Laboratory and field tests on a prefabricated steel-bar mesh-panel system for continuously-reinforced-concrete pavement (CRCP)Author 1: Norinobu KATAYAMA, Exe. director, Fujisaki co. ltd., Hiroshima, Hiroshima, Japan

Author 2: Kazuhiko FUJISAKI, President, Fujisaki co. ltd., Hiroshima, Hiroshima, Japan

Author 3: Takehisa UENO, Factory manager, Fujisaki co. ltd., Aki Takata, Hiroshima, Japan

Author 4: Ryutaro ONISHI, Graduate student, Yamaguchi Univ., Ube, Yamaguchi, Japan

Author 5: Isamu YOSHITAKE, Assoc. Professor, Yamaguchi Univ., Ube, Yamaguchi, Japan

For the corresponding author: yositake@yamaguchi-u.ac.jp

KEYWORDS: continuously reinforced concrete pavement, steel-bar mesh panel, fatigue, prefabrication, productivity

Conflict of Interest: All authors report that there is no conflict of interest related to the manuscript.

1. ABSTRACT

The decline in the number of persons of working age is a social problem in Japan. This is a particularly serious concern for workers in the construction field; construction systems should be considered for productivity improvements. Prefabrication systems are an effective method for shortening construction cycles and times. In fact, various precast concrete members have been employed to realize more rapid construction and improvements in quality. Using precast concrete members is difficult because jointless roads are preferable for highway pavement. Continuously reinforced concrete pavement (CRCP), which has the advantages of concrete jointless construction and high ductility, is a suitable method for highway road construction. Typical Japanese highways built with CRCP reduce the amount of horizontal cracking by arranging transverse rebars at an angle of 60° to the main rebars. Note that rebar placement and bonding in conventional CRCP are troublesome and labor intensive owing to the long construction time required. We have developed prefabricated steel bar meshes for CRCP and can report some benefits relating to their practical application. To examine the fundamental properties of mesh panels, we conducted a laboratory experiment and a simulated field test. The primary concern of welded rebars are failures induced by cyclic loading. A flexural fatigue loading test using CRCP models was conducted. In addition, a comparative survey on conventional and prefabrication systems was performed in the simulated field test to quantify the constructability of CRCP and to observe the extent of cracking in concrete. This paper reports on our experimental investigation.

2. INTRODUCTION

A social concern in Japan is that the population of workers is decreasing. According to a Japanese government report [Statistics Bureau of Japan, 2019], the entire worker population is gradually increasing owing to an increase in the number of older workers; the population of younger workers, on the other hand, is gradually decreasing. Figure 1 presents the working population in Japan: the bar graph implies that the working population must decrease in the future. In particular, the decrease in the population of construction workers is becoming more acute. Figure 2 shows the working population involved in construction and the population ratio of construction workers to total workers. The graph shows that the population ratio of construction workers and engineers was almost 9% of the entire working population in 2002; the ratio has gradually decreased to approximately 6.5%. To continue constructing and maintaining civil infrastructure in an appropriate manner, it is thus necessary to improve productivity to mitigate the effect of the reduced size of the construction labor force.

A prefabrication system is an effective means of reducing the size of the labor force at a construction site, in addition to improving quality. Precast concrete members are often used instead of cast-in-place concrete to increase construction productivity. However, it may be difficult to employ such precast members for continuously reinforced concrete pavement (CRCP) because it aims to reduce the number of concrete joints and wide cracks. Cast-in-place concrete must be employed in general CRCP. It may be difficult to place reinforcing bars and band each rebar by hand at a large CRCP construction site. Note that in CRCP, transverse reinforcing bars are placed at a 60° angle to the main rebars to reduce horizontal full-depth cracking [JRA, 2006].

0

1000

2000

3000

4000

5000

6000

7000

8000

Jan-73 Jan-83 Jan-93 Jan-03 Jan-13

Wor

king

pup

ulat

ion

(104

)

15-24 25-34 35-44 45-54 55-64 65<

Fig.1 Variation of working population in Japan.

4%

6%

8%

10%

12%

0

1000

2000

3000

4000

5000

6000

7000

Jan-02 Jan-07 Jan-12 Jan-17

Pup

ulat

ion

(104

)

Construction Total Ratio

Fig.2 Population in construction fields.

0

100

200

300

400

500

600

700

800

900

1,000

0 10,000 20,000 30,000 40,000 50,000 60,000

Tota

l num

ber

of w

orke

rs

Construction area (m2)

Conv. Prefab.(days*person)

Fig.3 Total number of workers (prefabrication and conventional systems).

A fabrication system of prefabricated rebar panels for CRCP construction was developed to deal with difficult tasks (Katayama et al., 2018). The prefabrication system can reduce the amount of labor and cost expended compared to conventional CRCP construction even when cast-in-place concrete is used; prefabricated steel bar mesh-panels have been employed in various construction sites in Japan. Figure 3 presents the total number of workers associated with prefabrication systems, based on recent construction records. For comparison, the estimated number of workers

in conventional construction is also plotted. The graph confirms that a prefabrication system can significantly reduce the total number of workers at a CRCP construction site. This prefabrication system is an effective construction technique in a country like Japan where the population is decreasing.

