ACI CODE VERIFICATION FOR FRP EXTERNALLY REINFORCED SLABS

Usama Ebead

Department of Civil Engineering, University of Sherbrooke

2500 boulevard de l’Université, Quebec, Canada, J1K 2R1

Hesham Marzouk

Department of Civil Engineering, Memorial University of Newfoundland

300 Prince Philip Dr., St. John’s, Newfoundland, Canada, A1B 3X5

ABSTRACT

An ACI code verification of FRP externally reinforced two-way slabs is introduced. An implementation of the ACI-318 [1] and the ACI-440 F [2] is presented for the purpose of verification against experimental results. In the experimental work, two different types of FRP materials were evaluated; namely carbon FRP (CFRP) strips and glass FRP (GFRP) laminates. The externally reinforced or strengthened slabs had steel reinforcement ratios of 0.35% and 0.5%. Results show that the flexural capacity of two-way slabs can be increased to an average of 35.5% over that of the reference (unstrengthened) specimen. An increase of the initial stiffness is achieved; however, an apparent decrease in the overall ductility is evident. In addition, an average decrease in the values of the energy absorption of about 30% is observed. The estimated ultimate load capacity using the ACI code is in an accepted level of agreement with the experimental results.

Keywords: concrete; FRP; external reinforcement, strengthening; reinforcement; two-way slabs.

Introduction

FRP materials have been used for strengthening reinforced concrete beams, columns and one-way slabs. The flexural capacity of concrete beams can be increased by bonding FRP sheets, strips or laminates to the tension side [3,4,5]. In addition, the shear strength of concrete beam can be increased by gluing FRP laminates to the concrete web at locations of high shear stresses [6,7]. The ease of handling FRP materials provides the means to the extension of their applications for strengthening other structural elements. A few research works has been conducted on FRP strengthening of reinforced concrete slabs especially two-way slabs. Some research works dealt with the strengthening of one-way slabs using FRP materials in which slabs were treated in a very similar way to beams [8,9]. Using FRP materials to enhance two-way slabs in flexure is very desirable from the applicability point of view due to the ease of handling and installing FRP materials. FRP materials are not subject to either corrosion or rust on the long term. A drawback of using FRP materials for flexural strengthening is the potential for brittle failure that results in a decrease of the overall ductility.

In the following sections, a summary of an experimental program on two-way slabs strengthened using FRP is presented. Following that summary, ACI code verification is presented with a comparison between the experimental and the code implementation results.

SUMMARY OF Experimental program

Materials

The concrete mix was designed for an average target cylinder compressive strength of 35 MPa after 28 days. The mix proportion of 1.0 m3 is 1160 kg of gravel, 690 kg of sand, 350 kg of Portland cement and 175 litres of water. The actual cylinder compressive strengths ranged from 29 to 38 MPa as shown in Table 1. The steel reinforcement bars were CSA grade 400 deformed bars. The actual yield strength of the steel reinforcement ranged from 435 to 450 MPa. Two different diameters were used, namely 10 and 20mm for the slab and column, respectively.

Unidirectional GFRP laminates and CFRP strips were used for strengthening. The Sika CarboDur was used for the CFRP strips and the glass fibre fabric, SikaWrap Hex 100G, was used for the GFRP laminates. The thickness of one layer of the GFRP laminates and CFRP strips was 1.0 mm and 1.2 mm, respectively. Two different types of two-component adhesive epoxy resins were used for each type of the FRPs as per the manufacturer’s specifications. Sikadur 30 and Sikadur Hex 300 epoxy resins were used for the CFRP strips and the GFRP laminates, respectively. The properties of FRP materials and the epoxy resins provided by the manufacturers are listed in Tables 2 and 3, respectively.

Table 1: Compressive strength of concrete

Specimen Title / Cylinder compressive strength, MPa / Strengthening material / Reinforcement ratio / Preload, kN
Ref-0.35% / 30.0 / - / 0.35% / -
Ref-0.5% / 35.0 / - / 0.5% / -
GFRP-F-0.35% / 29.0 / GFRP laminates / 0.35% / 125
GFRP-F-0.5% / 38.0 / GFRP laminates / 0.5% / 165
CFRP-F-0.35% / 35.0 / CFRP strips / 0.35% / 125
CFRP-F-0.5% / 34.0 / CFRP strips / 0.35% / 165

Table 2: Properties of one layer of the FRP materials

FRP / Thickness, mm / Tensile strength, MPa / E , GPa / Elongation at break , % / Weight,
g/m2 / Fibre content / Fibre density
g/cm3
CFRP strips* / 1.2 / 2800 / 170 / >1.7 / 2240 / 68% / 1.5
GRFP laminates** / 1.0 / 600 / 26.1 / 2.24 / 913 / 50-80% / 2.54

