortars containing SRA, the temperature started increasing earlier, before 1 hour, and eventually became higher than the chamber temperature around 3 to 4 hours after placing, depending on the SRA content. The earlier rise in temperature of the specimens containing SRA is consistent with the reduction in evaporation rate demonstrated in Figure 5. 3.4 Settlement of mortars Results of settlement vs. mass loss are shown in Figure 7. Initially, all curves are linear with similar slope, followed by a sudden decrease and a plateau. In mortars with SRA, settlement stopped at 1.5 hours after time of placement, while in the plain mortar settlement stopped at about 2 hours, at a final value about 50% higher than in mortars with SRA. 0.0 Settlement (mm) Plain mortar -0.5 Mortar 1% Mortar 5% -1.0 1.5 hours -1.5 2 hours -2.0 -2.5 0 -1 -2 -3 2 -4 -5 Mass loss (kg/m ) Figure 7: Settlement of mortars exposed to evaporation vs. mass loss 3.4 Plastic shrinkage cracks The cracks in the mortars were first observed above the central stress riser at approximately 90 minutes after placement. Figure 8 shows the average (of three specimens) crack width distribution in mortars with different SRA content measured after 24 hours. The addition of SRA results in a slight decrease in the frequency of plastic cracks and a more substantial reduction in the width of the plastic shrinkage cracks. Page 7 Cumulative Distribution of Measured Cracks (%) 100 80 60 40 1% SRA 20 5% SRA 2% SRA Plain 0 0 0.4 0.8 1.2 1.6 2.0 Crack Width (mm) Figure 8: Influence of SRA on the width of the plastic shrinkage cracks 4. DISCUSSION In the following paragraph a global description of the process leading to formation of plastic shrinkage cracks in mortars is briefly discussed, highlighting the benefits of SRA. A more detailed analysis of the phenomenon is presented in [11]. Initially, the solid particles in the mortars sink, causing the mixture to densify and the bleed water to be transported to the surface. Initial evaporation of fresh mortars proceeds from a layer of bleed water spread across the whole surface of the sample; in fact the initial rate of evaporation of mortars is close to that of bulk water at the same temperature (Figure 5). Furthermore, it was shown [11] that the initial rate of evaporation from mortars is independent of SRA content, until the layer of bleed water is consumed (Figure 5). During the first minutes of drying, the temperature of the specimen drops to the wet-bulb temperature and remains almost constant for a period of time (Figure 6). It is also interesting to note that temperature reduction was almost constant through the 75 mm depth of the specimens. The temperature increased after approximately 1 hour from casting in the plain mortar, while mortars with 1% and 2% SRA showed an earlier temperature rise (Figure 6). Once the layer of bleed water at the surface is consumed by evaporation, air-liquid menisci are formed in the liquid between the solid particles on the surface [2]. These menisci cause tensile stress to develop in the pore fluid, leading to shrinkage. The viscous nature of the material at these early ages causes the majority of the shrinkage to occur in the vertical direction. As the system shrinks, the pore fluid is brought to the surface and evaporates. When menisci form at the surface, the pore fluid is subjected to tensile stresses. The capillary pressure caused by the menisci can be calculated as: 2γ ⋅ cos θ (2) P=− r where γ in N/m is the surface tension of the pore fluid, θ (rad) is the contact angle between pore fluid and solids, and r (in meters) is the radius of the menisci. In mortars with SRA, the surface tension of the pore fluid is substantially reduced, even to about half the value for pure water in the case of the 5% SRA addition. Since the capillary pressure is proportional to the Page 8 surface tension (Equation (2)), a proportional reduction of the capillary pressure developing upon drying is expected in mortars with SRA. The capillary pressure produced by the menisci on the surface compresses the mortar, whose solid skeleton consolidates under the growing pressure. As a consequence, the pore fluid is drawn to the upper surface of the mortar. The higher the capillary pressure, the more fluid is forced out of the porous network and this water becomes available for evaporation. In the SRA mortars, a lower capillary pressure develops, thereby explaining why their rate of evaporation is lower than the one of plain mortar (Figure 5) after the first hour of drying. Shrinkage of the mortar under the effect of the capillary pressure is resisted in part by the modulus of the solid network, but much greater resistance is caused by friction between the pore fluid and the network [12]. As a consequence, a pressure gradient builds up between the surface and the interior of the drying body. The permeability of the network gradually decreases as the network is drained and consolidated under the capillary pressure. The higher the evaporation, the more the porous body is consolidated, and the steeper is the pressure gradient at the surface. In Figure 7 the initial rate of the settlement versus mass loss curves is the same for all mortars regardless of the SRA content. Moreover, in the first period, 1 mm of settlement corresponds to the evaporation of 1 mm of water from the surface. This confirms that no empty pores are created in the mortars in the first phase of evaporation. The final settlement is considerably lower in mortars with SRA, as a consequence of lower surface tension which