3. RESEARCH SIGNIFICANCE

In the developed system, transverse steel bars are welded at an angle of 60° to the main reinforcement. One concern of welded rebars is failure due to repeated loads (i.e., from traffic). A flexural fatigue loading test using CRCP models was conducted to examine the properties of the system under fatigue load. The prepared models were a conventional CRCP and a steel bar mesh panel specimen. This paper reports the fatigue response of the mesh panel CRCP model by comparing it with the fatigue test of the conventional CRCP model. Additionally, a pseudo field test was conducted to examine construction times and the long-term behavior of CRCP. In addition to the laboratory test, a conventional rebar arrangement and the prefabricated steel bar mesh panel construction were compared in the field test. Based on these laboratory and field tests, this paper reports that CRCP using welded steel panels exhibited a fatigue durability identical to that of the conventional system; it can thus be concluded that the prefabrication system can improve CRCP construction times.

4. LABORATORY TEST

Test specimens

The study prepared three CRCP specimens each for conventional and prefabrication systems. The physical dimensions of the specimens were 1560 mm (length) × 550 mm (width) × 150 mm (thickness). The yield strength of the rebar was 345 MPa; the nominal diameters of the main and transverse rebars were 16 mm (D16) and 13 mm (D13), respectively. The designed slump and air were 6.5 cm and 4.5%, respectively. The specified flexural strength of the concrete was 4.5 MPa at an age of 28 days. The concrete used for test specimens was the same as that used for the pseudo field test; thus, the concrete was made by using a batcher at a ready-mixed concrete factory. Table 1 lists the materials and mixture proportion of the concrete.

Table 1 Materials and mixture proportion of pavement concrete.

Ordinary Portland cement 3.16 g/cm3 428 kg/m3

Water 1.00 g/cm3 171 kg/m3

Manufactured sand 2.56 g/cm3 402 kg/m3

Crushed sand 2.60 g/cm3 272 kg/m3

Crushed stone 5–13 mm; 2.70 g/cm3 365 kg/m3

Crushed stone 13–20 mm; 2.70 g/cm3 677 kg/m3

Water-reducing agent Lignin sulfonate + polycarboxylic acid 3.85 kg/m3

Figure 4 (a)–(d) present the preparations of CRCP specimens embedding prefabrication and conventional steel reinforcements. The main rebars were jointed at the center of the specimens to examine the jointed effect, the weakest point of the concrete slab. After the concrete-casting, all test specimens were covered with wet-cloth for curing and were stored in the room for a month.

(a) Concrete form and rebars (b) Concrete-casting

(c) Welded rebars (prefabrication) (d) Banded rebars (conventional)

Fig.4 CRCP specimens.

Test program

The study first conducted a monotonic loading test using these specimens to observe cracking load. After that, the study conducted a cyclic loading test. A sinusoidal load at 5 Hz was applied for up to 500,000 cycles in the cyclic loading test. The levels of maximum load for the repeated loading test were 80% of the cracking load, and the load range (i.e., minimum to maximum load)

was 26 kN. Figure 5 (a) and (b) present a schematic of flexural loading test and the test set-up, respectively. Two CRCP specimens each were used for the negative bending test and the others for the positive bending test. All tests in this study achieved 500,000 cycles of 80% cracking load. The monotonic loading test was also conducted to observe flexural behavior of the post-fatigue slab.

D16

D13

Loading jig

1000mm300mm15

0mm

(a) Schematic of the negative bending loading

(b) Test set-up

Fig.5 Flexural loading test.

Test results and discussion

Table 2 shows the cracking loads in the static loading test. The reinforcing bars were embedded in 1/3 of the slab thickness (near the neutral axis of flexure); the rebars hardly contributed to the increase in cracking load. Hence, significant differences in cracking load were not identified in

the monotonic loading test. As mentioned earlier, all tested specimens endured 500,000 cycles of 80% cracking load (Table 2). The study conducted the monotonic loading test by using post-fatigue slabs. The load was statically increased up to 150 kN with a maximum bending moment of 26.3 kN·m.

Table 2 Cracking loads and the load-range in the fatigue test.

Specimen Reinforcement Loading Cracking load Load-range FatigueCon-1 Conventional Negative 47 kN 12 - 38 kN 500,000<Con-2 Conventional Negative 42 kN 7.0 - 33 kN 500,000<Con-3 Conventional Positive 43 kN 8.0 - 34 kN 500,000<Pref-1 Prefabrication Negative 43 kN 8.0 - 34 kN 500,000<Pref-2 Prefabrication Negative 53 kN 16 - 42 kN 500,000<Pref-3 Prefabrication Positive 68 kN 28 - 54 kN 500,000<

A remarkable observation of the cyclic and post-fatigue loading tests is that all rebar strains were significantly less than the yielding strain, even under a positive/negative bending moment. No specimen tested in this study showed any sign of flexural failure in the post-fatigue loading test. It was confirmed that the reinforcements were hardly damaged owing to such positive/negative flexural loading. After the post-fatigue test, the cover concrete was removed by using a chipping device to observe the damage to the prefabricated steel mesh panel. Figure 6 shows the steel mesh panel of a tested CRCP specimen. It is noteworthy that the welded rebars were hardly damaged even under cyclic and post-fatigue loading. This observation implies that the extent of fatigue failure of welded steel reinforcements may be negligible for a prefabricated system. It can be concluded that the prefabrication system has a nearly identical structural performance to that of conventional CRCP.

