Olcay G. AYDOĞANa, Senem BİLİCİb, Abdullah H. AKCAc,d, Nilüfer ÖZYURTa
aBogazici University, bIstanbul Aydın University, cYıldız Technical University, dColumbia University
Abstract
Water is one of the most important sources for human life and we need water in every aspect of our lives. As a result of the rapid increase of the world population and the unconscious consumption of resources, water scarcity is experienced throughout the world. Therefore, in recent years, studies have been carried out for the sustainable use of water resources. Although these studies are being conducted, resources are gradually decreasing due to the increased need in water.
As in the other water-related sectors, sustainability studies in the construction sector are continuing. The construction/concrete sector is one of the sectors that consume significant amount of water. When the related statistics and research studies are examined, it is foreseen that important water shortages will be faced in the near future. In this case, it can be seen that using potable water as the mixing water for concrete will not be sustainable in the near future. As an alternative to potable water, seawater can be used as the mix water for concrete.
In many standards, the use of seawater in concrete is prohibited because it causes corrosion of steel reinforcement. However, previous studies show that seawater concrete can be used for unreinforced concrete applications or used together with polymer-based reinforcements which are becoming widespread today. However, the number of studies on how the use of seawater as the mixing water affects the strength and durability of concrete is limited. The aim of this study was set to investigate the effects of the use of seawater on the strength and durability of concrete. For this purpose, mechanical strength and durability tests of concrete samples produced by using potable water or seawater were carried out and the results were evaluated together with the results of the microstructure tests. In this article, strength results obtained in the first 6 months were given and interpreted.
1. Introduction
Water is an indispensable compound for life. In order for living beings to survive, they need to have access to sufficient clean water. Although 70% of the world is covered with water, only 2.5% of this water is fresh water. Approximately 70% of freshwater resources are hidden in glaciers [1]. This means that only 1% of the world’s water is accessible fresh water. In addition, this amount is not evenly distributed throughout the world. As can be seen in Figure 1, the continents of South America and Asia have more than half of the world’s water resources [2].
A similar imbalance is observed in our country. According to the report prepared by TUSİAD [3], about half of the annual rainfall is collected in only 5 of the 26 basins of our country. The remaining 21 basins share half of the annual rainfall. At the same time, there is an imbalance between the flow amount of the basins and the population it serves. Again in this report [3], it is stated that the Marmara basin, where 28% of the total population lives, has only 4% of the total flow. Imbalance in the basins and population distribution were shown in Figure 2.
Figure 1: Distribution of Population and Water Resources on Earth [2]
Figure 2: Current and Population Distribution in Turkey Basin [3]
Under these conditions, while sustainable use of water is needed for all living creatures, the actual picture is exactly the opposite. Especially after 1950s, water shortages on a global and regional scale have been experienced due to rapid population growth, industrial growth, increased living standards, unconscious water consumption and water pollution. The United Nations’ water report in March 2017 [4] states that two thirds of the world population suffers from water shortage during certain periods of the year. According to studies and forecasts, this problem will continue to increase every year. The World Wide Fund for Nature report [1] emphasized that the world population has increased threefold in the last century, but demand for water has increased sevenfold. According to the report prepared by USIAD [5], approximately 2.3 billion people in the world are suffering from lack of clean water. This situation will become more serious and 75% of the 9.3 billion world population is expected to face water shortage by 2050.
According to the United Nations report [6] published in 2019, the Middle East and the Arab region are the regions where water shortage is the most intense. In 2015, 51 million people (approximately 9% of the population) in this region were reported to lack basic drinking water services. This problem is expected to increase due to population growth and global warming. It is thought that wars may arise in this geography,
where Turkey is present, due to water scarcity.
While this is the picture in the world, the picture in our country is not different. Turkey, -contrary to popular belief – is not a water-rich country. According to TUSIAD report [3], annual water amount per capita in Turkey is around 1500 to 1735 m3. Countries that use water below 1000 m3 per capita per year are classified as “water poor countries”, those who consume water between 1000-3000 m3 are classified as “countries under the water constraint”, and those who use water above 3000 m3 are classified as “water rich countries”. Turkey is a country under the water constraint with 1600 m3 average annual water amount per capita. The most important reasons of water shortage are increase in demand, increase in agricultural and industrial activities, and decrease in water
quality as a result of pollution. The countries under the water constraint in TÜSİAD’s report were shown in Figure 3.
