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Cement & Concrete Research, 2016 (Accepted)

1

Influence of reinforcement spacers on mass transport properties and durability 1

of concrete structures 2

S. Alzyoud, H.S. Wong* & N.R. Buenfeld 3

Concrete Durability Group, Department of Civil and Environmental Engineering, Imperial College London, 4

SW7 2AZ, UK 5

Abstract 6

Spacers are ubiquitous in reinforced concrete, but their influence on durability is unclear. This paper presents 7

the first study on the effects of spacers on mass transport and microstructure of concrete. Samples with 8

different spacers, cover depths, aggregate sizes, curing ages and conditioning were subjected to diffusion, 9

permeation, absorption and chloride penetration, and to μXRF, BSE microscopy and image analysis. Results 10

show that spacers increase transport in all cases, the magnitude depending on spacer type and transport 11

mechanism. Plastic spacers produced the largest increase, followed by cementitious spacers and then steel 12

chairs. The negative effect is due to a porous spacer-concrete interface that spans the cover where 13

preferential transport occurs. Spacers may seem low value, small and inconsequential, but because they are 14

placed every ≤ 1 m along rebars, their overall effect on ingress of external media is significant. This is not 15

currently recognised by standards or by most practitioners. 16

Keywords: Durability (C); Interfacial transition zone (B); Microstructure (B); Transport properties (C); 17

Spacers 18

19

1. Introduction 20

Spacers are essential components in reinforced concrete structures. Their function is to secure steel 21

reinforcements in the correct position within the formwork to prevent movement prior to and during 22

concreting so that the required cover is obtained in the finished structure. The size of spacer determines the 23

size of the cover depth to reinforcement, which in structural design, is defined according to the severity of 24

exposure environment, required durability and fire resistance. Achieving adequate depth and quality of 25

concrete cover is critical because it protects embedded steel reinforcement from the external environment. It 26

is well-known that inadequate cover is the major factor causing premature corrosion of reinforcement, the 27

principal form of degradation of concrete structures. In structural design, it is assumed that achieving the 28

specified cover ensures that the as-built structure achieves the expected design performance in terms of 29

durability, fire resistance and serviceability (crack width). 30

Spacers are made of plastic, metal or cementitious materials, and are available in various sizes and 31

shapes (see Fig.1). In this paper, we will use a generic term “spacers”, but recognising that other terms may 32

be prevalent elsewhere, e.g. bar supports, wire chairs, bolsters, continuous runners, dowels etc. Although 33

many types of spacers are available commercially, they generally fall into one of six categories: a) plastic 34

spacers with integral clip-on action for horizontal rebars of 20 mm or less, b) plastic end spacers that fit ends 35

of rebar for end cover, c) plastic wheel/circular spacers for vertical rebars in columns and walls, d) 36

cementitious block spacers for bar size > 20 mm in heavily-reinforced sections; e) continuous line spacers 37

that are either cementitious or plastic, of constant cross-section in typically1 m lengths to support several 38

bars; and f) steel wire chairs that may be single, continuous or circular, to support the top horizontal rebar 39

from lower rebar or to separate layers of vertical rebars. Of all the available types, plastic spacers are the 40

most popular because they are cost effective and they do not need to be wire tied to rebar, which is labour 41

intensive. For further details on spacer types, readers can refer to refs. [1-6]. 42

The first comprehensive guidance on spacers was probably Concrete Society Report CS 101 in 1989 43

[7]. Prior to that, it was not uncommon to use any material available on-site including bricks, tiles, broken 44

concrete, and timber pieces to support reinforcement [4]. Another practice was to use site-made cement 45

mortar blocks as spacers, but this practice was not stopped in BS 8110-1 [8] because of poor quality control. 46

At present, spacers in the UK should be factory manufactured, conform to BS 7973-1 [9] and placed in 47

* Corresponding author. E-mail: [email protected]. Phone: + 44 20 7594 5956

mailto:[email protected]


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accordance with BS 7973-2 [10]. The general rule is that spacers should be fixed to reinforcing bars at a 48

spacing not exceeding 50d or 1000 mm, where d is the bar size, and in staggered rows for parallel bars. 49

Other similar recommendations are available for North American practice e.g. ACI 315-99 [11] and ACI SP-50

66 [12], and German practice in DIN EN 13670 [13]. 51

All spacers (except some wire chairs for top rebars) work on the principle of supporting reinforcement 52

from the nearest exposed surface, i.e. from the formwork or blinding. As such, spacers must interrupt the 53

concrete cover and replace a portion of the concrete in the cover zone. Inevitably, they form a link between 54

reinforcement and the external surface, and present a possibility of compromising the effectiveness of the 55

cover to protect embedded reinforcement (see Fig. 2). It is reasonable to suspect that the presence of spacers 56

potentially facilitates ingress of aggressive agents such as water, chloride, CO2 and oxygen either through the 57

spacer itself or its interface with concrete. If true, then spacers would act as weak links and accelerate 58

deterioration. Furthermore, spacers are placed every metre or less along the reinforcement and are left 59

permanently in the structure, and so a structure contains thousands of spacers and their combined effect 60

could be significant. Indeed, several reports and field investigations have observed a link between spacers 61

and reinforcement corrosion. Examples of such reports include refs. [1, 14, 15, 16, 17, 18, 19, 20, 21, 22] and 62

a common observation was that local rebar corrosion occurs at spacer locations. This was assumed to be due 63

to either poor quality of the spacer (itself being highly porous), or the concrete near spacers (poor 64

compaction) or the interface between concrete and spacer (e.g. debonding). 65

Despite concerns expressed over the effect of spacers on durability, to the best of our knowledge, there 66

has been no systematic or fundamental research carried out on this issue. Therefore, the present work seeks 67

to redress this by establishing the effect of several spacer types on mass transport properties and 68

microstructure of concrete. The overall aim of this study is to enhance the understanding of how spacers 69

influence the durability of concrete structures. 70

71

2. Experimental 72

2.1 Spacers 73

Samples of cementitious, plastic and steel spacers were requested from all leading manufacturers and 74

distributors in the UK in order to explore the available range of products. Approximately 110 types of 75

spacers were obtained and they varied in terms of material, height (cover size) and shape, as shown in Fig. 1. 76

From this, 7 spacer types representing the most commonly used in construction, were selected for testing. 77

Details of the selected spacers are given in Table 1. Bar or line spacers were rejected because these would 78

require the preparation and testing of large concrete elements, which is impractical and probably unnecessary 79

for the purpose of study. The selected steel spacers were continuous lattice chairs and these were sectioned at 80

nodes into 100 mm long segments. The spacers used in this study are indicated in Fig. 1. 81

The porosity of cementitious spacers was obtained by measuring the mass difference from a vacuum 82

saturated-surface dry condition to 105°C oven-dried condition, divided by the spacer volume. Plastic spacers 83

contain openings that might trap aggregate particles and block the movement of fresh concrete. Therefore, 84

the openings were measured to ensure that the selected spacer had sufficiently large openings to allow the 85

largest aggregate particles to nestle in them. Some of the plastic spacers were modified by either grinding 86

using a 120-grit size SiC paper or by scoring four 1 mm deep notches on the main flange. The purpose of this 87

is to increase the surface roughness of the spacer in order to improve its adhesion and bond to the concrete 88

matrix. Prior to use, the plastic and cementitious spacers were cleaned and dried, while the steel chair 89

segments were sand blasted to remove any rusting from the surface. The spacers were stored in the 90

laboratory to avoid any moisture or temperature variations. The ‘volume fraction’ in Table 1 denotes the 91

fraction of the disc test sample (Section 2.3) that is occupied by the spacer. 92

93

2.2 Concrete materials and mix proportions 94

Ordinary Portland cement CEM I was used for all mixes. Its oxide composition was 63.4% CaO, 95

