Experimental techniques

3.4.1.1 Compressive Strength

The compressive strength of concrete was determined from triplicate 100mm cubic concrete samples at the end of conditioning period (generally at the age of 28 days) using a CONTEST compressive strength testing machine.

3.4.1.2 Absolute and Effective Porosity

In order to determine the absolute porosity of concrete, triplicate 100mm cubes were immersed in water for 7 days at the end of conditioning period (generally at the age of 28-day) and the saturated surface-dry mass determined. The weight gains were recorded during immersion until specimens had no more than 0.01% weight change per day. After that, specimens were oven-dried at 105±5°C for 5 days to determine the oven-dry mass.

Subsequent weighing after 5 days of oven drying showed no more than 0.01% change of dry weight per day. The oven drying procedure was performed at the final stage of testing to prevent micro-cracking. The absolute porosity of concrete was calculated as follows:

(Ws–Wd)×ρ

Absolute porosity= ———————×100 3.1

V

where, ρ is density of water, V is the volume of sample, Wd is the oven-dry mass of the specimen and Wsis the saturated surface-dry mass of the specimen.

Thus, absolute porosity is the proportion of the amount of pore space to the volume of the sample. It is usually expressed as a percentage of the volume of the sample.

The effective porosity of a concrete is defined as a percentage of the volume of empty pore to the volume of the sample at any given time and is given by the following expression:

(Ws–Wi)×ρ

Effective porosity= —————— ×100 3.2

V

where,W is the initial mass_i

The effective porosity depends on the moisture content of the sample. Whilst it changes during the test, the absolute porosity remains unaffected by changes in moisture content.

The absolute porosity and effective porosities were determined for all mixes in each group of samples immediately prior to first exposure at an age of 28 days. The effective porosities were estimated at the beginning of each cycle based on weight changes experienced by the specimens during previous wetting and drying cycles.

3.4.1.3 Absorption test- Wetting and drying cycles

The absorption test procedure as developed by TRL [Emerson and Butler, 1997] is a cyclic regime to represent site conditioning. It consists of 2 days of wetting followed by 12 days of drying. This regime had been chosen as meteorological data indicated that, on average, the UK has 48 hours of rain in every 14 days. In some cases different wetting and drying regimes were applied which varies duration of the cycle, drying temperature or concentration of absorbing salt solution as listed in section 3.2.2.

During the wetting phase, specimens (triplicate 100mm cubes) are placed on synthetic foam located in plastic trays containing salt solution. The level of solution was kept at or just below the level of the top surface of the foam. The cubes were placed on the foams with their suction surface, i.e. the uncoated face, touching the liquid (Figures 3.1 and 3.2). Each tray was covered with polyethene to restrict evaporation losses from the top surface of cubes and also to reduce evaporation of the solution thereby helping to maintain the concentration of the salt solution during testing. On day 1 the weight gained is measured after: 15 minutes, 30 minutes, 1 hours, 2 hours, 3 hours, 4 hours and 7 hours. This is repeated on day 2 but less frequently. Before each weighing a damp tissue was used to remove liquid from the surface taking care that no liquid was drawn out from the pores.

During the drying phase, the cubes were placed on narrow supports on a table in the drying room with their suction surfaces facing down. The supports allow free circulation of air. The temperature of the room was maintained at 20°C and the relative humidity was 80±10%. On day 1 the weight losses were noted after 1 hour 45 minutes, 3 hours 45 minutes and 7 hours 45 minutes and less frequently on subsequent days. In some cases, specimens were placed in an oven at 30ºC or 40°C during the drying period.

The salt solution employed was a sodium chloride (NaCl) solution generally made to a concentration of 178.5 g/l (50% saturated NaCl solution). In some cases different concentrations of salt solution were utilized, namely 0%, 3% (10.7g/l NaCl), 5% (17.8g/l NaCl), 10% (35.7 g/l NaCl) and 100% (357g/l NaCl). In the present study the concentration of NaCl solution is presented as a percentage of saturated NaCl solution.