Fig.6 Welded steel-reinforcing bars (after the post-fatigue test).

5. FIELD TEST

CRCP construction test

To compare the constructability of both conventional and prefabrication systems, we conducted a field test at a factory site of Fujisaki corporation (Hiroshima, Japan). The paving area for each CRCP was 2 m (width) × 10 m (length) (Fig.7 (a)).

Table 3 shows the labor (workers × time) involved in placing rebars on the paving area. The labor used by the prefabrication system was almost 27% of that required by the conventional system. Even in such a small area, the prefabrication system can significantly decrease the labor needed for placing rebars.

(a) Concrete casting at the field test (b) Concrete pavement

Fig.7 Field test of CRCP construction.

Table 3 Labors for the field test (2 m × 10 m).

I. Conventional II. Prefabrication Ratio (II / I)Placement of rebars 3 persons × 100 min. 3 persons × 15 min. 0.15 (15%)Joint of rebars 3 persons × 50 min. 3 persons × 25 min. 0.5 (50%)Total 450 persons*min. 120 persons*min. 0.27 (27%)

Pavement concrete of 250 mm thickness was cast in the form shown in Fig.7 (a). The materials and mixture proportion of the concrete are given in Table 1. Figure 7 (b) shows the hardened concrete pavement models. The CRCP models had been exposed to the field environment for a year. Significant strain or visible cracks were not observed when the deformation of the CRCP

models was monitored. It is also noteworthy that evident differences in cracking behavior in the CRCP models were scarcely found in the field test.

Field measurement

The study conducted an additional field test that induced cracks due to the volume change of concrete. Concrete of 150 mm thickness was added to the CRCP models. Conventional and prefabricated steel bars were also embedded in this additional concrete. The volume change of the additional CRCP was limited by the steel anchors embedded in the existing CRCP models. Figure 8 (a) and (b) present the placement of a steel mesh panel and the casting of additional concrete, respectively. The additional concrete was installed in October 2018.

(a) Placement of prefabricated steel-mesh panel (b) Additional concrete

(c) First observed cracks (d) Horizontal crack occurred near the center of CRCP slab

Fig.8 Field test for crack-monitoring.

Cracks on the concrete surface (Fig.8 (c)) were first observed in April 2019. The surface cracks occurred in both CRCP slabs (i.e., for both conventional and prefabrication systems). Figure 8 (c) illustrates the surface cracks of CRCP made with prefabricated steel mesh panels. A small surface crack in the conventional CRCP gradually increased in size and developed into a visible horizontal crack across the slab. Figure 8 (d) presents the horizontal crack (0.25 mm wide) in the

conventional CRCP. Interestingly, such a large crack was not observed until September 2019. Therefore, further research on the deformation and degradation of CRCP should be conducted though field measurements. In addition, this study constructed long CRCP lanes with conventional and prefabrication systems in the factory. Figure 9 presents the constructed CRCP lanes. The dimensions of these are 0.15 m (depth) × 4 m (width) × 40 m (length) for each system. In the field test, long term performance can be investigated by comparing the cracking behaviors of these CRCP systems.

(a) Placement of reinforcing bars (b) CRCP lanes (left: prefabrication, right: conventional)

Fig.9 Construction of long CRCP lanes

6. SUMMARYThis study aimed to investigate the fundamental properties of developed steel mesh panels and conduct laboratory experiments and field tests. It reported that the steel mesh panels were hardly damaged by cyclic loadings experienced during the laboratory test. In addition, the paper reported on the excellent constructability of the prefabricated mesh panel system by investigating previous construction records and field tests. Further research should be performed through long term tests in the simulated field and in various field applications to confirm the applicability and efficiency of this system.

7. ACKNOWLEDGEMENTSThe authors wish to acknowledge Mr. Ota and his staff for their work on CRCP construction.

8. REFERENCESJapan Road Association (JRA), 2006, Specification for Road Pavement. (in Japanese).

Norinobu Katayama, Kazuhiko Fujisaki, Takanori Tsutsui, Hiroaki Matsuda and Isamu Yoshitake, 2018, Practical Application of Prefabricated Steel Bar Meshes for Continuously Reinforced Concrete Pavement, the 13th International Symposium on Concrete Roads, Berlin.

Statistics Bureau of Japan, 2019, https://www.stat.go.jp/english/index.html, Accessed: Sep-10th, 2019.