20.8% SiO2, 5.4% Al2O3, 2.4% Fe2O3, 1.5% MgO, 0.3% Na2O, 0.7% K2O, 2.9% SO3 and < 0.1% Cl. The 96

calculated Bogue composition was 53.1% C3S, 19.1% C2S, 10.8% C3A and 7.2% C4AF. The loss on ignition, 97

specific gravity and fineness of the cement were 2.1%, 3.06 and 291 m2/kg respectively. Tap water was used 98

as batch water. Thames Valley sand and gravel were used as fine aggregates and coarse aggregates 99


3

respectively. The maximum particle size was 5 mm for sand. For gravel, the maximum particle size was 100

either 10 or 20 mm. The particle size distributions of the fine and coarse aggregates are shown in Fig. 3. The 101

sieve analysis show that the sand complied with BS 882:1992 medium grading, while the gravel complied 102

with BS EN 12620:2002+A1 overall grading. The specific gravity, moisture content and absorption of the 103

aggregates are given in Table 2. 104

Two concrete mixes with free water/cement (w/c) ratio of 0.4 were prepared according to the 105

proportions given in Table 3. The mixes were designed to the absolute volume method and batch water was 106

adjusted to account for aggregate absorption so that the target free w/c ratio was achieved. The main variable 107

in the mixes was the maximum size of aggregate (MSA) which was either 10 or 20 mm. The total aggregate 108

content was fixed at 70%. Therefore, the cement paste fraction and hence total porosity were the same for all 109

samples allowing meaningful comparison. The sand to total aggregate mass fraction was 0.4. 110

111

2.3 Test samples 112

140 cylindrical samples were prepared in total, as summarised in Table 4. The cylindrical samples 113

have a diameter of 100 mm and thickness of either 25 mm or 50 mm, depending on the cover size produced 114

by the particular spacer. Samples from Mix C10 were cast directly in 100 mm diameter steel moulds as 115

shown in Fig. 4, while samples from Mix C20 were cast in large slabs (1500 ×600×50 mm) and then cored 116

to obtain the required size. The spacers were placed at 150 mm centres in the slab. The latter procedure was 117

carried out to ensure that a proper placement and compaction of the concrete could be achieved with Mix 118

C20, which contained larger aggregate particles (MSA 20 mm). A Hilti DD 120 diamond corer was used to 119

extract the 100 mm diameter cores from the slab at the end of curing. 120

It is absolutely critical to ensure that the spacers are well secured onto the mould or formwork to 121

prevent any relative movement when the fresh concrete is placed and compacted. This is because 122

displacement of the spacer may prevent a complete compaction of the concrete or may induce a negative 123

effect on the microstructure of the hardened concrete. In real structures, the weight of the reinforcement 124

should be sufficient to hold spacers in place during concreting. However, samples prepared in the laboratory 125

are relatively small and so do not contain sufficient reinforcement weight to achieve this. Therefore, several 126

timber pieces bolted tightly onto the base plate of the mould were used to clamp the spacer and rebar (high-127

yield steel, deformed, 12 mm) in place prior to concreting, following the assembly shown in Fig. 4. This 128

worked very well and checks showed no relative movement between the spacer and mould when the entire 129

assembly was vibrated. 130

131

2.4 Mixing, curing and conditioning 132

All materials were batched by weight. Cement and aggregates were first dry mixed in a 30-liter 133

capacity pan mixer for about 30s. Water was then added and mixed for a further 3 min. The slump values for 134

all mixes were in the range of 90 to 100 mm. A vibrating table with adjustable intensity was used for 135

compaction. The intensity of the vibrating table was adjusted to suit workability of the fresh concrete mix. 136

The 25 mm thick cylindrical samples were compacted in one layer while the 50 mm thick samples were 137

compacted in two layers. Each layer was vibrated until no significant release of air bubbles was observed. It 138

should be noted that all of the mixes were easily compacted and showed no indication of segregation, 139

bleeding or excessive voidage in the cross-section (Fig. 5). The freshly compacted samples were covered 140

with plastic sheet and wet hessian at room temperature for the first 24 hours and then de-molded. 141

Samples for oxygen diffusion, oxygen permeation, water absorption and microscopy were sealed 142

cured by wrapping in at least 6 layers of cling film, then sealed in polythene bags at 20°C for 3 and 28 days. 143

A short curing of 3 days was carried out to replicate typical site practice in addition to the “standard” 28-day 144

curing. Periodic checks by weighing found insignificant mass loss showing the effectiveness of the sealed 145

curing. At the end of curing, samples were unwrapped and labelled. Samples subjected to chloride diffusion 146

were cured in a fog room at 100% RH and 20˚C for 28 days. This was to ensure that the samples were well-147

hydrated before exposure to chloride solution. 148

Samples were conditioned using one of three regimes prior to transport testing by drying to “constant” 149

mass, taken as when the mass loss was less than 0.01% per day. The purpose of conditioning to equilibrium 150

is to ensure that the measured transport properties are not influenced by variation in moisture content. 151


4

However, samples were subjected to three conditioning regimes so that the influence of spacers under 152

different exposure environments can be examined. The conditioning regimes were: a) 20°C, 75% RH, b) 153

20°C, 55% RH, and c) 50°C oven drying to replicate hot weather condition. Drying at 20°C was carried out 154

in closed chambers over saturated NaCl or Mg(NO3)2 to achieve the required RH of 75% and 55% 155

respectively. The chambers contained fans to generate circulating air and soda lime to prevent carbonation. 156

The conditioning time required to reach constant mass ranged between 2 and 6 months, depending on drying 157

rate and sample thickness. Samples conditioned at 50 °C were cooled to room temperature in a vacuum 158

desiccator for 24 h to prevent moisture from re-entering the samples during cooling prior to testing. Samples 159

for chloride diffusion were conditioned at 20°C, 55% RH for 28 days, similar to the procedure described in 160

the AASHTO T259 salt ponding test. 161

162

2.5 Oxygen diffusion, oxygen permeation and water absorption 163

Samples from Series I, II and III (Table 4) were tested following the sequence of oxygen diffusion, 164

oxygen permeation and water absorption in three replicates. The same sample can be used for all three tests 165

because the results of each test are not influenced by the previous test. Details of the tests are given in Wong 166

et al. [23] and summarised here. For oxygen diffusion and permeation, the sample was fitted into a silicone 167

rubber ring in a steel cell. The curved side of the sample was sealed by applying 15 kN compression on the 168

silicone rubber ring, which expands and grips the sample at a lateral confining pressure of ~ 0.6 MPa. This 169

prevents leakage through the circumference as shown in Wu et al. [24]. Oxygen diffusivity was determined 170

by exposing the two opposite flat faces of the sample to oxygen and nitrogen at equal pressure. The oxygen 171

and nitrogen diffuse in opposite directions through the sample, and the oxygen concentration in the outflow 172

stream was measured at steady-state flow using a zirconia analyser to calculate diffusivity. 173

