a) Floor slab with high SO₃ concentration (No. 1); Significant deterioration accompanied with widely distributed cracks is often found in the bottom surface of reinforced concrete (RC) floor slabs of steel bridges aged about 40 years in cold regions with snow cover as shown in Figure 1 (27). These RC slabs are being replaced with those made of prestressed concrete (PC) floor slabs in these several years for the safety of traffic. The sample used in this study was a full depth core (55 mm diameter, 215 mm long) taken from a piece of removed RC slab.

b) Parapet exposed to cold weather (No. 2); The parapet had been placed around a dam lake in an inland mountainous area (Figure 2). The cast in situ concrete was covered with mortar. Numerous cracks with white exudate were found in the side faces of the parapet. Surface concrete has peeled, and white products were also found on the exposed faces. The sample in this study was a fragment of scaling concrete with mortar adhering to it.

c) PC pole with vertical cracks (No. 3); The PC pole in this study was of a tension-type which was required to have a high strength. As shown in Figure 3, two vertical cracks originating from the ground level occurred in the opposing positions in about ten years from the installation. The pole with a nominal strength of 80 N/mm², a W/C of 30.5% and a cement content of 525 kg/m³ had been manufactured by applying centrifugation in a centrifugal casting machine, followed by 4 hours of steam curing at 70°C (28).

d) Precast concrete product with cracks (No. 4); A precast concrete product which had been used outdoors was found to have web-like cracks in the surface. The concrete could have been steam cured during the manufacturing process. The sample was a fragment of concrete taken from the surface area.

The radioactive substances and other contaminated wastes, generated from the accident at the Fukushima Daiichi Nuclear Power Station and being processed for volume reduction, include incineration fly ash and others with a high content of chlorides. In order to investigate the risk of storing such fly ash in sealed precast containers, an experimental test was performed by filling a real-size concrete container with a paste of incineration fly ash (with addition of CaCl₂ and water) and applying the most severe conditions assumed for concrete containers (29). To make observation of concrete easier, a cylindrical specimen (100 mm diameter, 200 mm long) was placed in the precast concrete container filled with fly ash-water mixture with an addition of CaCl₂. The mix proportions (water: 150 kg/m³; cement: 370 kg/m³; lime based expansive admixture: 25 kg/m³) and the curing conditions (steam curing for 3 hours at temperatures up to 65°C) used for the cylinder were the same as those of the precast concrete container. The specimen after four months of exposure to fly ash had cracks and scaling in the circumferential surface as shown in Figure 4.

Thin section specimens were prepared from the samples, and polarizing microscopy using a polarizing microscope was performed to determine rock types of aggregate, shapes of cracks and occurrence of products.

The polished thin sections after the polarizing microscopy were treated by carbon vapor deposition, and scanning electron microscopy was performed by using an electron microscope. Microstructural observation was made with the obtained backscattered electron images (BEI).

Some products observed under the polarizing microscope were too fine to identify. In order to identify these products from their compositions, elemental quantitative analysis was performed by energy-dispersive X-ray spectroscopy (EDS) under the electron microscope, and ZAF correction was made to the analysis results. The target elements were SiO₂, TiO₂, Al₂O₃, Fe₂O₃, MnO, MgO, CaO, Na₂O, K₂O, SO₃ and P₂O₅. Chlorine (Cl) was also assayed for the fly ash-exposed concrete sample. EDS mapping of SO₃ was performed for the floor slab sample.

a) Floor slab with high SO₃ concentration (No. 1); Aggregate consisted of gravel and sand mainly comprised of granite, diorite, rhyolitic welded tuff and andesite. ASR cracks were found extending from aggregate particles of andesite, rhyolite or rhyolitic welded tuff into the cement paste (Figure 5a). The depth of carbonation was 20 mm in the bottom surface, while being 3 mm in the top surface which had been in contact with the asphalt.

b) Parapet exposed to cold weather (No. 2); Aggregate consisted of gravel and sand mainly comprised of siliceous shale, shale, sandstone and andesite. ASR cracks were found extending from the aggregate particles of andesite into the cement paste, and needle-like crystals were observed to fill the cracks in the cement paste (Figure 5b). Several parallel cracks and scaling were found in the concrete surface immediately below the mortar cover, with needle-like crystals filling the cracks (Figure 5c). No entrained air voids were found in the cement paste.

