Most of the standards that evaluate the resistance of concrete against freeze-thaw cycles (FTC) are based on the loss of weight due to scaling. Such procedures are useful but do not provide information about the microstructural deterioration of the concrete. The test procedure needs to be stopped after several FTCs for weighing the loss of material by scaling. This paper proposes the use of mercury-intrusion-porosimetry and thermogravimetric analysis for assessing the microstructural damage of concrete during FTCs. Continuous strain measurement can be performed without stopping the FTCs. The combination of the above techniques with the freeze-thaw resistance standards provides better and more precise information about concrete damage. The proposed procedure is applied to an ordinary concrete, a concrete with silica fume addition and one with an air-entraining agent. The test results showed that the three techniques used are suitable and useful to be employed as complementary to the standards.
The increasing concern about the environmental cost of production of building materials and the impact of construction of civil infrastructure on nature have been taken into account in the newest design recommendations of concrete structures. According to this, the current design lifespan for relevant infrastructure such as bridges has recently been extended to 100 years. Therefore, the design of concrete based on durability now plays a leading role in the project stage and construction of structures (
Two kinds of damage can be observed when concrete is subjected to FTCs: scaling of damaged material from the surface of the concrete and internal damage of the concrete. Assessment of durability of concrete is based on the choice of a type of test that reproduces the environment where the structure is located. There are various standards and recommendations that measure scaling resistance, with UNE-CEN/TS 12390-9 (
Both internal and surface damage of concrete can be related with concrete dilation during the FTCs. There are two factors that mainly affect strain in concrete specimens during FTCs: the thermal gradients generated within the specimens and the pore pressure induced by freezing and thawing of the pore water (
The previously explained process might be a reliable indicator of damage in concrete. As deterioration develops, residual strain begins to accumulate. Greater residual strain indicates greater internal damage. As a result, the internal deterioration of a concrete specimen under FTCs might be evaluated by studying the strain increment. Should this be proved, it may be of significant interest if applied to monitor the strain of certain areas of real concrete structures sited in freeze-thaw environments.
As is widely known, the microstructure of concrete is a key factor that determines the mechanical properties of concrete (
When employed in concrete air-entraining admixtures (AEA) offer excellent resistance to FTCs, improving plasticity, workability and increasing the durability of the concrete (
This paper shows that the use of mercury intrusion porosimetry and thermogravimetric analysis may be useful for a better understanding of the freeze-thaw microstructural damage of concrete. In addition, continuous strain measurement can be performed without stopping the FTCs. The combination of the above techniques with the freeze-thaw resistance (scaling) standards provides better and more precise information about concrete damage. The proposed procedure is applied to an ordinary concrete (RC), a concrete with silica fume addition (SFC) and one with air-entraining agent (AEC). The tests were performed according to the standards and the number of freeze-thaw cycles was extended beyond their standard duration.
Ordinary Portland cement (OPC) of strength grade 42.5 R and silica fume were employed. The most important components of cement and silica fume are shown in
Physical properties and chemical composition of cement and silica fume
Cementitious Materials | Physical properties | Chemical Composition (%) | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
Fineness (m2/kg) | Specific gravity (kg/dm3) | CaO | SiO2 | Al2O3 | Fe2O3 | SO3 | MgO | K2O | Na2O | |
Cement | 378 | 3.13 | 65.89 | 21.58 | 3.49 | 4.78 | 3.26 | 1.00 | – | – |
Silica Fume | 20000 | 2.00 | 0.50 | 93.00 | 0.20 | 0.10 | 2.00 | 0.30 | 0.40 | 0.20 |
Round siliceous gravel, with diameters ranging from five to 25.4 mm and with fineness modulus of 7.31 was employed in all formulations as coarse aggregates. River siliceous sand (fineness modulus of 2.87) was chosen as a fine aggregate in all the concrete mixes.