Oxygen permeability was determined by applying a gas pressure of 0.05, 0.15 and 0.25 MPa above 174

atmospheric, and measuring the steady-state outflow rates at each applied pressure. The apparent 175

permeability at each pressure was calculated according to Darcy’s law for compressible fluids and the 176

intrinsic permeability was determined by applying Klinkenberg’s correction for gas slippage. The recorded 177

mass of the discs before and after both tests showed negligible change. Water absorption was carried out by 178

placing the sample on two plastic strips in a tray of water to a depth of 2-3 mm, and monitoring the amount 179

of water uptake with time until full saturation was achieved. Mass measurement was done using an 180

electronic balance accurate to 0.01 g. The sorptivity coefficient was obtained from the slope of the regression 181

line of absorbed water per unit flow area against square-root time according to the classical unsaturated flow 182

theory [Hall, 1977]. The best-fit regression line was drawn across at least 10 readings taken during the first 7 183

hours of measurement. The coefficients of regression of the least squares fit for the Klinkenberg correction 184

and sorptivity determination were always greater than 0.985. 185

186

2.6 Chloride diffusion 187

Samples from Series IV (Table 4) were subjected to chloride penetration. Immediately after the 188

predetermined curing and conditioning regime, the curved sides of the samples were sealed with two layers 189

of waterproof adhesive tape to ensure unidirectional chloride ingress. The samples were then placed in a 190

shallow tray containing 3% by mass sodium chloride solution for 90 days. The depth of the solution was 191

about 5 mm above the exposed surface of the sample and the solution was refreshed periodically. The tray 192

was covered to minimise evaporation. At the end of the exposure, samples were sectioned in half using a 193

diamond abrasive cutter and analysed with Orbis PC micro X-ray fluorescence (μXRF) to determine the 194

depth and spatial distribution of chloride penetration. 195

Each sample was analysed to obtain three chloride profiles as shown in Fig 5: a) within the 196

cementitious spacer or concrete nestled in plastic spacer, b) along the spacer-concrete interface and c) near 197

the sample edge away from spacer. Each profile consists of 50 spot analyses carried out at every 1 mm 198

spacing to produce a detailed chloride concentration distribution from the exposed surface through the entire 199

thickness of the sample. The positions of the spots were carefully selected with the help of a high 200

magnification camera (75x) to ensure that the analysis avoided large aggregate particles or air voids that 201

would otherwise produce discontinuities (gaps) in the generated profiles. Prior to analysis, a parametric study 202

was conducted to determine the optimal operating conditions of the μXRF for detecting and measuring 203

chloride. The final analysis was carried out using a 30 μm beam size, 25 kV beam voltage, 12.8 μs, amplifier 204


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time constant and 500 ms dwell time. A dead time of ~40% was achieved with this configuration. A 25 μm 205

thick Al filter was used to enhanced the sensitivity for chloride. 206

Subsequently, the collected spectra were analysed to indentify all detectable elements using the 207

ORBIS Vision (v. 2.0) software. False peaks (sum and escape) were accounted and removed prior to 208

quantitative analysis. However, the spectrum contains several peaks partially obscured by the background 209

signal. Therefore, the signal-to-noise ratio of spectral peaks method [25] was used for background 210

subtraction. Finally, the collected signal for chlorine was converted to % wt of cement by referring to a 211

calibration curve obtained by carrying out the same spot analysis on a series of cement pastes at the same 212

w/c ratio and curing age that contained known amounts of admixed chloride [26]. The non-steady state 213

chloride diffusion coefficient was then estimated by fitting the measured chloride profiles to Fick’s second 214

law of diffusion. Microsoft Excel Solver was used to calculate the best-fit values for the diffusion coefficient 215

and surface chloride concentration that minimised the sum of the squares of residuals. The coefficients of 216

regression of the best-fit lines (R2) were above 0.9 for all cases. 217

218

2.7 Fluorescent epoxy impregnation 219

Another method used to examine the influence of spacers on transport properties was to pressure 220

impregnate samples from the exposed flat surface with fluorescein dyed epoxy. The cross-sections of the 221

samples were then examined to measure the depth and distribution of the intruded epoxy. Selected samples 222

from Series II & III (Table 4) were placed in a cell similar to the one used for oxygen permeability and 223

sealed to prevent side leakage. Fluorescein dyed epoxy was then poured onto the top exposed surface and 224

compressed air at 0.7 MPa above atmospheric pressure was applied and maintained for 6 hours to force the 225

epoxy into the sample. The impregnated sample was allowed to harden at room temperature for 2 days and 226

then sectioned with a diamond abrasive cutter in half from the centre. Each cross-section was flat ground 227

with 120-grit SiC paper. Fluorescence imaging was carried out using a digital SLR camera under 15W 228

ultraviolet lamp placed 35 mm above the sample to provide uniform illumination and fluorescence. Images 229

of the entire cross-section (100 × 50 mm) were captured at 2177 × 1085 pixel resolution in TIFF in a dark 230

room to avoid stray light reflection and to achieve a good contrast. The camera was operated at small 231

aperture to increase the depth of field and slow shutter speed to achieve adequate exposure. Full details of 232

the procedures are given in Wu et al. [24, 27]. 233

Image analysis was carried out using ImageJ to measure the area fraction and depth of epoxy 234

impregnation on three replicate samples. The areas intruded with epoxy appear bright green due to 235

fluorescence and these were segmented using colour thresholding based on hue, saturation and brightness 236

thresholding to generate a binary image. For all samples, the hue threshold range was set at 60-180 which 237

corresponds to the green colour angle on the hue circle. The entire brightness and saturation histograms (0-238

255) were utilised so that all shades of the green colour were included. The same thresholding settings were 239

applied to all samples. The accuracy of the segmentation was checked by cross-referencing the binary 240

images with the original fluorescence images. 241

242

2.8 Backscattered electron imaging 243

A field-emission scanning electron microscope operated in the backscattered electron (BSE) mode 244

was used to characterise the microstructure of the spacer-concrete interface in greater detail. Block samples 245

(40 × 20 × 10 mm) were extracted from cross-sections prepared as described in Section 2.7, impregnated 246

with ultra-low viscosity epoxy resin under vacuum and then pressurised to ensure full impregnation. The 247

blocks were then ground and polished at successively finer grades (68 µm, 30 µm, 18 µm, 14 µm, 9 µm, 6 248

µm, 3 µm, 1µm, and 0.25 µm) to achieve a flat and well-polished surface, then carbon coated with an 249

evaporative coater. BSE images were collected at 10 kV accelerating voltage and 10 mm working distance. 250

Fifty images were collected per sample around the spacer-concrete interface for samples containing 251

plastic and cementitious spacers. Thirty-six images per sample were collected for those containing steel 252

spacer; the decrease in number of images captured is due to the small size of steel spacer (5 mm wire), which 253

limits the available area for observation. All images were captured at 500× magnification, digitised to 2560 254

x 2048 pixels at pixel spacing of 0.094 μm, giving a field of view of 240 × 192 μm. These settings were 255

chosen to obtain a good sampling area and resolution to characterise the phases of interest. To ensure random 256


6

and unbiased sampling, images were captured at ~ every 1 mm distance along the spacer-concrete interface. 257

To isolate the effect of aggregates, images were selected such that they were located at least 50 μm away 258

from the nearest aggregate particle to avoid sampling the aggregate-paste “interfacial transition zone” (ITZ). 259

If an image contained aggregate particle close to the spacer interface, then the image was replaced by another 260

within the adjacent area. Brightness and contrast settings were calibrated to achieve a brightness histogram 261

that utilised the entire dynamic range (0-255). 262

Quantitative image analysis was carried out to examine the microstructure gradients at the spacer-263

concrete interface. First, the spacer edge was carefully traced on an enlarged image and segmented using 264

ImageJ. This was relatively straightforward because all spacers are clearly visible and can be accurately 265

segmented. Then, the pores and any cracks were segmented using the overflow method [28] by defining the 266

upper threshold level from the inflection point of the cumulative brightness histogram of the BSE image. The 267

unreacted cement particles were segmented using the minimum grey value between peaks for hydration 268

products and the unreacted cement as the lower threshold value. Finally, the spatial distribution of segmented 269

porosity and unreacted cement from the spacer-concrete interface were measured using the Euclidean 270

