Performing laboratory study of the behavior of reactive powder concrete on the shear of RC deep beams by the drilling core test

1 Introduction

In recent years, the techniques, and materials for repairing, refurbishing, and reinforcing reinforcement concrete (RC) beams have been widely applied due to their high-performance qualities. Reactive powder concrete (RPC) is a new type of concrete characterized by having particles with a diameter not exceeding 600 mm and with very high compression and tensile strength [1]. Many parameters affect the enclosure efficiency of concrete deep beams with RPC, including concrete strength, deflection, crack pattern, crack width, first crack load, failure load, and mode of failure. It was concluded that ultra high performance fiber reinforced concrete shows a large difference in the mixing ratio and strength according to the purpose of the research required, but still there is difficulty in developing a design method in general [2]. RPC materials have superior performance properties that can be characterized by very high resistance that can reach 150 MPa with high ductility and toughness due to the presence of steel fibers. Moreover, the reinforcement schemes have improved the shear strength of deep beams to greater than those provided by reinforcing the web steel [3]. The study proved that the deep beams failure mode is a diagonal compression failure, with a small shear span to depth ratio, which is affected by the orientation and the amount of web strengthening [4]. Deep beams are among the most structural members to be a shear failure, and therefore, structural reinforcement for shear failure will be a very important repair because of structural failure [5]. A study proved that the use of steel fibers with a U-shape as well as rebars in RC deep beams will increase the maximum shear strength of deep beams and will produce inelastic and stable behavior for long periods in comparison to the beam with the stirrups [6]. The improved compressive strength is likely to be the input resonance (Vf) of the tensile range of RPC given a validated view of concrete under uniaxial compression, load failure due to lateral tensile stress inflicted by the effects of Poisson’s rate, and a mechanical property of containerized RPC on Vf probe [7]. For high hardness, the incorporation of steel fibers can increase the cracking energy of RPC to 20,000–40,000 J/m². Compared with conventional concrete, the tear strength of RPC is an order of magnitude higher, and the refractive energy is two orders of magnitude higher [8,9,10]. In addition, the decrease in the w/b ratio achieves an optimal porous structure and is impermeable which contributes to the remarkable toughness of RPC [11,12]. As a result of its superior ductility, the energy absorption of RPC will provide greater structural precision [13]. RPC has high tensile strength, which means all major tensile stresses can be imported from the same concrete except for tensile stresses. Shear rejection and more additional rebar give the structural member endless freedom for the structure. RPC is also a material that has superior abrasion resistance and will be in demand where physical wear is more likely than ordinary concrete, such as bridge decks and industrial floors [14]. Recent studies have examined the effects of maximum coarse aggregate size and type on the workability of new UHPC and the mechanical behavior of hardened UHPC. The results of this study concluded that the workability and mechanical properties of C200 UHPC will be significantly affected by the type and size of the maximum coarse aggregate [15]. The axial compression test results showed that the steel and RPC acted cooperatively in the premier stage, and the ultimate failure mode of the columns was a split failure, mainly at the column ends, with a longitudinal main crack extending to the center of the column and forming [16]. Orthogonal ties have a distinctive effect in confining the RPC core when the proportion of the bearing plate area to the reactive powder core area surrounded by orthogonal ties exceeds 1.0 [17]. A recent study for the proposed prefabricated steel reactive powder concrete (PSRPC) has shown in tests that the ultimate failure mode of PSRPC is cracking at the column ends and the study revealed that throughout the loading process, the RPC and the section steel harmonize and fully demonstrate their strength; the side effects of the RPC on the section steel and the restraining effects of the composite stirrup sand section steel on the RPC core significantly enhance the ultimate bearing capacity of the PSRPC. These effects increase the strength of the RPC [18].