Figure 3: Countries Under Water Restriction [3]
WWF reports that Turkey is one of the countries under the water constraint with 1500-1600 m3 of annual water amount per capita [1]. It is estimated that this amount will decrease to 1120 m3/year in 2030.
Because water is such an important compound, water scarcity damages all habitats. Water-related diseases and deaths have been gradually increasing in the regions which suffer from water shortage. According to USIAD data [5], 7 million people die every year from water-related diseases in the world. Researches show that water scarcity has a profound impact not only on human health but also on regional and global economies. Water is an important input for many sectors, especially for the food industry. One of the main reasons for the increase in demand for water in the last century is the diversification of water usage areas within the industrial ground. Although the impacts of water resources are thought to be experienced locally, water security is now defined as a global problem. Therefore, any shortages will have a direct effect on the global economy. According to the WWF report [1], the drought in Russia in 2010 had a direct impact on grain prices in North Africa and the Middle East. Again in this report, it was stated that the water shortage experienced in Brazil and India in 2009 caused an increase in sugar prices worldwide.
In summary, both the world and our country are about to face serious water shortages. This problem will affect nature, human health, quality of life and economy at an increased pace. Therefore, sustainable use of water in every aspect of life is essential.
In line with these estimates, many researchers in different countries have focused on issues such as waste water management and more widespread use of seawater within the scope of sustainability issues. Within the scope of 2016 Water Report [7] published by the United Nations, the construction sector was listed as one of the sectors that need water at a moderate level and it was stated that these sectors use serious amount of water. Due to the fact that concrete is the most used building material in the world, this sector is among the sectors that use a significant amount of water. In order to reduce the use of potable water in concrete and to use potable water in a sustainable manner, the use of seawater as the mixing water in concrete is worth studying.
It is known that chloride ions in seawater cause corrosion in steel reinforcement. For this reason, many standards prohibit the use of seawater as the mixing water. However, developments in recent years shows that polymer based reinforcement can be replaced with conventional steel reinforcement in concrete. It is also believed
that these reinforcements can be widely used in applications where the problem of corrosion must be solved in the near future. In addition to corrosion, it is also important to investigate other impacts that may occur in concrete (sulfate damage, etc.). As it is known, seawater contains much more chloride and sulfate ions than tap water. While chloride has an effect on reinforcement, sulfate ions affect the matrix and damage to the internal structure.
However, the effects of sulfate from the use of seawater as mixing water and the results of sulphate attack from outside may vary. The effect of sulfate ions added in the fresh state can be very different. Using seawater as the mixing water and examining the changes in the internal structure of concrete is very important in terms of understanding the differences in strength and durability properties.
2. Literature Review
These developments have caused researchers to increase interest in seawater use in concrete, which was first brought up in the 1970s. In an article published by Nishida et al. [8], the number of studies on the use of sea water as the mixing water in concrete between 1974 and 2013 was reported. In recent years, with the effect of decreasing water resources and climate change, the interest in the issue has increased and this situation has been reflected in the researches.
Figure 4: Number of articles published between 1974 and 2013 on the
use of seawater as mixing water in concrete [8]
Mohammed et al. [9] observed that concrete specimens were subjected to tidal effects for 20 years and that early strengths of concrete produced by seawater were better than those produced by potable water and at the end of 20 years, the strength difference between seawater concrete and tap water concrete was insignificant. They attributed the improvement in early strength to the acceleration of hydration reactions by the effect of chloride in seawater.
Wegian [10] used seawater in concrete for mixing and curing and investigated its effects on mechanical properties. As a result of the study, it was reported that the strength of the samples which was cast by using seawater as the mixing water and cured in the seawater was high in early ages (7 and 14 days), but the strength decreased in older ages (28 and 90 days). In this study, there was no investigation about the performance of concrete in the long term.
