Concrete is a material that exhibits excellent behavior over time. However, due to the sheer volume of reinforced concrete constructions, any durability issue in the material that requires repair, rehabilitation or replacement intervention of the affected elements entails major economic and social impact. The prevention of deterioration of concrete structures began to be considered in all concrete codes and regulations from the last decades of the 20th century, yet a fully satisfactory solution remains far off.

Most concrete codes establish prescriptive methods for the control of concrete durability (1), that regulate the type of components (cement type, aggregates, additions) that can be used and sets limiting values for concrete mixes (typically maximum water/cement ratio and minimum cement content). Although this approach has been successful up to a certain point so far, the main drawback is that the actual composition of the concrete delivered on site cannot be determined through testing, meaning the requirements can be verified in-situ only through documentary control.

To overcome this issue, certain codes have tried to introduce a performance-based specification for concrete durability. This approach is based in the measurement of relevant properties related to the transport properties of the concrete pore structure that can therefore be an indirect index of durability. Durability indicators used in practice might be physical parameters such as permeability to liquids or gases, water absorption, sorptivity, porosity or abrasion resistance, or chemical, physico-chemical and electro-chemical parameters such as diffusivity, conductivity, resistivity or migration coefficients of harmful species like chlorides.

Spanish regulatory codes have set the standard in terms of the treatment of concrete durability, both with prescriptive methods and through the designation of a durability index. The property designated by the Spanish government technical bodies for the indirect measurement of durability is the permeability to water determined by the measurement of the depth of penetration of water under pressure.

The test for determining this property on hardened concrete was standardized by the European Committee for Standardization (2) in 2001. The test is based on applying water at a pressure of 500 kPa for 72 hours on one of the sides of a specimen of hardened concrete. The specimen is then divided into two halves and the penetration depth of the water’s front is measured. The standard specifies that, until the time of testing, the specimens must be cured while completely submerged in water.

This method was first used by Soroka at the beginning of the 1970s (3) in order to assess indirectly the durability of different concretes. Throughout the 1980s, it was adopted by different researchers to classify the quality of concrete from the point of view of its durability (4-6) due to its apparent simplicity in terms of implementation and interpretation of results. Although the rationale for the method used by these authors was the same, there were differences in their procedures in the curing regime prior to the test (submerged in water, humid chamber) similarly in the number of pressure steps and the magnitude of the applied pressure.

With a view to reducing the variability introduced by the above-mentioned factors, the method was standardized for the first time in Germany in 1978 (7), and later included in an ISO standard (8). In Spain, the adoption of the method is mainly due to the work carried out by the Centro de Estudios y Experimentación de Obras Públicas (CEDEX) in the 1990s. The laboratory work carried out by this organization (9-11) resulted in the drafting of a Spanish standard in 1990 (12). This standard features significant differences vis-à-vis the current European version of the test: it was established that, 24 hours before the test, the specimens had to be dried in an oven at 50±5ºC; during the test, three pressure steps were applied (100 kPa for 48 hours, 300 kPa and 700 kPa for 24 hours each) instead of a single pressure step; and finally, in addition to the maximum water penetration depth, the average depth was also determined.

The Structural Concrete Instruction (EHE) issued in 1998 (13) passed new legislation in Spain that established the determination of the depth of water penetration according to (12) as an experimental verification of compliance with the durability requirements of minimum cement content and maximum water/cement ratio. This verification had to be carried out for concretes subjected to exposure classes with risk of chloride corrosion, chemical attack, freeze-thaw or abrasion. The limits considered sufficient to guarantee the impermeability of concrete were 50 mm for the maximum depth, and 30 mm for the average depth, for all the mentioned environments and obtained as the average of three specimens. For individual specimens, the maximum depth allowed is 65 mm and the average depth is 40 mm).

The EHE was replaced in 2008 by the Structural Concrete Instruction EHE-08 (14), and this in 2021 by the Structural Code (15). The Structural Code maintains the determination of the penetration depth as proof that the concrete has sufficient impermeability to guarantee its durability during the structure’s service life. The verification must be carried out using the single pressure step of the current European standard, but the EHE-08 introduced the modification, maintained in the Structural Code that, prior to carrying out the test, the specimens must be subjected to a drying period of 72 hours in an oven at 50±5ºC. The limits are the same as in the previous EHE Instruction, applicable to the same types of environments, though updated to the exposure classes defined in the European standard EN 206 (16) (corrosion by marine chlorides in aerial or submerged elements, corrosion by non-marine chlorides, freeze-thaw, abrasion, and weak and moderate chemical attack), with the exception of elements situated in the tidal range, severe chemical attack or pre-stressed elements subjected to moderate chemical attack, for which the limits are established at 30 mm for the maximum depth determined as the average of three specimens and 20 mm for the average depth (40 mm and 27 mm for individual specimens, respectively).

