Continuous prefabricated cross-beams manufactured by using off-shuttering molding technologies

The paper describes a new constructive solution of prefabricated cross-beams manufactured be using off-shuttering molding technologies. Prefabricated elements of such cross-beams are pre-stressed trough-shaped parts. Together with hollow slab panels, stacked on the upper surfaces of their vertical walls, they serve as permanent shuttering for installing required crosswise and longitudinal (for support moments) reinforcement and for further pouring of cast concrete. The study provides a methodology and gives tests results of three series of prototype samples and of one full-sized prefabricated cross-beam. The purpose of the tests is to study the actual work of prefabricated cross-beams of the proposed construction. To analyze the test results, the authors performed check calculations of prototype samples according to the existing Russian design regulations of reinforced concrete structures. The obtained data analysis leads to a conclusion that the proposed construction of prefabricated crossbeams is quite reliable and makes it possible to give recommendations on their calculation and design.


Introduction
Now in Russia there are large-scale processes of the technical re-equipment of enterprises producing reinforced concrete structures. Among the most dynamically introduced technologies, it is possible to distinguish the off shuttering molding technology [1], which makes it possible to produce various designs of constant cross-section length, reinforced with pre-stressed high-strength reinforcement. In this case, the cross-section can be of almost any shape (rectangular, T-shaped, I-shaped, U-shaped, with and without pinholes, etc.), and the product length is limited only by the requirements for the load bearing capacity of the manufactured structure. The authors proposed the construction of precast reinforced concrete frame, where in addition to hollow-core slabs manufactured by the off shuttering molding technology with continuous precast crossbeams [2], precast parts of which are manufactured as hollow-core slabs according to off shuttering molding technology, unlike cross beams for a similar frame, considered in the paper [3] and manufactured by conveyor technology. An example of the use of beams of T-section, MATEC Web of Conferences 196, 02021 (2018) https://doi.org/10.1051/matecconf/201819602021 XXVII R-S-P Seminar 2018, Theoretical Foundation of Civil Engineering manufactured by the off shuttering molding technology, can be the construction of a precast cross beam, mentioned above [4].
The proposed crossbeam ( Figure 1) consists of pre-stressed concrete parts with complex geometric shape, installed between the columns, and monolithic concrete which combines prefabricated elements (columns and beam parts) into a continuous single precast monolithic structure. Thus, the part and the slabs stacked on it serve as permanent shuttering for concrete, the part being a kind of pre stressed reinforcing element. To prevent the concrete from flowing into the slab pinholes plugs are inserted. Span parts in the proposed precast cross beam are taken up by the longitudinal pre stressed reinforcement part. For the take up of support elements in the nearby zone, the upper non-tensioning longitudinal reinforcement passing through the column ( Figure 5) is installed. The shearing strains are taken by the vertical reinforcing bars of frames. When the span poi4-4nts obtained from the results of static analysis framework, exceed the maximum cross beam strength, and the cross beam is reinforced only by pre stressed reinforcement parts on the bottom part (before laying monolithic concrete) additional untensioned longitudinal reinforcement can be set. 1prefabricated beam part; 2 -monolithic concrete; 3hollow-core plank; 4plastic plug; 5, 6lower and upper pre-reinforced wire reinforcement with 5 mm diameter, сarbon steel wire for reinforcement (Russian standards -Вр1400: 7)additional (if needed) lower untensioned reinforcement А400: 8reinforcement frames from А400 cores.

Materials and methods
In order to study the function and destruction nature of the proposed precast cross beams and prefabricated pretension part acting as an independent structure, as well as to study their strength in normal and inclined cross sections, special actual size prototype samples were manufactured and tested. The three series of samples were made and tested (two samples of the first series and three samples of the second and third series) and a cross beam part. Before the tests, the actual strength of concrete of each sample was determined by the methods of destructive and non-destructive testing (model N Schmidt hammer) (in cubes 100x100x100 mm).
The main geometric characteristics of the samples are shown in Figures 2 and 3, and the reinforcement parameters and strength characteristics of the concrete in the samples in Table 1. 1 -the prefabricated cross beam part; 2 -monolithic concrete; 3 -hollow core slab; 4 -plastic plug; 5, 6lower and upper pre-reinforced wire reinforcement with 5 mm diameter, сarbon steel wire for reinforcement (Russian standards -Вр1400: 7)additional (if needed) lower А400: 8reinforcement frames from А400 cores.
The test load for all samples was applied in steps up to destruction with an exposure time of 10 minutes at each stage. In the process of testing, the observation of the appearance and development of normal, inclined and contact cracks was made (between the beam part and monolithic concrete). The deflection in the middle of the span and the drafts of the supports were measured by the PAO-6 flexometers, the deformation of the concrete by resistance strain gages with a base of 50 mm and the Hugenberger strainmeters with a base of 100 mm, deformation of the reinforcemen by strain gages with a base of 10 mm.  Notations adopted in Table 1: A sp -the cross-sectional area of the lower pre stressed reinforcement of the cross beam part; a sp -the distance from the center of gravity of the A sp reinforcement to the bottom surface of the cross beam part; A s -the cross-sectional area of an unstressed stretched reinforcement (the lower in the span, the upper on the support); a s -the distance from the center of gravity of the reinforcement A s of the cross beam to its nearest horizontal side; q sw -the force in the clamps per unit length, determined by formula (8.59) [3], replacing R sw by 0.8 * R sn , where R sn = 400 MPa -the standard value of the resistance of reinforcement class A400; R bm , R btm -the average strength of concrete to axial compression and axial tension, respectively, taken as a result of determining the actual strength of concrete samples.
crossbeams and when testing the crossbeam part at the top of its edges. The distance between the supports of the upper distributor beam is 1600 mm, the two lower ones -600 mm each, the calculated span of the samples and the beam part were taken equal to 6850 mm. Samples of the second and third series were intended to study the operation of the precast cross-beam in the joint zone with the column, with maximum shearing force and the reference torque. They were made of column fragments 1.4 m high and 1.4 m pre-stressed parts (Figure 3), connected by monolithic concrete and tested using a cantilever scheme ( Figure 5). Load N 0 was created by a hydraulic jack and transmitted directly to one of the consoles. The opposite console was fixed from vertical displacements in a place symmetrical to the place where the load N 0 was applied. The places of application of the load and the consoles fastening when testing of the samples of 2-d series are chosen in such a way that the samples destruction occurs from the action of the supporting zone and the samples of the 3-d series -from the action of shearing force. The distance from the column side to the center of application of the force N 0 for the samples of the second series was 1.3 m, and for the samples of the third series -1.0 m.

