COMPARATIVE STUDY THROUGH TESTS OF THE DIFFERENT CONCRETE PAVEMENT SYSTEMS
Jose M. Lago Carrera, Senior industrial Engineer, Prosistemas, Porriño, Pontevedra, Spain / PhD student, University of Vigo, Pontevedra, Spain
Aida Badaoui, PhD professor, University of Vigo, Vigo, Pontevedra, Spain
KEYWORDS: Concrete pavement, fiber reinforce concrete pavement, continuously reinforced concrete pavement, post-tensioned, shrinkage-compensating concrete
Authors declare no conflict of interest with the material in this manuscript.
i
1. ABSTRACT
When the design of a concrete pavement is addressed, one of the most important points is to determine which type is the most appropriate to satisfy the requirements.
Based on the classification of concrete pavements indicated in ACI 360[1] (Plain concrete, continuously reinforced (bars or welded wire reinforcement), fiber reinforced concrete, shrinkage- compensating and post-tensioned), a series of tests have been developed that allow us to establish a comparison at the level of load capacity.
To evaluate the capacity to bending and shear a battery of tests were performed including a beam test and square slab testing. Different systems were designed: Continuously reinforced, steel and macro-synthetic fibers, combined solutions, post-tensioned and shrinkage-compensating concrete pavement.
The results show that with an equal amount of steel the load capacity of the continuously reinforced is more than 50% higher than the fiber, and this difference increases when increasing thickness. With equal volume of fiber, the capacity with steel fiber is 25% higher than the synthetic. In Shrinkage-compensating, an expansion of 300 microns/meter was generated, increasing the flexural strength at 0.3 Mpa, improving load capacity and energy absorption. In the post-tensioning system, an effective post-tensioning tension of 2 Mpa was introduced and, consequently, this increase was observed in the appearance of the first crack. In the square slab test the formation and development of the yield lines was observed.
Based on the results of the tests and having analyzed the ACI 360 and TR34 to determine the moment capacity of fiber-reinforced concrete and continuously reinforced, it is considered that in the case of continuously reinforced the guides should incorporate a nonlinear method with plasticity considering the ultimate tensile strength of steel. In this way, the comparison between both systems would be closer to reality.
Considering only moment capacity and punching shear capacity, it is concluded that solutions with continuous reinforcement (bars or welded wire reinforcement) are more effective for high loads than fibers.
1
2. INTRODUCTION
Different guides to design industrial concrete pavements show the possibility to design with different reinforcement systems or even without using reinforcement. A clear example are guidelines ACI 360 [1] and TR34 [2]. These guides show different models to calculate the mechanical capacity taking into account the reinforcement system.
The mentioned design guides in some cases consider elastic systems and in other plastic. In the case of continuous reinforcement, guides consider an elastic calculation, where the yield strength is established as a design value, however, in the case of fiber reinforced concrete, a plastic behavior is considered taking into account the residual flexural strength. According to previous comments, it seems clear that it is not easy to establish a comparison of the real capacity in the ultimate limit state between the different types of pavements.
Several trials were carried out with different types of pavements according to the classification indicated in the guide ACI 360.10[1], because this guide includes a good range of systems, (Plain concrete, continuously reinforced, Fiber reinforced concrete (steel and polymeric fibers), shrinkage-compensating concrete, post-tensioned concrete pavements), and is a worldwide reference in this field. The aim of these trials is to establish a comparison taking into account the mechanical capacity in order to facilitate the decisions on the type of pavement to be used depending on the loading requirements.
In order to evaluate the capacity to bending and shear, a matrix of trials was designed which included the beam test according to EN 14651[3] and the square slab testing according to EN 14488-5[4]. These methods were selected among others for the following reasons:
EN 14651[3] allows keeping the criteria of residual flexural strength in the fibers and evaluating at the same time the different types of reinforcement of pavements.
On the other hand, square slab testing is of great interest because it shows a comparative level of load capacity, and information about the real failure patterns (yield lines) in case of industrial pavements.