In order to achieve proper workability, a superplasticizer was poured while mixing in all the formulations manufactured. In one formulation an air entraining agent was used. This additive also enhanced concrete workability.
To assess the suitability of MIP, TG and the continuous measurement of strain when applied to concrete subjected to FTC, three types of concrete with different resistance to this kind of environmental exposure were manufactured: One concrete mix containing air entrainment agent (AEC); a second concrete mix with silica fume as a partial replacement of cement (SFC); and a third concrete mix without additives or any cement substitution was performed with ordinary Portland cement (RC). Concretes mixtures had a water/binder materials ratio (w/cm) ranging from 0.40 to 0.45. All concrete formulations were manufactured using a vertical-axis planetary mixer with a capacity of 100 L. Concrete formulations are presented in
Concrete mix proportions
Concrete mix | Cement (Kg/m3) | Silica Fume (Kg/m3) | Sand (Kg/m3) | Gravel (Kg/m3) | Water (Kg/m3) | Superplasticizer (Kg/m3) | AEA (Kg/m3) | water/binder |
---|---|---|---|---|---|---|---|---|
RC | 360 | – | 680 | 1160 | 162 | 1.1 | – | 0.45 |
SFC | 360 | 40 | 553 | 1239 | 166 | 1.8 | – | 0.42 |
AEC | 450 | 610 | 1190 | 180 | 1.8 | 0.45 | 0.40 |
For each type of concrete, seven cylinder specimens (Φ=15 cm h=30 cm) were cast in typical steel moulds; eight 150×150 mm2 and thickness of 70 mm prismatic specimens were also cast in steel cube moulds of 150 mm with a centred vertical plate of methacrylate (10 mm) which separated the mould into two halves. This process was performed in a laboratory with a temperature of 20 °C. The specimens were demoulded after 24 hours. Once demoulded, cube specimens were placed in a water bath, at 20±2 °C temperature for seven days, and then stored in a climate chamber (20 °C/65% RH) for surface drying for 21 days according to the standard procedures (
Both fresh and hardened concrete properties were evaluated. In addition, the salt scaling of the concretes had been studied and the results obtained were related with the testing techniques proposed in this work: instead of measuring length changes at certain freeze-thaw cycles, a new continuous strain measure system was developed and applied in order to register the changes of length that take place in the specimens during the cycles. Furthermore, the changes in the microstructure of the three concretes during the FTCs were studied by using mercury intrusion porosimetry (MIP), for assessing the pore structure, performing thermogravimetric (TG) analysis and examining the hydration products.
Slump tests were performed according to the ASTM C 143 standard (
Prismatic specimens of three concrete mixes were subjected to FTCs, according to the test method for the freeze-thaw resistance UNE-CEN/TS 12390-9 (
Freeze-thaw test setup. Measurements in mm.
Then, the specimens were exposed to the freeze-thaw cycles in a CCK temperature controlled chest. The evolution of temperature vs. time for each freeze-thaw cycle is shown in
Freeze-thaw cycle.
Scaling damage was measured at different intervals of freeze-thaw cycles; following the guide of UNE-CEN/TS 12390-9 (
Where
The strain of the surface of the prismatic concrete samples (150×150×70 mm) during the freeze-thaw cycles was measured through using 50 mm-long commercial strain gauges (HBM, model KLY41-50/120). They were glued on the middle part of the lateral surface of the specimens as shown in
Strain measure setup.
MIP tests were performed with a Micromeritics porosimeter, Autopore IV 9500 model, which reached a pressure of 228 MPa, and measured the diameter of pores from 0.006 μm to 175 μm. This pressure interval evaluates capillarity and air porosity. Solid specimens between 2–4 g were extracted from the prismatic samples through using a cylindrical head which ensures that there was no damage made to the samples. A sketch of the process carried out to obtain the samples can be seen in
Porosimetry sample extraction.