Distance Mapping approach [29] at a resolution of one pixel strip width. 271

272

3. Results 273

3.1 Sample density and compaction 274

It was absolutely critical to ensure that the samples were properly compacted so that any observed 275

effects of the spacers on mass transport properties are not attributable to excessive voidage from poor 276

compaction. As mentioned previously, the spacers were tightly secured to the sample mould to prevent 277

movement during concrete placement and compaction. However, the samples are relatively small and the 278

spacer may occupy up to ~25% of the sample volume (Table 1). Furthermore, plastic spacers contain small 279

openings that may create problems during concrete placement and compaction. To check this, the density of 280

the hardened concrete sample was measured and compared against its theoretical density, calculated from the 281

mix design and spacer density. Measurements were made on three replicate samples. The difference between 282

measured and theoretical density was found to be very small and ranged between 0.03% and 0.5%, with an 283

average difference of 0.3%. Furthermore, all of the examined cross-sections show that the samples were 284

indeed well compacted with no sign of excessive voidage or segregation. Some examples of sample cross-285

sections are shown in Figs. 2, 5 & 10. 286

287

3.2 Oxygen diffusivity, oxygen permeability and water sorptivity 288

The measured oxygen diffusivity, oxygen permeability and water sorptivity coefficients are presented 289

in Figs. 6-8. Each data point represents the average of measurements on three replicate samples and the error 290

bar indicates standard error 𝜎 √𝑛⁄ , where σ is the standard deviation and n is the number of replicates. The 291

results show that increasing curing age from 3 to 28 days caused a reduction in transport properties in all 292

cases, as expected. Transport properties decreased by about 7% to 37% when comparing the data obtained at 293

3d curing (Fig. 6a) to that at 28d curing (Fig. 6b) for the same sample type and thickness. It is also evident 294

that the height of spacer (sample thickness) has a huge influence on the measured permeability, but not on 295

diffusivity and sorptivity. The 25 mm thick samples (t/MSA = 2.5) consistently gave much higher 296

permeability, by about 38% to 52%, compared to the corresponding 50 mm thick samples (t/MSA = 5) for 297

the same mix, spacer type and curing age. This is due to drying-induced surface microcracking and the 298

related size effects on permeability as reported in Wu et al. [27]. 299

Interestingly, there is a clear and consistent trend that samples containing spacers have higher 300

transport properties than their respective control samples. Overall, the increase in transport properties as a 301

result of spacers ranged between 10% and 300%. The average increases in oxygen diffusivity, oxygen 302

permeability and water sorptivity were 57%, 138% and 27% respectively. However, the increase in transport 303

property is dependent on spacer type. Samples containing plastic spacers consistently gave the highest 304

diffusivity, permeability and sorptivity coefficients followed by samples with cementitious spacers, then 305

steel spacers. The control samples had the lowest transport coefficients in all cases. For example, the average 306

increases in diffusivity, permeability and sorptivity for samples containing plastic spacers were 85%, 193% 307

and 41% respectively. For samples containing cementitious spacers, the average increases in diffusivity, 308


7

permeability and sorptivity were 37%, 125% and 18% respectively. For samples with steel spacers, the 309

average increases in diffusivity, permeability and sorptivity were 25%, 36% and 10% respectively. 310

It should be noted here that the percentage differences in transport properties between samples with 311

and without spacer presented in this paper were obtained from 100 mm diameter cylindrical test sample with 312

a centrally placed spacer. Furthermore, the spacer occupies between 6 and 25% vol. of the test sample (Table 313

1) depending on spacer type. Therefore, it is expected that the percentage influence of a particular spacer on 314

the average transport property depends on the size of the test sample relative to the spacer. For example, it is 315

expected that the measured increase in transport property for samples containing spacers would be higher if a 316

smaller test sample were used, and vice-versa. 317

Fig. 7 & Fig. 8 show that conditioning regime prior to testing has a major influence on the measured 318

transport properties. Increasing the severity of drying increases the accessible porosity, therefore oven drying 319

at 50 °C consistently produced the highest transport coefficients, followed by room temperature (20°C) 320

drying at 55% RH and 75% RH. For samples conditioned at 50°C, the average increases in diffusivity, 321

permeability and sorptivity of samples containing plastic spacers relative to the control were 102%, 162% 322

and 44% respectively. For samples containing cementitious spacers, the corresponding values were 87%, 323

96% and 25% respectively. In contrast, samples containing steel spacers showed modest increases in 324

diffusivity, permeability and sorptivity of 21%, 22% and 9% respectively. 325

It is important to note that the effect of spacers on transport is also evident in samples that were 326

subjected to gentle drying at 20°C, 75% RH. For example, the average increases in diffusivity, permeability 327

and sorptivity for samples containing plastic spacers relative to the control after 20°C, 75% RH drying were 328

52%, 243% and 37% respectively. The corresponding values for samples containing cementitious spacers 329

were 57%, 107% and 19% respectively, and for samples containing steel spacers, 25%, 31% and 8% 330

respectively. The results in Fig. 7 also show that samples containing the modified plastic spacer PS(a) and 331

PS (b) gave a noticeable improvement in transport properties. For example, the average decreases in 332

diffusivity, permeability and sorptivity relative to the samples containing unmodified plastic spacer (PS) 333

after conditioning at 50°C were 14%, 38% and 11% respectively. 334

Cementitious spacers are inherently porous, and this may be an important factor. Therefore additional 335

testing was carried out to determine the sorptivity and total water absorption of the cementitious spacer. The 336

results show that the average sorptivity of the cementitious spacer was 31.9 g/m2.min0.5, which was ~39% 337

that of the control sample (C10-Co50, 3d-cured) after conditioning at 20°C, 55%RH. It was also observed 338

that the absorption of the cementitious spacer to saturation was only 18% of the control sample. These results 339

suggest that the observed increase in transport properties of samples containing cementitious spacers is due 340

to factors other than the porosity of the spacer itself. 341

342

3.3 Chloride penetration 343

Chloride concentration profiles measured at 1 mm increments along the spacer-concrete interface, at 344

the centre and edge of the sample are shown in Fig. 9. For samples containing plastic spacers, the highest 345

chloride concentration and penetration depth occurred along the spacer-concrete interface and at the centre, 346

i.e. concrete nestled within the spacer. The lowest chloride concentration profile was obtained near the edge 347

of sample away from the spacer, and this was very similar in magnitude to that obtained from the control 348

sample. For samples containing cementitious spacers, the highest chloride concentration profile also 349

occurred along the spacer-concrete interface. In contrast, the lowest chloride concentration profile was 350

obtained from the centre of the sample, i.e. within the spacer. This is consistent with the fact that the porosity 351

and transport property of the cementitious spacer are much lower than those of the concrete. 352

The average non-steady state diffusion coefficient Deff and surface chloride concentration Cs are 353

presented in Table 5. It can be seen that the chloride diffusion coefficient and surface chloride concentration 354

for samples with spacers were highest at the spacer-concrete interface, for both plastic and cementitious 355

spacers. The lowest values were found at the centre of the cementitious spacer. Interestingly, the values 356

obtained at the edge of the samples containing spacers were very similar to those measured for the control 357

samples without spacers. Plastic spacers produced a greater effect on chloride penetration compared to 358

cementitious spacers. At the spacer-concrete interface, the chloride diffusion coefficient and surface chloride 359

concentration were higher than the control sample by 33% and 203% respectively. 360


