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Figure 4.26 a, b and c shows the expansion of the OPC, AAS and AAFA concrete prisms exposed to

38ºC. In each figure four curves can be seen, representing different conditions in which the concrete bars were subjected. One set of bars were kept at 23ºC and 100% RH chamber all the test (represented as 23ºC in the figure). Other set of bars were cured according to ASTM C1293 standard (after demolding, the prisms were placed in a sealed container at 38ºC and 100% RH) (represented as 38ºC/1D in the figure). In the last conditions, the prisms were kept in a humidity chamber at 23ºC during 28 days and 90 days respectively prior to put them in a sealed container at 38ºC (represented as 38ºC/28D and 38ºC/90D respectively in the figure). In these special cases, the zero time is when the prisms are placed in the sealed container at 38ºC and 100% RH, and it has already an initial value showing the expansion that occurred during the period in the humidity chamber prior to place them at 38ºC.

128 For the OPC concrete prisms (figure 4.26 a) it can be observed that the expansion of the prisms kept the whole time in a humidity chamber at 23ºC is practically constant until 300 days. Instead, it is observed a shrinkage between 100 and 300 days. At 300 days, it seems that the prisms start to expand, although more data is needed to confirm that trend. The prisms that followed the standard conditions (38ºC/1D in the figure) experimented an expansion after 50 days, exceeding the limit compliance of the standard (0.04%) after 100 days. Afterwards, the expansion was practically constant until 360 days. For the prisms that were put in the sealed container at 38ºC after a 28 and 90 days period in a humidity chamber (38ºC/28D and 38ºC/90D in the figure), it can be observed that the expansion is lower than those put at 38ºC after demolding. The later the prisms were put at 38ºC, the lower the expansion was. Nevertheless, in both cases the limit compliance was exceeded.

In the case of the AAS concrete prisms (figure 4.26 b) none of the set of prisms exceeded the limit compliance after 360 days. Instead, it can be seen that all the sets of prisms suffered a shrinkage during the first 2 months approximately. The lowest shrinkage was found for the prisms kept in a humidity chamber at 23ºC all the time. After this age, the latter prisms still continued with a shrinkage trend. However, the prisms that were at 38ºC started to expand after the first two months. It can be seen that the prisms with higher expansion were the prisms put in the sealed container at 38ºC after demolding, with a total of 0.01%. Then it was found that the prisms kept during 90 days in a humidity chamber at 23ºC expanded again to kept around 0.00% (still in shrinkage values) and those kept during 28 days expanded to reach around -0.01% (again still in shrinkage values).

In the case of AAFA concrete prisms (figure 4.26 c) it can be seen a shrinkage during all the test (360 days), with some minor fluctuations. Two different periods can be observed. The first period around 60 to 100 days, in which all the prisms experimented a low shrinkage, and the second period afterwards in which there was a fast shrinkage. In all the conditions, the prisms seem to stabilize around 275 days. It has been found the same shrinkage for the prisms kept the whole time in a humidity chamber at 23ºC and for the prisms put in the sealed container at 38C after demolding. Hence, both set of prisms got - 0.03% of expansion. For the other set of prisms, the shrinkage was the same, being of -0.04% of expansion.

129

a) OPC (1.25% Na2O) b) AAS (5.00% Na2O)

c) AAFA (7.00% Na2O)

Figure 4.26 Expansion of OPC (a), AAS (b) and AAFA (c) concrete prisms exposed at different conditions: at 23ºC/100% RH all the time; at 38ºC/100%RH (ASTM C1293) after demolding, after 28

days and after 90 days at 23ºC/100% RH.

4.5.3.2 Mass change

The mass change of the same prisms used to follow the expansion was also measured. The results can be seen in figure 4.27. As in the case of the expansion, the zero value represents the point in which the prisms were placed at the test conditions, i.e. sealed container at 38ºC (less the set of prisms kept all the time at 23ºC).

In the case of the OPC concrete prisms (figure 4.27 a), all the set of prisms experimented an increase of the mass during time. The prisms that were kept in a humidity chamber at 23ºC the whole time experimented a higher increase. The prisms put in the sealed container at 38ºC after demolding experimented a more progressive increase of the mass, being quite similar to that obtained by the prisms in a humidity chamber after 360 days. In the case of the prisms kept during 28 and 90 days in a humidity

130 chamber, it can be seen that at 0 days (when the prims were put at 38C) the mass has already increased. This, as explained before, is the mass change of the prisms during the period in a humidity chamber. Thus, it can be seen that in both cases, the prisms suffered a mass loss at the beginning of the conditions in the sealed container at 38ºC. This is logic because at these new conditions the humidity is lower than that of the humidity chamber (contact with fog), and the prisms experienced an initial mass loss. Afterwards, there was an increase in both conditions.

In the case of AAS, all the sets of prisms experimented the same mass gain after 360 days. The mass gain was around 0.6-0.7%.

a) OPC (1.25% Na2O) b) AAS (5.00% Na2O)

c) AAFA (7.00% Na2O)

Figure 4.27 Mass change of OPC (a), AAS (b) and AAFA (c) concrete prisms exposed at different conditions: at 23ºC/100% RH the whole time; at 38ºC/100%RH (ASTM C1293) after demolding, after

28 days and after 90 days at 23ºC/100% RH.

