Durability

168 𝜎_𝑎 = Stress at fcu

𝜎_𝑏 = Stress at basic stress (0.5N/mm²) 𝜀_𝑎 = Strain at fcu

𝜀_𝑏 = Strain at basic stress (0.5N/mm²)

The modulus results were finally converted to GPa by dividing by 1000000000.

169 Apparatus

 Oven capable of 50° C (± 2° C)

 Compressible rubber collars

 Permeability cell arrangement as show in Figure 3-19

 Gauges and pressure transducers with at least a 0.5 kPa accuracy

 Vernier calliper (0.02mm accuracy)

 Standard Oxygen supply and regulator capable of 120 kPa pressure regulation

 Desiccator

Figure 3-19: Oxygen permeability test apparatus (Comsiru, 2009)

Method

Specimens were cast and cut into 4 disks (70 mm diameter x 30 mm thickness) per specimen. After cutting, specimens were dried for 7 days in an oven at 50° C. Specimens were cooled in a desiccator for 2-4 hours. When testing, each disk was placed in the compressible collar with a rigid sleeve. The sample, collar and rigid sleeve were placed into the test chamber. A solid ring was placed over the rigid sleeve and a cover plate was placed

170

and tightened on top of the solid ring. The test was started 30 min after the sample was removed from the desiccator. The oxygen inlet and outlet were left open and the test chamber was purged of all gases other than oxygen for 5 seconds. The oxygen outlet was closed until pressure built up to 100 kPa then the inlet was closed. The initial pressure P0 at time t0 was recorded. The second reading was taken after 5min and subsequent time readings after every 5 kPa drop in pressure were recorded until the pressure reading reached 50kPa. The test was repeated for all four disks and the calculations below were used to calculate the oxygen permeability of the sample.

The time t0 readings were then plotted against pressure P0 readings taken and the slope of the line of regression was found (Comsiru, 2009).

The formulas used are shown below:

z =

∑[ln(^𝑃0

𝑃𝑡)]

2

∑[ln(^𝑃0

𝑃𝑡)𝑡]

z = slope of the regression line P0 = pressure at time t0[kPa]

Pt = pressure readings taken at time t after t0 [kPa]

t = time in seconds

The Darcy coefficient of permeability (k) was found by:

k = ^{𝜔𝑉𝑔𝑑𝑧}

𝑅𝐴𝜃

𝝎 = Molar mass of Oxygen = 32 g/mol

V = Volume of Oxygen under pressure in the permeameter [m³] g = gravitational acceleration = 9.81 m/s²

d = Average specimen thickness [mm]

z = slope of the regression line

R = universal gas constant 8.313 Nm/K mol

171 A = cross-sectional area of the specimen in [m²] 𝜽 = absolute temperature [K]

The Oxygen permeability index (OPI) was finally calculated from the negative log of the average k value for the four disks.

OPI = = - log10 [¼ (k1 + k2 + k3 + k4)]

k1, 2, 3, 4 = Darcy coefficients of sample disks 1, 2, 3 and 4.

The average of the four concrete disks tested for the OPI for the mixes at critical volume and the mixed waste combinations, were reported by Contest and used for this study.

Chloride conductivity (CC)

The chloride conductivity tests, developed in South Africa, gave an indication of the resistance of the concrete to the ingress of chloride ions by diffusion. Chloride conductivity was measured in milliSiemens per centimetre (mS/cm).

Apparatus

 Oven capable of 50° C (± 2° C)

 Vacuum saturation apparatus as shown by Figure 3-20

 Conduction cell with flexible collars shown by Figure 3-21

 DC power supply (90-12 V) , (0-1 A stabilised)

 Digital voltmeter and ammeter

 Electrical cables and plugs

 Scale accurate to 0.01g

 10 litre container with lid

 CP grade NaCl (99% purity)

 Desiccator

 5 M sodium chloride solution

172

Figure 3-20: Vacuum saturation apparatus (Comsiru, 2009)

Figure 3-21: Chloride conductivity apparatus (Comsiru, 2009)

Method

Specimens were cast and cut into 4 disks (70 mm diameter x 30 mm thickness) per specimen. After cutting, specimens were dried for 7 days in an oven at 50° C. Specimens were cooled in a desiccator for 2-4 hours. The samples were saturated using the vacuum saturation apparatus in Figure 3-20, for 3 hours under vacuum of -75 kPa to -80 kPa. The

173

vacuum tank was then isolated and NaCl solution was allowed to enter under a constant vacuum of -75 kPa to -80 kPa until the specimens were covered to a depth of 40 mm. After 1 hour the vacuum was released and samples were soaked for 18 hours. After soaking, the specimens were towel dried to saturated surface dry conditions and weighed.

