Combined effects of vertical spacers and segregation on mass transport properties of reinforced concrete

2.1

Materials and mix proportion

Plastic ‘A’ clip spacers (PS) and single cementitious spacers (CS) to achieve 50 mm cover were used in this study. The spacers (Fig. 1) were obtained from a major manufacturer. The plastic spacer was made of polyvinyl chloride (PVC) and has a surface area of 2560 mm2. The cementitious spacer was made of fibre-reinforced Portland cement mortar containing 50% replacement of cement with ground-granulated blastfurnace slag (GGBS) at a water/binder ratio of 0.35. The cementitious spacer has a water accessible porosity of 8.5% determined by vacuum saturation and a surface area of 1860 mm2. Single cover spacers were selected for this study because these are the most widely used spacers to support vertical reinforcements.

Plastic ‘A’ clip spacer and cementitious spacers for 50 mm single cover used in this study

Full size image

An ordinary Portland cement CEM I concrete mix was designed based on absolute volume method at free water/cement (w/c) ratio of 0.4. The cement complied with BS EN 197–1:2011 [29]. Thames Valley sand (< 5 mm) and gravel (< 10 mm) complying with medium grading [30] were used as fine and coarse aggregate respectively. The gravel had a specific gravity of 2.75 and 24-h absorption of 0.6%, while the sand had fineness modulus of 2.76, specific gravity of 2.51 and 24-h absorption of 0.62%. The total aggregate fraction was 70% vol. and sand to total aggregate mass ratio was 0.4. A superplasticiser (MasterGlenium SKY 920) was added to achieve good consistence and workability retention. Tap water was used as batching water, which was adjusted to account for aggregate absorption and water from the superplasticizer.

Several trial mixes were carried out to determine a viable mix at 0.4 w/c. The final mix proportions were 167 kg/m3 water, 418 kg/m3 CEM I, 728 kg/m3 sand, 1092 kg/m3 gravel and 0.75% wt. cement superplasticizer. This produced a slump of 120 mm when tested in accordance with BS EN 12,350–2: 2009 [31], which indicates a high workability mix (consistence class S3) suitable for reinforced concrete columns and walls [32, 33].

2.2

Sample preparation

Thirty-six rectangular columns (80 × 90 × 400 mm3) were cast vertically in specially fabricated timber formwork. Each column contained a high-yield steel reinforcing bar (Ø12 mm, ribbed) fixed to the formwork to achieve 50 mm cover using two spacers near the top and bottom as shown in Fig. 2. The column dimensions were selected to ensure sufficient clearance for concreting and to mimic the thickness of concrete layers placed on site, which are typically < 500 mm [24, 25]. Control reference specimens were prepared by casting columns without internal spacers. The formwork was clamped tightly using external threaded steel rods to achieve the required dimensions and to avoid leakage. Prior to casting, a thin layer of release agent was applied to internal surfaces to achieve a smooth finish and avoid defects (e.g. bugholes) that may influence the penetrability of the cover zone.

a Schematic diagram of the vertically cast column 80 × 90 × 400 mm3; b cross-section showing the position of spacers and rebar, and the extracted top and bottom samples for transport testing; c top view of the formwork during casting. Dimensions in mm

Full size image

All ingredients for the concrete mix were batched by weight. Cement, sand and gravel were first dry-mixed in a 50-L capacity pan mixer for 30 s to ensure the dry solids were well-mixed before addition of water. Batch water pre-mixed with the required amount of superplasticiser was then added and mixed for a further 3 min. The columns were compacted in four uniform layers using a vibrating table (Vibtec FFT 2000 × 1000) with adjustable frequency control. This was chosen for the study since compaction of vertical elements on-site is usually carried out with external vibration rather than internal. Vibration was continued until large entrapped air bubbles ceased to appear and the surface started to glisten.

Many trials were carried out to determine the frequency and period of vibration required to achieve good compaction around rebar and spacers by visual inspection and image analysis (Sect. 2.7). This was found to be 6.5 Hz for 40 s. In another set of experiments, replicate columns were prepared in the same manner, but compacted at a higher frequency of 8.5 Hz and longer duration of 90 s to vary the degree of compaction. It should be noted that none of the samples showed visible bleeding or segregation during preparation. In addition, periodic visual checks found that the entire assembly worked well and that the spacers and rebar did not move relative to the formwork during concreting, which could otherwise influence results.

