Combined effect of high curing temperature and crack width on chloride migration in reinforced concrete beams

Deterioration of reinforced concrete structures is a serious concern in the construction engineering, largely due to chloride induced corrosion of reinforcement. Chloride penetration is markedly influenced by one or several major factors at the same time such as cuing in combination with different crack widths which have spectacular effect on reinforced concrete structures. This research presents the results of an experimental investigation involving reinforced concrete beams with three different crack widths ranging from 0 to 0.2mm, curing temperatures of 20°C or 40°C and water-to-cement of 0.5. Chloride content profiles were determined under non-steady state diffusion at 20°C. Based on the obtained results, higher chloride content was obtained under condition of high curing temperature in combination with large crack more than 0.1mm and there are no significant differences between narrow crack width (less than 0.1 mm) and beams without crack (0 mm).


Introduction
Reinforced concrete structures are wildly used nowadays in ocean environment construction where they usually provide satisfactory performance during their service life [1]. However, some structures suffer performance deterioration due to the chloride attack which may corrode reinforcement bars in RC structures and lead to cover cracking, spalling or in extreme cases collapse of the structure [2]. Transport of chloride ion through concrete is a complicated process [3] which can be impacted by several factors. Curing temperature to which reinforced concretes are subjected is one of the factors affecting concrete resistance [4]. Concrete resistance does not depend only on the material compositions or the water to cement ratio but also on the curing temperature which is required for cement hydration to attain the needed durability [5]. Higher curing temperature will accelerate hydration process of the cement paste [6], which results in high volume of pores creating an easy path for harmful substances such as chloride ion [7], and it has a substantial effect on the strength and durability of concrete [8]. High curing temperature plays an important role in identifying chloride ingress rate [9] and results in high chloride ion penetration [10]. High curing temperature in combination with other factors such as applied loads and concrete cover cracking can have enormous effects on chloride diffusion [11]. Cracks create preferential pathways for chloride ion penetration and other aggressive agents [12]. Besides, it reduces the strength of RC structure and induces loss of serviceability [13]. Increasing the crack width leads to increase chloride diffusion [14]. It is of great importance to study the effect of cracks on chloride diffusion through concrete [15].
The combined impacts of high curing temperature and different crack widths on chloride penetration of reinforced concrete beams were experimental investigated in this paper. Total 54 RC beams were 2 1234567890''"" initially cured under the temperatures of 20°C or 40°C, respectively. A non-steady state rapid chloride ion migration test was performed on total of 54 reinforced concrete beam with different crack widths under 20°C. Chloride contents were then tested and the comprehensive impacts of different crack widths and curing temperatures on chloride migration were discussed here.

Beam specimens and testing program
Total 54 reinforced concrete (RC) beam specimens were divided into two groups, one group of 27 RC beam specimens were cured under the temperature of 20°C and another group of 27 beam specimens were cured under the temperature of 40°C. The RC beam specimens were demoulded after cast 24 h and then cured under the temperature of 40°C or 20°C, respectively. At the age of 28 days, the RC beam specimens were loaded by three points bending to make different widths of flexural cracks. Then rapid chloride migration test was proceeded and the chloride content were tested for the RC beam specimens with different widths of cracks under high curing temperature or normal curing temperature. The details of the experiments were introduced in the following.

Beam specimens
Total 54 beam specimens were manufactured in prefabrication factory with ordinary Portland cement (42.5) and water-to-cement of 0.5. All beam specimens were designed with the same geometry sizes, same concrete materials and same steel bars. The length of beam specimens was 400mm, the width and the height of cross section was 100mm and 120mm, respectively. Two reinforcing steel bars of 340mm in length and 10mm in diameter were embedded at a cover depth of 30mm as illustrated in Figure 1.

Initial curing conditions
After demoulding, both groups of RC beam specimens were cured under two different curing temperatures of 40°C or 20°C, respectively, until the age of 28d. The group cured in high temperature chamber of 40°C was labelled HT for short. Another group cured in normal laboratory temperature (around 20°C) was labelled NT for short.

Crack width by three-points loading test
After initial curing of RC beam specimens finished, the three points loading were applied on the RC beam specimens at the age of 28d using a hydraulic universal testing machine. The schematic diagram of RC beam specimens was shown in Figure 2. The static loading of three points beading was applying to RC beam specimens until different widths of around bending cracks appeared, then unloaded. The maximum width of bending crack at tensile extreme fibres of RC beams are tested by crack width measuring and recording instrument, here the real flexural cracks were made by three points loading.
The RC beam specimens were classified into three types according to the testing results of the crack widths. First type of crack widths ranges from 0 to 0.1mm was named narrow cracks or named 'N' for short. Second type of crack widths range from 0.1 to 0.2mm was named wide cracks or named 'W' for short. Third type of cracks, as a reference, the beam specimen without crack was named '0'.

