Assessment of early-age cracking of high-performance concrete in restrained ring specimens

Quang-phu NGUYEN* , Lin-hua JIANG Qiao ZHU

1. College of Mechanics and Materials, Hohai University, Nanjing 210098, P. R. China

2. Faculty of Civil Engineering, Water Resources University, Hanoi, Vietnam

1 Introduction

The volume of concrete changes due to autogenous, drying, or thermal shrinkage, with moisture variation, temperature variation, and chemical reaction of cementitious materials. When these volume changes are prevented, residual tensile stresses can develop inside the concrete material. If these residual stresses exceed the tensile strength of the concrete, cracking may occur.Since it is essential that water-retaining structures be free from cracks, the assessment of early-age cracking of concrete in this study will help to alleviate the problem to a great extent.

ASTM C 157/C 157M-03 (ASTM Committee C09 2003)is frequently used to measure the free shrinkage of a concrete mixture. However, the free shrinkage is not sufficient for predicting whether cracking will occur. Weiss et al. (2000)suggested that the potential for cracking is dependent on the interaction of several factors, including the magnitude of free shrinkage, rate of shrinkage, elastic modulus, degree of restraint, and fracture toughness.

Krause et al. (1996), AASHTO (2005), and ASTM Committee C09 (2004)used the restrained ring test to assess a mixture’s susceptibility to restrained shrinkage cracking. The restrained ring test has been used by numerous researchers to assess the potential for shrinkage cracking in concrete mixtures (Grzybowski and Shah 1990; Kolver 1994; Carlson and Reading 1988). The ring test consists of a concrete annulus that is cast around a steel ring. As the concrete ring dries, it shrinks. The steel ring restrains this shrinkage by reducing the tensile stress developed in the concrete. If these stresses are large enough, cracking may occur.

The purpose of this study was to determine whether the stress development in the concrete ring can be used to provide quantitative information to assess the potential for cracking and analyze early-age cracking in restrained ring specimens.

2 Residual stress development in restrained ring specimens due to uniform drying condition

Weiss and Shah (2002)and Hossain and Weiss (2004)suggested that the rings could be allowed to dry from the top and bottom so that the moisture loss would be uniform along the radius of the specimen. Then, the actual residual stresses that develop in restrained ring specimens would be in the radial direction, providing for uniform drying shrinkage.

Moon (2006)studied the shrink-fit approach to simulate the restraint of the concrete by the steel ring. The drying and autogenous shrinkage of the concrete causes the concrete ring to shrink. It can be assumed that there is a pressure acting on the outer surface of the steel ring that is equal to the pressure acting on the internal surface. To compensate for shrinkage, this pressure can be modified to compress the steel ring and expand the concrete ring. The shrink-fit approach is described in Fig. 1.

Fig. 1 Geometry of ring to determine elastic response(Δ USH is the shrinkage of the ring, ΔUS is the shrinkage of steel, and ΔUC is the shrinkage of concrete)

Weiss et al. (2000)computed the circumferential strain in the ring by dividing the radial displacement by the radius. The actual residual interface pressure (pr)can be computed as the pressure required to cause a strain that is equivalent to the measured strain in the steel (εs). The actual residual interface pressure at time t is calculated by Eq. (1)(Weiss and Fergeson 2001):

where εs(t)is the strain in the steel at time t and can be obtained experimentally using strain gages on the inner surface of the steel ring; ESis the elastic modulus of the steel; and ROSand RISare the outer and inner radiuses of the steel ring, respectively. This pressure acts on the steel ring, and a pressure with the same value but in the opposite direction acts on the concrete ring, so that the stress distribution in the concrete ring can be determined using the following equation (Timoshenko and Goodier 1987; Hossain and Weiss 2004):

where ROCis the outer radius of the concrete ring, and r is the radius coordinate. Substituting Eq. (1)into Eq. (2), we obtain

3 Materials, experimental program and test methods

3.1 Materials

All materials used in the experiments were supplied by the Jiangsu Bote Advanced Materials Co., Ltd., including fine aggregate, portland cement (C)with a 42.5 grade from the Nanjing Jiangnan Cement Plant, some mineral admixtures (silica fume (SF), Class C fly ash(FA), and Grade 100 slag), and a high-range water-reducing admixture (PCA-I). The chemical composition and physical properties of these materials can be obtained from Nguyen et al.(2008).

3.2 Experimental program

To understand the influence of the mW/mBon the residual stress development and cracking in the restrained ring specimens, different mortar mixtures, mW/mB= 0.22 and 0.40, were prepared.Three mineral admixtures were used in this study: SF, FA, and slag. The mineral admixture that contained 25% FA and 25% slag was used for the mW/mB= 0.40 mixture, and the one that contained 15% SF and 25% FA was used for the mW/mB= 0.22 mixture. The contents of ingredients in the high-performance mortar mixture are summarized in Table 1.

Table 1 Content of ingredients in high-performance mortar mixtures with different mW/mB

3.3 Test methods

The mixtures were mixed in a forced mixer. After mixing, the mixture was cast and placed in the molds, vibrated, and then sealed with a plastic film for 24 hours. The specimens were removed from the molds and stored thereafter at 20 ℃ and 50% relative humidity. The specimens were always connected with the data acquisition system in order to measure strain in the steel rings.

