DURABILITY TEST METHODS FOR SERVICE LIFE DESIGN OF CONCRETE STRUCTURES – EXPOSED TO COASTAL

ENVIRONMENT

V V Arora & Puneet Kaura National Council for Cement and Building Materials, India

ABSTRACT Across the globe, chloride induced corrosion is chiefly responsible for early service life in concrete structures exposed to coastal environment. Their exist number of test methods e.g. RCPT, NT build 492 etc which are used as tool for quantifying the magnitude of chloride penetration in concrete. But interrelationship between such types of test methods with the change in cement type is still implicit in nature. The present paper illustrates the importance of short term and long term durability test methods under chloride aggressive environment. Blended cement (fly ash based and slag based) and non blended cement (ordinary Portland cement) have been used and thereby comparisons have been made keeping all other elements constant. On the basis of chloride diffusion coefficients so obtained during the study as per the ISO 16204 chloride ingress model, service life of concrete structures could be predicted for concrete structures located along coastal areas. Keywords; RCPT, NT build 492, Electrical Resistivity and Air permeability 1.0 Introduction Reinforcement corrosion affects the safety, performance and serviceability of concrete structures by reducing the load bearing capacity and ductility. Predominately two types of concrete deterioration mechanism dominates the corrosion process during the service life of a concrete structure [1, 2] • Chloride induced corrosion. • Carbonation induced corrosion.

The corrosion of steel reinforcement bars in reinforced concrete structures exposed to marine environment usually attributed towards the aggressive behavior of the chloride ions. Various researches had been reported worldwide where the impact of chloride environment had created fissure among country’s infrastructure. To mitigate the effect of chloride ions penetration into the concrete, industrial wastes like fly ash and slag have been seen as a huge potential. Numerous studies had been carried out in past where partial replacement of ordinary Portland cement with fly ash and slag had provided sufficient resistance barrier for the chloride ions movement into the concrete[3,4,5,6,7]. Ingress of chloride ions into the concrete body is primarily governed by the phenomenon known as diffusion which is generally slow in nature [8, 9]. Most of the conventional chloride diffusion tests depend upon the required degree of chloride ions saturation which is a time consuming task [10, 11]. Whether it is steady state diffusion tests methods or immersion test method; both generally last for months and laborious in nature. In order to obtain rapid information regarding the ability of the concrete to resist chloride ion ingress, numerous attempts had been made to develop accelerated test

procedures. This task had been achieved through accelerating the movement of chloride ions by applying an external source of voltage potential. This had led to the development of the rapid chloride penetrability test method (RCPT) and NT build 492 (non-steady state chloride migration test). Both test methods requires less time and less expensive in comparison to conventional chloride diffusion test methods. With further advancement in technology, techniques like air permeability and electrical resistivity had come into existence with the aim of providing instantaneous solutions to the futuristic problems that could arise because of the changes occurring into the concrete during its life cycle. And it had been found that with the passage of time, these techniques had established themselves very well as durability indictors. Nevertheless, the question arises up to what extend such test methods/techniques could be utilized suitably for service life prediction of concrete structures. The credibility criteria of such test methods depend upon the relationship that could be explored between natural process of chloride diffusion and accelerated migration processes involving steady and non-steady state chloride ion transportation. In the last few decades, various studies in this regard had been conducted where researchers had tried to develop correlation between the two processes of chloride ion mobility. Yang et.al[12,13] and Sugiyama et.al[14] studies had clearly indicated the use of steady state migration technique as a replacement to conventional long term method of chloride ion diffusion whereas as non -steady state migration method ( RCPT) studied by Andrade et.al [11] and Yang was not able to generate satisfactory relationship between the charged passed and chloride migration coefficient. Since steady state migration test methods are time consuming generally takes 1-2 months. Therefore, requirement of our present system is accelerated test methods. So, our present study is confined to test methods like RCPT, NT build 492 and electrical resistivity which are accelerated chloride migration non-steady state test methods/ technique. NT build 492 and electrical resistivity tests have shown considerable relationship when plotted against Chloride diffusion coefficient obtained from immersion test but again the effectiveness of these test methods is greatly affected by the cement type and extent to which supplementary cementitious materials are added to concrete mix. McNally et.al [15] found that rapid migration non- steady state test method i.e. NT build 492 carried out at an age of 28 days was not able to give satisfactory result as desired in case of concrete containing supplementary cementitious material like fly ash and ggbs when they compared the chloride diffusion coefficients so obtained with that from immersion test and ponding test. The failure of NT build 492 in this case was mainly attributed towards the age at which the test had been conducted as the presence of fly ash and ggbs requires greater time period for hydration, so care and experience is required while applying the rapid migration test in case of concrete substituted with SCM’s. The present investigation involves use of short term tests method / techniques like RCPT, NT build 492, electrical resistivity and air permeability on the concrete specimen with maturity age of 56 days. Early age testing was avoided because of the presence of SCM’s that would affect the results whereas the chloride diffusion test through immersion had been conducted on 28 days matured concrete samples followed by 90 days exposure to 3 % Nacl solution. This paper illustrates the behavior of blended cement containing fly ash and slag under chloride aggressive environment and persistence efforts have been made to formulate relationship between short term durability test methods with long term durability test method in conjunction to the cement type, so as to predict service life of concrete structures under coastal environment well before its execution.

