Study of the Mechanical Performance of a Recycled Aggregate Concrete with Admixture Addition ()
2. Materials and Equipments
2.1. Sand
The sand used in this study was a clean siliceous and fine sand of fraction 0/5 mm from Boussaada region [12]. Its characteristics are reported in Table 1 and the grading curve is shown in Figure 1.
2.2. Gravel@NolistTemp# Ordinary gravel was obtained from crushed limestone rock and delivered from the quarry of COSIDER El Euch region (250 km south of Algiers).The gravel fraction used in this study was 8/16, 16/25 in proportions of 40% and 60% respectively [13]. The characteristics are shown in
Table 1 and the grading curve in
Figure 1.
2.3. Recycled Aggregate
Pieces of old concrete specimens were crushed using a steel hammer provided by the civil engineering laboratory. The size of the particles maintained for this investigation was 8/16 an 16/25 at a percentage of 40% and 60%. The characteristics of the recycled aggregate are reported in Table 1 and Figure 2.
2.4. Cement
The cement used was type CPJ 42.5 delivered from Ain
Table 1. Some characteristics of the sand, ordinary crushed aggregates (CA) and recycled aggregate (RA).
Figure 1. Grain size distribution of the sand and crushed aggregate (ca).
Figure 2. Grain size distribution of recycled aggregate (ra).
Kebira cement plant and widely used in the construction sector in Algeria in accordance with standard [14].
2.5. Admixtures
The admixtures is a local superplasticizer product Medaplast (120) manufactured by Granitex in Oued Smar near Algiers. It is a brownwish emulsion having a relative density of 1.2 and a pH ranging from 8 to 8.5 [15].
2.6. Water
A tap water from the civil engineering laboratory was used for concrete batching.
2.7. Concrete Mix
The concrete mix proportions used were for a grade 350 mix, determined by the absolute volume [16].
Cement 324 kg/m3
Sand 565 kg/m3
Gravel 8/16 565 kg/m3 (40%)
16/25 760 kg/m3 (60%)
Water (total) 180 L (This quantity takes into account the degree of aggregate absorption).
3. Testing
3.1. Workability
The method used to assess the workability for both ordinary and recycled concrete was the slump test method [11]. A workability of about 50 mm was maintained for all mixes.
3.2. Compressive Strength
Compression tests were carried out on cubic specimens (150 × 150 × 150) mm3 Tests were done using the hydraulic press model “STRASSENTEST FHF” [17]. The specimens were centred on the tray of the press and a continuous load was applied. The ultimate compression load for each concrete cube was recorded at 28 days age (Tables 2 and 3).
3.3. Rebound Hammer Test
The specimens were placed in the centre of the hydraulic machine press; a continuous load was applied and maintained within a range of 10 to 20 KN. The rebound hammer test was carried out on five different points spaced at 2 cm intervals on both faces of the cubic specimens [18]. The final result from the test was calculated using the following equation:
(1)
with: n—Number of tests carried out on both faces of the cube; S—The recorded value of rebound hammer.
3.4. Ultrasonic Method
The pulse velocity test was carried out on the two opposite sides of the specimens (150 × 150 × 150) mm3 using direct transmission [19]. The transit time t in µs was recorded and the velocity V is measured as; (see Tables 2 and 3).
(2)
V—Velocity in km/sec.
d—The distance between the two transducers.
t—The transit time in µs.
4. Analysis of Results
4.1. Presentation of Results
In this study the concrete mixes have been prepared according to the method of absolute volume [16]. The following percentages of admixtures 0%, 1%, 1.5%, and 2% were chosen. The cubic specimens (150 × 150 × 150)
Table 2. Results of mechanical strength and density for recycled aggregate (RAC) at 28 days of age.
Table 3. Results of mechanical strength and density for Crushed Aggregate Concrete (CAC) at 28 days of age.
mm3 were cast and cured in water to be tested at 28 days age. Initially specimens measuring 150 × 150 × 150 mm3 with different admixture dosages were studied by the means of the non destructive tests (ultrasonic and rebound hammer) and finally the compressive tests were carried out on these concrete cubes. The value of compressive strength Rc is compared to Rsu the combined value obtained of ultrasonic (V) method and the rebound hammer reading (S) (see Tables 2 and 3).
The due strength Rsu is then assessed according to the formula of Feret [16].
(3)
n0 = 3.64, S—Rebound hammer reading.
n1 = 0.023, Rsu—Ultrasonic-rebound hammer resistance (N/mm2).
n2 = 0.56, V—Velocity Km/sec.
5. Results and Discussion
5.1. Compressive Strength
According to Figure 3 the behaviour of RAC shows the same trend of strength development for all dosages of admixture at 28 days. However for a dosage of 1% admixture the recycled concrete gives a lower compressive strength compared to normal concrete with crushed stone aggregate. The values of strength registered were 29.98 and 46.32, respectively.
For admixture contents over 1.5% the recycled concrete showed similar comparative values to ordinary concrete. Thus, performance of RAC similar to ordinary
Figure 3. The compressive strength Rc in function of the admixture content for recycled (RAC) and normal concrete (CAC) (compression test) at 28 days age.
concrete can be achieved with the incorporation of admixtures. Khaldoun Rahal, reported that the trend in the development of compressive and tensile strength and strain at peak stress in recycled aggregate showed; were similar compared to natural aggregate [4].
5.2. Rebound Hammer Results
Although the rebound hammer test results are affected by many factors, such as the mix characteristics (cement type, content and the aggregate type) or the member characteristics (mass, density, surface type, age, curing type and surface carbonation), only the main factor of aggregate type is considered in this study [10,16]. The rebound hammer test results in Figure 4 show that the 0% admixtures dosage gives the lowest value of strength. It is noted that there is a slight increase of strength for higher percentages admixture of 1%, 1.5% and 2%.
The recycled concrete RAC proved to give lower reading results compared to normal concrete (CAC). This could be related to the reliability of rebound hammer indirect test that is not convenient for accurate measurements of real strength for concrete structures compared to compression test.
5.3. Ultrasonic Tests
Figure 5 illustrates that what ever the type of aggregate used, strength development is similar for normal concrete and RAC. Furthermore the compressive strength of ordinary concrete is superior to the results given for RAC. The effect of admixtures on the mechanical strength is more advantageous for recycled aggregate concrete. The contribution of the admixtures to increasing strength is marked. This is due to the improvement of workability of fresh concrete, as well as water reduction in concrete mix resulting in an improvement of the resistance of concrete matrix.