Material Characterization for Sustainable Concrete Paving Blocks
2. Experimental Program 1. Materials
The materials used in this series of experiments are listed as follows: natural coarse aggregate, natural fine aggregate, ordinary Portland cement, RCCA (contain crushed concrete, small amount of crushed stone and crushed bricks), RCFA, CR, CG, and GGBS. Figure1shows the recycled wastes used in this experiment. Table2shows the information of the construction wastes, which contains the size range, the source of each material listed in the table. The detail properties of GGBS provided by the supplier are shown in Table3.
(a) (b) (c) (d) (e) Figure 1.Recycled wastes. (a) RCCA; (b) RFCA; (c) crumb rubber; (d) crushed glass; (e) GGBS.
Table 2.Size range and sources of construction wastes.
Materials Size Range Source
Recycled concrete coarse aggregate 5.0–25.0 mm Crushed concrete Recycled concrete fine aggregate 0.1–4.0 mm Crushed concrete
Crumb rubber 1.0–2.0 mm Recycled tires
Crushed glass 5.0–25.0 mm Crushed toughened glass
Table 3.Properties of ground granulated blast furnace slag (GGBS).
Properties Unit Results National Code
GB/T-18046 [19]
Density g/cm3 2.940 ≥2.80
Specific surface area m2/kg 455.000 400.00–500.00
Mass loss on ignition % 0.600 ≤3.00
MgO percentage % 9.910 ≤14.00
SO3percentage % 1.825 ≤4.00
Cl−1percentage % 0.012 ≤0.06
Water percentage % 0.010 ≤1.00
Activity index (7 days) % 90.000 ≥75.00
Activity index (28 days) % 98.000 ≥95.00
2.2. Mix Proportion
In this project, the original mix proportion was designed according to the “mass method” in Chinese standard JGJ55-2011 [20]. The particular mix proportions for this series of experiments are shown in Table4. The range of replacement levels for each recycled material was determined as follows:
Recycled concrete coarse aggregate, or RCCA for short, is one of the most common recycled waste used for recycled aggregate concrete (RAC) products. According to the previous articles and experiments’ results, five replacement percentages of RCCA were selected in this experiment.
The concrete paving blocks which were cast by replacing the natural coarse aggregate with different replacement levels of 20%, 40%, 60%, 80%, and 100% [21] by weight of the total coarse aggregate content.
Recycled concrete fine aggregates (RCFA) were also considered as a type of recycled wastes to be used in this experiment. Unlike the RCCA, previous researchers and experimental results [6]
indicated that a high replacement level of RCFA would lead to a sharp decline of concrete product’s properties. The limitation of the replacement percentage of RCFA was suggested as 30% [22]. Therefore, the replacement levels of RCFA in the experiment were determined as 10%, 20% and 30%.
Both crumb rubber (CR) and crushed glass (CG) were barely used as aggregates in the concrete paving blocks. Therefore, several trial experiments were undertaken to estimate the ranges of replacement levels. Due to the rubber particles having strong rebound resilience [23], the specimens cannot maintain the original dimensions after being taken out from the brick making machine if the rubber content is too high, considering which the replacement levels of CR were selected as 1%, 2%, and 3%. Similarly to RCCA, crushed glass was also used to replace the coarse aggregates in the blocks.
According to the previous articles and trial test results [24], the final replacement levels of CG were decided as 10%, 20%, 30% and 40%. GGBS was used to replace the cement with replacement levels selected as 30%, 50% and 70% according to the previous test results [14,25].
In Table4, “RL” represents “replacement level”, “NCA” represents “natural coarse aggregate”, and “NFA” represents “natural fine aggregate”. The replacement levels of RCCA in the experiment were determined as 20%, 40%, 60%, 80%, and 100%, labeled as RCCB-X, with “X” indicating the replacement level of RCCA in the blocks. The replacement levels of RCFA in the experiment were determined as 10%, 20%, and 30%, labeled as RCFB-X. The replacement levels of CB were selected as 1%, 2%, and 3%, and labeled as CRB-X. The replacement levels of CG were decided as 10%, 20%, 30%, and 40%, and named as CGB-X. The replacement levels of GGBS were selected as 30%, 50%, and 70%
and labeled as GGBS-X.
