CORROSION AND DURABILITY OF POLYMER MODIFIED CONCRETE

Moetaz M. El-Hawary*, Kuwait Institute for Scientific Research, Kuwait

Ali Abdul-Jaleel, Kuwait Institute for Scientific Research, Kuwait Thamer AI-Yaqoub, Kuwait Institute for Scientific Research, Kuwait

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29th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 25 - 26 August 2004, Singapore

CORROSION AND DURABILITY OF POLYMER MODIFIED CONCRETE

Moetaz M. El-Hawary·, Kuwait Institute for Scientific Research, Kuwait Ali Abdul-Jaleel, Kuwait Institute for Scientific Research, Kuwait

Thamer AI-Yaqoub, Kuwait Institute for Scientific Research, Kuwait

Abstract

Corrosion of reinforcement is considered the main type of concrete deterioration in Kuwait as its rate increases with temperature, humidity and the presence of chloride.

Polymer and cement may be used together to form what is known as Polymer Modified Concrete. One of the advantages of introducing polymer in concrete is to increase its corrosion resistance. Epoxy coated bars has been used to reduce corrosion but was found to localize corrosion in certain areas as they usually get scratched. The main objective of this proposed work is to investigate the corrosion resistance of reinforced Polymer Modified Concrete in the hot marine environment and the possibility of introducing epoxy in concrete to improve its durability. . Different percentages (0, 10,20,40,60, 100%) of cement were replaced by epoxy. Cylinders, cubes and special reinforced prisms were utilized. Specimens were put in the testing tanks of a specially manufactured accelerated marine durability system, where they were exposed to cycles of sea water wetting and hot air drying. Specimens were examined after 90 and 150 cycles of exposure. The specimens were then tested to investigate the effect of different polymer percentage and number of exposure cycles on the compressive strength, absorption, chloride penetration and steel corrosion. Corrosion was investigated using the half cell corrosion meter beside the actual determination of loss in bar weight and diameter.

The introduction of epoxy in the concrete mix was found to increase the corrosion resistance, reduce permeability, reduce chloride penetration and increase strength. The improvements were found to increase with the increase in epoxy percentage.

Keywords: Epoxy, polymer modified concrete, corrosion, durability, accelerated testing

1. Introduction Due to the harsh environment in Kuwait, concrete usually experience some type of

premature deterioration. Corrosion, which is an electrochemical process is one of the most common causes of deterioration in Kuwait. The heat, humidity and high concentration of carbon dioxide due to industry and large number of vehicles in Kuwait, promotes corrosion. The high concentration of chlorides in the Gulf, which could be transferred to structures through ground water or water-laden wind also increases the rate of corrosion in Kuwait. Many techniques have been tried to reduce corrosion either by improving the quality of the concrete matrix, increasing the cover thickness, using

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different types of reinforcing materials such as FRP or by the introduction of a sacrificial material. One of the most common corrosion control techniques is the use of epoxy coated bars. The coated bars may lead to more dangerous localized corrosion as bars often get scratched . The use of modified concrete (PPCC) will reduce corrosion without the need to pre-treat the steel bars and without the fear of scratching bars during transportation and erection. Polymer Portland Cement Concrete ( Polymer Modified Concrete), in which a percentage of cement is replaced by polymer, is cheaper than polymer concrete, requires the same technology as conventional concrete and it can "breath" while possessing about the same advantages as polymer concrete. PPCC has been used in patching and overlaying deteriorated bridge decks. The excellent bond strength, freeze-thaw resistance, resistance to penetration of chlorides and its ease of application have made it a widely used material. PPCC has been also used in producing prefabricated brick panels, in industrial floors, in sewer pipes and manholes ahd in piling. The PPCC may also be used in structural members where high ductility is required .

