Question

According to Turkish Standards; How to calculate Shrinkage and Shrinkage of Concrete? (Can be viewed from TS500)
Please explain by solving one numerical sample.

Answer 1

There are various methods According to Turkish Standards to calculate Shrinkage and Shrinkage of concrete are as follows :

1. Drying Shrinkage and Weight Loss

2.Restrained Shrinkage Cracking

and many more methods.....

1. Drying Shrinkage and Weight Loss :

In order to observe the drying shrinkage and weight loss of the SCCs, three 70 × 70 × 280 mm prisms according to ASTM C157 were used as shown in Figure . As soon as the prisms are demoulded, the gage length was fixed on each specimen by the means of the glued pins on the face of prisms. The length change was measured via a dial gage extensometer with 200 mm gage length and 0.002 strain for measuring. Measurements were implemented for the first 3 weeks as every 24 h and then 3 times a week. Meanwhile, measurements of weight loss were operated on the identical prism, too. After the gage length was fixed on each specimen via the glued pins on the face of prisms, its initial weight was recorded to monitor the weight loss during drying period. Then, the prisms specimens were subject to drying at 23 ± 2°C and 50 ± 5% relative humidity for about 56 days. The test results for each property were evaluated by averaging the measurement of three prism specimens

Result :

Drying shrinkage can be defined as the volumetric change due to the drying of concrete. Firstly, the loss of free water occurs, which causes little to no shrinkage. As the drying of the concrete continues, the adsorbed water is taken away. This adsorbed water is held by hydrostatic tension in the small capillaries (<50 nm). The loss of this water produces tensile stresses, which cause the concrete to shrink. The shrinkage due to this water loss is significantly larger than that associated with the loss of free water. Drying shrinkage is a long-lasting process that depends on w/c, degree of hydration, curing temperature, relative humidity, duration of drying, aggregate properties, admixture, and cement composition [31, 32, 42]. The typical drying shrinkage versus time curves for SCCs incorporating with RAs are depicted in Figures 12(a), 12(b), 12(c), and 12(d) for NAs (Series I), RCA (Series II), RFA (Series III), and RAs (Series IV) concretes, respectively. Also, the maximum drying shrinkage deformation of SCCs at the end of the 56 days drying period

It can be noted that drying shrinkage rate reduced gradually with the time passed for all mixtures. However, Figure 12 showed that the presence of RAs in concrete led to increasing shrinkage. According to the curves of Series IV, Series III, and Series II, drying shrinkage of concretes containing RCA and/or RFA decreased in this order, but all of the concretes showed higher shrinkage than the control mixtures (Series I). As seen in Figure 12, the lowest shrinkage strains of 315 microstrains at 56 days were measured for 0.3RCA0RFA0SF10 while 0.43RCA100RFA100SF0 had the highest shrinkage value which was found to be 727 microstrains. The high rate of drying shrinkage in RAs concrete should have been connected with the fact that the adhering mortar on the surface of the recycled aggregate, which normally contributes to a considerable absorption of water, could have contributed to the high drying shrinkage. Similar test results were observed by other researchers such that the high absorption as well as the lower modulus of elasticity of RAs was accompanied by the higher shrinkage strain in concrete as verified from the increase in drying shrinkage with increasing volume of RAs in SCCs [34, 42–45]. In addition, SCCs containing RAs were prone to having higher shrinkage strain due to their lower tensile strength with respect to reference mixes. Another factor that may have affected the shrinkage of SCCs is the absorption ability of the RAs. A higher absorption value indicates a higher percent of aggregate voids filled with water, which may lead to an increase in drying shrinkage. Aggregates with high absorption properties are associated with high shrinkage strain in concrete, and this is confirmed by the increase of drying shrinkage with the utilization of RAs and the increase of w/b ratio [42]. From the results of the properties of the RAs, the water absorption of the RAs was about four times more than that of NAs; this therefore mostly caused a higher drying shrinkage observed in the concrete containing RAs. However, it is well known that the self-desiccation increases with decreasing w/b ratio, and this will cause an increase in the total shrinkage. With increasing the degree of hydration, products of hydrates fill the pore spaces and the amount of movable water decreases; consequently, the amount of drying shrinkage will decrease [46]. For instance, there was an increase in 56-day drying shrinkage by as high as 47%, 54%, 56%, and 53% for 0.3RCA100RFA100SF0, 0.3RCA100RFA100SF10, 0.43RCA100RFA100SF0, and 0.43RCA100RFA100SF10 mixes, respectively, compared to SCCs incorporating NAs (Series I). Moreover, the internal curing effect of RAs as used in SSD condition is to decrease the early age drying shrinkage for SCCs when compared to conventional RAs concretes [12].