The weight gain measurements are used to calculate sorptivities. This involved plotting the cumulative salt solution absorbed (g) against the square root of time (h)- h is hour- for the first four hours of absorption where there was a linear relationship between these two quantities. The slope of the line is weight sorptivity.

Distance sorptivity was calculated using weight sorptivity and effective porosity of specimens using following expression:

Volume sorptivity (mm³/√h)

Distance sorptivity (mm/h) = —————————————————— ×100 3.3

Effective porosity (%) × surface area (mm²)

Weight sorptivity (g/h) Where, Volume sorptivity (mm³/√h) = ———————————

Water density (g/mm³)

The depth of salt solution penetration is determined via the volume of solution absorbed and effective porosity of concrete as follows:

Volume of solution absorbed (mm³)

Salt solution penetration depth (mm): —————————————————————×100 3.4 Effective porosity (% volume) × surface area (mm²)

Weight gain (g) Where, Volume of solution absorbed (mm³) = ———————

Water density (g/mm³)

Figure 3.1: Schematic of set-up for wetting phases and sorptivity test

Figure 3.2: Concrete specimens located on synthetic foam in a tray containing salt solution

3.4.1.4 Chloride profiling

In general the cubes were removed from the salt solution after the 1^st, 6^th, 12^th, 18^th and 24^th cycle and the chloride profiles determined by analysing dust samples removed at successive depths from the concrete surface.

The chloride profiles of specimens immersed in salt solution continuously for 6 months were determined after the first, third and sixth months of immersion.

Concrete specimen

Salt solution

Synthetic foam

Generally no replicates were used for chloride profiling. Testing five sets of triplicate specimens showed that the chloride profiles are consistent [Appendix III- Figure A.1-A.5].

Sampling

The dust samples were obtained by grinding the cubes (Figure 3.3) and collecting the incremental dust samples which were subsequently analysed using a calibrated chloride meter (Figure 3.4).

Figure 3.3: Grinding machine

Figure 3.4: Chloride meter (Jenway PCLM3)

The surface grinding equipment consists of a diamond grinding wheel on a horizontal spindle and an adjustable flat-bed on which the concrete specimen is firmly clamped. The height of the flat-bed and the location of the concrete sample can be arranged and fitted by the use of three adjustment wheels.

The concrete dust was collected by vacuum suction in a disposable paper filter which was fitted tightly on the mouth of a pipe by elastic bands and connected to a vacuum. The mouth of the vacuum collector was positioned close to the grinding wheel.

The cubes were firmly clamped onto the flat-bed of the grinder, with the face exposed to the salt solution horizontal and uppermost. The dust was collected in 2mm depth increments.

The collected dust was transferred to a small plastic bag and the bag sealed and labeled. The region of grinding was a vertical line to the face exposed to the salt solution in order to eliminate any effects of mix segregation within the mould.

Analysing the chloride content

The chloride content was determined by electrochemical titration using a chloride meter (Jenway PCLM3). The procedure involves constructing a calibration curve of electrode potential (E) vs. log [Cl] by immersing the chloride selective and reference electrodes in chloride solutions of known concentration and recording the meter reading. The electrodes can then be immersed in the unknown solution and the measured electrode potential used to deduce the chloride concentration of the solution by means of the calibration curve.

The method is an application of the Nernst equation which states that the electrode potential, as measured against a reference electrode, in a solution containing chloride ions is directly proportional to the logarithm of its chloride ion concentration.

The procedure for sample preparation is as follows: 1-2 grammes of each concrete dust sample are weighed to 0.001g on an analytical balance into a 150ml beaker. Using a pipette, approximately 8 ml 1M acetic acid solution is poured over the ground sample and the solid sample dispersed in the acid. A watch glass is placed on the mouth of the beaker which is then heated to boil the liquid and then simmer at near boiling point for 30 minutes.

The suspension of dissolved cement paste is transferred to a 10ml volumetric flask. The volume is made up to 10 ml with 1 M acetic acid. Then, the suspension is filtered to remove any undissolved solids and a 500μl sample is taken from the filtered liquid and tested using the chloride meter.