C OMPOSITION OF THE CONCRETE PORE SOLUTION

The test set up and the test procedures must also aim to avoid loss of water during storage and measuring. Important parameters in this respect are quality control (e.g. always use water tight lids), strict measuring procedures (measure quickly with as little moisture loss as possible), pre-cooling or not before measuring (the prisms will dry during cooling because moisture will move from the warmer inner part to the colder outer parts) and storage temperature (the higher storage temperature, the more drying during cooling). As a quality control, the mass of prisms should always be measured, evaluated and reported.

If the mass increase during the test is too low, the test results should be questioned.

One important question has to be asked: What is the most correct “reference length” to apply in ASR testing; the length after de-moulding, the shortest length after some shrinkage has occurred or the length after a pre-curing period? The magnitude of the irreversible shrinkage will also influence this question.

7.2 Composition of the concrete pore solution Available alkalis

The content of alkalis, i.e. sodium (Na⁺) and potassium (K⁺), in the concrete pore solution plays a major role for development of ASR. An increased alkali content leads to disso-lution of more hydroxyl ions (OH^-) from Ca(OH)2 and the pH of the pore solution in-creases. This higher alkalinity subsequently leads to dissolution of more reactive silica (SiO2) from alkali reactive aggregates.

It has become standard practice to express the alkali content in terms of "sodium oxide equivalent" (Na2O_eq). There is, however, some discrepancy in the literature whether cements having a similar Na2O-equivalent, but different K/Na ratios, perform differently regarding ASR. This may be of importance if reference CEM I cements are selected for laboratory performance testing.

As the degree of reaction is a function of the alkalinity of the pore solution, a high degree of alkali leaching during laboratory testing will lead to a poor laboratory/field correlation.

The reported fixation of alkalis, alkalis not available for alkali leaching, should be looked into further, along with the potential enrichment and concentration of alkalis in concrete structures. The issue of recycling of alkalis in the concrete should also be investigated further, as well as the main question of how to overcome the problem with alkali leaching. The feasibility of accelerated laboratory tests may depend on this.

Alkali leaching

Only a limited number of papers have quantitatively reported the extent of alkali leaching revealed with various concrete prism methods. Thus, more research is needed to contri-bute data in this area. Research studies have however revealed cases where up to 35% of the alkalis originally in the concrete found their way into water reservoir after 1 year (Thomas et al., 2006), while other studies present alkali leaching in the order of 2-10% of the original alkali content (Wigum, 2010; Lindgård, 2010). It is also revealed that alkali leaching is highest for specimens with the highest original alkali content.

It is clear that various storage conditions will influence the amount of alkali leaching, and potentially synergic effects between various conditions may occur. In particular the prism size, the storage temperature and any application of wrapping of the prisms may have a major influence on the extent of alkali leaching (Lindgård, 2010). Another issue of interest for further research is the extent of alkali leaching vs. the concrete alkali content.

Alkali release from aggregates

Some aggregate types may release significant amounts of alkalis to the concrete pore water and thus influence the ASR. Most of the 17 tested aggregate types from Canada contributed with alkalis in the range 0.45 to 0.70 kg Na2Oeq per m³ of concrete, but the amount varied from about 0.1 to 1.6 kg Na2Oeq alkalis per m³ of concrete dependent on aggregate type (Bérubé et al., 2000).

Ideker et al. (2010) showed that the contribution of alkalis from a “non-reactive” sand resulted in increased concentration of K⁺ in the pore solution, elevated pore solution pH and a higher rate of expansion at early age compared to other “non-reactive” sands tested.

The difference was most pronounced for the 60°C CPT. The choice of “non-reactive”

sand in laboratory testing may thus influence the outcome of the tests.

The influence of w/b ratio, alkali boosting and pH on the extent of alkali release is also discussed in the report (chapter 3.5).