3. RESEARCH SIGNIFICANCE
Concrete pavements are plate structures with a thickness that is relatively small compared to the in-plane dimensions and with out-of-plane loadings with complex failure patterns. Guides and codes use different design systems depending on the type of reinforcement, so they don't show a clear comparison of real mechanical capacity. This is the main reason why it is considered a key point to carry out a matrix of trials where the different pavements types can be analyzed in order to establish comparisons and to facilitate the decision about which type is the most appropriate to satisfy the requirements.
2
4. METHODOLOGY
The key point is to clearly define the tests, concrete pattern and the different reinforcement system along with the cured and environmental conditions in order to guaranty the test reproducibility and reach a conclusion in the comparative analysis.
The concrete pattern was established taking into account Eurocode 2 [5], and the different systems were designed: Continuously reinforced concrete according to Eurocode 2 [5] in different ratios and positions, steel fibers and polymeric macrofibers in quantity according to ACI 544 [6] and ACI 360 [1], combined solutions according to TR34 [2], post-tensioned according to PTI DC10.1- 08 [7] and shrinkage-compensating according to ACI 223[8]. The trials were performed in order to conclude the key indicators.
4.1 Product
Concrete was produced according to EN 206 [9] both in the specification and production, type HA-25/F/20IIa (C25/30 according to Eurocode). This is a concrete with compressive strength in cylinder specimen of 25 Mpa with a fluid consistency, with aggregate maximum size of 20 mm. Table 1 indicates the composition of the concrete and reference normative for every material used. Sand moisture was raised at the same level of sand moisture absorption. Additives and microfibers used were “Mapei” Brand. These are commonly used in concrete pavements in accordance with the technical data sheet specifications in order to achieve a fluid consistency concrete with a ratio w/c of 0,55.
Dosage
Material quantity Unit Standard
AF 0/4-T-G-L 320 Kg/m3 EN 12620 [10]
AF 0/5-T-G-L 640 Kg/m3 EN 12620 [10]
AG 5/11-T-G-L 150 Kg/m3 EN 12620[10]
AG 11/22-T-G-L 740 Kg/m3 EN 12620 [10]
Cement I 52,5 R 300 Kg/m3 EN 197 [11]
Mapefibre NS12 - Mapei 0.3 Kg/m3 EN 14889-2 [12]
Mapefluid AC 40 - Mapei 1.76 dm3/m3 EN 934-2 [13]
Dynamon floor 3 - Mapei 4.32 dm3/m3 EN 934-2 [13]
Water 163 dm3/m3 EN 206[9]
Table 1. Concrete pattern.
3
4.2 Test
As previously explained, two types of tests were performed: according to EN 14651[3] and EN 14488-5[4]. Beam test EN 14651[3] is a 3-point bending test where in the middle of the span the prism is notched, the depth of the notch is 25 mm. The beam size is 600x150x150. Test EN 14488- 5[4] is a square slab supported by 4 sides, with a size 600x600x100 and with a central load. Two test specimens were made for every test. Test specimens were kept in a moisture control room with relative humidity > 95% and temperature 20ºC ± 2ºC and were tested at 28 days.
There were complimentary tests to control the concrete such as EN 12350-2 [14] slump-test, EN 12350-7[15] air content-pressure methods, EN 12350-6[16] fresh concrete-density and EN 12390- 3[17] Compressive strength of test specimens.
Expansion was measured according to UNI 8148[18] in the case of shrinkage-compensating concrete. Design was calculated to achieve an expansion of 300 microns/meter. In order to avoid dry shrinkage during the test, concrete specimens were immersed in water until the day of the tests (28 days).
To determine the values for each type of test, it was used values commonly used in the construction of concrete pavements.
4.2.1 Continuously reinforced
Two amounts of reinforcement were considered, one slightly lower than the minimum mechanical value and another slightly higher. The minimum steel ratio based in the modulus of rupture of concrete was determined for both the beam and plate test. The formulation to determine the minimum value is indicated below.
푤 퐴 푓 = 푓 Equation 1 푠 푦푑 푧 푐푡,푚,푓푙
As Area of steel fyd design strength of reinforcement w Section modulus z Mechanical lever arm fct,m,fl Average flexural strength of the concrete.