2.0 Experimental Program 2.1 Cement The cements used were Ordinary Portland cement (OPC-43, confirming to IS 8112:2013), Portland Pozzolana cement (PPC, confirming to IS 1489 part1:1991) and Portland Slag cement (PSC, confirming to IS 455:1989). 2.2 Aggregates The coarse aggregates (20 mm and 10 mm) and the sand (Zone II), confirming to IS 383 were used in all the concrete mixes. Some of the physical properties of aggregates so used are mentioned in table 2.

Table 2: Physical properties of aggregates Property Sand

(Natural) Coarse Aggregate 20 mm 10 mm

Specific gravity 2.61 2.83 2.83 Water absorption (%) 0.8 0.40 0.40 2.3 Admixture Super plasticizer normal type confirming to IS 9103 was used in concrete mix design. 2.4 Concrete mix composition The research was primarily conducted on six water/ cement ratios (0.36, 0.40, 0.45, 0.50, 0.55, and 0.60) with three different varieties of cement i.e. OPC-43 and PPC and PSC, correspondingly resulting into 18 concrete mixes. The concrete mix proportion for all the six w/c ratio’s had been described in table 3 whereas the 28 days compressive strength of the concrete mixes was mentioned in table 4.

Table 3: Concrete Mix proportion Sr no

w/c ratio

Type of cement

Mix Constituents Air content (%) Cement

(Kg/m3) Sand

(Kg/m3) Coarse

aggregate (Kg/m3)

Water (Kg/m3)

Dose of admixture(%

by Wt of Cement)

1 0.36 OPC-43 444 736 1161 160 0.5 1.2 2 PPC 444 701 1152 160 1 3 3 PSC 444 728 1147 160 1 1.2 4 0.4 OPC-43 400 770 1165 160 0.4 1.5 5 PPC 400 727 1170 160 1 2.9 6 PSC 400 762 1153 160 1 1.1 7 0.45 OPC-43 356 805 1169 160 0.4 2

8 PPC 356 764 1170 160 1 2.1 9 PSC 356 799 1160 160 0.6 1.2 10 0.5 OPC-43 320 837 1167 160 0.35 1.8 11 PPC 320 803 1179 160 1 2 12 PSC 320 831 1158 160 0.6 1.1 13 0.55 OPC-43 300 849 1160 165 0.2 1.7 14 PPC 300 788 1178 165 0.8 2 15 PSC 300 863 1131 165 0.55 1.2 16 0.6 OPC-43 280 863 1155 168 0 2.2 17 PPC 300 801 1130 180 0.4 2 18 PSC 280 877 1127 168 0 1.4

Table 4: Compressive strength (in MPa) at 28 days

0.36 0.4 0.45 0.5 0.55 0.6

OPC-43 52.63 45.97 42.33 38.26 33.65 28.06

PPC 59.02 53.65 43.72 37.79 35.16 33.7

PSC 48.64 47.57 34.71 33.48 30.67 28.48 2.5 Specimen preparation For each concrete mix, a number of cylindrical specimens (dia =100mm, height= 200mm) and concrete slabs (300x300x100mm) were cast. After 24 hours, the concrete specimens were demoulded and water cured for 28 days. After 28 days of water curing, all the concrete specimens either cylinder or slabs were shifted to laboratory environment of RH= 65 ± 5 % and temp = 27 ± 2 °C till the age of testing. Concrete specimens for NT build 492, RCPT and Chloride immersion tests were obtained by sawing the cylindrical specimen whereas slabs were used for the determination of electrical resistivity and coefficient of air permeability. 3.0 Test Conducted 3.1 NT Build 492 This is a non-steady state chloride migration test which requires cylindrical concrete specimens of 100 mm diameter and a thickness of 50 mm. An external electrical potential was applied across the sample in order to force the chloride ions to migrate into the concrete. The solutions used in the cathode and anode are a 10% NaCl and a 0.3 N NaOH respectively. This test was initiated by applying a 30 V potential difference across the concrete specimen and thereby measuring the amount of current generated. Then, the applied voltage was adjusted and the test duration is determined. After the test, the sample was split and a silver nitrate