The task group “Releasable alkalis” in RILEM TC 219-ACS is presently trying to develop a reliable test procedure to measure the extent of alkali release from various aggregate types.

Binder type

The binder type is influencing the composition of the concrete pore solution that is main-ly dependent on the alkalis available in the clinker. Low-CaO fmain-ly ashes reduce the pore solution alkalinity beyond mere dilution, whereas ground granulated blast furnace slag (ggbfs) can release alkalis to the pore solution. The extent is much less than that of clinker, and is almost independent of the alkali content of the ggbfs. Silica fume reacts fast and decreases the alkali concentration in the pore solution within the first two days of hydration, whereas the alkali concentration increases after 28 days up to 2 or 3 years, due to a reaction of calcium with the alkali-silica-gel and a release of alkalis to the pore solution.

The storage temperature and the length of the pre-curing period at 20°C may have a significant influence on the outcome of a performance test, in particular when SCMs are present. Both Fournier et al. (2004) and Schmidt et al. (2009) have documented that the concentration of sulphates in the pore solution is increased when elevating the storage temperature. Thus, the concentration of OH^- is reduced correspondingly. Primarily as a result of the drop in OH^- concentration, addition of only 10% fly ash to the binder was apparently able to suppress the expansion below the critical expansion limits for a highly reactive aggregate when exposed to 60ºC one day after casting (Schmidt et al., 2009).

When pre-cured at 20ºC for at least 28 days before starting the 60ºC CPT, the concrete prisms with 10% fly ash expanded far beyond the critical limits.

The cement type or cement-SCM-combination influences the permeability of the concrete and thereby the rate of alkali leaching, the development of the OH^- concentration and the rate of expansion.

The rate of alkali leaching is much higher in small test specimens compared with massive concrete blocks or real structures in the field. This will influence the laboratory/field correlation.

Mix design/casting

The alkali content is one of the most important factors for ASR. Hence, all parameters that influence the alkali content have to be considered carefully, especially if the influ-ence is likely to change the relationship between the laboratory and the field. Thus, well established, but non-performance test methods should not be adopted carelessly, for example by boosting the alkali level.

For a performance test, the same ”alkali-conditions” (i.e. identical type and amount of cement, chemical admixtures and any external alkalis; insignificant alkali leaching etc.) should preferably be used as will be used in the field.

Curing becomes more important for a performance test, especially when testing concretes with SCMs. With increasing curing time, temperature and RH, the hydration degree of the binder will increase, hence the paste will become denser, alkali leaching will be reduced, but the ingress of water and (possibly) de-icing solutions will also be reduced (Stark et al., 2008; Giebson, 2010b).

Increasing w/b ratio will result in a higher porosity (increasing the amount of capillary pores) and consequently internal transport processes will be accelerated, alkali leaching will increase and water or possibly other solutions will penetrate more easily (Kamali et al., 2003; Kamali et al., 2008). A decreasing w/b ratio will increase the OH^– concentration in the pore solution and vice versa (Stark et al., 2008). As discussed above, a reduced w/b ratio will also enhance the self-desiccation and reduce the internal RH in the concrete prisms. If the laboratory testing is performed with w/b ratios different from what will be applied in field, the laboratory/field correlation may be negatively influenced.

Air entrainment is important for concretes exposed to freeze-thaw cycles and de-icing solutions. In the literature, contradictory conclusions exist with respect to influence of the concrete air content on the ASR expansion. Jensen et al. (1984) and Hobbs (1988) showed that air entrainment addition was effective in decreasing the expansion due to ASR. The effect seems to be mechanical; air bubbles accommodate a portion of the ASR gel. Boyd et al. (2000) stated that the addition of a lightweight aggregate may show a similar behaviour. The same may be assumed to be the consequence if prisms for ASR testing are poorly compacted. However, other researchers have reported less or no influence of increased air content on the ASR expansion; Fournier et al. (2009) and FIB (unpublished results).

7.3 Properties of hydration products formed