2 Experimental program

The test program includes two samples of simple reinforced concrete beams one with a reference beam and the other with RPC, with an overall beam length (L) of 1,200 mm, overall height (H) of 500 mm, thickness of 150 mm, and shear span to effective depth ratio (a/d) of 0.77. Using a hydraulic jack under a four-point loading configuration, all beams were tested as shown in Figure 1. Three Ø16 mm diameter rebar was used as longitudinal tensile strength for the bottom zone, two Ø12 mm as longitudinal tension reinforcement for the top zone, and Ø12 mm @200 mm c/c for stirrups. The concrete cover was 40 mm.

Figure 1

Specifics of tested reinforced CDBs with and without RPC.

3 Specimens preparation

In this study, plywood with a thickness of 20 mm was used to manufacture the sample mold and it was according to the required dimensions. Oil is used to coat the mold to ensure that the concrete does not stick, and the mold is removed easily. The rebar of the deep beams was lowered into these molds, and small concrete cubes measuring 2.5 cm were placed to achieve the required concrete cover. Mixing materials were weighed for normal concrete and RPC, according to the weight ratios of the required concrete mix. To mix the concrete components, a small mixer with a capacity of 0.25 cubic meters was used. Twelve cylinders (100 × 200) mm for each series were cast to specify the concrete properties. All things mentioned above are shown in Figure 2.

Figure 2

Specimens preparation and materials of RPC.

4 Concrete mix design

Before starting the concrete mix, a slump test was taken for the concrete mix, and it was the limits of the required specification as shown in Figure 3. Several experimental mixtures were used in this study before the mixture required for RPC was obtained. All deep beam samples as well as samples (cubes, cylinders, and drill cores) were 1∶1∶0.08 (cement:fine sand:silica fume) with a W/C of 0.25. The superplasticizer was used at the rate of 2% by weight of the cement, and for the steel fibers, the percentage of the weight was 0.7% in all mixtures. The values of the mechanical properties of specimens cast in the laboratory based on the codes to determine the required concrete properties (Table 3):

• 200 mm high × 200 mm diameter cylinder based on ASTM C39 [26]. for compressive Strength and based on ASTM C496 [27]. (The split-cylinder test) for tensile strength.

• 150 mm × 150 mm cubes based on ASTM C39 [26]. for compressive strength.

• 150 mm diameter × 150 mm long drilling core samples for getting real mechanical properties based on ASTM C42/C42M-18a [28]. such as (compressive strength, modulus of elasticity, Poison’s ratio, stress–strain curve, and longitudinal–lateral strain).

Figure 3

Slump test.

5 Testing procedure

After the completion of the curing process of the deep beams at the age of 28 days, they were removed from the water. To find out the crack propagation behavior, the specimens of deep beams were cleaned, white-painted, and marked. Using a jack hydraulic machine with two points load as shown in Figure 4, all deep beams were tested to study their behavior. To note the development of cracks, strains, and deflections of the beam, the test machine was stopped at every 10 kN. The deflection dial gauge was placed in the middle of the deep beam between the two points of the applied load and in contact with the lower face. When the cracks began to develop with the gradual increase in the load, the maximum failure load and load removal were recorded to take pictures of the final crack pattern. In addition, after the failure, six core drill specimens were taken from the undamaged regions (areas without cracks) of deep beams’ body as shown in Figure 5. The specimens taken from the deep beam body with normal concrete and deep beam with RPC have been divided into three specimens each. The specimens were subjected to testing using the compression strength machine. In conjunction with testing specimens under the compression machine, a digital caliper and mini digital thickness gauge were used, which have been manufactured manually to get the real mechanical properties, especially the stress–strain curve as shown in Figure 6.

Figure 4

Universal testing machine.

Figure 5

Taking drilling core samples.

Figure 6

Testing compression machine with two gauges to calculate the strains.

