PSI - Issue 30

A.A. Vasilieva et al. / Procedia Structural Integrity 30 (2020) 186–192

189

Vasilieva A.A. et al. / Structural Integrity Procedia 00 (2020) 000–000

4

with constant initial distribution of concentration and boundary conditions

( , 0) c x c

const, (0, ) ( , ) c t c l t

(2)

const.

 

  

0 

0

If ( ) M t denotes the amount of absorbed moisture

l

0 c x t dx M lc M l      0 0 ( , ) , , eq

(3)

M t

( )

,

0

then in terms of (3) the solution of the initial-boundary value problem (1), (2) has the form (Crank (1975))

    

     

 

  

2 exp (2 1)    

2 n D t l 2

 

) 1 8  

eq   

M t

( M M M

( )

.

(4)

0

0

2

2

(2 1) n 



n

0



eq D M ) model. The accurate evaluation of the ( ,

eq D M ) is critical

Thus, the Fickian model is two-parameter ( ,

to understand the physics of the absorption process and the related reliability issues. eq M determines the moisture absorption capacity of the specimen and D determines how fast the moisture could diffuse into of the specimen. The specimens of BFRP rebar have the shape of a cylinder (see Fig. 1a). In the case where the influence of volumetric factors can be neglected, the solution of the diffusion problem in the long cylinder (Crank (1975)) is not very different from the solution of the diffusion problem in a plane-parallel plate with constant boundary conditions (4) with the replacement l R  . 3. Results and Discussion Kinetics of moisture uptake experiment for BFRP rebar specimens that have been exposed for 54 months in the cold climate of the Northern Russia (“climatic specimens”) and that have been stored in warehouse (“warehouse specimens”) are shown in Fig. 2 and in Fig. 3. The effect of climate aging on moisture uptake of BFRP rebar revealed by comparing this kinetics. The specimens absorbed a small amount of moisture <0.35 %. The low level of moisture saturation did not lead to a swelling effect. In addition, with a high degree of the binder curing and low sorption capacity, chemical hydrolysis reactions are unlikely. The kinetics of the climatic and warehouse specimens are similar, with the exception of the end kinetics of the climatic specimens, where there is a decrease in the moisture weight gain as the result of loss of dehydrated polyamide yarns. The kinetics of the climatic specimens are monotonic. The kinetics of the warehouse specimens have a variation. When analyzing the experimental data, D and eq M are optimized so that the difference between the experimentally determined mass M and its calculated value obtained on the basis of equation (4) is minimized. The initial condition is determined in the form 0 0 c  in (2), since the specimens were previously dried. Fitted nonlinear regression parameters that ensured the convergence of an infinite series in (4) are listed in Table 1. This convergence means that our diffusion model adequately describes the kinetics of moisture uptake of the specimens.