2.3

Curing and conditioning

The freshly cast columns were covered with plastic sheeting and wet hessian, and kept at room temperature for the first 24 h. Afterwards, they were demoulded and cured in a fog room at 21 °C, 100% RH for 28 days. Prior to curing, any externally exposed rebars were encased with cement grout (w/c 0.30) to prevent premature corrosion. After curing, each column was sectioned using a diamond abrasive cutter to extract two rectangular samples (80 × 90 × 90 mm3) from near the top and bottom of the column, as illustrated in Fig. 2b. These rectangular samples were used for further testing while the offcuts were discarded.

Moisture state has a huge influence on measured transport properties. Therefore, it is important to condition samples to a uniform state prior to testing to ensure meaningful results. The cut samples were conditioned to constant mass by drying at either 21 °C, 75% RH or at 50 °C, 7% RH to achieve two moisture states. The 21 °C, 75% RH regime was chosen to represent mild drying and to minimise shrinkage-induced microcracking [34] whilst the 50 °C, 7% RH regime was carried out to simulate severe drying in hot weather conditions. Drying was carried out in either an enclosed box in a temperature-controlled lab (21 ± 1 °C) or in an oven; both contained fans to circulate air and soda lime to prevent carbonation. Saturated NaCl solution was used to maintain 75% RH at 21 °C. This was monitored regularly, and salt solution replaced when required.

Samples were weighed periodically, and “equilibrium” was assumed when the mass loss was less than 0.01% per day. This required approximately 3–4 months of conditioning time. Oven-dried samples were cooled to room temperature (21 ± 1 °C) in a vacuum desiccator containing silica gel for 24 h to prevent condensation or moisture re-entering the samples during cooling, prior to testing. The mass before and after cooling was recorded and the difference was always less than 0.01%, implying constant moisture content.

2.4

Capillary absorption, accessible porosity and electrical conductivity

The conditioned samples were then tested for capillary absorption (water sorptivity) and electrical conductivity in three replicates per measurement. Absorption was carried out using conventional gravimetric measurement in a simple vertical capillary rise setup. The cast surface containing the spacer-concrete interface (Fig. 2b) was tested to simulate the direction of water ingress in real structures. Each sample was supported on two plastic rods (to allow free access of water) and then partially immersed in a tray containing shallow water to a depth of ~ 3 mm to simulate unidirectional absorption. Prior to testing, the sides of the sample were sealed with waterproof tape to prevent absorption from these surfaces.

The water uptake over time was measured with an electronic balance accurate to 0.01 g. Weighing was carried out at frequent intervals: typically 5, 10, 15, 20, 30, 40, 50 and 60 min, then every 15 min for the next hour, and then approximately every hour for the next 5 h. Subsequently, daily readings were taken until the sample approached saturation. Prior to each weighing, excess surface water was removed with dampened cloth and weighed quickly (within 30 s) without stopping the clock. The tray was covered with a loose-fitting transparent lid to prevent sample drying, but care was taken to ensure that no condensates formed underneath the lid that might drop onto the samples. Water level was maintained as required.

The cumulative absorption per unit inflow area i (g/m2) was plotted against the square-root of time t (min). For one-dimensional absorption into an unsaturated semi-infinite medium, cumulative absorption is given by \(i=S\sqrt{t}+a\), where S is the sorptivity coefficient (g/m2.min0.5) and a is a small fitting constant arising mainly from surface effects [35,36,37]. The sorptivity coefficient S was determined from the slope of the best-fit line drawn across at least ten readings taken during the first seven hours of measurement. In all cases, the R2 value of the linear regression was greater than 0.95.

Immediately after the capillary absorption test, samples were fully immersed in water and placed under vacuum for 4 h, then left submerged for 24 h to achieve saturation. The accessible porosity ϕ (%) was estimated from the mass difference between vacuum saturated surface dry condition and preconditioned state, divided by the sample volume.

The vacuum saturated-surface dry samples were then tested for bulk electrical conductivity. Samples were clamped between two larger brass plates and connected to an LCR databridge. A generous amount of a salt-free electrode gel was applied to ensure proper electrical contact between the brass plates and sample surfaces. Electrical conduction was tested in the same direction as capillary absorption through the cover in order to examine the effect of spacers. Electrical resistance R (Ω) was measured at alternating current of 1 kHz frequency to minimise polarisation effects. The resistance typically stabilises within one minute; three readings were taken and averaged. Electrical conductivity \({\sigma }_{e}\) (S/m) was then calculated as the reciprocal of electrical resistivity (\(\rho =RA/l)\), where A (m2) and l (m) are the sample cross-sectional area and length respectively.

