REFLEXIONES Y RECOMENDACIONES FINALES

Concrete Cubes

4.3.1.1. Mixing

Three pan mixers were used throughout this study with capacities of 0.01 m3, 0.02 m3 and 0.1 m3, which were chosen depending on the volume of the concrete batch needed. The concrete mixes were done in accordance with BS 1881-125:1986[242]. The aggregates were added in the following order: initially about half of the coarse aggregate, then the fine aggregate and the remainder of the coarse aggregate. The mixer was then started for 15 to 30 seconds. The mixing continued after adding about half of the total water for two to three minutes. All the cementitious materials were then added and the mixing was continued. Then the remaining water was added after 30 seconds, continuing mixing until two to three minutes after all the materials were added.

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4.3.1.2. Casting

To measure the compressive strength of the concrete, samples were cast into 100mm 3-gang cube moulds placed on a vibrating table. The moulds were half filled as shown in Figure 4.3a, vibrated and then filled up to the top before they were vibrated again to have a sufficient level of compaction as shown in Figure 4.3b. The time for vibrating was dependent on the workability of the concrete mixes.

a). Moulds were half filled b) Moulds filled to top then vibrated Figure 4.3: Concrete was cast into 100 mm 3-gang moulds

and placed on a vibrating table

4.3.1.3. Curing

The concrete moulds were then wrapped with cling film immediately after casting. Some of them were then submerged in water tanks set at 500C and the remainder of the wrapped specimens were then placed in a controlled humidity/ temperature environmental chamber called Temperature Applied Science (TAS) for adiabatic curing (the curing procedures are discussed in Section 4.3.6). In addition, concrete cubes cured at 200C were covered with a damp hessian and polythene sheeting and left on the vibrating table. After 24 hours, they were demoulded and then transferred to a water tank and placed in another environmental chamber set at 200C, as shown in Figure 4.4. Concrete cubes that had been cured at other temperatures (500C and adiabatic) were also demoulded

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after 24 hours. They were returned back into their curing place immediately after demoulding. They were subsequently tested for compressive strength at 0.25, 0.5, 1, 2, 4, 8, 16, 32, 64, 128, 256 and 365-days.

Figure 4.4: Environmental chamber set at 200C for standard curing

4.3.2 Mixing, Casting and Curing Procedures for Measurement the Strength of

Mortar Cubes

4.3.2.1. Mixing

Mortar equivalent to the concrete mixtures under investigation were prepared in accordance with ASTM C-1074[6]. A quantity of 0.30 m3 of each mortar mixture was prepared using a horizontal pan mixer. Materials were added in the order: cement/ggbs, sand and water mixed with the superplasticisers (if they were used). The mortar was mixed for three minutes[40].

4.3.2.2. Casting

To investigate the strength development of mortar cured at different curing temperatures, the mortar was cast into 50 mm 3-gang cube moulds placed on a vibrating table. Similar to the way that was concrete cast, the moulds were filled in two phases and compacted.

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4.3.2.3. Curing

The specimens were wrapped in cling film immediately after casting. They were then transferred to water tanks set at 30, 40 and 500C (shown in Figure 4.5). Mortars that were cured under adiabatic conditions were placed in a controlled humidity/temperature environmental chamber. While the mortar cured at 100C was cured in the Temperature Match Curing (TMC) tank (shown in Figure 4.6), which could be set to the temperature of 100C. As with the concrete cubes, all the mortar cubes were demoulded after one day and returned back to their curing temperature. Mortar cubes cured at 200C were covered with a damp hessian and polythene sheeting and left on the vibrating table. After 24 hours, they were demoulded and then transferred to a water tank set at 200C, in a similar way to the concretes cured at the same temperatures. They were subsequently tested for compressive strength at 0.25, 0.5, 1, 2, 4, 8, 16, 32, 64, 128, 256 and 365-days.

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Figure 4.6: Temperature Matched Curing (TMC) tank for curing

4.3.3. Mixing, Casting and Curing Procedures for Measurement the Strength

of Mortars Cured Under Rapid Temperature Rise

Five mortar mixes of grade C45 with different levels of GGBS (0, 20, 35, 50 and 70%) were prepared to investigate the effect of changed curing temperatures on the strength development of the mortars. The mixing and casting procedures were the same as those of mortars cured at different isothermal temperatures. Mortar cubes were cured inside two tanks with different isothermal temperatures i.e. 20 and 500C. There were five different curing regimes, as presented in Table 4.3. Three cube specimens of each mix were prepared for each testing age.

Table 4.3: Curing regimes for mortars with changing curing temperatures

No Curing regime Testing age (days)

1 Isothermal 20 0C (Control) 0.5, 1, 2, 3, 4, 5, 7, 14, 28, 56 _{and 91} 2 ₂₀0_{C 1-day and then 50}0_{C onwards 2, 3, 4, 5, 7, 14, 28, 56 and 91}

3 ₂₀0_{C 2-days and then 50}0_{C onwards 3, 4, 5, 7, 14, 28, 56 and 91}

4 ₂₀0_{C 3-days and then 50}0_{C onwards 4, 5, 7, 14, 28, 56 and 91}

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All mortar cubes, except for the specimens cured at 500C from the beginning, were left for 24 hours on the vibrating table, covered with damp hessian and polythene sheeting. They were demoulded after 24 hours, when 27 cube specimens were transferred into a tank set to 500C. The remaining cubes were transferred into a water tank placed in a controlled chamber at 200C. After 2-days, another 24 cube specimens were transferred from 20 into 500C. Finally, after 3- days, 21 cube specimens were transferred from 20 to 500C. They were subsequently tested for compressive strength at the ages shown in Table 4.3.