Long-Term Flexural Behaviour of Cracked Reinforced Concrete Beams with Recycled Aggregate

Portland high strength cement (C52.5 N) was used for all mixes of this research. The cement is manufactured to meet the requirements of BS EN 197–1. According to the standard limits of BS EN 882, natural river sand was used as a fine aggregate (FA) with a maximum particle size of 5 mm. Two types of coarse aggregate were used: a natural coarse aggregate (NA) composed of un-crushed limestone with a maximum particle size of 20 mm (NA), and a recycled aggregate (RA) composed of crushed, washed construction and demolition waste (CDW) also with a maximum size of 20 mm. The composition of the CDW as provided by the supplier is presented in Fig. 1.

The grading and properties of both types of aggregate conformed to the standard requirements of BS EN 882 and BS EN 8500 as shown in Fig. 2 and Table 1. In terms of the grading, the main concern in this research was to ensure that the grading of the two types of aggregate fell between the maximum and minimum grading lines defined in BS EN 882 (see Fig. 2). By doing so any effects on creep due to the differences in grading would be minimised. In addition, the longitudinal and shear reinforcement steel used in this research had a yield stress of 500 MPa and a modulus of elasticity of 200GPa.

Three different concrete mixes were designed and used for this research with three CDW replacement levels. The mixing water compensation method was followed in order to ensure all of the mixes had the same effective water-to-cement ratio (0.42). Additional water (AW) was added to the RAC mixes to compensate for the high-water absorption capacity of CDW (according to the final measured absorption ratio of CDW) and to achieve the required workability. The amount of additional water was calculated based on the amount of CDW in the mix and its water content. Details of the mix proportions and slump tests are summarised in Table 2.

In the first stage of this research, six square-section prisms (120 × 120x1200 mm) reinforced with a central bar ∅16 mm were prepared and cast for the long-term tension stiffening tests as shown in Fig. 3. There were two specimens of each mix. The first specimen was subjected to a sustained load for 35 days (The test was stopped when the stabilisation in the trend of results occurred.), and the second specimen was stored next to the first one and was used to measure shrinkage. A special rig was built for this test in the George Earl Laboratory at the University of Leeds which is similar to that used by Scott and Beeby in 2005 (2005).

In the first specimen, three strain gauges were placed at 300-mm intervals along the length of the steel bar and were connected to a data logger to measure the strains in the bar over time. In addition, seven demountable mechanical strain gauges (DEMEC) were fixed on opposite sides of the concrete specimens every 150 mm in order to measure the surface strains of the concrete over time as shown in Fig. 4.

A direct tensile load of 45 kN was applied to the steel bar throughout the test using a hydraulic jack and load cell. The applied load corresponded to a stress of 200 MPa in the steel bar at the crack and matched the load applied during the beam test. The measured strains were used for calculating the reduction in tension stiffening of the concrete over time. Scott and Beeby’s Method 1 was followed for the tension stiffening calculations; the details of this method are presented in Scott and Beeby (2005). The calculations of concrete tensile stresses were based on the following relationship: where \(\sigma_{c}\) is the concrete stress; \(F\) is the applied force; \({F}_{s}\) is the force in the reinforcing bar; and \({A}_{c}\) is the cross-sectional area of the concrete section. Method 1 required the assumption of strain compatibility between the steel and surrounding concrete. Average reinforcement stresses were then calculated over the sample length by assuming that average reinforcement strains were the same as the average surface strains computed from the hand-held mechanical strain gauge results. Calculation of average bar forces, and hence concrete stresses, was then straightforward. The final calculations were based on the loss of tensile stress in the concrete over time and the results were presented as a ratio between the measured long-term concrete stress and the initial stress which was measured directly after applying the load.

In the second stage, full-scale beams were tested, long term, in flexure. All beams were prepared with the same longitudinal and shear reinforcement ratios using three different replacement levels of CDW (0%, 50% and 100%). All of the beams had a rectangular cross-section with a width of 300 mm and a height of 150 mm, and were 4.2 m in length, as shown in Fig. 5.