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361

3.4 Epoxy impregnation depth 362

Fig. 10 shows cross-sections of samples impregnated with fluorescent epoxy resin from the bottom 363

exposed surface and their respective binary images to highlight the epoxy intruded areas. The measured area 364

fraction and epoxy impregnation for samples with different spacer types, MSA and conditioning regimes are 365

presented in Fig. 11. The results are the average of three replicates and error bars shown on the figure 366

represent +/- one standard error of the average. It can be seen that the presence of spacers increases the 367

amount and depth of epoxy impregnation. Samples containing plastic spacers consistently showed highest 368

epoxy penetration followed by samples with cementitious spacers and then steel spacers. The control 369

samples had the lowest amount of epoxy penetration. 370

It can also be seen that the amount of epoxy intrusion increased with increase in severity of drying, as 371

expected. These trends are similar to those observed for mass transport properties presented in the preceding 372

sections. When conditioned at 50°C, samples with plastic spacer showed epoxy impregnation that almost 373

reached the full thickness of the sample (50 mm). The area fraction and depth of impregnation were greater 374

than those of the control sample by 127% and 273% respectively. Fig. 10d shows that most of the epoxy 375

intrusion occurred along the spacer-concrete interface and through the concrete nestled inside the plastic 376

spacer. The presence of steel spacer had the least effect on the area and depth of epoxy impregnation, 377

increasing by only 22% and 6% relative to the control on average. 378

379

3.5 Microstructure of the spacer-concrete interface 380

The distributions of unreacted cement and detectable porosity, measured at 0.1 μm intervals up to a 381

distance of 80 μm from the spacer boundary, are shown in Fig. 12 (a) and (b) respectively. Results are the 382

average of fifty frames (thirty-six for steel spacer) and expressed as the area percentage of the cement paste 383

area. The results are compared to the microstructural gradients measured at the aggregate-paste interfacial 384

transition zone (ITZ) and bulk paste of the control sample. It can be seen that the unreacted cement fraction 385

increased steadily from < 0.5% at the spacer boundary to around 14% at the “bulk” cement paste region and 386

remained relatively constant beyond 50 μm away from the spacer boundary, for all spacer types. In contrast, 387

the detectable porosity was highest at the spacer boundary, and decreased with increasing distance from the 388

spacer boundary. Beyond about 50 μm from the spacer boundary, the porosity for all sample types was 389

relatively stable and achieved a similar value to that of the bulk paste porosity. The porosity at the spacer-390

concrete interface is approximately 3 to 6 times that of the bulk paste region, depending on spacer type. 391

It is evident that plastic spacers had the greatest effect on the microstructure of the spacer-concrete 392

interface. Nevertheless, all spacer types produced strong microstructure gradients that are reminiscent of the 393

aggregate-paste interfacial transition zone. Measurements made on the control sample along the bulk cement 394

paste region showed no significant gradients in unreacted cement and porosity, as expected. When all of the 395

acquired BSE images of the spacer-concrete interface and their individual gradients where compared, it was 396

observed that the microstructure is heterogeneous and spatially variable. Although the averaged porosity and 397

unreacted cement distributions displayed clear trends, the individual result from each location was quite 398

variable. The microstructure at some locations was highly porous with a visible bond crack forming between 399

the spacer and concrete, while others were seemingly unaffected by the presence of spacers. Furthermore, 400

fine microcracks of ~4 to 6 μm width were detected on the exposed face, along the spacer-concrete interface 401

in some of the samples containing plastic and cementitious spacers. Several example BSE images of highly 402

porous spacer-concrete interface are shown in Figs. 12 (c) to (d). 403

404

4. Discussion 405

Mass transport properties 406

Spacers are either non-porous/impermeable (e.g. plastic and steel spacers), or have lower porosity 407

(cementitious spacers) to that of the concrete around it. In samples containing spacers, the spacer occupies 408

and replaces a substantial volume of the concrete, ~25% for samples containing cementitious spacers and 409

~14% for samples containing plastic spacers. As such, the spacers should act as obstacles to transport by 410

reducing the cross-sectional area available for flow and this should decrease overall transport. However, the 411


9

results from this study collectively show that the inclusion of spacers always increases overall transport. The 412

spacers facilitate transport to gas, chloride ions and water, regardless of the mechanism, i.e. under 413

concentration gradient, pressure gradient or capillary suction. 414

The magnitude of increase in transport mainly depends on spacer type and transport mechanism. 415

Curing age, conditioning regime and thickness of the sample or spacer did not produce a huge impact on 416

how spacers influence transport properties. In other words, the negative effect of spacers was observed for all 417

curing ages and conditioning regimes, even for thick samples (50 mm) that were reasonably well cured (28 418

day) and subjected to very mild drying at 20°C, 75% RH. However, plastic spacers consistently produced the 419

largest increase in transport, followed by cementitious spacers and steel spacers. In terms of transport 420

mechanism, permeability was affected the most, with increases of up to 300% observed in some samples. 421

Unfortunately, it is not possible to compare our results with other studies since there are no 422

precedents. However, it is interesting to note that the magnitude of increase in transport due to spacers is of 423

the same order of magnitude as that caused by drying-induced microcracking or air voids [24, 27, 30]. The 424

transport properties of the control samples (no spacers) obtained in this study are in good agreement with 425

available experimental data from samples of similar mix proportions, curing age and conditioning regime. 426

The measured chloride diffusion coefficients and surface concentrations are slightly higher than those 427

reported in earlier studies such as [31, 32, 33]. This could be due to the samples being dried prior to exposure 428

to salt solution. Samples from Mix C20 with maximum aggregate particle size of 20 mm were cored from a 429

larger slab to ensure proper placement and compaction, but coring could be damaging and this may have an 430

effect on the measured transport properties. However, the results from the cored samples were not hugely 431

different to those from cast samples with smaller aggregate particle sizes (10 mm). This indicates any 432

damage caused by coring is likely to be minimal. 433

Microstructure of the spacer-concrete interface 434

The results clearly show that inclusion of a spacer disturbs concrete microstructure by decreasing the 435

cement content and increasing the porosity at the vicinity of spacer. The deficiency in cement content at the 436

spacer-concrete interface is due to difficulties in packing cement particles close to the spacer surface, i.e. the 437

“wall effect”. The increased porosity can be explained by higher initial water content at the interface due to 438

the wall effect and bleeding along the spacer surface or entrapment of bleed water beneath horizontal 439

surfaces of the plastic spacer. Furthermore, debonding and microcracking occurred at the interface, and this 440

is probably due to differential volumetric changes (drying shrinkage or thermal) or relative movements 441

between spacer and concrete. 442

For all cases, the zone around the spacer with affected microstructure extends to around 50 μm from 443

the spacer boundary, with respect to both unreacted cement and porosity gradients. We note that the 444

characteristics of these gradients and the width of the affected zone are similar to those of the aggregate-445

cement paste “interfacial transition zone” [34, 35, 36, 37], as well the steel-concrete interface [38, 39] and air 446

void-paste interface [30]. The width of the affected zone should be related to the size of cement particles and 447

so it is expected to decrease if finer cement particles or supplementary cementitious materials such as 448

microsilica are used. However, this needs to be verified. 449

The results show that a zone of weakness exists in the concrete near spacers that has higher 450

water/binder ratio and porosity compared to the concrete located farther away. The increase in transport 451

properties recorded for samples with spacers is caused by the porous spacer-concrete interface. This is also 452

confirmed by the fluorescent epoxy impregnation (Fig. 10 & 11) and μXRF results (Fig. 9) showing a 453

preferential transport path along the spacer-concrete interface. The observed increase in surface chloride 454

content is most probably caused by the increase in the porosity at the spacer-concrete interface. The 455

performance of samples with plastic spacers was the worst across all properties measured. This is probably 456

due to a weak bond between plastic and concrete. There is also a greater mismatch in material properties in 457

terms of drying shrinkage and thermal expansion/contraction. In addition, samples with plastic spacers have 458

the highest amount of spacer-concrete interface; the surface area of a plastic spacer is 3-4 times that of a 459

cementitious spacer and 23 times that of a steel spacer (Table 1). 460

Comparison to aggregate-paste ITZ 461

Although the microstructure gradients at the spacer-concrete interface and aggregate-paste ITZ are 462

similar, their effect on the overall transport property and durability is probably very different. This is because 463

the spacer-concrete interface forms a continuous link that spans the full cover and so directly exposes 464