The dimensions of the ring setup and the boundary conditions are shown in Fig. 2. The ring test used in this study was similar the AASHTO ring (AASHTO 2005), and the ring used in the experiments of Hossain (Hossain and Weiss 2004), with a 37.5 mm, 75 mm, or 112.5 mm thick mortar annulus cast around a steel ring, which had a steel wall thickness of 6 mm, 19 mm, or 30 mm. Four strain gages were attached at the mid-height of the inner surface of the steel ring and connected to the data acquisition system. Steel strain was monitored over time.The average strain information monitored by the strain gages was used to determine the residual stress development in mortar rings.

Fig. 2 Geometry of ring specimen (Unit: mm)

Mortar rings were sealed along their circumference. The boundary conditions were such that, by permitting drying from only the top and bottom surface of the ring, moisture could only be lost along one parallel plane. As the mortar shrunk, the steel ring was pressurized at the outer surface.

4 Residual stress development and cracking in restrained ring specimens

This study examined the influences of the steel thickness and the wall thickness on the residual stress of restrained ring specimens. The behavior of the rings only had one boundary condition: drying from the top and bottom of the ring. The steel thickness and mortar thickness were varied in two series. The strain that developed in the steel ring was measured; the maximum residual tensile stresses were calculated from Eq. (2), and then the age of cracking was determined.

Two series of restrained ring specimens were prepared to study the effects of the steel thickness and mortar thickness on early-age stress development and cracking in restrained ring specimens. The study cases are listed in Table 2.

Table 2 Steel thickness and mortar thickness in some study cases

In the two series the mortar rings had an inner diameter of 300 mm and a height of 75 mm.In the first series, the mortar thickness used for all experiments was constant, and in the second series the steel thickness was constant.

Thirty minutes after the first contact between cement and water, the strain data began to be measured every ten minutes. This was done to capture the early-age strains that developed during the first 24 hours. However, to prevent moisture loss from specimens, we sealed all of them after casting.

The stresses in mortar rings were calculated using Eq. (2)for the two series. The stress development and age of cracking in the restrained ring specimens are shown in Fig. 3 and Fig. 4.

Fig. 3 Stress development in restrained ring specimens for various steel thicknesses of series 1

Fig. 4 Stress development in restrained ring specimens for various mortar thicknesses of series 2

Fig. 3 shows the actual maximum residual stresses of series 1 that were computed for the two mixtures tested in this study (mW/mB= 0.22 and 0.40). The results show that with thicker steel rings the degree of restraint and the stress level were higher as compared with the thinner rings. The figures show the abrupt change of stress corresponding to age of cracking in the restrained ring specimens. The time the crack occurred coincided with the age of a visible crack in experiments. A higher rate of stress development was observed in the lower mW/mBmixture.Observing the specimens with thicker steel rings, we found that the crack appeared earlier than in specimens cast around thinner steel rings, despite having similar average stress.

Fig. 4 illustrates the influence of the mortar wall thickness on stress development in the test specimens that dried from the top and bottom. The degree of restraint is the same (the steel thickness was 19 mm). The thinner mortar wall showed an earlier age of cracking. With thicknesses of 37.5 mm, 75 mm, and 112.5 mm, respectively, the ages of cracking were 3.4 days,8.0 days, and 9.8 days with the mW/mB= 0.22 mixture; and 7.1 days, 12.6 days, and 16 days with the mW/mB= 0.40 mixture. The thicker rings had slightly higher maximum stresses. Despite of different mortar wall thicknesses, it can be noticed that there is no dramatic difference between the maximum stresses that develop in the mortar rings.

The age of cracking of restrained ring specimens for the two series are shown in Table 3.It can be seen that the ring specimens with thicker steel rings provide a higher degree of restraint, resulting in higher interface pressure and earlier cracking. With steel thickness of 6 mm, 19 mm, and 30 mm, the ages of cracking were, respectively, 12 days, 8 days, and 5.4 days with the mW/mB= 0.22 mixture; and 22.5 days, 12.6 days, and 7.1 days with the mW/mB= 0.40 mixture. Table 3 shows that the rings with a thicker mortar wall, cracked later. With the mW/mB=0.22 mixture, mortar wall thicknesses of 37.5 mm, 75 mm, and 112.5 mm cracked at 3.4 days,8.0 days, and 9.8 days, respectively; similarly, with the mW/mB= 0.40 mixture, the ages of cracking were 7.1 days, 12.6 days, and 16.0 days, respectively. The reason may be that the ring specimens have the same degree of restraint and different mortar wall thicknesses, so with the thicker mortar wall there are lower stresses in the concrete and a higher stress level is required to cause a crack in mortar ring specimens.

Table 3 Age of cracking of restrained ring specimens for two series

5 Conclusions

The figures of stress development in the restrained ring specimens show that there was an abrupt change in stress corresponding to the age of cracking in the specimens, which coincided with the age a visible crack was observed. A higher rate of stresses development was observed in the lower mW/mBmixture.

The thicker mortar rings had slightly higher maximum stresses. From the results we can see that with the decrease in the mW/mBmixture, the cracking happens earlier and the area of cracking is nearer to the inner surface of the mortar ring.

The average strain information from the strain gages attached to the interface of ring tests can be applied as an input for finite element modeling (FEM)analysis. Restrained ring tests using FEM can be used to provide quantitative information on early-age stress development and early-age cracking of the concrete.

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