TG tests were carried out with the aim of studying the hydrated compounds generated during hardening, curing and freeze-thaw tests in the three types of concretes. TG is probably the best method for measuring the reduction in calcium hydroxide content and assessing the pozzolanic activity in hardening cementitious materials. Tests were carried out in these concretes before and after 28 freeze-thaw cycles.
The DTG/TG test was performed by using the ASTM E1131: 2008 standard, on thermogravimetric and compositional analysis of solids and liquids. The equipment used was the simultaneous thermal analyser Setaram brand, model Labsys Evo with a balance accurate to 0.1 mg. The dynamic heating ramp varied between 40 °C and 1100 °C. The heating rate was 10 °C/min and the crucibles used were made of alumina. The reference material was α-alumina (α-Al2O3) previously calcined at 1500 °C. The test was conducted under N2 atmosphere. All tests were made by using approximately 55 mg of previously ground sample.
The hydration of the concrete mixtures was stopped by soaking the samples in methanol for two hours in order to replace the capillary water. To minimise the possible influence of methanol, the samples were dried afterwards in a desiccator over silica gel for one week before testing (
The results of the slump and air content test are shown in
Concrete fresh properties
Concrete mix | Slump (mm) | Air content (%) |
---|---|---|
RC | 75 | 2.6 |
SFC | 80 | 2.8 |
AEC | 80 | 7.5 |
Concrete properties
Properties | RC | SFC | AEC |
---|---|---|---|
Compressive strength seven days (MPa) | 24.2 | 38.5 | 28.0 |
Compressive strength 28 days (MPa) | 28.5 | 41.5 | 32.4 |
Tensile strength (MPa) | 2.9 | 3.9 | 3.1 |
Modulus of elasticity (GPa) | 26.2 | 29.0 | 26.2 |
SFC had the highest compressive strength, 31% higher than RC. This effect was caused by the lower w/c ratio and the effect of silica fume in cement hydration reactions. Silica fume particles react with the calcium hydroxide, producing an additional calcium silicate hydrate gel that improves the mechanical properties of concrete. Silica fume not only plays this role, but also alters the microstructure of hydrated cement products, leading to bonds (mainly formed by particles of calcium silicate) with higher strength between cement matrix and the aggregate (
AEC had higher compressive strength than RC. These results point out that even though AEC concretes suffer the deleterious effect of air-entraining additives (
The indirect tensile strength test showed the same trends as those previously mentioned for compressive strength. There were significant differences between SFC and the rest of concretes while between AEC and RC there were only differences of 8%. Similar values of the modulus of elasticity of the different concrete mixes were obtained. As expected, and due to the densifying effect of silica fume particles, SFC was stiffer than the rest of concretes though there was only a slight variation of 10% between the modulus of elasticity of RC and SFC (
Permeability tests were performed in one cylindrical 15Ø×30 cm specimen of each formulation in accordance with the EN 12390-8 standard (
Silica fume improves concrete in two ways: it promotes the pozzolanic reactions that take place during cement hydration and, due to the small size of its particles, also acts as microfiller. Silica fume particles, due to an extremely small size, are able to occupy small voids in the concrete microstructure. This phenomenon reduces the permeability of concrete; it improves the paste to aggregate bond and also increases the density of the cement matrix (
Water penetration in concrete
Concrete mix | Water Permeability–Penetration depth (mm) |
---|---|
RC | 13.8 |
SFC | 11.8 |
AEC | 10.8 |
Entrained air in concrete produces discrete nearly spherical bubbles in the cement paste. These are approximately 50 micrometer in diameter and result in the formation of extremely few channels for the flow of water and an insignificant increase in permeability (
The measured cumulative scaling after four, six, 14 and 28 freeze-thaw cycles according to UNE-CEN/TS 12390-9 EX (
Cumulative Scaling during 28 Freeze-thaw cycles.
These results can be visually confirmed by comparison of the photographs of the test surfaces of the concrete samples shown in
Test surfaces of concrete specimens before and after 28 freeze-thaw cycles.