10

embedded reinforcing steel to external aggressive species. The ITZ on the other hand, envelopes aggregate 465

particles that are discontinuous. At high aggregate contents, the ITZ fraction may increase to the point where 466

adjacent ITZs overlap and interconnect. However, the effect of this is balanced by reduction in cement paste 467

content and total porosity of the concrete, and the increase in cement paste tortuosity. Furthermore, 468

conservation of water in the system dictates that increasing ITZ fraction will decrease porosity of the bulk 469

paste and therefore, its transport property. Indeed, experimental and numerical studies have shown that the 470

net effect of ITZ on bulk transport properties is small, even though the ITZs themselves have a greater local 471

porosity (and transport property) and are interconnected [e.g. 31, 40, 41, 42, 43, 44, 27]. Therefore, the effect 472

of the ITZ is clearly different to that of the spacer-concrete interface despite both having similar 473

microstructure characteristics in terms of porosity and unreacted cement gradients. 474

Implications 475

It is well-known that poor quality and/or inadequate concrete cover thickness is a major factor causing 476

premature degradation in concrete structures worldwide resulting in huge repair and replacement costs. 477

Spacers may seem low value, small and inconsequential, but because they are placed every metre or less 478

along the length of steel reinforcement and are left permanently in the cover zone, their overall effect could 479

be significant. The results from this study suggest that for any concrete structure, the locations where 480

reinforcements are supported by spacers would experience a greater exposure to external environment and 481

ingress of aggressive species. The assumed protection afforded by the design concrete cover is therefore not 482

achieved at the location of spacer. For instance, if a spacer increases diffusivity by a factor of four, the effect 483

of this is broadly similar to reducing the local cover by half. This leads to onset of carbonation-induced or 484

chloride-induced corrosion in quarter of the expected time, compromising long-term durability. 485

At present, it seems there is very little emphasis on the potential effects of spacers on long-term 486

durability, whether in research, design or construction. There is no guidance concerning how one could 487

reduce, prevent or mitigate the negative effects of spacers. It should be noted that codes of practice do 488

recognise that spacers must be durable and should not cause corrosion of reinforcement, spalling of concrete 489

cover, cracking or allow free passage of moisture, e.g. BS 7973-1:2001 [9] and BS EN 13670: 2010 [45]. 490

However, there are no recommendations on performance testing to ensure the quality of spacers or the 491

concrete placed near spacers or the bond between spacer and concrete. There is very little differentiation 492

between spacer types in terms of how their material, shape or execution may influence long-term durability. 493

For example, in BS 7973-1:2001, spacers are classified mainly according to their load capacity. The 494

emphases in current standards are on fixity, stability and load bearing capacity of the spacer, as well as their 495

impact on aesthetic appearance of the finished structure, rather than durability. It is also not common practice 496

for designers to specify spacers or include them in drawings or contract documents. Very often, the 497

responsibility for selecting spacer type to be used is left to the contractor. If guidelines are not provided, then 498

contractors may base their decision on cost, which may not be in the best interests of the durability of the 499

structure. 500

Single spacers were used in this study for the sake of convenience because it allowed testing to be 501

carried out on relatively small samples. However, continuous/bar/line spacers are commonly used, for 502

example in bridge decks and large slabs, to support several bars per spacer. These continuous spacers are 503

attractive because they reduce bar fixing time, but they create a large spacer-concrete interface area 504

compared to that of single cover spacers and thus could be more problematic in terms of cracking and effect 505

on durability. Furthermore, the samples in this study were tested in constant/controlled environment and 506

under non-loaded conditions. Real structures are subjected to moisture, temperature and load cycles. These 507

factors will increase the likelihood of debonding and cracking at the spacer-concrete interface, thus 508

exacerbating the effect of spacers on transport and microstructure. 509

510

5. Conclusion 511

The effect of reinforcement spacers on mass transport and microstructure of concrete was investigated via an 512

experimental programme involving 140 samples. Tests were carried out on 100 mm diameter samples with a 513

centrally placed spacer that is either plastic, cementitious or steel wire chair. Other test variables include 514

cover depth (25 and 50 mm), maximum aggregate particle size (10 and 20 mm), curing age (3 and 28 days) 515

and conditioning regime (20°C/75% RH, 20°C/55% RH and 50°C oven). The main conclusions are as 516

follows: 517


11

a) Presence of spacers always increases transport properties despite the spacers themselves containing little 518

or no porosity, and the concrete being fully compacted. Spacers increase penetration of gas, chloride ions 519

and water, regardless of the mechanism, i.e. permeation, diffusion or capillary absorption. The magnitude 520

of increase in transport depends on spacer type and transport mechanism. 521

b) Permeability was affected the most, with increases of up to 300% observed for a 100mm diameter 522

concrete sample with a centrally placed spacer. The average increase in oxygen diffusivity, oxygen 523

permeability and water sorptivity for all samples containing spacers were 57%, 138% and 27% 524

respectively. At the spacer-concrete interface, the chloride diffusion coefficient and surface chloride 525

concentration were higher than the control by 33% and 203% respectively. 526

c) The negative effect of spacers on transport was observed for all curing ages and conditioning regimes, 527

even for thick samples (50 mm) that were well cured (28 day) and subjected to very mild drying at 20°C, 528

75% RH. 529

d) The spacer-concrete interface contains higher porosity, lower cement content, and therefore higher 530

water/binder ratio compared to the concrete farther away from the spacer. Debonding and microcracking 531

were also observed at the spacer-concrete interface. The width of the disturbed microstructure is around 532

50 μm from the spacer. The porosity at the spacer-concrete interface is 3 to 6 times that of the bulk paste 533

region depending on spacer type. 534

e) The increase in transport recorded for samples with spacers is caused by the porous spacer-concrete 535

interface, which forms a continuous link spanning the full cover depth and provides an easy path for 536

ingress of aggressive species to reach embedded reinforcement. This is confirmed by fluorescent epoxy 537

impregnation and μXRF showing a preferential transport path along the spacer-concrete interface. The 538

increase in surface chloride concentration is linked to the higher porosity at the spacer-concrete interface. 539

f) Plastic spacers consistently produced the largest increase in transport and the highest porosity at the 540

spacer-concrete interface, followed by cementitious spacers and steel spacers. The performance of 541

samples with plastic spacers was the worst across all properties measured. This is probably due to a weak 542

bond between plastic and concrete. There is also a greater mismatch in material properties in terms of 543

drying shrinkage and thermal expansion/contraction. 544

g) Spacers may seem low value, small and inconsequential, but because they are placed every 1 m or less 545

along the length of steel reinforcing bar and are left permanently in the cover zone, their overall effect on 546

the penetration of external media into the cover zone of concrete structures is significant. This is not 547

currently recognised by codes of practice or by most practitioners. 548

549

Acknowledgements 550

We would like to thank Mr. Andrew Morris for his help with the laboratory work. 551

552

References 553

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Concrete Technology Today, September, 3. 555

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C568, Construction Industry Research and Information Association, London, 90 pp. 558

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Steel Institute, 288 pp. 562

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12

[8] BS 8110-1 (1997), Structural use of concrete. Part 1: Code of practice for reinforced concrete. British 565

Standard Institution, London, 1997. 566

[9] BS 7973-1: 2001, Spacers and chairs for steel reinforcement and their specification. Part 1: Product 567

performance requirements, British Standards Institution. 568

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application of spacers and chairs and tying of reinforcement, British Standards Institution. 570

[11] ACI 315-99 (1999), Details and detailing of concrete reinforcement, American Concrete Institute, 44 571

pp. 572

[12] ACI SP-66 (2004), ACI Detailing Manual, American Concrete Institute, 219 pp. 573

[13] DIN EN 13670:2011-03, Execution of concrete structures, Deutsches Institut für Normung. 574

[14] Levitt, M. & Herbert, M.R. (1970), Performance of spacers in reinforced concrete. Civil Engineering, 575

August, 1-5. 576

[15] Jahren, P. (1994), Carat - a development programme for new reinforcement chairs, Betong - Industrien 577

2/94, Oslo, 25-27. 578

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durability of reinforced concrete beams in long-term exposure to marine environment, Proceedings of 580

