Mechanical behavior of recycled aggregate concrete using cement and alkaline binder

Concrete using recycled aggregate (BTCLTC) has been and is being studied by many studies on material properties. However, there are very few studies on the properties of reinforced concrete structures on reinforced concrete structures. This paper presents the research results on the mechanical behavior of concrete samples and reinforced concrete beam structures using recycled aggregates combined with a cement binder or an activated alkaline slag binder. The mechanical characteristics such as compressive strength, flexural strength and elastic modulus of the reinforced concrete samples have been studied and compared with the mechanical behavior of concrete using natural aggregates with compressive strength. 30 MPa.
>> Mechanical behavior of recycled aggregate concrete using cement and alkaline binder (P1)

3. Results and discussion

3.1 Mechanical properties of concrete

Compressive strength: The development of compressive strength of concrete is compared in Figure 2, the value shown is the average value of three 100×100×100 mm cubes. It can be seen that the compressive strength of all concrete samples increases with curing time, however, the compressive strength of reinforced concrete is lower than that of concrete using CLTN at most ages. experiment. The compressive strength at 28 days of the DCI sample was 35.5 MPa (converted compressive strength reached M300 mark), while the compressive strength of BTCLTC was only 26.9 MPa (reduced by 24.2%). This can be caused by 2 reasons: (1) The old mortar component has a porous structure that adheres to the old CLTN grain and (2) the CLBTTC grain itself also has many defects and cracks that appear during the process. process of crushing concrete waste. These have resulted in the mechanical and mechanical properties of the CLBTTC particles often being worse than that of CLTN [20].

When using CKDXK with 7% alkali content, the compressive strength of BTCLTC is significantly improved. The compressive strength of concrete BTCLTC-XK7% after 28 days of curing has reached 37.1 MPa; 35.9% increase compared to the BTCLTC sample using only Portland cement. The increase in compressive strength of concrete with CKDXK is due to the influence of alkali activation and puzolenic effect of XLCNM [20]. The CLBTTC particles always have an adherent cement mortar with many voids and Ca(OH)2 is available in the pores. When concrete using CLBTTC in combination with CKDXK, there will be two effects that can contribute to improving the compressive strength of concrete, which are [12,17,19]: (1) Part of the NFNC particles will penetrate into the concrete. voids and hollow structure of the adhesive grout in the CLBTTC granules, then improves the interface transition zone (ITZ) between the alkaline slag binder rock and the CLBTTC grain surface better by performing alkaline reactions activation, pozzolanic reaction right at the voids and defects [17,19]; (2) The existing cracks and defects in the CLBTTC granules will also be filled and healed by the hydration products of CKDXK [12] and of the pozzolanic reaction between XLCNM and Ca(OH)2 available in the granules. voids, small cracks or due to continued hydration of old cement clinker, increasing the density of the aggregate particle structure as well as increasing the strength of the binder rock.

Bending strength and elastic modulus

Similar to compressive strength, flexural strength and elastic modulus of reinforced concrete at 28 days also decreased sharply compared to reinforced concrete (reduced by 22.2% and 20.3% respectively), the values ​​in Figure 3 are average value on 03 prism samples 10×10×40cm (for flexural strength) and 03 cylindrical samples D×H = 15×30cm (for elastic modulus). When using CKDXK with 7% alkali content to replace cement, the flexural strength and elastic modulus of BTCLTC improved markedly (35.7% and 14.3%), equivalent to and even exceeded. including the flexural strength of the DCI sample, but the elastic modulus is still about 9% lower than that of the DCI sample (Figure 3). This proves that the low elastic modulus of CLBTTC particles, although improved, is still lower than the elastic modulus of CLTN [21].

3.2 Bending behavior of reinforced concrete beams

Relationship between load and strain (deflection)
The relationship between flexural load and deflection of reinforced concrete beams using reinforced concrete and reinforced concrete is similar and the failure mechanism is plastic failure (Figure 4). Before the first crack appears (point A), the relationship line is linear and shows the elastic behavior of the beam. Concrete with the same grade will have similar bending behavior (DCII and BTCLTC, DCI and BTCLTC-XK 7%). This proves that predictive models of flexural behavior of reinforced concrete can be used for BTCLTC [10]. Specifically, the part showing the load-deflection relationship is linear for the low reinforced concrete beam, which proves that the reinforced concrete beam has a lower crack resistance moment than that of the DCI girder. This is because the reinforced concrete has a lower elastic moment than the DCI concrete. Furthermore, in the RC beam, there are always three transitional interface zones (between the CLTN and the old mortar in the CLBTTC grain, between the new mortar and the old mortar or the old aggregate), while in the CI beam there is only one ITZ zone between the CLTN and the old aggregates. adhesives. When using 7% CKDXK instead of portland cement, the crack resistance moment of 7% BTCLTC-XK beams has been significantly improved (reaching 6.1 kN) and equivalent to that of DCI beams (reaching 6.5 kN), significantly higher than that of BTCLTC and DCII beams.

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