PSI - Issue 45

Yipu Guo et al. / Procedia Structural Integrity 45 (2023) 66–73

69

4

Yipu Guo et al. / Structural Integrity Procedia 00 (2019) 000 – 000

3.2 Peak strain As shown in Fig. 2 (a), the peak strain ( ) increases with the increasing lateral confining pressure, which is mainly attributed to two aspects. On the one hand, the constraint effect of lateral confining pressure increases the elastic interval of the specimen. On the other hand, plastic deformation is also increased owing to the continuous squeeze of internal pores and mortar and the development of microcracks (Chen et al. 2022). Fig. 2 (b) compares the peak strain of Multi-RAC after sequential recycling cycles. Generally, RACI and RACII show the obvious larger peak strain, whereas RACIII has a slightly larger peak strain than the counterpart of NAC, respectively, except for the RACII under 5 MPa confining pressure. Several mechanisms may coexist that are responsible for the increased peak strain for Multi-RAC. Firstly, Multi-RA is formed by both fragments of natural aggregate and adhered old mortar where the volume fraction of adhered mortar incrementally increases as the increased number of recycling cycles (Silva et al., 2021). The increased volume of adhered mortar will improve the porosity of the corresponding concrete and thus increase its deformation capacity. The angularity and surface roughness of RA particles lead to greater friction force and aggregate interlock force during the crack development of RAC, accordingly improving its deformation capacity (Chen et al., 2022). In terms of Multi-RAC, the angularity and surface roughness of Multi-RA were reported to be increased with the increasing number of recycling cycles (Zhu et al., 2016). With the increase of confining pressure, the aggregate locking stress is further increased, resulting in the gradual increase of deformation ability (Chen et al., 2022). According to the widely accepted aggregate interlocking model proposed by Walraven (Walraven, 1980), the aggregate interlocking action should be closely correlated to the natural aggregate fraction in Multi-RCA. After 3 sequential recycling cycles, the volume fraction of natural aggregate will be critically reduced, typically less than 30% based on previous studies (Thomas et al., 2020; Zhu et al., 2016). The projected contact areas between mortar and natural aggregates will be reduced as a result of the critically reduced volume of natural aggregate in Multi-RCA, therefore weakening the aggregate interlocking action and impeding the deformation ability of the specimen. This should explain why the peak strain of RACIII is relatively smaller than the counterparts of RACI and RACII.

6

30

NAC RACI RACII RACIII

NAC RACII

RACI RACIII

5

25

20

4

e v (10 -3 )

15

e v /e 0

3

10

2

5

1

0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0

0

5

10

15

20

s w (MPa)

s w /¦ co

(a) Relationship between peak strain and confining pressure

(b) Peak strain variation

Fig. 2. Correlation between peak strain and confining pressure.

3.3 Elastic modulus The variations of elastic modulus in terms of increasing lateral confining pressure for NAC and 3 generations of Multi-RAC is illustrated in Fig. 3. Generally, the elastic modulus firstly increases and then decreases. Under the small confining pressure, the elastic modulus increases since the lateral confining pressure hinder the initiation and propagation of longitudinal cracks. However, when the specimen is subjected to larger confining pressure, the internal damages start to accumulate due to excessive triaxial pressure during the loading stage one (hydrostatic state) before