results in lower consolidation. Consequently, the addition of SRA may reduce the risk of plastic shrinkage cracking due to differential settlement. As evaporation progresses, under the increasing consolidation pressure, the network of the solid particles becomes progressively stiffer and the slope of the settlement-capillary pressure curve decreases accordingly [2]. A critical point is reached, where the vertical deformation of the mortar stops, because the capillary pressure is no longer able to compress the solid skeleton of the mortar and draw water to the surface. At this point, the menisci reach a breakthrough radius and recede into the interior of the sample [12]. The minimum radius of the menisci in a fresh cementitious material, before a pore structure has formed, is related to the dimension of the cement particles. The cement used in this investigation had an average cement particle diameter of about 20-30 μm. Considering different packing arrangements of monosize spheres, the radius of the menisci will be about 5 to 7 times smaller than the particle size [12]. With these assumptions, a maximum capillary pressure of -30 kPa is calculated with Equation (2) when the pore fluid is water. This value compares well with -20 kPa measured on cement paste by Wittmann [2]. If a paste with 5% SRA addition is considered, the surface tension is reduced from 0.072 to 0.039 N/m and the calculated maximum capillary pressure would be reduced to -16 kPa. According to these calculations, the capillary pressure in the surface of the mortars at the critical point is lowered by SRA addition. Mortars containing SRA are subjected to lower internal stress and are therefore less consolidated when the critical point is reached. As a consequence of being less consolidated, mortars with SRA have higher permeability at the critical point, and the gradient of the capillary pressure at the surface is less steep than in plain mortars. It has been observed that cracking of cementitious materials happens at the critical point, when the maximum pressure is reached and the settlement-pressure curve shows a plateau [2]. Also cracking of gels occurs at the same point [12]. Both in gels and in cementitious materials, the probability of cracking is related to the rate of drying and cracking can be avoided by slowing down the evaporation rate [3,12]. Page 9 According to Scherer [25], the stress acting on the solid network at the surface of a drying body can be calculated as: ~ 1 − 2ν σx = ⋅ P − P −φ ⋅ P (3) 1 −ν where ν is Poisson’s ratio for the solid network, P (Pa) is the capillary pressure at the surface of the drying body, P (Pa) is the volume-weighted average of the capillary pressure in the pore fluid, and φ is the volume fraction of fluid. If the pressure at the surface is close to the average pressure in the body (i.e., the pressure gradient at the surface is not high), the stress at ( ) ~ the surface σ x is compressive, and cracking does not occur. When the pressure gradient at the ~ surface is high, the global stress σ x acting on the surface of the mortar becomes tensile. If flaws, such as large pores, are present on the surface of the mortar, their edges would be pulled apart by the global tensile stress, generating and propagating larger cracks [25]. A high rate of evaporation causes a steep gradient of the capillary pressure at the surface and therefore a high tensile stress at the surface (Eq. (3)), which would induce plastic shrinkage cracks. The lower rate of evaporation obtained by addition of SRA (Figure 5) should therefore reduce the tensile stress at the surface and ultimately reduce the probability of plastic shrinkage cracking. This is indeed confirmed by the experimental results shown in Figure 8. 5. CONCLUSIONS Addition of SRA reduces the width of plastic cracks, evaporation and settlement in mortars exposed to evaporation immediately after casting. By reducing the surface tension in the pore fluid, SRA’s seem to be advantageous in several ways: 1) they reduce evaporation; 2) they reduce settlement; and 3) they reduce the stresses that develop at the surface of the mortar. Each of these benefits of SRA reduces the potential for the development of plastic shrinkage cracks. ACKNOWLEDGEMENTS Financial support from Degussa Chemical Corporation is gratefully acknowledged. In addition, this work was performed in the Materials Sensing and Simulation Laboratory and as such the authors gratefully acknowledge the support that has made that lab possible. REFERENCES [1] Powers, T.C., 'The Properties of Fresh Concrete', (John Wiley & Sons, New York, 1968). [2] Wittmann, F.H., 'On the Action of Capillary Pressure on Fresh Concrete', Cem. Concr. Res. 6 (1) (1976) 49-56. [3] Cohen, M.D., Olek, J. and Dolch, W.L., 'Mechanisms of Plastic Shrinkage Cracking in Portland Cement and Portland Cement-Silica Fume Paste and Mortar', Cem. Concr. 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[23] Pease, B. J., Shah, H. R., and Weiss, W. J., 'Shrinkage Behavior and Residual Stress Development in Mortar Containing Shrinkage Reducing Admixture (SRA's)', Shrinkage and Creep of Concrete, ACI SP 227, ed. Gardner, J., and Weiss, W. J., Farmington Hills, MI, 2005 [24] Ai, H., and Young, J.F., 'Mechanisms of Shrinkage Reduction Using a Chemical Admixture', Proceedings of the 10th Int. Con. on the Chemistry of Cement, ed. Justnes, H., Gothenburg, Sweden, Vol. 3, 1997. [25] Scherer, G.W., 'Crack-tip stress in gels', J. Non-Cryst. Sol. 144 (2-3) (1992) 210-216. Page 11
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