For the long-term tests, a stabilised cracking pattern was achieved in the simply supported beams by subjecting them to a sustained load of 23 KN (equivalent to a sustained moment of 17.2kNm (including the self-weight of the beam) and an average stress in the compression zone of around 7 MPa); the sustained load was maintained for a period of 90 days. The beam mid-span deflection was measured using LVDT and recorded by a data logger. Surface strain measurements were also taken at four different heights on both sides of the beams. DEMEC points were placed every 150 mm along the constant moment zone. Electrical resistance strain (ERS) gauges were placed on the top surface at the mid-span point of the three longitudinal reinforcing bars; these gauges were connected to a data logger and allowed automatic recording of the strains over the 90 days of the test. (The test was stopped when the stabilisation in the trend of results occurred.) The development of crack patterns and propagation were observed and marked on the beams; crack widths were measured using a hand-held optical microscope.

For each mix, six cubes (100 × 100x100 mm), four cylinders (150 × 300 mm), three prisms (100 × 100x500 mm) and four small prisms (75 × 75x200 mm) were prepared and cast. These specimens were tested to measure the compressive strength, splitting tensile strength, flexural strength, modulus of elasticity, creep and shrinkage of the resulting concrete. All of the beams and specimens were compacted immediately after casting and were left for 24 h before being removed from their moulds and transferred to the curing room for 28 days. Fig. 6 shows the test specimens and experimental set-up.

The measured mechanical properties of the concrete are summarised in Table 3. It is evident that due to the incorporation of CDW as RA, there was a reduction in all of the mechanical properties. In comparison with normal concrete, the results showed a reduction of 5 and 10% in the compressive strength, 15 and 30% in the splitting tensile strength, 10 and 22% in the flexural strength and 20 and 36% in the modulus of elasticity when 50 and 100%, respectively of the natural aggregate was replaced. As reported in the literature, these reductions are mainly attributed to the poor quality of the recycled aggregate. This poor quality is mainly due to the presence of old cement paste and other materials such as clay bricks and tiles (Brito & Saikia, 2012; Etxeberria et al., 2007; Juan & Gutiérrez, 2009; Kishore, 2007; Kosior-Kazberuk & Grzywa, 2014; Malešev et al., 2010; Rahal, 2007; Sryh & Forth, 2015; Xiao et al., 2005; Yang et al., 2008).

Fig. 7 shows the relationship between the applied load and short-term, mid-span deflection of the beams. The measurements were taken up to an applied load of 23 KN (the value of the sustained load). The results indicate that the overall load–deflection behaviour of the RAC beam was similar to the behaviour of the conventional concrete beam (NC). The maximum mid-span deflection increased by about 8 and 15% when 50 and 100% of CDW was added, respectively. For the two replacement levels (50 and 100%), the initial cracking loads measured for the beams containing recycled aggregate were about 15 and 30%, respectively less than that recorded in the normal concrete beams.

In terms of cracking, the RAC beams showed different cracking patterns to the NC beams. Wider and more closely spaced cracks were observed in the RAC specimens as detailed in Table 4 and Fig. 8. The results of this study were, therefore, similar to those reported earlier from previous investigations (Ajdukiewicz and Kliszczewicz, 2007; American Concrete Institute, 2004; Ignjatović et al., 2013; Maruyama et al., 2004; Sato et al., 2007). A reduced crack spacing and hence a greater number of cracks can clearly be linked to the lower tension stiffening and tensile strength of the RAC; as can the lower initial cracking loads which all can make the defection to be higher.

Creep and shrinkage significantly affect the long-term deformations of concrete structures. As mentioned before, one of the main problems with using recycled aggregate is their low properties and high porosity, which is the result of the presence of old cement paste and other materials such as clay bricks and tiles. The old cement paste which adheres to the natural particles and the presence of bricks and tiles particles can decrease the volume of the aggregate in the mixture and increase the water content which leads to increased creep and shrinkage. In addition, the lower elastic properties of recycled aggregate can significantly influence the results of creep and shrinkage.