It is evident in Figure 12 that the effects of replacing the cement by GGBFS and SF were to reduce the free shrinkage of SCCs remarkably at a low w/b ratio. When the shrinkage strains measured at 56 days were considered, the shrinkage of the concrete with ternary use of cement + GGBFS + SF exhibited the highest reduction in comparison to the SCCs without incorporating SF. For instance, drying shrinkage measurement of 0.3RCA0RFA0SF10 was 24.1% lower than that of 0.3RCA0RFA0SF0. The test results agreed with the findings of the study conducted by Li and Yao which showed that the ultrafine mineral admixtures like GGBFS and SF can substantially promote hydration of cement and increase the amount of crystal hydrates and C-S-H gel hydrates in cement paste, which offers a hardened concrete with a stronger structure and higher resistance to deformation caused by applied forces. Moreover, this ultrafine material may fill small pores and voids harmful to the structure of concrete. That might be the mechanism of reducing effect of ultrafine mineral admixtures on drying shrinkage of concrete. Güneyisi et al. [16] reached the results that SF modified concretes exhibited a lower shrinkage in comparison to the plain concretes. Moreover, the drying shrinkage rates of the concretes had a decreasing tendency with passing drying time, particularly for the SF concretes.

The results of weight loss with time due to the drying for SCCs , and 13(d) for Series I to IV, respectively. Additionally, Table 6 gives the maximum weight loss values of SCCs. Similar to drying shrinkage test results, SCCs incorporated with RAs exhibited a higher weight loss in comparison with the control mixture regardless of the size of RAs. During 56 days of drying period, the difference of weight loss between SCCs mixtures became more distinguishable after one week. The differences were then observed to have an increasing tendency with increasing drying time. However, for the same mix specification, higher amount of RAs as fine and/or coarse grade gave rise to greater water loss inasmuch as the RAs were used in SSD condition, which in turn increased the unit water content. As a result, the increased weight loss led to a higher total drying shrinkage of SCCs.

2. Restrained Shrinkage Cracking :

Concrete is expected to crack whenever the tensile stress induced by the constraint for the free shrinkage surpasses its tensile strength. The crack developments and the shrinkage cracking age of the restrained shrinkage specimens are shown in Figure 15 while the maximum crack width of SCCs

there was a marked effect of aggregate type on the restrained shrinkage cracking performance of SCCs. For all 16 SCCs mixes, there were small differences for cracking initiate time. It can be seen from Figure 15 that initial cracking of the specimen was observed at the 6th to 10th days for control mixes (Series I); however, the crack initialization of Series IV concrete was observed to occur between the 10th and 13th days. Despite the fact that Series IV mixes had a higher free shrinkage and lower tensile strength, the cracking time extended for approximately 3 to 4 days. RAs were used in SSD condition and this provided water into the drying matrix at the very early ages; thus the cracking time would be extended by reducing the autogenous shrinkage [42]. Water within the prewetted RA is preferred to be drawn from the RA to the surrounding paste because of the larger pore sizes in the RA relative to those in the hydrating cement paste and the emptying of these larger pores produces a much lower capillary stress, which reduces both the measured strain and the propensity for early-age cracking [46, 47]. Furthermore, the lower elastic moduli of SCCs with RAs helped in extending the cracking time with respect to SCCs with NAs.

Though the cracking time extended, the crack opening for SCCs with RAs was faster in the first day due to the weakness of RAs. The lowest crack propagation was observed for 0.3RCA0RFA0SF10 mix as 1.25 mm, irrespective of drying time. The maximum crack width at the end of 56 days was found to be 2.4 mm for mix 0.43RCA100RFA100SF0 in Series IV. On the other hand, considering the overall drying period, SCCs incorporating RFAs (Series III) exhibited higher crack propagation with respect to Series I and II, respectively. For example, utilizing RAs as a fine grade resulted in a crack width ranging from 1.3 to 2.1 mm depending on the w/b ratio and incorporating of SF while these values increased to the range of 1.7 to 2.4 mm for fine and coarse grade (Series IV). The increment observed in the cracking width for SCCs with RAs mostly resulted from the increase in free shrinkage and the decrease in both tensile strength and modulus of elasticity. However, in the present study, restrained shrinkage of SCCs was also affected by the paste parameters like w/b ratio and microfine particles incorporated like SF. For instance, increasing the w/b ratio from 0.3 to 0.43 caused an increase in the restrained shrinkage crack width values by 28.5% for 0.3RCA0RFA0SF0 compared with 0.43RCA0RFA0SF0 mix. On the other hand, the incorporation of SF decreased the width of restrained shrinkage crack values by about 12% for 0.3RCA0RFA0SF0 compared with 0.3RCA0RFA0SF10. The trend of crack width decrement was valid for other SCCs mixes. The final crack widths of SF concretes were less than those of control concrete

As shown in Figure 14, irrespective of the aggregate type, w/b ratio, and incorporation or not of SF, the crack due to restrained shrinkage of SCCs exhibited a good linear correlation (approximately 77% regression factor) with compressive strength as for drying shrinkage.