Materials Performance

MAR 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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57 MATERIALS PERFORMANCE: VOL. 57, NO. 3 MARCH 2018 FIGURE 4 SEM images of the control concrete S0 (left) and SAPs-containing concrete S7 (right) at 28 days of cure time. when the water froze, and relieved the hydrostatic pressure and osmotic pressure. Therefore, the freeze thaw resistance is greatly improved by adding SAPs. Shrinkage Ratio Testing The variations in the shrinkage ratios of the concretes after aging were studied, and the results are shown in Figure 2. There was a similar curve variation phenomenon over the 28 day cure time for the control con crete and the concretes modified with dif ferent SAPs content. The shrinkage ratio of concrete modified with SAPs improved with age as did the control concrete. How ever, when compared to the control con crete, the shrinkage ratio of modified con cretes tended to decline as the addition of SAPs increased. At three days, the shrinkage ratio of S1, S3, S5, S7, and S9 decreased by 3.41, 4.02, 9.53, 10.60, and 15.95%, respectively, com pared with the control concrete S0. When the cure time was increased to 28 days, the corresponding results were 4.18, 5.12, 9.89, 11.46, and 16.72%. It was concluded that SAPs became saturated by absorbing some additional water. Further, water was con sum ed durin g th e c ontinuin g c em ent hydration and hardening. These two fac tors combined changed the penetration pressure of the concrete. With time, water was released from the SAPs, and the internal RH of the concrete was maintained at a certain degree, which reduced the negative pressure of the con crete pore solution. The shrinkage defor mation of the concrete, therefore, showed a gradually decreasing trend. Sulfate Wet-Dry Cycle Testing The influence of sulfate wet dry cycles on the attack resistance coefficient of com pressive strength in concrete with and without the SAPs was evaluated, and the results are shown in Figure 3. Increasing th e numb er of sul fat e w et dr y c ycl e s decreased the sulfate attack resistance coefficient of compressive strength of con crete. The results imply, however, that the SAPs abated the sulfate attack's adverse effects on the durability of concrete. During the process of concrete wet dry cycles, the SAPs gradually swelled by absorbing water, and the internal pores of mortar were blocked, which prevented the sulfates from entering the interior of the concrete. Another important conclusion from Figure 3 was that the concrete's sulfate attack resistance coefficient of compres sive strength increased as the amount of added SAPs increased. It is possible that numerous microbubbles were produced that improved the pore structures of the concrete and provided a buffer space for ettringite (hydrous calcium aluminum sul fate) and gypsum (calcium sulfate dihy drate) crystals, which decreased the impact of expanding ettringite and gypsum crys tals on the concrete. Therefore, the durabil ity of concrete was significantly improved and would provide improved corrosion protection of embedded metals. Microscopy Examination To further understand the effects of the SAPs, the concrete specimens modified with SAPs were investigated with SEM. Fig ure 4 illustrates the change in observable morphology of the control concrete sample (S0) and the SAPs modified concrete sam ple (S7) at 28 days of cure time. Figure 4 shows that the interior of the S7 specimen includes some cement hydration products. The SEM images show that the lamellar structures appear to be calcium hydroxide [Ca(OH) 2 ] cr ystals and calcium silicate hydrate. The acicular structures appear to be ettringite. The SEM images of concrete with and without SAPs showed that the amount of hydration products of Ca(OH) 2 crystals, calcium silicate hydrate, and ettr ingite in the concrete with added SAPs exceeded that of the control concrete. In summar y, the saturated SAPs gradually rel eased s om e wat er to al low fur th er cement hydration, resulting in the improve ment of hydration products so that the durability of the concrete was significantly improved. Conclusions The inf luence of differing amounts of SAPs in concrete was investigated using freeze thaw cycles, shrinkage tests, and Durability of Concrete Modified with Superabsorbent Polymers

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