Rapid Chloride Migration Test
The chloride penetration resistance of cracked RC beam specimens was tested by means of the nonsteady-state rapid chloride migration (RCM) test, pushing chloride ion to penetrate through cracked concrete cover of RC beam specimens. The principal of RCM is similar to NTbuild492 [16], also called 'Nordtest' and was developed by several authors [17]. The schematic diagram of RCM test of RC beam developed here was shown in Figure 3, a gluing PVC pipe of 40mm high and 60mm diameter was put on tensile cracked or un-cracked surface and impenetrable all the other cracked area with epoxy resin to assure one way of penetration. To make sure the saturation of RC beam with water, it was immerged in pure water for 4 to 5 days except the covered PVC area. An electrical potential difference of 20 to 30 V was applied axially across the tensile cover depth, using steel tablet dipped in NACL solution as anode and steel bar as cathode which forced the chloride ion outside to migrate into concrete cover, as illustrated in Figure 3. The RCM test for each RC beam specimens lasted 24 hours.

Sampling and Chloride Testing
After rapid chloride migration test was performed, a cylinder sample with a depth of 30 mm was drilled at the cracked area which was covered by the PVC pipe in order to evaluate chloride penetration. Each drilled cylinder sample was then subdivided along the cover depth into 5 slices at a thickness of 6 mm. The sampling of concrete from RC beam was given in Figure 4. The slices were grounded into fine powdery particles to test free chloride content.

Chloride content profiles
After rapid chloride migration test was performed, the results of total chloride content with three types of crack widths under high curing temperature or under normal curing temperature were shown in Figure  5a and 5b, respectively. Notice: Attention has to be drawn on chloride content of RC beam specimens  named '0' (without crack): beam specimens suffered a significant rate of chloride penetration that can be clarified by presence of micro-crack. As illustrated in Figure 5, the chloride content profiles show much higher values near the exposed surface and it increase significantly when increasing crack width in both considered curing temperatures. Both groups share the same variation trend of chloride content which is high at the beam exposure surface and steeply decreases reaching 10 mm of depth. Thereafter, chloride content decreasing rate deaccelerate for a depth range from 10 to 15 mm, then it becomes almost stable reaching the steel bar surface for all concrete beam specimens.
The effect of crack width at both curing temperatures groups was significant, where increasing crack width, imperatively, increase chloride content. High rate of chloride content was observed under condition of crack width large than 0.1mm, this indicate that crack width would be considered as one of the major factors affecting chloride incursion in concrete structures. For high curing temperature, chloride content distribution was higher than that of normal curing temperature. According to [9], curing at high temperature results in high volume of pores wider than 150 nm which leads to higher rate of chloride ion flow comparing to normal curing temperature; this point to that un increasing in chloride content due to an augmentation in curing temperature can be marked as one of major factors influencing chloride penetration in concrete. Chloride content is inversely proportional to depth, whenever depth increases, chloride content value decreases.
(a) (b) Figure 6 demonstrates a comparison of chloride concentration versus depth for similar crack widths in both considered curing temperatures. Usually, cracks form flow paths inciting higher concrete permeability and makes easier incursion of aggressive agent through concrete resulting in deterioration and loss of severability [18]. For the three considered crack widths, chloride content has the same tendency of distribution, increasing depth, chloride content value presents no substantial changes for both considered curing temperatures. Beam specimens with wider crack width 'W' display higher chloride concentration than that of 'N' and '0' specimens.

Chloride Content at the Surface Steel Bar
The steel bar surface located at the maximum depth of concrete cover as shown in Figures (5, and 7) demonstrate the variation of chloride concentrations at the steel bar surface in both curing temperature groups for the three considered crack widths. The experimental data of Figure 7 were magnified 10 times in comparison with those of Figure 5. As it is illustrated in Figure 7, beam specimens with wide crack 'W' exhibit high chloride content values than crack width 'N' and '0', which leads to early corrosion of steel bar. The RC beam specimens named HT (curing in high temperature) display higher chloride content compared to those of NT (curing in normal temperature) RC beam specimens; this can be explained by the fact that curing in high curing temperature results in an acceleration in hydration  process of the cementitious material [19]. Also, it can be explained by the fact that raising the curing temperature leads to modify the porosity's characteristics by loss of free and absorbed water [20]. As it is shown in Figure 5, for the same curing temperature, wider crack results in high chloride content.

Conclusion
The ensuing conclusions are based on the test results stated in this paper: Chloride concentration of reinforced concrete with different cracks is extremely dependent on the crack width. For both curing temperature, high values of chloride content were obtained in RC beams with crack width larger than 0.1 mm (wide crack) and much less in RC beams with crack width less than 0.1mm (narrow crack). This indicates that wide crack results in high chloride penetration rate.
Reinforced concrete beams under high curing temperature condition (40°C) displayed higher chloride content compared to normal curing temperature (20°C). This indicates that curing temperature have an obvious impact on chloride penetration.
For RC beam specimens with crack width 'N' (less than 0.1mm) and with crack width '0' (without crack), the chloride content profiles were almost equal for both curing temperatures. This shows that the effect of narrow crack width (less than 0.1mm) is nearly the same as without crack (0mm).