Epoxy is the most familiar and well known type of polymer to civil engineers. The word epoxy is related to the structure of the polymer as it is derived form the Greek language and is composed of "epi" which means outside and "oxy" from oxygen as it has oxygen on the outside. Epoxy is widely available in the Kuwaiti market, cheap and most importantly water based which means it could be mixed easily with the water based cement mortar. Polyester for example is cheaper than epoxy but it requires the introduction of some type of surfactant to allow for the production of a homogeneous mixture when mixed with the water based cement mortar. This will increase the cost and complicate the production technology. Epoxy is utilized for the production of PPCC in this work.

Extensive work was conducted on polymer- concrete materials applications including comparison between epoxy and other repair materials[1], effect of fire on epoxy repaired structures[2], physical properties of epoxy resins such as creep[3] amd impact loading on epoxy repaired concrete[4]. The studies performed on polymer concrete include flexural behavior of polymer concrete[5] and the properties of some types of polyester concrete[6].

Polymers may be also used in polymer impregnated concrete in which a monomer impregnate concrete pores, either fully or partially and in polymer modified concrete in which the binder consists of both polymer and cementitious matrices [7,8]. The structural design and behavior of polymer concrete and polymer modified concrete have drawn attention[9] .

. The durability of the different types of polymer-concrete materials in the hot marine environment of Kuwait is of major concern. Some work was conducted by the author, and others, on the behavior of conventional concrete at high temperature [10], where the relations between temperature and concrete compressive strength, steel tensile strength and other properties of concrete were established and quantified. Work was also conducted on the durability of an epoxy mortar system[11], where the compressive, tensile and flexural strength were evaluated for polymer mortars subjected to sulfuric acid, hydrochloric acid and sodium chloride solution at different temperatures, and on the effect of temperature on the mechanical behavior of polymer concrete[12], in which compressive, tensile and flexure strength of polymer concrete along with the stress-strain relations were evaluated. An extensive research was also conducted on the behavior and durability of conventional concrete elements repaired using different types of polymers and subjected to sea water, elevated temperatures and tidal zone in Kuwait[13] . The tidal zone effect was captured by hanging specimens in fishing nets tied to a pier in such a way that they get immersed in sea water during the high tide periods only.

Due to the hot marine environment of Kuwait, corrosion of steel reinforcement is the main cause for the loss of durability in concrete structures. Corrosion is an electrochemical process that propagates rapidly in the presence of humidity, high temperature, carbon dioxide and chloride ions. The research work on corrosion is extensive and it includes the effect of chlorides and sulphates[14], where the effect of concentrations of sodium chloride and sodium sulphate solutions on the corrosion potential of concrete was evaluated. The corrosion protection including cathodic protection[15], where many methods of corrosion protection, such as cathodic protection, galvanizes steel, stainless steel and the use of inhibiting admixtures were discussed. The corrosion in high performance concrete was also discussed and evaluated[16] . Other investigated subjects include the effect of epoxy coating[17], where it was found out that the use of calcium nitrite as a corrosion inhibitor with epoxy coated bars will increase durability and reduce corrosion risk, while in the latter an analytical model was used to evaluate bond strength of epoxy coated bars. The behavior of epoxy coated bars in marine

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environment was discussed by the author[18] where it was found that immersing concrete specimens with epoxy coated bars in sea water does not reduce their bond strength. The main objective of this proposed work is to investigate the corrosion resistance of reinforced Polymer Portland Cement Concrete in the hot marine environment and the possibility of introducing epoxy in concrete to improve its durability.

2. Materials and mix proportions The used mix consists of 445 Kg of Portland cement, 800 Kg of sand, 645 Kg of 3/8 inch and

640 Kg of % inch coarse aggregates. Water cement ratio of 0.45 was used. Six different percentages (0, 10, 20, 40, 60, 100%) of cement were replaced by epoxy, where the 0% represent the conventional concrete and the 100% represent polymer concrete . Two types of epoxy were considered for this work, the fist is Conbextra EPGPTM which is based on solvent free epoxy resin recommended by the manufacture as free flow grout, while the second is SPEC COATTM which is also based on solvent free epoxy resin recommended for repair work. The results reported in this paper are related to the first material only as the second, being recommended for overhead hand repair jobs, rendered mixes with zero slump that cannot be utilized for general applications.