2.3. Specimens Preparation
The specimens were designed as concrete paving blocks with typical dimensions. Figure2 demonstrates the process of manufacturing the concrete paving blocks in this project. The whole procedures, including preparing the concrete mixture, casting concrete paving blocks and curing, were in accordance with the actual industrial production standards by using a small scale brick-making
machine as shown in Figure3with maximum pressing force of 80 kN. Before the casting phase, all the materials used in the experiments were completely dried in the oven for 24 h. At the concrete mixture preparation phase, water was added after the coarse aggregates, cement, and sand mixing in the mixer for two minutes, and then the whole materials were mixed for another three minutes. For one batch of the concrete mixture, the total mass was 60 kg for casting 20 concrete paving blocks. For one batch of blocks (20 blocks), five specimens were used to test the compressive strength, five specimens for tensile splitting strength, and the other three for water absorption, slip resistance tests, and abrasion resistance [15]. After casting, all the specimens were cured in a thermostatic chamber for 28 days until testing. The temperature in the chamber remained at 20±2◦C, and the humidity was controlled at 50% by the temperature and humidity controller.
Table 4.Mix proportions (kg/m3) of concrete paving blocks.
Mix RL Water Cement NCA NFA RCCA RCFA Rubber Glass GGBS
RCCB-0 0% 152 380 959 959 0 0 0 0 0
RCCB-20 20% 152 380 767 959 192 0 0 0 0
RCCB-40 40% 152 380 575 959 384 0 0 0 0
RCCB-60 60% 152 380 384 959 575 0 0 0 0
RCCB-80 80% 152 380 192 959 767 0 0 0 0
RCCB-100 100% 152 380 0 959 959 0 0 0 0
RCFB-10 10% 152 380 959 863 0 96 0 0 0
RCFB-20 20% 152 380 959 767 0 192 0 0 0
RCFB-30 30% 152 380 959 671 0 288 0 0 0
CGB-10 10% 152 380 863 959 0 0 0 96 0
CGB-20 20% 152 380 767 959 0 0 0 192 0
CGB-30 30% 152 380 671 959 0 0 0 288 0
CGB-40 40% 152 380 575 959 0 0 0 384 0
CRB-1 1% 152 380 959 949 0 0 10 0 0
CRB-2 2% 152 380 959 940 0 0 19 0 0
CRB-3 3% 152 380 959 930 0 0 29 0 0
GGBS-30 30% 152 266 959 959 0 0 0 0 114
GGBS-50 50% 152 190 959 959 0 0 0 0 190
GGBS-70 70% 152 114 959 959 0 0 0 0 266
Figure 2.The process of manufacturing the concrete paving blocks.
(a) (b) (c) Figure 3. Full view and detailed images of the brick making machine. (a) hydraulic brick making machine; (b) two moulds for deforming the concrete paving blocks; and (c) baseboard for load application.
2.4. Experiments
As the essential component of the concrete mixture, the properties of different types of raw material were tested, including sieving analysis [7] and aggregate water absorption [26].
The blocks’ strength tests and coarse aggregates’ aggregate crushed value test were conducted on a compression testing machine as shown in Figure4a. In order to obtain the compressive strength of blocks, the applied load on the pedestal was controlled by force under a constant rate of 3 kN/min, and, for the tensile splitting test, the rate was 0.1 kN/min with reference to BS EN 1338: 2003 [15].
Slip resistance was measured using pendulum friction test equipment as shown in Figure4b.
Specimens were adjusted to ensure the contact surface was horizontal before testing. Water absorption of blocks was also determined in accordance with BS EN 1338: 2003 [15]. Blocks were immersed in water at a temperature of 20±5◦C for three days before testing.
Abrasion resistance of blocks was tested by a special test machine: CM-B type unglazed brick abrasion testing machine. Figure4c shows the image of the testing machine. According to BS EN 1338:
2003 [15], the “wide wheel abrasion test” method was selected in this series of experiments.
(a) (b) (c)
Figure 4.Test equipment in this series of experiments; (a) compression testing machine; (b) pendulum friction test equipment; (c) unglazed brick abrasion testing machine.
3. Results and Discussion