In most of the investigations, mineral additives such as slag, fly ash, metakaolin were used to bind chloride ion and increase final strength. It has been reported that these mineral admixtures are combined with chloride to contribute to the binding of chloride ions and to increase final strength as a result of pozzolonic reactions. Katano et al. [11], used seawater as mixing water and undrawn sea sand as aggregate, and tried different combinations of fly ash, slag and silica fume together with ordinary Portland cement (OPC). They stated that compressive strength of concretes produced by seawater was higher than concrete produced with potable water, especially at early ages. They reported that the increases in strength obtained in the first 28 days varied between 3% and 70% according to the mineral additive ratio used. However, they stated that these increases decreased to less significant levels on the 90th day and ranged between 2% and 15%.
Otsuki et al. [12] investigated the effect of usage seawater as mixing water on concrete. They used Ordinary Portland cement, High Early Strength cement, Moderate cement, Blast Furnace Slag cement and Aluminate cement as binders separately and exposed the samples in the tidal zone for 20 years starting from 7 days. In order to mathematically express the differences in compressive strength, they used the compressive strength ratio (seawater-produced concrete / potable water-produced concrete), which indicated that the proportions for all concretes ranged from 0.9 to 1.1, it means that the type of mixing water had no significant effect on the compressive strength. (They also reported that this result was also valid to concrete produced with Portland Cement).
Shi et al. [13] used a small amount of metakaolin (in the range of 0‐6%) as mineral additives and artificial seawater as mixing water in their study. They observed that the increase in compressive strength of artificial seawater concretes was higher than that of potable water concretes as a result of the acceleration effect of the seawater ions. Therefore, more CH were produced and then more CSH were produced due to pozzolonic reaction. In addition, they reported that the chloride resistance increased with the amount of metakaolin and that the negative effects that may arise with the use of seawater could be eliminated by metakaolin addition.
As can be seen, while the problem is so serious, there are limited studies on the subject. The general opinion of the studies is that seawater increases the hydration rate due to the chloride effect and therefore the strength at early ages is improved and the strength at later ages is similar to that produced by tap water. In this study, it was aimed to investigate the changes in strength and durability properties by using potable water and seawater together with OPC and sulphate resistant cement. In addition, it was aimed to investigate the effects of seawater on fiber matrix adherence by using macro polymer based fibers. In this article, the strength results obtained in the first 6 months of this project were given.
3. Experimental Study
3.1. Materials
Two types of cement [Portland Cement (CEM I 42.5R) and Sulfate Resistant Cement (CEM IV / B)] were used to understand the effect of sulfate ions of seawater in concrete. Polycarboxylic ether based chemical plasticizer was used to achieve S4 consistency class. Four different types of aggregates were used, including two fine and two coarse aggregate types, with high quartz minerals (> 85%). Tap water and seawater were used as two different mixing water. Seawater was taken from the coast of Istanbul-Kilyos. The results of the analysis of the mixing waters were given in Table 1.
Table 1: Analysis results of tap water and the sea water used
In this study, polypropylene based structural fibers, with a length of 40 mm and diameter of 0.72 mm were used. The tensile strength of the fibers was 550 MPa.
3.2. Mix Design
Mixture codes were shown in Table 2.
Table 2: Sample Codes and Descriptions
Mix design was given in Table 3.
Table 3: Mix Design (kg/m3)
3.3. Experiments
Slump and density tests were performed on fresh state. Compressive strength tests, modulus of elasticity and flexural tests were applied on hardened concrete. Compressive strength and elastic modulus tests were performed at 7th, 28th and 180th days, and flexural tests were performed at 28th and 180th days.
4. Results and Discussion
4.1. Fresh State Results
The slump test was performed according to EN 12350-2 and the density test was performed according to EN 12350-6 standards. The slump values and density results were shown in Table 4.
Table 4: Fresh State Results
As can be seen from Table 4, all mixtures satisfied the S4 consistency class. When the fresh state density results were examined, it was seen that all mixtures produced had similar density.
4.2. Hardened State Results
4.2.1. Compressive Strength
Compressive strength test was performed according to EN 12390-3 standard. The test results were shown in Figure 5.