3. Accelerated durability testing apparatus Durability assessment and evaluation usually requires long time durations. An accelerated durability testing apparatus was designed and assembled in laboratory. The system consists of two tanks for keeping specimens, a sea water storage tank, a pump, a control panel and hot air blowers. The system is shown in figures 1,2. Specimens are subjected to alternate cycles of wetting in sea water and drying under hot air to simulate the hot marine environment in Kuwait.

Index: I-Testing tanks 2-Seawater reservoir 3-Pump 4-Valve

5 6 5 5-Hot air distributor •n n 1 1 4

=

6-Control

F~ 2

Figure 1: Schematic of accelerated marine durability testing system

4. Samples preparations and testing Three types of specimens were used, cylinders of 100mm in diameter and 200mm in height, cubes of size 100mm and special reinforced prisms of 70*1 00*200mm. The prism sample is shown in figure 3. Special plywood moulds were manufactured for this type of samples with a hole in the side to fix the steel bar in place. Ten mm diameter reinforcing steel bars were used and the exposed portions of the bars were protected with epoxy to allow for steel deterioration through the concrete only.

The coarse and fine aggregates were mixed and put in mixer . Water was added to cement and poured on mixture. Epoxy resin and hardener were mixed for two minutes before being added to the mix. The mixture was mixed for five minutes before being pored into moulds. Moulds were treated with grease to allow easy dismantling . Samples were vibrated on the vibrating table, cured in humidity chamber for 28 days before being put in the accelerated durability testing tanks.

For each mix the slump and the compression strength at 7 and 28 days were evaluated. Those values are given in table 1 .

Samples put in the accelerated durability system were subjected to cycles of sea water wetting followed by hot air drying. Samples were subjected to 1 hour of immersion in sea water at room

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Figure 2: Accelerated Durability System

200mm

Figure 3: Corrosion Specimen

Table 1: Summary of Compressive Strength Results

Mix

No.

Epoxy Slump Compressive Strength (k~/cm2)

Type ( %) (cm) 7 days 28 days 1 B (control) - 0 3.0 285.3 374.6 1 B-10 Contbextra EPGP 10 9.0 251 .6 314.1 1B-20 Contbextra EPGP 20 16.0 270.4 387.3 1B-40 Contbextra EPG P 40 20.5 275 .8 384.6 1 B-60 Contbextra EPGP 60 Flow 205.5 265.2 1B-1 00 Contbextra EPGP 100 Flow 491.5 520.9

temperature followed by 2 hours and 15 minutes of hot air drying at 45°C making the duration of each cycle 3 hours and 15 minutes. The used water was obtained from the Gulf, filtered and subjected to UV. TDS (total dissolved salts) and PH of the sea water in testing tanks were monitored regularly and were kept constant. Samples were tested for corrosion after 0, 90 and 150 cycles. Samples were tested for corrosion using The Great Dane™ which measures the potentials and the electrical resistance between the reinforcement and the concrete surface. Samples were wetted and the measuring cell was used to measure the potential in mV and the resistance in kOhm. After 150 cycles samples were broken and corrosion was also evaluated through visual observation and through

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evaluation of the reduction in both the diameter and the weight of each steel bar, which were evaluated before casting the samples . Possibility of corrosion was also evaluated through the chloride penetration test. In this test the electrical conductance of concrete is determined to provide a rapid indication of the concrete resistance to the penetration of the chloride ions. Higher rate of penetration increase the possibility of corrosion. A disk of depth 50mm was cut from each cylinder and was tested for chloride penetration according to ASTM C1202. Test was terminated as temperature reached 60°C.

As a measure of deterioration resistance, cubes were tested for density, water absorption according to ASTM C413 and compressive strength after 150 cycles of wetting and drying. Three samples were tested for each type of test, epoxy percentage and exposure period. Average results are reported.