Despite the method’s relative simplicity, doubts have been cast surrounding it due to the high variability of test results (17), and the influence of a large number of factors on the results (18). In this sense, it seems clear that the previous treatment can have a significant bearing on the depth of the penetration fronts for the same type of concrete, since in addition to the mechanism of transport of water by permeability in a porous medium due to the pressure gradient, the previous drying established by the Structural Code can lead to an additional transport mechanism, namely capillarity, which should lead to greater water penetration and can contribute to variability. Another issue pointed out since the method was used for the first time is the difficulty to correctly visualize the penetration front, especially in specimens cured using total immersion in water. The foregoing is why it is believed that the previous treatment was introduced in order to ensure the correct visualization of the front once the specimen is divided into two halves.

Another durability indicator that could be used to measure the durability performance of concrete is resistivity (19). Electrical resistivity can be defined as the ratio between an applied voltage and the resulting electrical current circulating in a unit cell of a material. The resistivity of concrete mainly depends on its porosity, water content and the chemical species present in the pore solution. One of the first authors in developing a testing apparatus for the measuring of resistivity in solid materials was Wenner (20). This method uses a probe fitted with four electrodes and has been successfully applied for the assessment of the corrosion of steel (21). One of the advantages of the four-electrodes probe is its non-destructiveness, which makes it especially suitable for the evaluation of existing structures (22, 23).

A further method for the determination of resistivity in concrete consists in two electrode plates directly applied to a concrete specimen (24, 25). This test, known as the direct method, determines the bulk electrical resistivity of concrete, yielding fewer variable results than the four-electrodes test, thus meaning it is considered as the reference testing method for resistivity in concrete.

Standards for both test methods for the determination of resistivity in concrete have been issued in Spain (26, 27), and at the time of writing this article (end of 2021), a work item is underway in CEN for the publication of a European standard collecting the two methods. However, although several publications try to promote the use of electrical resistivity as a quality control tool (28), this technique is usually not included in the current concrete codes and regulations like the recently published Spanish Structural Code.

As stated before, resistivity is mainly influenced by the moisture content of concrete. As such, in order to successfully characterize the concrete pore network connectivity, resistivity measurement should be performed on completely water-saturated specimens. For this reason, both testing methods establish that resistivity must be measured on concrete specimens cured in water

This work aims to evaluate the influence of three factors, the nature of the aggregate, the water/cement ratio and the previous treatment (curing submerged in water or previous drying in an oven as per the Structural Code procedure), in the results of penetration of water under pressure obtained on concretes manufactured with aggregates of a granitic, limestone and siliceous nature, common materials for the manufacture of concrete in large areas of the Iberian Peninsula. This interest is due to the difficulties highlighted by manufacturers of concrete prepared in order to meet the requirement established in the previous Instruction EHE-08, requirements that have been transposed to the new Structural Code.

Alongside the above, to aid in the search for a possible alternative index that could be used with guarantee to control the durability requirements, the work tries to evaluate the influence of two factors, aggregate and water/cement ratio, on the measurement of resistivity on saturated specimens, since it makes little sense to determine this property on dried specimens.

The water penetration test under pressure carried out on concretes manufactured from granitic, limestone and siliceous aggregates proved to be sensitive to changes both in the water/cement ratio and to the treatment by prior oven-drying established in the Spanish concrete codes. The results obtained on limestone and siliceous aggregates are similar, though not the values measured on granite aggregates, which present differences with respect to the other two types. The results showed a great dispersion, especially in the cases in which the specimens were previously dried according to the procedure stipulated in the Structural Code. Surprisingly, the test did not result in water penetration depth values outside the limits established in the Code, even when testing concretes with a water/cement ratio higher than the maximum water content limits established in the Code and when they were subjected to pre-drying.

The power observed in the test campaign does not allow to draw sufficiently conclusive statistically significant inferences regarding the effect of the interactions between various factors. Likewise, the difference in behavior observed between concretes with different types of aggregate (especially granitic) or the obtaining of penetration values below those expected in the regulation, suggests it is advisable to carry out future studies to bolster the results, increasing for example the power of the campaign by increasing the number of samples, extending it to a greater number of levels per factor or studying the influence of other factors such as the content of fines or the possible presence of entrapped air, among others.

Regarding resistivity, the results carried out on concretes manufactured from limestone and siliceous aggregates show that this test is also sensitive to changes in water/cement ratio. Results were similar for both types of aggregates. The variability of the results is also large, especially for siliceous aggregates, yet this variability does not confound the changes in water/cement ratio.

This study similarly stresses the danger of using indirect relationships between various properties of concrete such as compressive strength and indirect indexes of durability, correlated in occasions but without causality (34). This type of relationship can lead to incorrect deductions implying, for example, that concretes with higher compression strength are less permeable. In this work, granitic concrete showed higher compressive strength than siliceous concrete, while its water permeability was clearly higher.