**1. Introduction**

The recycling of construction waste (RCW) refers to the crushing of construction waste to obtain di fferent types of products and reuse them as resources. An important product of RCW is recycled aggregate, which itself is attached to old mortar that is di fficult to peel due to the limitation of crushing technology. Therefore, it can be approximated that recycled aggregate is a two-phase material composed of the old mortar and the natural aggregate wrapped therewith. Recycled aggregate concrete (RAC) refers to recycled aggregate, which is made by crushing and classifying waste concrete and mixing according to a certain proportion, and partially or completely replacing the natural aggregate to make new concrete referred to as RAC [1]. RAC not only digests waste concrete and solves the problem of a serious shortage of natural resources, but also meets the requirements of existing specifications under reasonable design. It is a green building material worthy of promotion. At present, domestic

and foreign countries have given eager attention and long-term exploration of RAC, especially towards the shrinkage and creep of the concrete, which can cause structural deterioration and safety problems. Based on a number of research reports on concrete shrinkage and creep [2–4], the effects of concrete shrinkage and creep mainly include the increase of beam deflection and the reduction of structural bearing capacity. In addition, the shrinkage and creep of the concrete can also cause prestressing loss of the prestressed members and secondary internal forces that create the structure. In the local area of the concrete, due to the shrinkage of internal stress, it is easy for problems such as cracks on the outer surface of the member to be created. In view of the fact that RAC has properties close to that of ordinary concrete [5–12], the structural adverse effects caused by shrinkage and creep are similar or even more serious than in natural concrete. Therefore, the research on the shrinkage and creep of RAC has an irreplaceable significance.

Many scholars have conducted relevant research on the importance of the shrinkage and creep of RAC. Domingo-Cabo et al. [13] studied the shrinkage and creep tests of RAC with different substitution rates. The results show that with the increase of the replacement rate of recycled aggregates, the shrinkage and creep deformation of RAC increases. However, there is a lack of in-depth exploration of the impact of factors. Geng et al. [14] conducted experimental research on the creep behavior of RAC with different water–cement ratios of base concrete. The results show that the creep properties of recycled base concrete with a low water–cement ratio are significantly affected by recycled coarse aggregate (RCA), and the establishment of a prediction model for the creep of recycled base concrete needs to consider the influence of the water–cement ratio. However, the effects of different recycled aggregate-attached mortars on shrinkage and creep have not been deeply considered. Adam et al. [15] studied the shrinkage and creep of RAC based on three factors: curing condition, loading age, and axial stress level. The experimental results show that the corresponding average ratios of the shrinkage strain of RAC of a 50% and 100% substitution rate are 1.21 and 1.71, compared to ordinary concrete. Guo et al. [16] devoted himself to the study of the effect of old adhesive mortar on the creep of RAC. Based on the experimental data, it is proposed that the RAC with a 100% substitution rate develops more rapidly and the amplitude is 1.6 times larger than that of traditional concrete. Miguel et al. [17] explored the effect of recycled aggregates from Portuguese construction and demolition waste on the shrinkage and creep properties of concrete. The results showed that the creep coefficient of the 12 recycled aggregates tested at 91 days was 0.22 to 1.63. This proves that the shrinkage prediction model of ordinary concrete is not suitable for RAC, so research on a prediction model of the shrinkage and creep of RAC has become the focus of solving the problem of the shrinkage and creep of RAC.

Fathifazl et al. [18] pointed out that the prediction model of RAC creep should consider the influence of mortar attached to the aggregate surface. Based on the MC90 model, the predicted values of the model agree well with the experimental values. Brito et al. [19] studied the Gómez-Soberón test data, and the concrete creep coefficient was corrected considering the difference in apparent density or water absorption between recycled aggregate and natural aggregate. Based on the European standard EC2 model, Brito (D) and Brito (R) obtained two kinds of RAC creep prediction models. The prediction accuracy of the two models is good. Tošic et al. [20] explored the effect of recycled aggregates on the creep of concrete. It was pointed out that when using the creep prediction model of fib Model Code 2010 to predict the creep coefficient of RAC, the creep coefficient of RAC is underestimated relative to the performance of the model on the accompanying natural aggregate concrete (NAC). At the same time, compared with NAC, RAC shows a larger creep coefficient, and the average increase in the coefficient of creep is 39% for RAC at full replacement rate. Silva et al. [21] studied the correction coefficient of the concrete creep of different recycled aggregates and a prediction model suitable for recycled aggregate concrete. Compared with conventional concrete, the use of recycled aggregate to absorb part of the mixed water and cement slurry for coating produces a stronger ITZ (Interface transition zone), which can reduce the creep strain by up to 23%. Liu et al. [22] explored the calculation method of the long-term deformation of recycled concrete beams based on the creep adjustment coefficient. At the

same time, the adjustment coe fficient of the mortar creep bond was proposed. Three typical ordinary concrete shrinkage and creep prediction models were modified and used for long-term deformation calculation of recycled concrete beams. Luo et al. [23] proposed a model for the shrinkage and creep of RAC that considered the two factors of the regenerated aggregate grade and substitution rate, and the regression coe fficient was used to obtain the influence coe fficient expression. This can quantitatively calculate the influence coe fficient of the shrinkage and creep of recycled aggregates with di fferent quality and di fferent substitution rates. Luo et al. [24] studied the influence coe fficient of aggregate pretreatment on the creep behavior of RAC. Xiao et al. [25] corrected the q2 and q4 parameters in the B3 model based on the test results. Considering the influence of the replacement rate of the RCA, the calculated results of the modified model were in good agreemen<sup>t</sup> with the experimental results.

However, the existing method of proposing the correction coe fficient by a certain influencing factor fails to establish the prediction model of the shrinkage and creep of RAC. In particular, for the e ffect of recycled aggregate-attached mortar on concrete shrinkage and creep, there is a lack of predictive models with universal significance. In this paper, the key di fference between recycled aggregate and natural aggregate-attachment mortar is taken as the entry point, and the coe fficient of increase and the coe fficient of contraction increase of the attached mortar are proposed. Based on the two increasing factors, the shrinkage and creep model of RAC is established, and the applicability of the model is checked.

#### **2. Shrinkage and Creep Test of Recycled Aggregate Concrete**