In this research, four small prism samples (75 × 75x200mm) were tested to assess the effect of RA on the compressive creep and shrinkage strains. Specimens were stored in a controlled environment at 20 °C and a relative humidity of 45%, similar to the average temperature and humidity in the laboratory where the beams were tested.

For the compressive creep test, the specimens were subjected to a constant compressive stress of 10 MPa (which is equivalent to 0.2ƒ^`_c of the control specimen). The compressive creep strains and coefficients were calculated based on the average results from the two specimens after removing the elastic strain and taking into account shrinkage. The creep and shrinkage data are presented in Fig. 9, the curves of the test results showed that the compressive creep coefficient and the shrinkage of the RAC increased by about 15 and 30%, and 18 and 38%, respectively, as a result of incorporating 50 and 100% recycled aggregate. These increases reflect the importance of understanding the long-term behaviour of RAC members.

Fig. 10 illustrates the effect of using CDW on the long-term loss of tension stiffening. The reduction over time of the ratio between the long-term concrete stress and the initial concrete stress was used to represent the loss of tension stiffening. The figure also shows the loss in the actual concrete tensile stress as a function of time for the NC and RAC specimens. As can be seen from the figure, there is a very rapid loss of tension stiffening in all of the specimens during the early stage of the test. Up until day 5, the normal concrete specimen and recycled aggregate concrete specimens exhibit similar reductions of around 25% from the initial applied stress. After day 5, the recycled aggregate specimens showed a higher rate of loss of tension stiffening in comparison to the normal concrete specimen.

The figure also shows that the stress in the normal concrete specimen stabilised earlier than that in the RAC specimens which only occurred at the 20th and 25th days for the 50 and 100% samples, respectively. After stabilisation occurred, the rate of loss of tension stiffening remained constant for all of the specimens. On the final day of testing (35th day), the tensile stress in the normal concrete had reduced by 36.2% while the stress in the RAC specimens had reduced by 42.4 and 49.1% for the 50 and 100% recycled aggregate specimens, respectively. This means that by substituting 50 and 100% of the aggregate, the loss of tension stiffening increased by 6.2 and 12.9%, respectively. Table 5 summarises the average short and long-term tensile stresses for the NC and RAC specimens.

In general, it can be concluded that the higher the replacement percentage of CDW, the greater the loss of tension stiffening. This can be attributed to the effect of the inclusion of the CDW as recycled aggregate on the mechanical properties of the concrete, in particular the tensile strength which is the most significant factor affecting tension stiffening. Furthermore, the effect of recycled aggregate on the microstructure of the Interfacial Transition Zone (ITZ) can influence the micro-cracking of the concrete matrix under load which can also cause the observed reduction in tension stiffening as discussed earlier in the section regarding the results of the mechanical properties.

Fig. 11 shows a graph of the long-term mid-span deflection versus time for all of the tested beams. In general, the specimens all showed a similar trend in deflection over the period of time, i.e. the deflection increased continuously. It is clear from the results that the increase in long-term deflection was much more rapid during the first 20 days of loading. The majority of the long-term deflection (about 65%) occurred during this time. Furthermore, the results indicate that increasing the replacement ratio of CDW resulted in significant increases in long-term deflection. The long-term deflection increased by 20 and 38% as a result of replacing 50 and 100% of the aggregate, respectively. This was attributed to the effect of recycled aggregate on the properties of the concrete, in particular the time-dependent deformation (e.g. creep and shrinkage).

In terms of crack development, the cracks in the RAC beams extended further throughout the height of the section than those in the NC beams as shown in Fig. 8. Also, the cracks were wider in the RAC beams under sustained load, as summarised in Table 4, which agrees with the observations of Choi and Yun (Choi & Yun, 2013). It can be noticed from the results that the percentage of the increase in the crack width decreased by increasing the replacement ratio. This can be owed to the increased number of cracks with increasing the replacement ratio of CDW which reduced the spaces between the cracks and hence distributed the development of width on more cracks.