The results of flexural and equivalent cube compressive strength tests are shown in Table . The flexural and compressive strength of mortar containing wood shavings decreased as the fine aggregates replacement increased. This decrease is attributed to the weaken bonding of mortar cement paste and wood shavings, as compared to the bonding of mortar cement paste and conventional aggregates.

Table 3: Mechanical properties test results.

It is shown that the resulting decrease in strength of mortar containing wood shavings is not attributed only on the impact of the replacement of fine aggregates with wood shavings. The resulting significant increase in the specific proportion of cement in the final mixture is expected to have a positive impact in the value of strength. Therefore the result of strength value decrease is the combination of the simultaneous and adverse impact of the two above phenomena. It seems that a decision on fine aggregate replacement by wood shavings should not only rely on a calculation according to the volumes of these two materials as they appear before mixing. This calculation should be according to the apparent volume of each mixture constituent volume as the conditions in which this appears within the mixture.

As it is shown in Figure 6, in all cases, compressive strength of mortar containing ayous shavings was found greater than compressive strength of mortar containing beech shavings. The mean difference calculated from 12 groups of six specimens, each, having the same value for fine aggregates fraction and specimen age, was found to be 20±7%. This finding has no significant statistical relation to either the values of fine aggregate replacement fraction (Pearson r=0.255, significance p=0.423) or the values of specimen age (Pearson r=0.217, significance p=0.498).

Figure 6: Comparison of compressive strength test results from groups of six specimens with specified fine aggregates fraction and specimen age (each group corresponding to one figure point).

According to the experimental results and following an equation based on one initially proposed by Freiesleben Hansen and Pedersen [18], compressive strength is given as a function of fine aggregate replacement fraction (*W*) and specimen age (*t*) by Eq. (5):

$$CS\mathrm{(}t,\text{\hspace{0.17em}}W\mathrm{)}=\mathrm{(}C{S}_{\infty ,1}-\text{\hspace{0.17em}}k{W}^{n}\mathrm{)}\mathrm{exp}[-{\mathrm{(}\tau /t\mathrm{)}}^{a}]$$(5)

where *CS*(*t*, *W*) is the compressive strength at age *t* (days) when fine aggregates replacement fraction is *W*, *CS*_{∞,1} is the limiting compressive strength for the reference (maximum asymptotic value of the strength for the function that fits the data), *n* is a shape parameter for the function of compressive strength when fine aggregates replacement fraction is *W*, *k* is compressive strength reduction parameter such that *kW*^{n} equals to the reduction of the specimen limiting strength due to a fine aggregate replacement equal to *W*, *τ* is a time constant and *a* is a shape parameter for the sigmoidal function of compressive strength with specimen age *t*, *CS*_{∞,1}–*kW*^{n} corresponds to the limiting compressive strength of a specimen with a fine aggregate replacement fraction equal to *W*.

This means that for a given specimen age the relation between compressive strength and fine aggregate replacement is a function of fine aggregate replacement fraction to a power *n* (Figure 7A). Simultaneously, for a given fine aggregates replacement fraction, compressive strength is a function of age, which corresponds to a sigmoidal curve (Figure 7B).

Figure 7: (A) Compressive strength versus fine aggregates replacement fraction, (B) compressive strength versus specimen age.

The regression procedure using Eq. (2) based on the experimental results of the present study provided a statistically significant model (Pearson r=0.96) with parameter values: *CS*_{∞,1}=74±3 MPa, *k*=55±4 MPa, *n*=0.8±0.1, *a*=0.7±0.2 and *τ*=3.1±0.6 days.

In Eq. (5) the parameter of wood shaving type was not investigated, although the statistical significance of that result was satisfying enough in order to be used as a general model for predicting the loss of limiting strength when any kind of wood shaving is used for fine aggregate replacement. Making one step more, the parameter of wood shaving type was introduced in Eq. (5), forming Eq. (6):

$$CS\mathrm{(}t,\text{\hspace{0.17em}}W\mathrm{)}=[C{S}_{\infty ,2}-\mathrm{(}{k}_{1}{m}_{1}+{k}_{2}{m}_{2}\mathrm{)}{W}^{n}]\mathrm{exp}[-{\mathrm{(}\tau /t\mathrm{)}}^{a}]$$(6)

where *m*_{1}, *m*_{2} equal to one when wood shaving type is ayous or beech, respectively, otherwise each equal to zero. The combination *m*_{1}=0 and *m*_{2}=0 corresponds to the case of reference specimens (no use of wood shavings). *k*_{1} and *k*_{2} are shape parameters, similar with *k* in Eq. (5).

This equation has been tried for only one type of wood per mixture and not for two types together in the same mortar mixture. When two or more types of wood shavings are to be used simultaneously in the same mortar mixture, then the use of Eq. (5) is suggested, but it is also suggested that further studies should be done for multiple kind of wood shavings in the same mortar mixture, mainly in order to investigate the significance in which this factor contributes to the uncertainty of Eq. (5) parameters. Any combination of *m*_{1} or *m*_{2} other than values of 0 and 1 has not been studied and it is suggested for future investigation.

The regression procedure using Eq. (6) based on the experimental results of the present study provided a statistically significant model (Pearson r=0.976) with parameter values: *CS*_{∞,2}=74±3 MPa, *k*_{1}=48±3 MPa, *k*_{2}=60±3 MPa, *n*=0.76±0.07, *a*=0.7±0.1 and *τ*=3.1±0.5 days (Figure 8).

Figure 8: Compressive strength versus fine aggregates replacement fraction (A) only for ayous wood shavings and (B) only for beech wood shavings.

The results of UPV tests are shown in Figure 9.

Figure 9: Ultrasonic pulse velocity versus fine aggregates replacement fraction.

UPV linearly decreases as fine aggregates replacement fraction increases. This is attributed to the different properties of wood compared to the properties of conventional fine aggregates. The importance of UPV is that it is significantly correlated with mortar elastic properties. A regression model was applied to the experimental data using Eq. (7):

$$\begin{array}{l}\text{UPV}\mathrm{(}t,\text{\hspace{0.17em}}W\mathrm{)}=[{\text{UPV}}_{\infty}+\mathrm{(}{l}_{1}{m}_{1}+{l}_{2}{m}_{2}\mathrm{)}W]\\ \text{}[1-\mathrm{exp}\mathrm{(}-t/{t}_{0}\mathrm{)}]\end{array}$$(7)

where UPV_{∞} is the limiting UPV for the reference which is the maximum asymptotic value of UPV for the function that fits the data, UPV_{∞} ·[1−exp(−*t*/*t*_{0})] is the UPV of the reference (*W*=0) for the specified age of curing (*t*), *m*_{1} and *m*_{2} equal to one when wood shaving type is ayous or beech, respectively, otherwise equal to zero, *l*_{1}, *l*_{2} are shape parameters and *t*_{0} is a time constant.

The regression procedure using Eq. (4) based on the experimental results of the present study for UPV provided a statistically significant model (Pearson r=0.981) with parameter values UPV_{∞} =(5.33±0.08)·10^{3} m/s, *l*_{1}=(−1.73±0.17)·10^{3} m/s, *l*_{2}=(−2.18±0.16)·10^{3} m/s, *t*_{0}=11.4±0.7 days.

Observation by means of a stereoscope shows a homogeneous mixture in which wood shavings are well wrapped (Figure 10).

Figure 10: Stereoscope images of mortar with (A) 70% by volume replacement of fine aggregates by ayous shavings and (B) 20% by volume replacement of fine aggregates by beech shavings.

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