* Sika CarboDur ** Sika Wrap

Table 3: Properties of the epoxy adhesive

Property / Epoxy for strips* / Epoxy for laminates**
Tensile strength, Mpa / 24.8 / 72.4
Elongation at break, % / 1.00 / 4.8
Elastic modulus, Gpa / 4.5 / 3.1

*Sikadur 30 **Sikadur Hex 300

Test slabs

The tested specimens were square with 1900-mm side length and 150-mm thicknesses. The test specimens were simply supported along the four edges with corners free to lift and were loaded through the column stub. A layout of the tested slabs is shown in Fig. 1 that shows also reinforcement details of the tested specimens.

Two unstrengthened specimens were used as reference specimens, namely, Ref-0.35% and Ref-0.5% of reinforcement ratios of 0.35 and 0.5%, respectively. Specimens CFRP-F-0.35% and CFRP-F-0.5% had steel reinforcement ratios of 0.35 and 0.5%, respectively, and were strengthened using CFRP strips. Similarly, specimens GFRP-F-0.35% and GFRP-F-0.5% had reinforcement ratios 0.35 and 0.5%, respectively and were strengthened using GFRP laminates.

A minimum concrete cover of 25 mm was maintained for all specimens at compression and tension sides. Column stubs were square of 250-mm side dimension and were located at the slab center. The column stubs were extended on the compression side to a distance 850-mm from the concrete surface to allow for the application of the load.

Fig. 1: Details of a typical test specimen

Test set up and instrumentation

The specimens were tested using a large reaction steel frame. A 10-ton capacity crane was used to lift and install the specimens vertically inside the frame. Rubber pieces were placed between the back surface of the tested slabs and the supporting edges of the frame. A hydraulic actuator facing the specimen was used to apply a uniform load through the column stub. A load cell was used to measure the load using four calibrated electrical resistance strain gages fixed to the inner cylinder of the load cell. The actuator had a maximum load capacity of 700 kN and a maximum stroke of 150 mm.

Linear Variable Displacement Transformers (LVDT’s) were built in the front actuator to measure the deflection of slabs. The loads were applied using displacement control to avoid the uncontrolled failure at the maximum loads. The displacement rate for the actuator was 0.25 mm/min. A displacement function of the ramp type was applied through computerized function generator. Eight-mm length electrical resistance strain gages having a resistance of 120 ± 0.3% and a gage factor equal to 2.070 ± 0.5% were used to measure the steel reinforcement strains at locations shown in Fig. 1.

The LVDT’s and the electrical strain gages were connected through a master panel to a data acquisition system. The analog electrical signals of loads, deflections and steel strains were converted through the data acquisition system to digital signals and were stored in digital computer files.

Load application and testing procedure

The unstrengthened reference specimens; Ref-0.35% and Ref-0.5% were loaded through the column stub until failure to estimate the ultimate load carrying capacity. The ultimate load carrying capacity of the reference specimens was 250 and 330 kN, respectively. Fifty percent of the ultimate load carrying capacity of the reference specimens was used as an initial loading for the specimens prior to strengthening. Hence, the specimens with reinforcement ratios of 0.35 and 0.5% to be strengthened were loaded prior to strengthening with initial loads of 125 and 165 kN as initial loading. Fifty percent of the load represents a level of load on a building in field where strengthening may be required. The applied loads were completely released to represent a state of shoring two-way slabs in the field prior to strengthening. Afterwards, the specimens were removed from the loading frame for strengthening according to the strengthening procedure detailed below. After one week of curing, the specimens were relocated at the loading frame and were subjected to the load until failure.

Strengthening procedure

The concrete surface to be strengthened was roughened carefully using a vibrating hammer to improve the bond characteristics between concrete and CFRP strips and GFRP laminates. Dust and fine materials caused from the roughening process were removed carefully from the concrete surfaces. In addition, for CFRP strips, a special solvent was used to remove all grease, waxes, foreign particles and other bond inhibiting materials from the bonded surface as specified by the manufacturer. The two-part epoxy resin was applied on the concrete surfaces and the strengthening materials. Afterwards, the FRP strengthening materials were bonded to the concrete surface according to the type of strengthening.