Results
The cross beam parts test results. The first normal cracks in the part appeared in the middle of the span in the lower zone at a load of N 0 = 4.2 tons (60% of the fracture). The width of the crack was 0.05 mm, the height of crack propagation from the lower face was 50-80 mm, the average distance between the cracks along the length of the part was 250 mm, the deflection in the middle of the span was 18.4 mm. With gradual load increasing, there was the development of existing cracks and the formation of new normal ones occurred between the initially formed cracks, with an average crack pitch of 125 mm. At a load of 6 tons (85.7% of the destructive one), the width of crack opening reached 0.3 mm, and the deflection in the middle of the span was 57.4 mm. At a load of N 0 u = 7 t, there was a brittle sample fracture in the middle zone over the compressed concrete.

The results of the first series samples testing
The sample 1-1-400. The destruction of the sample occurred at a load of N 0 u = 270 kN in compressed concrete in the cross section in the middle of the span. At a load of 140 kN (51.7% of the destructive), in the middle of the span in the lower stretched zone, the first normal cracks appeared with a width disclosure of 0.2 mm to a height of 50-85 mm. The distance between the cracks along the length of the specimen was 250 mm. Under this load, the deflection in the middle of the span was 7.8 mm. Later, the existing cracks and the formation of new normal cracks between the existing ones took place, with the minimum MATEC Web of Conferences 196, 02021 (2018) https://doi.org/10.1051/matecconf/201819602021 XXVII R-S-P Seminar 2018, Theoretical Foundation of Civil Engineering crack pitch being 125 mm, and the height development did not exceed 250 mm (ie, normal cracks did not develop into the upper monolithic part). With a load close to destructive one, the width of crack opening reached 0.2-0.4 mm, the deflection in the middle of the span was 32.8 mm. It should be noted that during the test of the sample there was no cracking between the beam part and monolithic concrete.
Sample 1-2-310. The sample destruction occurred with compressed concrete at a load of N 0 u = 170 kN. Under the load of 9 kN (52.9% of the destructive), in the middle of the span in the stretched zone, the first normal cracks appeared with a 0.2 mm wide opening to a height of 50-85 mm. The distance between the cracks along the length of the specimen was 250 mm. The deflection in the middle of the span was 12.7 mm. Later, the existing and the formation of new normal cracks took place, with the minimum crack pitch being 125 mm, and the height development did not exceed 250 mm (that is, the formed normal cracks did not spread to the upper monolithic part and the height development did not exceed 250 mm). Under the load of 15 kN (88.2% of the destructive), the crack opening was up to 0.3 mm, the deflection in the middle of the span was 38 mm. As in the previous sample, the formation of cracks between the beam part and monolithic concrete did not occur. The sample 2-3-400 was destroyed under the load of N 0 u = 329 kN. The destruction nature ( Figure 6) was different from the previous two. By the time of destruction in the sample, a sharp increase in the width of the inclined cracks occurred, followed by concrete crushing in the middle zone of the upper monolithic part. More over the monolithic concrete was destroyed over the upper working armature. When the destructive loads were reached in all the samples of the third series, the inclined cracks were sharply opened, followed by the destruction of the concrete of the cross beam part (Figure 7). By nature, it is a typical destruction on concrete strips between inclined cracks.

Discussion
The results of tests and check calculations for all samples are presented in Table 2. The check calculations of the tested prefabricated cross beams for determining theoretical destructive loads as well as comparing them with experimental values are performed both for reinforced concrete T-sections in accordance with hardening tests 8.1.8 ÷ 8.1.13 and 8.1.32, 8.1.33 [5] with the replacement of the calculated resistances of concrete to the corresponding average values, and the calculated resistances of the reinforcement to the corresponding normative values. https://doi.org/10.1051/matecconf/201819602021 XXVII R-S-P Seminar 2018, Theoretical Foundation of Civil Engineering Analysis of the data given in Table 2 shows that the calculation of the strength of normal sections of a precast cross beams can be carried out with sufficient reliability by the