NaCl solution was filled into the plastic tubes shown in Figure 3-21, and the samples were placed in a flexible collar. The use of a highly conductivity (5M) NaCl solution ensured that the test was independent of the concrete pore solution and primarily assessed the physical resistance of the concrete microstructure to chloride penetration (Alexander, et al., 1999).

The sample and collar were then inserted into the cell and the voltmeter and ammeter were connected. The DC power supply was then adjusted until there was an approximate voltage of 10 V across the specimen. The voltage and ammeter readings were subsequently taken immediately and after 15 minutes (Comsiru, 2009). The test was carried out at a constant room temperature of 25°C because conductivity increases with increasing temperature.

The chloride conductivity was calculated as follows.

σ = ^id

VA

σ = Chloride conductivity in milliSiemens per centimetre (mS/cm)

i = Electric current (mA) V = voltage difference (V)

d = average thickness of the specimen (cm) A = cross-sectional area (cm²)

The average of the four concrete disks tested for CC for the mixes at critical volume and the mixed waste combinations, were reported by Contest and used for this study.

Water sorptivity (WS)

The water sorptivity test measured the unidirectional ingress of water in a preconditioned concrete sample.

174 Apparatus

 Oven capable of 50 ° C ± 2° C

 Vacuum saturation

 20 mm deep tray to hold specimens

 Paper towels

 Measuring scale (0.01g-accuracy)

 Sealant

 Desiccator

 Stopwatch

 Tap water saturated with calcium hydroxide. (5 grams of Ca (OH) 2 per 1 litre of water), maintained at 23 ± 2°C.

Method

The disk specimens used for the oxygen permeability test were utilised for the water sorptivity tests.

The WS test was carried out at a room temperature of 23°C .Ten rolls of paper towels were laid out in a tray and were saturated as shown by Figure 3-22 on page 175. Calcium hydroxide solution was poured into the tray with a water layer visible on the top surface. The final water level had to be approx. 2mm up the side of the specimen.

Sealant was applied to the sides of the specimens and the dry mass of the specimens were recorded. Specimens were placed on the paper towels and the stopwatch was started.

Specimens were weighed after 3, 5, 7, 9, 12, 16, 20 and 25 minutes. The samples were then removed and placed in a vacuum tank. The samples were saturated using the vacuum saturation apparatus in Figure 3-20 on page 172, for 3 hours under vacuum of -75 kPa to - 80 kPa. The vacuum tank was then isolated and NaCl solution was allowed to enter constant vacuum of -75 kPa to -80 kPa until the specimens were covered to a depth of 40 mm. After 1 hour the vacuum was released and samples were soaked for 18 hours. After soaking the specimens were towel dried to statured surface dry and weighed (Msv). The calculations to obtain WS were as follows (Comsiru, 2009):

Line of best-fit (F) by linear aggression analysis.

175 F = ^{∑[√𝑡𝑖−𝑇]−[𝑀𝑤𝑡𝑖−𝑀𝑤𝑡]}

∑[√𝑡𝑖−𝑇]²

Mwti = the mass at any given time in grams

ti = the time in hours corresponding to the mass gain reading 𝑀_𝑤𝑡 = ^{∑ 𝑀𝑤𝑡𝑖}

𝑛

T = ^{∑ √𝑡𝑖}

𝑛

n = number of data points

Water Sorptivity (WS) was calculated as follows.

WS = ^{𝐹 𝑑}

𝑀_𝑠𝑣−𝑀_𝑠0

WS = water sorptivity [mm/√ℎ] F = line of best-fit [g/√ℎ]

d = average specimen thickness (mm) Msv = vacuum saturated mass (g) Ms0 = initial mass (g)

Figure 3-22: Specimens in tank with paper towels (Comsiru, 2009)

176

The average of the four disk tested for WS for the mixes at critical volume and the mixed waste combinations were reported by Contest and used for this study.