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P.H., Banthia, N., Buckland, P.J., Sixth International Conference on Short and Medium Span Bridges, 585

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[21] Tang, L. & Utgenannt, P. (2009), A field study of critical chloride content in reinforced concrete with 591

blended binder, Materials and Corrosion, 60, 617-622. 592

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[23] Wong, H.S., Buenfeld, N.R., Hill, J., Harris, A.W. (2007), Mass transport properties of mature 595

wasteform grouts, Adv. Cem. Res., 19, 35-46. 596

[24] Wu, Z., Wong, H.S., & Buenfeld, N.R. (2014), Effect of confining pressure and microcracks on mass 597

transport properties of concrete, Adv. Appl. Ceram., 113 (8) 485-495. 598

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S., Trejos, T. & Valadez, M. (2014), Signal‐to‐noise ratios in forensic glass analysis by micro X‐ray 600

fluorescence spectrometry, X‐Ray Spectrometry, 43, 13-21. 601

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London, PhD thesis (in preparation). 603

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size effects on mass transport properties of concrete, Cem. Concr. Res., 68, 35-48. 605

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from backscattered electron images. Cem. Concr. Res., 36, 1083-1090. 607

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gradients at interfaces in composite materials, Cem. Concr. Res., 36, 1091-1097. 609


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voids on the microstructure and mass transport properties of concrete, Cem. Concr. Res., 41, 1067-611

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concrete: three laboratory studies, Cem. Concr. Res., 36, 200-207. 616

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accelerated chloride migration test, Materials and Structures, 38, 313-320. 618

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concrete, Heron, 38 (1) 5-69. 620

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in high strength concretes, Advances in Cement Research, 1, 230-237. 624

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cement paste and aggregate in concrete, Interface Science, 12, 411-421. 626

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steel-concrete interface, Corrosion Science, 43 (4) 605-610. 628

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interfaces in steel reinforced concrete, Cem. Concr. Res., 37 (12) 1613-1623. 630

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concrete diffusivity: Identification of significant variables, Cement, Concrete and Aggregates, 20, 129-632

139. 633

[41] Carcasses, M., Petit, J.Y. & Olliver, J.P. (1998), Gas permeability of mortars in relation with the 634

microstructure of interfacial transition zone (ITZ), in: A. Katz, A. Bentur. M. Alexander, G. Arliguie 635

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92. 637

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diffusivity of mortars, Adv. Cem. Based Mater., 7, 60-65. 639

[43] Wong, H. S., Zobel, M., Buenfeld, N. R. & Zimmerman, R. W. (2009), Influence of the interfacial 640

transition zone and microcracking on the diffusivity, permeability and sorptivity of cement-based 641

materials after drying, Mag. Concr. Res., 61, 571-589. 642

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[46] BS 4482: 2005, Steel wire for the reinforcement of concrete products - specification, British Standards 647

Institution. 648

649

650

651

652

653

654

655


14

656

657

Table 1 Properties of spacers used in this study 658

Spacer

ID*

Cover size

(mm)

Porosity

(%)

Surface area

(m2)

Vol. fraction

(%) ** Notes

CS-25 25 15.9 0.00472 25.6 Cementitious spacer made of polypropylene

fibre-reinforced Portland cement mortar

containing 50% GGBS replacement at 0.35

w/cm ratio. With centre hole for tie wire. CS-50 50 16.2 0.00963 25.1

PS-25 25 0 0.00165 13.2 Clip-on “A” spacer made of recycled plastic.

Smooth surface. PS-50 50 0 0.00256 14.1

PS-50 (a) 50 0 (0.00256) 14.1 Clip-on “A” spacer made of recycled plastic.

Surface roughened by grinding with 120-grit

SiC paper (a) or by scoring notches on the

main flange (b). PS-50 (b) 50 0 (0.00256) 14.1

SS-50 50 0 0.00041 6.1

Continuous lattice steel wire chairs made of 5

mm diameter cold reduced wire conforming

to BS 4482:2005 [46]. Sectioned to 100 mm

segments.

* CS = cementitious spacer, PS = plastic spacer, SS = steel spacer. 659

** Expressed as volume fraction of the test sample (100 × 50 mm or 100 × 25 mm discs). 660

661

662

Table 2 Specific gravity at saturated-surface dry condition (SSD), moisture content and absorption of 663

the aggregates used. 664

Aggregate type MSA (mm) Specific gravity

(SSD) Moisture content (%)

24-hr moisture

absorption (%)

Sand (5.0) 5 2.60 0.25 0.62

Gravel (10) 10 2.51 0.40-0.43 0.82-0.86

Gravel (20) 20 2.56 0.41-0.46 0.80-0.84

665

666

Table 3 Concrete mix proportions. 667

Mix ID Cement

(kg/m3)

Water

(kg/m3) Free w/c

MSA

(mm)

Sand

(kg/m3)

Gravel

(kg/m3)

Aggregate vol.

fraction (%)

C10 413 172 0.4 10 726 1090 70

C20 413 172 0.4 20 733 1098 70


15

668

Table 4 Summary of the cylindrical test samples (100 mm). 669

Series Sample ID Mix Spacer Thickness

(mm)

Curing

(days) Conditioning Type

I C10-Co25

C10

None 25

3 & 28

(sealed) 20°C, 55% RH

C10-Co50 None 50

C10-CS25 CS-25 25

C10-CS50 CS-50 50 Cast

C10-PS25 PS-25 25

C10-PS50 PS-50 50

C10-SS50 SS-50 50

II C10-Co50

C10

None

50 28

(sealed)

20°C, 75% RH

20°C, 55% RH

50°C oven

Cast

C10-CS50 CS-50

C10-PS50 PS-50

C10-PS50a PS-50a

C10-PS50b PS-50b

C10-SS50 SS-50

III C20-Co50

C20

None

50 28

(sealed)

20°C, 75% RH

20°C, 55% RH

50°C oven

Cored C20-CS50 CS-50

C20-PS50 PS-50

C20-SS50 SS-50

IV C10-Co50

C10

None

50 28

(100% RH) 20°C, 55% RH Cast C10-CS50 CS-50

C10-PS50 PS-50

670

Table 5 Chloride diffusion coefficient (Deff) and surface chloride concentration (Cs) obtained from 671

fitting chloride profiles to Fick’s second law. 672

Sample ID Spacer Location Cs (%) Deff (×10-12 m2/s)

C10-Co50 None - 0.34 (0.071) 12.6 (0.12)

C10-PS50 Plastic Interface 1.03 (0.057) 16.8 (0.18)

C10-PS50 Plastic Centre 0.76 (0.062) 14.4 (0.31)

C10-PS50 Plastic Edge 0.35 (0.091) 12.7 (0.89)

C10-CS50 Cementitious Interface 0.69 (0.041) 14.1 (0.22)

C10-CS50 Cementitious Centre 0.29 (0.059) 11.7 (0.34)

C10-CS50 Cementitious Edge 0.33 (0.088) 12.7 (0.94)

673


16

Figure 1 Typical examples of spacers used in reinforced concrete: a) plastic clip-on “A” shaped, b) 674

plastic wheel/circular, c) plastic tower, d) cementitious single spacer, e) cementitious line/bar spacer, f) 675

steel wire chairs and g) steel wire continuous lattices. 676

677

678

679

680

681

682

683

684

685

686

687

688

689

690

691

692

693

(a)

(b)

(c)

(d)

(e)

(f)

(g)

PS-50

PS-25

PS-50a

PS-50b

CS-50

CS-25

SS-50


17

Figure 2 Cross-section of reinforced concrete showing the placement of steel reinforcement on a 694

cementitious spacer and plastic clip-on “A” spacer to achieve 50 mm cover. Aggressive species (e.g. 695

water, oxygen and chloride) may penetrate through the porous hardened cement paste (A), concrete-696

spacer interface (B) or the porous spacer (C). 697

698

699

700

701

702

Rebar

Cementitious

spacer

10 mm

(Exposed surface) (H2O, O2, Cl2 etc)