Finally, for the AAFA concrete prisms, different behaviors can be observed, although in all the cases there was a mass gain. Thus in all the cases, the mass increased. These mass increases were produced

131 during the first two weeks, and afterwards remained practically constant. The prisms with higher mass increase were those kept the whole time in a humidity chamber at 23ºC, with 1.2%. The set of prisms that followed the ASTM C1293 standard conditions (38ºC after demolding), experimented a mass gain after 360 days close to the formers, with 1.0%. In the case of the set of prisms with a pre-conditioning period in a humidity chamber prior to expose at 38ºC conditions (38ºC/28D and 38ºC/90D in the figure), it is observed a mass loss during the first months in the sealed container at 38ºC, as observed with the OPC concrete prisms. As explained before, in the new conditions at 38ºC, the prisms have lost part of the initial humidity as these conditions are different to those in the humidity chamber. After the first month, it can be observed a mass loss trend in the case of the 38ºC/28D prisms that became constant after 200 days. For the 38ºC/90D prisms, the mass kept constant all the test after the first month.

4.5.3.3 Discussion

In this experiment, ASR was performed by means of the concrete bar test method described in the ASTM C1293 standard for OPC, AAS and AAFA. In this case, a siliceous-dolomite limestone, considered as highly reactive with alkalis (Spratt), has been used as the coarse aggregate. Normal silica sand, considered to be non-reactive, has been used as the fine aggregate of the concretes.

In this test, not only the susceptibility to ASR of different systems (with very big differences on the alkali content) have been studied but the test conditions in which the experiment was carried out too. Thus, the effect of the curing time prior to subject the concrete prisms to 38ºC (according to ASTM C1293 standard) has been also analyzed.

As shown in the results, the OPC concretes prisms have shown to exceed the standard limit compliance of 0.04% at 360 days in all the situations in which the prisms were subjected to 38ºC conditions. The prisms which were kept all time at 23ºC did not expand as fast as the others, as has been already demonstrated for OPC (Wigum et al. 2006). In this system, it has been found that temperature of the test conditions had a main role in the expansion rate and extent (Wigum et al. 2006). At ambient temperature, it has been found lower expansion rate, but higher expansion extent during years. In this study, it can be seen that after one year (the end of the test), the expansion is lower comparing with prisms subjected to 38ºC. Hence, the results of this test are in line with literature, showing a low expansion for the prisms kept at 23ºC after 1 year.

Similar trend was observed for AAS. But in this case, none of the studied concrete prisms exceeded the compliance limit of 0.04%. For this system, it was found that the prisms kept at normal temperature all time, i.e. 23ºC, experimented a shrinkage rather than an expansion. It can be seen that at the end of the test according to standard, one year, the prisms continued to shrink. It has already been found in other

132 studies that AAS systems have tendency to shrink, even in humidity conditions (Duran Atiş et al. 2009; Chi et al. 2012; Ye & Radlinska 2016). In the other cases studies, in which prisms were subjected to 38ºC, it was found that as the prisms were kept longer at 23ºC prior to place them at 38ºC, the expansion at 1 year was lower. However, prisms cured during 90 days at 23ºC got higher expansion than those cured during 28 days at the same conditions. This could be due to the higher shrinkage that this prisms suffered during the first month at 38ºC in the sealed container. It has to be pointed out that there is a direct effect of changing the conditions of the prisms from the humidity chamber at 23ºC to the sealed container. This change can produce a partial desiccation of the samples prior to stabilize to the new conditions, producing this shrinkage. This effect was also observed in a lesser extent in the OPC prisms. According to this results, it is shown again, as it was found in the OPC system, that the curing time at normal temperature prior to subject the prisms at 38ºC is beneficial in terms of the expansion extent observed at one year. This demonstrate that following the ASTM C1293 standard procedure, higher expansion would be observed at one year. This will confirm that alkalis in pore solution continue to be fixed in the structure with curing time, decreasing the allowable alkalis to participate in the ASR. However, the same time as alkalis are fixed to the structure during time, the natural shrinkage of these systems would balance the effect of the expansion by ASR.

Finally, in the case of the AAFA system, different behavior that those found in the OPC and AAS was observed. For this system, it was not found any sign of expansion but a high shrinkage. Despite the test conditions used (in terms of curing time and temperature), in all cases it was found similar shrinkage trend and extent. In the case of the prisms that were subjected to 38ºC after a period in humidity conditions, the shrinkage extent was little higher.

An interesting point to note comparing the three systems is that, as happened in the accelerated ASR test method in mortar (section 4.5.2), the OPC concrete prisms got the highest expansion despite its lowest alkali content (1.25% of Na2Oeq comparing to 5.00% of AAS and 7.10% of AAFA).

Furthermore, in the case of AAFA, there was a shrinkage instead, that stabilize after 200 days.

4.5.3.4 Conclusions

The following conclusions can be drawn from the results of ASR by means of the concrete bar test method (ASTM C1293):

- OPC concrete prisms experimented higher expansion than the AAS and AAFA concretes prisms, despite the higher alkali content of the last.

133 - OPC concrete prisms got the higher expansion when they were subjected to 38ºC after demolding (ASTM C1293 procedure). Keeping the prisms in humidity conditions at 23ºC showed to be beneficial for the expansion extent after 1 year.

- AAS concrete prisms passed the test in all the curing regimes studied. The worst condition was when the prisms were subjected to 38ºC after demolding and the best when the prisms were kept all time at 23ºC. However, it was found shrinkage in all cases at the beginning of the test, and during all the test for the prisms kept all time at 23ºC. This shrinkage could be partly responsible of the lower expansion of AAS comparing to OPC.

- AAS concretes cured longer at 23ºC before being subjected to 38ºC showed to have lower expansion than the prisms that were exposed at this temperature after demolding. This could be due to a higher fixation of alkalis in the microstructure, and thus, less free alkalis are allowed to participate in the ASR.

- AAFA concretes showed in all the cases a shrinkage instead an expansion. Thus, this systems are very favorable for preventing ASR.