w/c Type of cement

solution was sprayed on the concrete surface. The penetration depth was measured from the chloride precipitation, and the migration coefficient was determined through Nernst Planck Equation. For research purpose, the test was conducted on the concrete specimens which were air cured for 28 days after 28 days of water curing. 3.2 Rapid Chloride Penetration Test (ASTM C1202) In this test method, a steady external electrical potential of 60 volts D.C potential was applied to the concrete specimen of 50 mm thick and 100 mm diameter for period of 6 hours. The anode and cathode were filled with 0.30 N sodium hydroxide and 3.0% sodium chloride solutions respectively. The total charge passed during the 6 hour test was recorded and used as a measure to chloride ion penetration in concrete. The test was conducted on set of concrete specimen at an age of 56 days which includes 28 days water and 28 days air curing. 3.3 Chloride immersion test, unidirectional diffusion (ISO 1920 Part 11) In this method, cylindrical specimens (dia = 100mm and length =75 mm) were immersed into the 3% Nacl solution for 90days and the chloride profile was measured immediately after the exposure by grinding off material into at least eight depth intervals parallel to the exposed surface followed by titration analysis in order to determine acid soluble chloride content in each concrete powder. Through non-linear regression analysis by least squares curve fitting as shown in Figure 1 for w/c 0.40 and 0.50, the non-steady state chloride diffusion coefficients (Cd) were determined by the following equation which is also known as Fick’s second law of diffusion.

×××−−=

tCd

xerfCCCtxC iss

2 )(),(

Similarly, chloride diffusion coefficients (Cd) were obtained for other w/c ratios.

Where Cx is the chloride content measured at average depth x and exposure time t, % by mass of concrete; X is the depth below the exposed surface to the midpoint of the ground layer, in (mm)

Figure 1- The chloride profile for w/c 0.40 and 0.50 obtained after 90 days of immersion test

Cd = 145mm2/year

Cd = 115mm2/Year

3.4 Electrical Resistivity (Four point Wenner Probe method) The electrical resistivity, inverse of conductivity, is the property of the material that reflects the ability to transport electrical charge [16]. In this method four equally spaced probes were applied to the concrete slabs (300x300x100 mm) in a line. The two outer probes induce the current to the specimen and the two inner electrodes measure the resulting potential drop. 3.5 Air Permeability This test was conducted on the concrete slabs (330x300x100 mm) which were water cured for 28 days followed by 28 days conditioning in laboratory environment which includes pre-conditioning for 3 days in an environmental chamber under a controlled temperature and RH of 50 °C and 80 %, respectively followed by 10 days storage in sealed container at 27 ± 2 ºC. The concrete slabs were tested at the age of 56 days. Vacuum is created inside the two-chamber vacuum cell (Fig 2) which is sealed onto the concrete surface by means of a pair of concentric soft rings, creating two separate chambers. At a time between 35 and 60 s valve 2 is closed and the pneumatic system of the inner chamber is isolated from the pump. The air in the pores of the material flows through the cover concrete into the inner chamber, raising its pressure Pi. The rate of pressure rise ∆Pi with time (measurement starts at to =60 s) is directly linked to the coefficient of air permeability of the cover concrete.

Figure 2- Sketch of air-permeability test

4.0 Results and Discussion 4.1 Effect of w/c ratio on durability test methods a) Effect of w/c on long term durability test method

Figure 3 shows the relationship between chloride diffusion coefficients Cd obtained from immersion test with w/c.

Figure 3- Relationship between w/c and chloride diffusion coefficient Cd b) Effect of w/c on short term durability test methods Figure 4 shows the relationship between chloride diffusion coefficients (Cd (NT)) obtained from accelerated migration test NT build 492 with w/c.

Figure 4- Relationship between w/c and chloride diffusion coefficient Cd (NT)

Figure 5- Relationship between w/c and RCPT (coulombs) Figure 3 and 4 shows a very good correlation between w/c ratio and chloride diffusion coefficient value Cd and Cd (NT) obtained from different test methods for PPC concrete and OPC-43 concrete. The Cd (NT) value for PSC concrete shows no trend with the change in w/c ratios, almost similar trend had been observed when PSC concrete was tested for Rapid chloride ion penetration test whereas OPC-43 and PPC concrete shows satisfactory relationship for different w/c ratios when tested for RCPT as shown in figure 5.