However, extension of the duration of the test beyond the 28 freeze-thaw cycles stated by this standard test procedure was considered of interest. Previous studies (
Scaling after 70 cycles.
However, and in contrast with the widely known tendencies observed in RC and AEC, the behaviour of SFC showed a clear change immediately after exceeding 28 cycles. The scaling/cycle ratio rose continuously between 28–42, 42–56 and 56–70 cycles. Upon finishing the 70 cycles, SFC registered a higher degree of damage than RC.
Previous studies have pointed out that silica fume increases the early strength of concrete. In
However, after cycle 28 the damage of the surface of SFC samples starts. A slight increment of the scaling/cycle ratio that promotes the damage in the sample until cycle 42 is identifiable. This tendency continued, and was even amplified, between cycle 42 and 56. This phenomenon was also evident between cycles 56 and 70, increasing the cycle/damage ratio rapidly. The scaling ratio is so steep that the cumulative scaling at the end of the 70 cycles in SFC exceeded the RC values.
This is also confirmed in
Test surfaces of concrete specimens after 70 FTCs.
In
Strain registered during two cycles.
Strains were registered continuously only during the first 56 cycles, as the damage induced by freeze-thaw cycles reached the strain gauges position and the gauges were peeled off from the surface of the RC and SFC samples. For the sake of clarity the continuous measurement of strain has been replaced in
Strain measurements.
In RC between cycle 0 and 4 microcracks appear in the tested surface which shows a noticeable length change, as can be seen in
No permanent strains were registered by the gauges in any cycle of the freeze-thaw test in the AEC specimens.
As can be seen in
While SFC internal damage does not either cause an increment of strain or scaling in the first 20 cycles of the test, microcracks appear inside the sample. Once microcracks are widespread and distributed in the sample and the tested surface damaged, cracks from the tested surface grow rapidly from it into the sample and coalesce with the microcracks previously generated. Therefore, damage evolution (strain increments and scaling) occurred at higher ratios because the cement matrix was previously damaged. The strain/cycle ratio changed between cycle 20 and 30 and was constant beyond. The scaling/cycle ratio continued to grow as the test continued, with the reason being that the distance between the crack lips grew and the volume of the damaged sample therefore increased. These effects were registered by the continuous measurement of the strains of the samples.
A detailed study on the changes of the porous network of all formulations was carried out by using MIP tests, before the start of freeze-thaw test at 28 days and after 42 FTCs. The evolution of the total porosity, critical pore diameter and average pore diameter are shown in
Analysis results of porosity and pore characteristics according to the types of concrete
Concrete mix | Total Porosity (%) | Critical diameter (nm) | Average pore diameter (nm) | |||
---|---|---|---|---|---|---|
28 days | after F-T | 28 days | after F-T | 28 days | after F-T | |
RC | 15.05 | 17.85 | 64.30 | 62.30 | 3.69E-02 | 4.65E-02 |
SFC | 9.01 | 13.25 | 40.00 | 17.96 | 3.39E-02 | 2.89E-02 |
AEC | 13.87 | 15.92 | 62.88 | 51.03 | 5.12E-02 | 4.53E-02 |
The evolution of the pore distribution of the concrete formulations before and after the freeze-thaw cycles is shown in
Pore size distribution of RC.
Pore size distribution of SFC.
Pore size distribution of AEC.
The volume of mercury intruded is the area under the porosimetry results shown in
After 42 freeze-thaw cycles there were significant differences in the results obtained in MIP tests when compared with those obtained at 28 days (before the FTCs). As the left part of the curve, which represents the smaller fraction of pores (between 3.100 and 2.102 nm), shifted to the left the diameter of these pores were reduced. However, the amount of pores of the biggest diameters increased dramatically. This increment can be attributed to the cracks that appeared during the freeze-thaw tests. This result is in accordance with the results of those previously obtained (
During the cycles, the hydration reactions continued to take place in cement paste that led to a reduction in the diameter of the pores. As a result of this hydration process, the critical diameter of the pores was smaller than before the cycles. Furthermore, the average diameter was smaller than before the cycles. The effect of the cement compounds hydrated during the cycles outbalanced that of the cracks induced by ice during the freeze-thaw test.