2.5

Imaging water absorption

The imaging method described by Wu et al. [34] was used to study water ingress during capillary absorption. Following electrical conductivity measurements, samples were sectioned in half with a diamond abrasive cutter from the centre to expose the spacer and rebar, as shown in Fig. 3. The sectioning was carried out at a slow feed rate to avoid damaging the sample. The cross-sections were then conditioned at 50 °C, 7% RH and capillary absorption (sorptivity) was repeated by using water dyed with 1 wt% fluorescein (C20H12O5). The dye acts as a tracer that enhances the visibility of the water penetration front. The test was carried out in a dark room to enhance contrast and the sample cross-section was imaged regularly with a digital camera. The camera was operated at a small aperture to increase the depth of field and high ISO to obtain adequate exposure. Images were captured at 5–10 min interval for the first hour, then at 15–30 min interval for the next two hours, then hourly for the next 6 h and daily for up to 3 days. This setup mimics the sorptivity test and allows continuous imaging without disturbing the sample and water absorption process.

Sample cross-sections extracted from column with plastic spacer (left) and cementitious spacer (right) for transport testing

Full size image

2.6

Epoxy impregnation and fluorescence imaging

Replicate samples were pressure impregnated with low-viscosity fluorescein-dyed epoxy resin to study preferential transport paths. This was carried out by adapting the procedures described by Wu et al. [38, 39]. Prior to impregnation, samples were dried at 40° C and the edges were sealed with several layers of adhesive waterproof tape to prevent side leakage. The sample was then evacuated and de-aired in a vacuum chamber at  − 1 bar for 3 h. Epoxy was prepared by mixing the resin (Stuers EpoFix) with fluorescein dye (Struers EpoDye) at 0.05 wt. % using a magnetic stirrer for 24 h and then heated to 40 °C to reduce viscosity. The fluorescein-dyed epoxy resin was then mixed with hardener at 25:3 mass ratio. The resin was diluted with 5 wt% toluene to reduce viscosity further.

Without breaking the vacuum, the low viscosity fluorescent resin was fed into the chamber and onto the sample, covering the entire test surface (containing the spacer-concrete interface). Vacuum was then released and the sample transferred into a pressure chamber. Compressed air at 3.5 bar pressure was applied and maintained overnight to force the epoxy into the sample. After three days of curing at room temperature, the impregnated sample was sectioned in half with diamond abrasive cutter and flat ground with 120-grit SiC paper. The entire cross-section (~ 90 × 50 mm) was imaged with an optical stereo microscope (Olympus SZX10) under ultraviolet light. A series of overlapping images was captured, aligned and stitched to produce a large montage of the cross-section. Images were then analysed with Fiji/ImageJ [40] to determine the depth and spatial distribution of epoxy impregnation.

2.7

Image analysis to estimate segregation

The cut surfaces of the rectangular samples (Fig. 2b) were imaged and analysed for segregation by measuring the area fraction of exposed aggregate particles with image analysis, using a method adapted from previous studies [41,42,43]. The entire cut surface was imaged with an optical stereo microscope following the approached described in the preceding section with bright field illumination. Colour images were collected at constant magnification, brightness and contrast settings for repeatability. The aggregate particles were clearly resolved from their colour and shape information; therefore no further image processing was required to enhance contrast.

Segmentation of the aggregate particles was carried out using Fiji/ImageJ [40]. The first step was to enlarge the montage so that individual aggregate particle boundaries are clearly visible. Then, a pixel-wide line was carefully drawn onto the montage tracing the boundaries of the aggregate particles. This was done manually, and its accuracy was checked frequently by cross-referencing with the actual sample under the microscope. This is time consuming but yields accurate results (Fig. 4). Although automated edge detection methods exist, our experience found that these are less reliable when applied to complex aggregates that display a range of colour and mineralogy.

Example image analysis showing segmented aggregate particles at different heights on the same column compacted at 8.5 Hz, 90 s. (I, II, III, IV correspond to sections shown in Fig. 2b)

Full size image

The smallest detectable aggregate size with this approach was 0.4 mm (diameter). Inevitably, some of the fine aggregate particles were excluded, this was estimated to be ~ 10–20% based on the particle size distribution from sieve analysis. Once all the aggregates particles were selected and accurately marked, its total area fraction (%) was calculated with image analysis as the area of the aggregate particles divided by the sample cross-sectional area. Figure 4 provides examples of segmented images with the aggregate particles marked with a yellow boundary.