6 Results and discussion

The experimental results obtained are summarized in Table 4, including the first cracking load, final load, final shear strength, failure mode, and crack patterns. Moreover, load-deflection, shear crack width, and stress–strain curves of these beams are analyzed and studied. They are illustrated in Figures 7–11. The CDB (reference specimen) failed in shear. It was noticed that in the middle of the deep beam region, the appearance of the first flexural crack for the load period of 80 kN, and when the load was increased to a load of 140 kN, we noticed the appearance of another crack. With the increase in the load, more propagation of flexural cracks was observed but they stopped at 240 kN load. On the other hand, when loading 160 kN, it was noticed that the first shear crack appeared, which extended from the inner edge of the support toward the point load. When the load was increased little by little until it reached 200 kN, another primary shear crack appeared in the middle of the support base. It was also observed that the type of bending crack and shear spread on increasing the load. Upon failure at full load, the main shear crack occurred at an angle of 58° as shown in Figure 7. The failure occurred in the control deep beam CDB when the load reached 440 kN or the shear load reached 220 kN. Through the results, it was observed that the beam suddenly failed, and the failure mode was the shear-compression failure type. The specimen RPCDB (with RPC) failed in shear. The appearance of the first bending crack was observed in the center of the sample when the load reading was about 80 kN. Other small cracks appeared widely between the two supports starting from the bottom toward the load points when the load gradually started to increase. Other diagonal shear cracks formed (in the left as well as the right shear span of the beam) were observed first at different points at a load of 260 kN and then started to extend towards the loading points. With the increase in the load, these cracks rapidly spread and widened. The type of failure that occurred in the beam was shear failure type and its location was at the edge of the beam, at the support, and was crushing and dislodging part of the deep beam extending from the support towards the loading point as shown in Figure 8. Through the readings, we note that the failure of the RPCDB sample was at a load of 740 kN or a shear load of 370 kN. Through the results, it was observed that the beam suddenly failed, and the failure mode was the shear-compression failure type. Also, from the significant decrease in shear strength of the reference beam, when compared CDB to the RPCDB, the presence of reactive concrete powder reduces the shear capacity. In addition, the sample RPCDB had decreased shear strength by 68% compared to the shear strength of the CDB reference beam. The behavior of the cracks was represented by the emergence and extension from the support point to the load point, at an angle of approximately 58°, and distributed on both types of deep beams.

Figure 7

Failure mode and crack pattern for CDB.

Figure 8

Failure mode and crack pattern for RPCDB.

Figure 9

Deflection profile for beams in CDB and RPCDB.

Figure 10

Crack width profile for beams in CDB and RPCDB.

Figure 11

Stress–strain curve for beam CDB.

Figure 9 shows the deflection behavior of CDB and RPCDB deep beams. Deflection readings were recorded at each overload until sample failure. Depending on the deflection instrument, a deflection value of 8.05 mm was read for the RPCDB beam at the highest peak but only a value of 7.29 mm for the CDB beam was recorded. However, after maximum load, the behavior of the highest peak in the graph for RPCDB and CDB beams showed similar patterns, represented by a decrease in load but the deflection will start to gradually increase. The RPCDB beam had a more gradual post-peak increase in deflection compared to CDB. Before complete failure, the RPCDB beam was followed up with a maximum deflection reading of 9.40 mm while a reading of 8.45 mm was recorded for CDB.

Using a special microscope, the readings of the shear crack width of the deep beams were recorded during the loading stages. For the reference beam CDB, the first shear crack was observed at 160 kN, and the crack width was read with a value of 0.15 mm. Upon reaching 65% of the final shear load (285 kN), it was observed that the crack width continued to expand with each load and the crack diagonal increased to 2 mm. On the other hand, at a load of 260 kN, the occurrence of the first shear crack of the RPCDB beam was confirmed, and a crack width reading of 0.15 mm was recorded with increasing loading, and a shear diagonal crack reading of 1.8 mm was recorded at the load of 480 kN. Figure 10 shows the evolution of shear crack widths for the CDB and RPCDB loading samples. From observation, the beam (RPCDB) delayed the appearance of the first shear crack by (78%) with respect to the load compared with the beam (CDB) due to the higher shear capacity of specimen RPCDB. Moreover, the beam (RPCDB) reduced the first shear crack width by (11%) compared with the beam (CDB) at (65%) of the ultimate shear load.