The strengthening material was located at the tension side of the slab and was extended to a location 50 mm before the support. Two 300-mm width layers of GFRP laminates were bonded to the slab surface in both directions of specimens GFRP-F-0.35% and GFRP-F-0.5%. Specimens CFRP-F-0.35% and CFRP-F-0.5% were strengthened using three adjacent CFRP strips 100-mm width each so that the strengthened width is 300 mm. Additional transverse layers of CFRP strips were bonded at the end of the FRP materials to improve the end anchorage of the FRP strips or laminates with concrete surface. The anchorage layers were 100 mm wide and 500 mm long. Fig. 2 shows the strengthening details of the specimens.

Fig. 2: The strengthening details of the specimens

Test results and discussion

Deformational properties and load capacity

The load-deflection relationship was recorded using the data acquisition system. The slope of the load-deflection curve was higher than that of the corresponding reference specimens. Moreover, the average deflection at the ultimate load of the specimens was about 0.61 that of the corresponding reference specimens. In general, the strengthened specimens experienced smaller deformation compared to the corresponding reference specimens due to the effect of the FRP materials on the overall behaviour of the slabs. Fig.3 shows the load deflection relationship for the tested specimens.

The ultimate load carrying capacity will be referred to as the load capacity. The strengthened specimens showed higher load capacity than that of the corresponding reference specimens. Specimens CFRP-F-0.35% and GFRP-F-0.35% showed an increase of 44.4 and 38%, respectively in the load capacity over that of the reference specimen, Ref-0.35%. Moreover, Specimens CFRP-F-0.5% and GFRP-F-0.5% showed an increase of 36.4 and 25.8 %, respectively in the load capacity over that of the reference specimen, Ref-0.5%. The load capacity of the corresponding reference specimens was influenced by the reinforcement ratio that is in accordance with earlier research work [10]. The load capacity of specimen Ref-0.5% was 1.32 times that of specimen Ref-0.35%.

The stiffness of a slab at any loading point is the slope of the load-deflection curve at that point. The initial stiffness, K, was evaluated numerically as the slope of the load-deflection curve within the first 5-mm deflection. This is an approximation made to avoid the misleading initial readings when there is a relaxation of the load actuator. The strengthened specimens showed higher initial stiffness over that of the reference specimens. The average initial stiffness of Specimens CFRP-F-0.35% and GFRP-F-0.35% was about 2.37 times that of the reference specimen, Ref-0.35%. Moreover, the average initial stiffness of Specimens CFRP-F-0.5% and GFRP-F-0.5% was about 1.99 times that of the reference specimen, Ref-0.5%.

The energy absorption is the area under the load-deflection curve for a tested specimen. This area was evaluated numerically based on the available values of load and the corresponding values of deflection. At the maximum load, it was clearly noticed that the strengthening technique contributed to a decrease in the energy absorption of the strengthened specimens. An average decrease in the values of the energy absorption of about 30% for the strengthened specimens was observed.

Values of deflection at the cracking load, dcr, the ultimate load, Pu, the Deflection at the ultimate load, du, the initial stiffness, K, and energy absorption, Y for each slab are summarized in Table. 4.

Fig. 3: Load deflection relationship of the tested slabs

Table 4: Ultimate capacities and deflection characterestics of the tested slabs

Title / Deflection at cracking load,
dcr,
mm / Ultimate load,
Pu,
KN / Deflection at ultimate load,
du,
mm / Energy Absorption’
y,
kN.mm / Stiffness,
K, kN/mm
Ref-0.35% / 7.00 / 250 / 42.01 / 9346 / 8.42
Ref-0.5% / 6.25 / 330 / 35.57 / 9445 / 12.54
CFRP-F-0.35% / 7.25 / 361 / 18.08 / 7821 / 15.54
GFRP-F-0.35% / 7.69 / 345 / 27.72 / 4597 / 24.42
CFRP-F-0.5% / 6.03 / 450 / 21.03 / 6686 / 26.76
GFRP-F-0.5% / 6.35 / 415 / 26.71 / 7475 / 23.15

Failure characteristics

Reference specimens, Ref-0.35% and Ref-0.5%, failure mode was classified as flexural-ductile. Flexural reinforcement yielded and the two specimens showed relatively large deflection values before reaching the ultimate load.

Fig. 4 shows a typical flexural failure mode of GFRP and CFRP strengthened specimens after failure. It is evident that the FRP materials contributed to an increase of the capacity until the bond between the FRP material and concrete failed. De-bonding cracks appeared at a late stage of loading that resulted in a separation of the strengthening materials. These cracks were located along the edges of the strengthening material length. This indicates that end anchorage severed to a certain extent preventing a premature bond failure at the cut-off end of FRP materials. Followed the appearance of these cracks, the specimens failed due to accelerated concrete flexural failure after the FRP de-bonded from the slabs without rupture of the FRP material.