5. Results and discussion The relation between the epoxy percentage and the potential difference after 0, 90 and 150 cycles of sea water immersion and subsequent hot air drying is shown in figure 4. As expected the increase in the epoxy percentages increase the corrosion resistance. The corrosion also increases with the number of sea water exposure cycles. The resistance readings were less than 10 kOhm for all epoxy percentages except for 60% where the average resistance was 97.5 kOhm for 90 cycles exposure and 54.5 kOhm for 150 cycles. The high resistance indicates that either concrete is dry, existence of membrane or existence of air voids . As concrete was wet and no membrane was used, air voids are expected to be present. This indicates that cement was coated by epoxy and little hydration took place, leaving voids after drying. This also could explain the low compressive strength at high epoxy percentages.

-500 T-450

-400

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Figure 4: Relation between epoxy percentage and potential difference for various exposure cycles

The reductions in the bar weights with the epoxy percentages after 150 cycles of sea water exposure are shown in figures 5. It can be seen that the introduction of epoxy, even at low ratios, increase the corrosion resistance.

The variation of compressive strength is shown in figure 6. The introduction of epoxy increased the compressive strength except at high percentages, as discussed before for this type of epoxy.

Permeability is considered a good indication for the general rate of deterioration in concrete. Low permeability concrete has higher resistance to all types of chemical deterioration as no chemical reaction will take place in concrete without the presence of moisture which generally enters concrete through the pore system. Water permeability for all epoxy percentages is shown in figure 7. The increase in durability due to the introduction of epoxy is obvious.

As chloride ions penetration is the main cause of corrosion in marine environment regions, chloride penetration rates were recorded for each epoxy percentage after 150 cycles of sea water exposure, as shown in figure 8. The introduction of epoxy effectively reduced the rate of chloride penetration in most cases and hence reduced the possibility of corrosion.

241

The introduction of 20% epoxy as cement replacement has reduced the electrical potential by 52%, 41 % and 30% for 0, 90 and 150 cycles of exposure, respectively, with an average reduction of 41 %, while increasing the compressive strength by about 5%, reducing permeability by 40 .9%and reducing chloride penetration after 150 exposure cycles by 30%, moving it to below 4000 Columb which is the high chloride penetrability limit according to ASTM C1202.

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-; 0.4 o ~ o 0.3 ~

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Figure 5: Reductions in weights of steel bars with epoxy percentage

N 600 -.,..--,.....,......,=-.....,..""..........",.-,-,.,.,..,-----,-,-...,...-,::-:-=-=0-."--...........,,.----.

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500

400 ~

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Figure 6: Variation of compressive strength with epoxy percentage

242

- -

8.00

7.00

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1.00

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.0

0 20 40 60 80 100

Epoxy %

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8 4000

J3000

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Figure 8: Variation of Chloride penetrability with epoxy percentage after 150 cycles

6. Conclusions The following conclusions and recommendations are extracted from the study presented above:

1, To avoid the generally very slow durability testing, an accelerated technique has been established for testing concrete durability in the hot marine environment in much shorter periods than conventional methods.

2. In addition to the already established advantages of the polymer modified concrete, polymers may be introduced while mixing to reduce corrosion risks in concrete while avoiding the common problems usually arise when epoxy coated bars are used,

3. Additives that alter epoxy behavior, such as those used to increase durability, may affect the compatibility between the polymer and cement paste and hence may alter the behavior of the polymer modified concrete.

4. The increase in the polymer percentage correspondingly increases the corrosion resistance of the resulting polymer modified concrete .

5. Introduction of epoxy increases the concrete durability in general as it reduces the concrete permeability.

6. Chloride ions penetrability was also reduced as a result of introducing epoxy in concrete mixes. The reduction is proportional to the epoxy percentage as cement replacement.

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7. The prices of polymers are diminishing and their use in construction is inevitable. The replacement of 20% of cement with epoxy is recommended to achieve acceptable corrosion resistance in hot marine environments.

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