From the drilled core test based on ASTM C42/C42M-18a [19] applied on beams RPCDB and CDB, real and realistic data for those deep beams were obtained, which were translated in the form of stress–strain curves which include longitudinal and lateral stress–strain curves for concrete compression zone of beams CDB and RPCDB, as shown in Figures 11 and 12. A longitudinal stress–strain curve of the concrete compressive zone of CDB and RPCDB samples was combined with one aggregation curve to compare the results between these two samples as shown in Figure 13. In Figure 13, the stress–strain curve of the CDB reference sample is shown, which shows the drop at the loading end. However, the behavior of the RPCDB sample is very different compared to that of the CDB sample of the higher stress and decreased strain of the RPCDB and this is evident by the high peak and little extension of the curve. The Young’s modulus increased from 27,932 MPa for reference mixings to 43,029 MPa for mixings that have 10% silica fume powder. In addition, the flexural strength rises by rising silica fume content to 12.5%, after which the value decreases [32].

Figure 12

Stress–strain curve for beam RPCDB.

Figure 13

Stress–strain curve for beam RPCDB and CDB.

From the split-cylinder test based on ASTM C496 [18], the stress-strain results were obtained for the concrete tensile zone for CDB and RPCDB samples as shown in Figure 14.

Figure 14

Stress–strain curve for tension zone in beams RPCDB and CDB.

In Figure 14, for the stress–strain curves for the stress in the tension zone for CDB and RPCDB samples, it can be observed that the tension stress of beam RPCDB is higher than beam CDB. Moreover, the strain of the sample beam RPCDB is higher than that of the sample CDB. The increase in tensile stress for the RPCDB sample at maximum stress compared with the CDB sample is 10%, while the increase in tensile strain for the RPCDB sample at maximum stress is 29%.

All the results of stress-strain for samples RPCDB and CDB were compared based on the results of previous studies as shown in Figure 15 and it showed an acceptable agreement with those results, which proves the validity of these laboratory results.

Figure 15

Stress-strain for (a) tension and (b) compression zone in concrete.

7 Conclusion

1. The presence of the RPC caused an exponential increase in the final strength of the RPCDB sample. The maximum strength of the RPCDB sample in this research increased by about 68% from the final strength of the CDB.

2. The presence of RPC caused an exponential increase in the first shear crack capacity of the deep beams. The RPCDB sample increased by approximately 62.5% of the maximum capacity of the CDB.

3. Through this study, we can observe that the value of the drilling core compressive ( f dc ′ ) applied based on ASTM C42/C42M-18a [28] has given a real and very different value compared with the two values of f c ′ and f cu ′ based on ASTM C39 [26]. For the CDB specimen, where f dc ′ differs from f΄ _c and f cu ′ by 30% and 55%, respectively. For the RPCDB specimen, where f dc ′ differs with f c ′ and f cu ′ by 12% and 20%, respectively.

4. The resistance of the RPCDB sample increased at all stages of loading and thus led to an increase in the deflection at the corresponding loads. The maximum deflection overload of the RPCDB was increased by about 10–11% compared with CDB specimen.

5. From this study, the beam (RPCDB) delayed the appearance of the first shear crack by 78% with respect to the load compared with the beam (CDB) due to the higher shear capacity of specimen RPCDB. Moreover, the beam (RPCDB) reduced the first shear crack width by 11% compared with the beam (CDB) at 65% of the ultimate shear load.

6. From the drilled core test based on ASTM C42/C42M-18a [28] applied on beams RPCDB and CDB real has observed that the stress results of RPCDB have increased by 118% at the maximum stress compared with CDB. In contrast, the strain scores for CDB increased by 22.5% at maximum stress compared to RPCDB.

7. From the stress-strain results of split-cylinder test based on ASTM C496 [27], the increase in tensile stress for the RPCDB sample at maximum stress compared with the CDB sample is 10%, while the increase in tensile strain for the RPCDB sample at maximum stress is 29%.