Figure 5: Compressive strength of the samples at 7th, 28th and 180th days
When the results were considered, the use of seawater as the mixing water did not have a significant effect on the strength at an early age. The 7th day strength of the samples produced by sulfate resistance cement was relatively low compared to the samples produced by OPC because of the pozzolonic reactions. However, this difference was closed in later years. 180th day strengths were similar in all concrete samples, there was no significant difference.
4.2.2. Flexural Strength and Toughness
Bending tests were performed according to JCI-S-001 and JCI-S-002 standards at 28th and 180th days. The flexural strength and toughness values obtained from the experiments were given in Figure 6 and Figure 7.
Figure 6: Flexural strength of the samples at 28th and 180th days
Figure 7: Toughness of the samples at 28th and 180th days
As can be seen from Figure 6, it was found that most of the samples produced with seawater (except SRC-TW-F) had higher flexural strength compared to concrete produced by using tap water. This was similar for both fibrous and non-fibrous concrete. On the other hand, when the toughness values were examined, it was seen that while the toughness values of non-fibrous specimens were similar both in tap water and seawater concretes, the toughness values decreased when seawater was used in fibrous concrete. The mechanisms affecting the flexural strength and toughness values are different. While the flexural strength value is more dependent on the strength of the concrete matrix, the toughness value is a parameter related to the fiber-matrix adherence. Microstructure analysis showed that fibermatrix adherence was good in both concrete types. Figure 8 showed SEM images, which illustrated fiber-matrix adherence of concrete produced by potable water and seawater. When the pictures were examined, it was understood that the adherence was strong in all types of concrete from the matrix pieces remaining in the perforated parts of the fibers. However, in concrete produced using potable water, it can be said that the cement matrix adhered better to the fibers. In order to understand this situation, further microstructure analyzes are continuing. It is also planned to examine the adherence by conducting direct tensile tests on new concrete samples.
Figure 8: Fiber surfaces after stripped from cement matrix (a: PC-TW-F) (b: PC-SW-F)
4.2.3. Elastic Modulus
Elastic modulus tests were carried out in accordance with EN 12390- 13 standard. Experimental results were shown in Figure 9.
Figure 9: Elastic modulus of the samples at 7th, 28th and 180th days
As shown in Figure 9, there was no significant difference and / or orientation between the elastic modulus of the samples produced by seawater and the samples produced by tap water in the first 6 months. At the same time, the fibers and cement type had no significant effect on the modulus of elasticity. The development of
the samples was similar.
5. Conclusion
In this study, it was aimed to investigate the strength and durability characteristics of seawater concrete at early ages (first 6 months). In addition, the effect of synthetic macro-polymeric fibers on the mechanical properties and adherence between fibers-matrix were studied. It is possible to summarize the results as follows:
– All mixtures provided the S4 consistency class. No significant effect of the mixing water, cement type and fiber addition on fresh density was observed.
– When the results of the first 6 months of strength were evaluated, it was observed that the use of potable water or seawater as the mixing water did not cause a significant change in the compressive strength and elastic modulus. When the flexural strength and toughness values were examined, it was seen that flexural strength values were relatively higher and toughness values were lower for the concrete specimens produced by using seawater when compared to the specimens cast by using tap water. When seawater was used together with Portland cement, toughness values at 28th and 180th days were 19% and 10% less than the samples produced with potable water, and in the case of samples produced with sulfate resistant cement, the toughness values were 36% and 27% less compared to the samples using potable water.
It is thought that fiber – matrix adherence may be slightly lower in seawater concretes and this may cause a decrease in toughness. More detailed results can be obtained from the ongoing microstructure experiments.
Acknowledgement
This study was fund by Bogazici University Scientific Research Projects Coordination Unit under project number 14861P. The support of AKÇANSA, Boğaziçi Beton and BASF Turkey are highly appreciated. The authors would also like to acknowledge Mr. Ümit Melep for his contributions during experimental studies.
References:
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[2] United Nations, 2003, World Water Development Report
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[4] United Nations, 2017, World Water Development Report
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[6] United Nations, 2019, World Water Development Report
[7] United Nations, 2016, World Water Development Report
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