Only slight differences were found when comparing the MIP results before and after the freeze-thaw cycles in AEC as shown in
In the DTG/TG tests the components of the paste are decomposed in a certain range of temperatures while heating. This temperature is characteristic for each chemical component. The amount of mass loss is determined using the TG curve. To assess the beginning and the end of a decomposing process the DTG curve (derivative curve of TG) is employed. The peak areas of the DTG curve that can be seen in
DTG/TG analysis of a standard concrete.
As can be seen in
Degree of cement hydration in the three concrete mixes.
Evolution of hydration products according to the types of concrete
Product | RC | AEC | SFC | |||
---|---|---|---|---|---|---|
Before FTCs | After FTCs | Before FTCs | After FTCs | Before FTCs | After FTCs | |
Gel | 12.31 | 12.89 | 12.08 | 16.03 | 11.70 | 14.52 |
Portlandite | 20.26 | 21.00 | 20.02 | 24.97 | 15.70 | 21.22 |
Hydration degree | 61.04 | 64.36 | 59.72 | 82.99 | 57.62 | 73.86 |
The comparison of the results before and after FCTs show that, for the three concretes, water gel, portlandite content and the degree of hydration increased during FCTs. Which is a consequence of the normal evolution of the hydration of concrete. The concrete with air entrained agent (AEC) showed higher content of gel, portlandite and a significant increase in the degree of hydration of concrete due to freeze-thaw cycles. Which may be caused by the higher content of cement and the modified network of pores generated by the air bubbles that help to maintain more water inside the concrete. This aspect should be studied in depth and it is out of the scope of this paper.
All the data gathered in this study open the spectrum to a new way of monitoring concrete structures. By using strain gauges, a continuous measurement of the harmful consequences of the freeze-thaw cycles in the structure can be performed. Such measurement can be correlated with laboratory results obtaining precise information of the residual resistance of the concrete element to the environment. In sites where a continuous monitoring of the structure cannot be introduced, the onset of the damage can be determined by performing PIM analysis as has been shown. This early detection of damage would allow taking actions to prevent further damage.
According to the results, silica fume addition generates concretes capable of being subjected to midterm freeze-thaw exposition without being damaged. However, silica fume additions do not avoid either the internal or the damage of the surface introduced by long-term freeze-thaw environments and are even more damaged that RC. The damaging processes of freezing environments act faster in concrete manufactured with silica fume once the damaging effects have appeared.
Strain measurement is a reliable and easy parameter in evaluating concrete resistance to freeze-thaw environments. There is a clear correlation between strain measurements and scaling. Results obtained for both parameters have proved to be consistent. In addition, continuous measurement of strain avoids possible human errors in manual length measurements. This technique is shown as being viable and provides advantages over the weight of the mass released by peeling surface.
Porosimetry techniques have revealed the differences in the porous network of the three types of concrete. Frost damaging effects have been clearly observed by comparing porosimetry tests results before and after the freeze-thaw cycles in RC and SFC. Air entraining agents did not affect the connected pore network of concrete. TG results confirmed the results of the MIP tests. The hydration process of cement continued even during FTCs, refining the smallest sizes of the pore structure of concrete. As expected AEC enhance the concrete scaling resistance even in long-term expositions to FTCs.
The authors gratefully acknowledge the financial support for the research provided by the Spanish Ministerio de Economía y Competitividad under grant DPI2011-24876. H.L. Romero also wishes to express his gratitude to the Fundación Agustín de Betancourt for the grant provided.