c) PC pole with vertical cracks (No. 3); In the PC pole manufactured by centrifugal casting, a larger amount of coarse aggregate was present near the surface, and larger amounts of fine aggregate and cement were present in the inside. The coarse aggregate was gravel mainly comprised of diorite, granite, gabbro, gneiss and rhyolitic welded tuff, and the fine aggregate was sand mainly comprised of rhyolitic welded tuff, granitic rocks and andesite. ASR was found around the coarse aggregate particles of rhyolitic welded tuff near the surface of the pole (Figure 5d), and around the fine aggregate particles of andesite in the inside (Figure 5e). ASR was more significant in the inside of the pole where the andesite content was higher. In the inside, ASR cracks were propagating into the cement paste, and they were filled with needle-like crystals that formed to replace the ASR gels (Figure 5e). Most of the ASR cracks developed concentrically parallel to the circumference of the pole, but some cracks without products were also found growing in the direction perpendicular to them.

d) Precast concrete product (No. 4); Coarse aggregate consisted mainly of crushed stone of sandstone and shale, and fine aggregate was sand comprised of granite-derived crystalline and rock fragments. No ASR was observed in the concrete under the polarizing microscope, while there were gaps filled with needle-like crystals between aggregate particles and cement paste (Figure 5f).

a) Floor slab with high SO₃ concentration (No. 1); Figure 6 shows an EDS map of SO₃ in the bottom surface of the slab. High concentrations of SO₃ were found at around 180 to 195 mm from the top surface of the slab, with some variations due to the inhomogeneity within the observation area where cement paste and aggregate particles were present. No cracks were found in this area. Cement hydrates were found to have needle- or plate-like crystals at the center, and formation of ettringite (Figure 7a) was confirmed by the EDS analysis (Table 1).

The ideal composition formula of ettringite is 3CaO, Al₂O₃, 3CaSO₄, 32H₂O (CaO: 26.81%, Al₂O₃: 8.12%, SO₃: 19.14%, H₂O: 45.93%), and the total EDS analysis value is low due to the presence of bound water. Since some of the bound water in ettringite undergoes dehydration in the electron microscope, a high vacuum condition, the total of CaO+A_l2O₃+SO₃ is 54.07% in the ideal equation, but the actual total analysis value of the three elements is as high as 72%.

b) Parapet exposed to cold weather (No. 2); The needle-like crystals filling the cracks caused by scaling were confirmed to be ettringite by the SEM observation (Figure 7b) and EDS analysis (Table 1). Ettringite was also found in the cracks caused by ASR in the cement paste.

c) PC pole with vertical cracks (No. 3); The needle-like crystals (Figure 7c) were observed in the form of cross section of their slices under the electron microscope, and they were confirmed to be ettringite by the EDS analysis (Table 1).

d) Precast concrete product (No. 4); Numerous fine cracks (Figure 7d, 7e) were observed in the cement paste under the electron microscope. Needle-like crystals were found filling the fine cracks as well as the gaps between aggregate particles and the cement paste, and they were confirmed to be ettringite by the EDS analysis (Table 1).

Cracks and scaling were found along the circumference near the surface of the specimen as shown in Figure 8a. The cracks were extending along the peripheries of aggregate particles into the cement paste. These surface cracks were observed within a depth range of about 3 mm from the circumferential surface of the specimen. Needle-like crystals were found to be present in the cracks under the polarizing microscope (Figure 8b). No ASR was confirmed.

The needle-like crystals in the cracks were found to intersect at almost right angles each other under the electron microscope (Figure 8c). Its composition was confirmed to be 3CaO•CaCl₂•15H₂O by the EDS analysis (Table 2). The total EDS analysis value of 3CaO•CaCl₂•15H₂O is low due to the presence of water. Like ettringite, some of the bound water in 3CaO•CaCl₂•15H₂O undergoes dehydration in the electron microscope, then the actual analysis value becomes a little higher than the ideal total of CaO+CaCl₂ in the ideal equation, which is 50.8%.

As one of the concrete deterioration phenomena caused by deicing salt, chemical deterioration is known to occur in concrete immersed in a highly concentrated solution of CaCl₂ of 30°C or below, being accelerated by the action of dry and wet cycles (33). One of the possibilities for the deterioration of concrete is the leaching of Ca(OH)₂ from cement paste making the paste porous (29). The other possibilities is the crystal growth pressure caused by the formation of 3CaO•CaCl₂•15H₂O (hereafter, the 3-1-15 crystalline phase), which is formed by the reaction of Equation [1] (18):

The 3-1-15 crystalline phase is considered to result in volumetric expansion to about two times Ca(OH)₂, pressure of which acts on the hardened concrete and ultimately causes expansion failure (33).

Reported strain of the precast container indicated expansion in winter and overall contraction in other seasons (33). Therefore, the reason of the parallel cracks at the concrete surface can be attributed to frost attack due to the freezing and thawing and combined formation of the 3-1-15 crystalline phase within cracks.