A C B

Cover

depth

Rebar

Plastic spacer

10 mm

(Exposed surface) (H2O, O2, Cl2 etc)

C B

Cover

depth

A


18

Figure 3 Particle size distribution of fine and coarse aggregate (MSA = maximum size of aggregate). 703

704

705

Figure 4 Set-up for preparing cylindrical test samples of 100 mm diameter containing various types of 706

spacers. 707

708

709

710

711

712

713

714

715

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10 100

Cu

mu

lati

ve

pas

sin

g (%

)

Sieve size (mm)

Sand (MSA:5)

Gravel (MSA: 10)

Gravel (MSA: 20)

Rebar

Cementitious

spacer

Plastic

spacer

Control sample

(no spacer)

Rebar

Steel spacer

Timber

Timber


19

(a)

(b)

Figure 5 Cross-section of samples containing a) cementitious spacer (CS-50) and b) plastic spacer (PS-716

50). Chloride concentration profiles within the spacer, along the spacer-concrete interface and near 717

the sample edge away from spacer, were measured using micro X-ray fluorescence. Locations of the 718

spot analyses are indicated with crosses (). 719

720

721

722

723

724

725

726

727

728

729

730

731

732

733

734

735

736

737

738

739

740

Inte

rface

spo

ts

Ed

ge sp

ots

Cen

tre

spo

tsCen

tre sp

ots

Inte

rface

spo

ts

Ed

ge sp

ots

10 mm

10 mm 10 mm

(Surface exposed to chloride) (Surface exposed to chloride)


20

a) 3-day cured

b) 28-day cured

Figure 6 Effect of spacer type and height on oxygen diffusivity, oxygen permeability and water 741

sorptivity of samples from Series I after curing for 3 and 28 days. Samples were conditioned at 20°C, 742

55% RH prior to transport testing. Notation: Co = control (no spacer), SS = steel spacer, CS = 743

cementitious spacer, PS = plastic spacer. 744

745

746

747

748

749

750

751

752

753

3

4

5

6

7

Co SS CS PS

O2

Dif

fust

ivit

y (x

10

-8m

2 /s)

Sample type

25mm

50mm

0.0

0.4

0.8

1.2

1.6

2.0

Co SS CS PS

O2

Pe

rme

abili

ty

(x1

0-1

6m

2 )

Sample type

25mm

50mm

50

60

70

80

90

100

Co SS CS PS

Sorp

itvi

ty

(g/m

2 .m

in0.

5)

Sampe type

25mm

50mm

2

3

4

5

Co SS CS PS

O2

Dif

fust

ivit

y (x

10

-8m

2/s

)

Sample type

25mm

50mm

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Co SS CS PS

O2

Pe

rme

abili

ty

(x1

0-1

6m

2 )

Sample type

25mm

50mm

50

55

60

65

70

75

80

85

Co SS CS PS

Sorp

itvi

ty

(g/m

2 .m

in0

.5)

Sample type

25mm

50mm


21

754

Figure 7 Effect of spacer type and drying regime on oxygen diffusivity, oxygen permeability and water 755

sorptivity of concretes from Series II (10 mm maximum aggregate size, 50 mm thick, 28 day cured). 756

757

0

2

4

6

8

10

12

14

16

18

Co SS CS PS PS(a) PS(b)

O2

dif

fust

ivit

y (x

10

-8m

2 /s)

Sample type

20 C 75% RH

20 C 55% RH

50 C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4


O2

pe

rme

abili

ty (

x 1

0-1

6m

2 )

Sample type

20 C 75% RH

20 C 55% RH

50 C

50

60

70

80

90

100

110


Sorp

itvi

ty

(g/m

2 .m

in0

.5)

Sample type

20 C 75% RH

20 C 55% RH

50 C


22

Figure 8 Effect of spacer type and drying regime on oxygen diffusivity, oxygen permeability and water 758

sorptivity of concretes from Series III (20 mm maximum aggregate size, 50 mm thick, 28 day cured). 759

760

761

762

a) Plastic spacer

b) Cementitious spacer

Figure 9 Chloride penetration profiles of samples from Series IV measured within the spacer (centre), 763

along the spacer-concrete interface and near the edge of sample at 1 mm spacing as shown in Fig. 5. 764

Results are compared to the control sample (no spacer). 765

766

767

768

769

770

0

2

4

6

8

10

12

14

16

18

Co SS CS PS

O2

dif

fust

ivit

y (x

10

-8m

2 /s)

Sample type

20 C 75% RH

20 C 55% RH

50 C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Co SS CS PS

O2

pe

rme

abili

ty (

x10

-16

m2 )

Sample type

20 C 75% RH

20 C 55% RH

50 C

50

60

70

80

90

100

110

120

130

140

Co SS CS PS

Sorp

itvi

ty

(g/m

2.m

in0.

5)

Sample type

20 C 75% RH

20 C 55% RH

50 C

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 5 10 15 20 25 30 35 40 45 50

% C

l b

y m

as

s o

f c

em

en

t

Depth (mm)

C10-PS50-Interface

C10-PS50-Edge

C10-PS50-Centre

C10-Co50

0.0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20 25 30 35 40 45 50

% C

l b

y m

as

s o

f c

em

en

t

Depth (mm)

C10-CS50-Interface

C10-CS50-Edge

C10-CS50-Centre

C10-Co50


23

a) Control sample (no spacer)

b) Sample with steel spacer

c) Sample with cementitious spacer

d) Sample with plastic spacer

Figure 10 Cross-sections of fluorescent epoxy impregnated samples imaged under UV light and their 771

respective binary images showing the epoxy intruded areas. Samples are a) C10-Co50-28d-50°C, b) 772

C10-SS50-28d-50°C, c) C10-CS50-28d-50°C and d) C10-PS50-28d-50°C. 773

774

775

776

777

778

779

10 mm

10 mm

10 mm

10 mm


24

(a)

(b)

Figure 11 Effect of spacer and conditioning regime on a) area fraction and b) maximum depth of 780

epoxy penetration. Samples are from Series II & III. 781

782

783

784

785

786

787

788

789

790

791

792

793

794

795

796

797

798

799

800

801

802

803

804

805

2

3

4

5

6

7

8

Co SS CS PS

Epo

xy im

pre

nat

ed

are

a (

%)

Sample type

20 C 75% RH (C10)

20 C 55% RH (C10)

20 C 55% RH (C20)

50 C (C10)

0

10

20

30

40

50

Co SS CS PS

Max

imp

regn

atio

n d

ep

th (

mm

)

Sample type

20 C 75% RH (C10)

20 C 55% RH (C10)

20 C 55% RH (C20)

50 C (C10)


25

(a)

(b)

(c)

(d)

(e)

Figure 12 Effect of spacers on the average distribution of unreacted cement (a) and porosity (b) from 806

the spacer-concrete interface. Results are compared to that of the aggregate-paste interface (ITZ) and 807

bulk paste of the control sample. Figures (c) to (d) show example BSE images of very porous spacer-808

concrete interface. Samples are from Series II & III that were 28-day cured and conditioned at 20°C, 809

55%RH. 810

811

812

813

814

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60 70 80

An

hyd

rou

s ce

me

nt

(%

)

Distance from interface (μm)

CS-50

PS-50

SS-50

Co (bulk paste)

Co (Aggregate ITZ)0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80

De

tect

able

po

rosi

ty (

%)

Distance from interface (μm)

CS-50

PS-50

SS-50

Co (bulk paste)

Co (Aggregate ITZ)

50 μm 50 μm 50 μm

Pla

stic

sp

ace

r

Cem

enti

tio

us

spa

cer

Ste

el s

pa

cer

influence of reinforcement spacers on mass transport ... · 51 66 [12], and german practice in din...

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