Materials Performance

NOV 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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41 MATERIALS PERFORMANCE: VOL. 57, NO. 11 NOVEMBER 2018 H poles employ a low-quality concrete, have low durability in environmental expo- sure, and after about five years, concrete degradation is visible in the lower part of the pole through color change of the con- crete. To resolve this deleterious situation, the following modifications were made in the H poles: 1. Use superplasticizer for reducing the water-to-cement ratio of the mix and increase the concrete strength. 2. Add silica fume as a super pozzolan material in the concrete mix design to reduce concrete permeability. 3. Increase the thickness of concrete cover over the rebar in the lower part of the H pole by converting the H cross section in the lower part to a cube cross section. 4. Apply epoxy-glass fiber coating in the slightly higher and lower part of the buried atmospheric zone to prevent the accumulation of corrosive agent and color changes from groundwater. Changing the line of production of the H pole so that it can produce prestressed H concrete poles came with a considerable expense. Although it was a good service- ability proposal, it was not accepted due to financial considerations. Results and Discussion Aggregate Tests The aggregates and water commonly used in preparing the concrete used for the H poles were investigated in terms of the amounts of corrosive agents, including chloride and sulfate. The chloride content of water was measured according to ASTM D1411-09, 5 and was 40 ppm . Based on ASTM D512-12, 6 the maximum permitted chloride content of water for use in new concrete is 1,000 ppm, so the typical mix water was within the permitted limit. Also, the chloride and sulfate content of aggre- gates were measured according to the grav- imetric method. Their values are shown in Table 1. Based on the Iran Concrete Regula- tion, 7 they were less than the maximum allowed content, which are 0.4 and 0.04% for chloride and sulfate, respectively. Sand equivalent (SE) value is another important factor that was considered and investigated. If SE value is low, it means there is a significant amount of particles <75 µm in the sand. Often these are clays that can c over th e a g gregat e sur fac e, thereby preventing sufficient adhesion of cement gel to the surface of the aggregate during the curing process. This reduces concrete strength and increases its perme- ability. The SE value was measured accord- ing to ASSHTO T176. 8 It was 73; therefore, it wasn't suitable for application in the con- crete mix design. The sand was washed with water to remove the clay particles and the SE value increased to 86. In addition, if concrete aggregates are reactive to the alkaline environment pro- duced during curing, it subsequently can produce an expansive silica gel in the pres- ence of moisture. The expansive gel pro- duces an internal pressure within the con- crete that creates surface cracks. This is cal l ed th e al kali si lica reaction (ASR) problem. ASR is a chemical reaction between dif- ferent active amorphous silica minerals of aggregate and the alkali pore solution of concrete. This reaction produces a dark gel around the aggregate that has high activity for the reaction with moisture and water. When the gel adsorbs moisture, it expands and produces cracks in concrete. 9 When ASR starts in concrete, it generally can't be controlled. ASR cracks facilitate penetra- tion of corrosive agents, like chloride or carbon dioxide (CO 2 ), into concrete that can initiate corrosion at the concrete-rebar interface. Like the ASR gel, the corrosion products are expansive and accelerate crack grow th and spalling that further expose the embedded rebar to corrosion. Another reason for concrete pole cracks i s delayed ettringit e formation (D EF). Ettringite is normally produced in early cement hydration. If the curing tempera- ture is above 70 to 80 °C, formation of ettr- ingite mineral is prevented. 9 For accelera- tion of curing, the concrete poles generally are cured in heated water vapor, so they can develop a DEF condition. After several years, and in lower parts of a concrete pole in contact with water or moisture, ettring- ite can be formed. Its formation is accom- panied by expansion and can cause con- c re t e c ra c k s . L i k e A S R , D E F c ra c k s accelerate penetration of corrosion agents, (a) (b) FIGURE 1 Two different types of concrete poles: (a) H cross section, and (b) round cross section or O section. TABLE 1. CHLORIDE AND SULFATE CONTENT OF THE AGGREGATE USED IN CONCRETE MIX DESIGN Aggregate Total % of Chloride Total % of Sulfate 12-25 mm Manufactured aggregate 0.01 0.14 6-12 mm Manufactured aggregate 0.01 0.16 6-12 mm Natural aggregate Less than 0.01 0.1 0-6 mm Sand 0.03 0.23

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