Figure 6- Relationship between w/c and electrical resistivity Figure 6, indicates a good correlation between electrical resistivity test values for blended concrete (fly ash and slag) and OPC concrete. It could also be seen that PPC and PSC concrete performance is better than OPC concrete in controlling the migration of electrical charge and performance of PPC concrete was better among the two.

Figure 7- Relationship between w/c and air permeability coefficient (KT)

Above figure 7, shows a very good correlation between w/c and air permeability coefficient values (KT). It had been observed that air permeability coefficient values for PPC and PSC concrete were considerably at lower side in comparison to OPC concrete at all w/c ratios. 4.2 Cd (chloride immersion test) vs Cd (NT build 492) The values of chloride coefficient obtained from immersion test is shown in figure 8 and compared with charge passed obtained from RCPT test for the concrete mixes of different w/c ratios. From Figures 8 shows a fair, not so good correlation between Cd with RCPT (coulombs)

Figure 8- Graph between chloride diffusion coefficient Cd and RCPT (coulombs) - Fair correlation

In Figure 9, the values of chloride diffusion coefficient obtained from accelerated migration tests NT build 492 (Cd (NT)) is graphically correlated with Chloride diffusion coefficient obtained from chloride immersion

Figure 9- Relationship between Cd (NT) and Cd 4.3 Cd (chloride immersion test) vs KT (Air permeability coefficient) The air permeability coefficient value reflects the pore geometry of the concrete specimen which is one of the crucial element in determining durability of cover concrete. Figure 10 shows a very good correlation between Cd (chloride diffusion coefficient) and KT (coefficient of air permeability value) when tested at the age of 56 days.

Figure 10- Graph between chloride diffusion coefficient Cd and KT- Good correlation

In Torrent permeability tester, air permeability coefficients for concrete specimen depend upon the quantum of vacuum so developed. So, in order to achieve desire relationship between chloride diffusion coefficient and air permeability coefficient values, the concrete specimens should be sufficiently dry so as to get pores within the concrete matrix at most free from water. But completely drying out the concrete specimens would not be a healthy practice as dry concrete actually does not exist in real situation [17, 18]. Therefore, a system was required to replicate the field conditions in laboratory which was achieved through air drying the specimen under specific relative humidity as well as temperature conditions. In NCB laboratory, air permeability coefficient values were generated from the concrete specimens which were water cured for 28 days followed by 28 days air curing which includes pre-conditioning regime also. It was found that keeping the concrete specimens into air for a time duration of 28 days which includes pre- conditioning regime also had led to the generation of intermediate moisture conditions with the opening of pores which were earlier occupied by the water. This had led to the generation of condition that actually prevails during the service of concrete structure. 4.4 Cd (chloride immersion test) vs Electrical Resistivity In Figure 11, the chloride diffusion coefficient values obtained from chloride immersion test is correlated graphically with the electrical resistivity values obtained from concrete specimens at the age of 56 days which includes 28 days of water curing followed by 28 days of air curing. Electrical resistivity of concrete containing blended cement increases with the increase in age. Therefore, correlation curve so generated between Cd and electrical resistivity at later age was more suitable rather than generating the correlation curve at 28 days or earlier ages.

Figure 11- Graph between Cd and Electrical Resistivity – Good Correlation

Conclusion 1. The w/c ratio has considerable effect on the long term durability test and short term test methods

Effect of w/c is quite evident in OPC concrete and PPC concrete whereas in PSC concrete dependency on w/c is found to be absent. The chloride diffusion coefficients obtained either from chloride immersion test (Cd) or from NT build 492 (Cd (NT)) shows an increasing trend with increase in w/c ratio which is true in case of OPC concrete and PPC concrete. But in case of PSC concrete dependency of chloride diffusion coefficient on w/c is much smaller.

2. Electrical resistivity and air permeability coefficient are greatly affected not only by the w/c ratios but effect of binder either slag or fly ash considerably influences the test values.

3. Non steady state migration test RCPT shows a fair trend with chloride diffusion coefficient generated from immersion test.

4. No Satisfactory correlation was found between chloride diffusion coefficient obtained from Chloride immersion test (Cd) and NT build 492 ( Cd(NT)).

5. Electrical Resistivity test and air permeability test correlate very well with chloride diffusion coefficient. These tests can be used for the determination of chloride ingress in concrete but such tests are affected by the conditioning of specimen. Therefore due attention is required while using electrical resistivity test and air permeability as a substitute for chloride ingression.

6. Further research work has to be done to obtain more data in order to further enhance the correlations so developed.

7. Time of accelerated tests can be further reduced by adopting accelerating curing techniques (developed for blended cement specifically). Work in this direction is in progress at NCB.

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