4
Table 2 and 3 indicate the values of steel for each of the tests as well as minimum steel ratio based in the modulus of rupture.
EN 14651
Minimum Steel ratio Test
Reinforcement (1) 0.32 cm2 0.28 cm2 1 Ø 6
Reinforcement (2) 0.32 cm2 0.56 cm2 2 Ø 6
Table 2. Steel quantity in the beam test.
EN 14488
Minimum Steel ratio Test
Reinforcement (1) 1.2 cm2 1.13 cm2 Ø 6 c 15
Reinforcement (2) 1.2 cm2 1,41 cm2 Ø 6 c 12
Table 3. Steel quantity in the square slab test.
• Bars were placed in the case of the beam with a 30 mm cover concrete from the underside. • In the case of square slab, Cover is 25 mm in one direction and 31 mm in the perpendicular. • Reinforcement bars is B500SD, minimum yield strength 500 Mpa and ultimate tensile strength of steel 575 Mpa conforming to EN 10080[19].
4.2.2 Fibers
It was established a value commonly used for fiber and it was taken a fiber amount in weight similar to the steel used in the continuously reinforcement solution, reinforcement (1). Steel fiber used was Arcelor HE+1/60, 60 mm length fiber, diameter 1mm with a yield strength of 1500Mpa. Regarding polymeric fiber, it was selected a prestigious structural macrofiber, and a dosage, in volume, similar to the one used in the steel fiber. It was used the polymeric fiber “FortaFerro”, length 54mm length of polypropylene.
4.2.3 Combined solutions
Combined solutions were carried out with a reinforcement (2) and 20 kg/m3 of steel and with the same continuous reinforcement and 3 kg/m3 in the case of polymeric fiber.
5
4.2.4 Shrinkage-compensating concrete
In the case of shrinkage-compensating concrete, an addition type G according to ACI 223 [8] was added. A percentage of 10% was used according to the weight of cement (30 kg/m3). The addition used was an expanding agent “Mapei” brand.
4.2.5 Post-tensioned
Cables with 15 mm diameter and yield strength 1560 Mpa were used according to pr·EN 10138[20]. The load needed in the specimens correspond with a 2Mpa compression. Losses were considered. Concrete resistance was checked in order to confirm that it is higher than 15 Mpa at the time of tensioning, using two cables in each direction for the square slab specimens, and one cable for the beam specimen, both situated mid-section. A non-adherent cable was used, and the load to get the tensioned, and consequently pre-compressed concrete, was applied with an hydraulic jack according to PTI DC10.1-08 [7]. It was applied 81,84KN load in the cable for the square slab test, and 55,59 KN for the beam test.
Table 4 shows a summary of the typologies tested to make the comparison, the indicator to measure and compare was the load depending on the crack opening in the case of the beam test and the load depending on the deformation for the case of square slab.
Slab type EN 14651 EN 14488 Nº specimens Nº specimens Unreinforced concrete 2 2 Continuously Reinforcement (1) 2 2 reinforced Reinforcement (2) 2 2 Steel fiber 2 2 Fibers Synthetic fiber 2 2 Steel fiber + Reinforcement (2) 2 2 Combined Synthetic fiber + Reinforcement (2) 2 2 Shrinkage-compensating concrete (Reinforcement (2)) 2 2 Post-tensioned 2 2
Table 4. Number of specimens by test and pavement type.
6
5. RESULTS
Figures 1 and 2 show a comparative between the average values obtained through the beam and square slab test of the following typologies: unreinforced concrete, continuously reinforced (1), steel fiber and macro polymeric fiber, which are the most commonly used systems.
Regarding the reinforcement (1), concrete rupture is observed at a point very close to the unreinforced concrete for the beam test, and after rupture, the capacity maintains nearby the maximum point obtained, which is a result close to that expected from reinforced concrete with a minimum flexural reinforcement. In the case of square slab, rupture lines increase the capacity until a failure point, which is produced by punching shear. In the case of steel fibers and polymeric fibers, once the maximum value is reached there is a decrease on the capacity in the case of the “beam test”, on the other hand in the case of the square slab a slight increase is observed in the capacity but with lower values compared with the reinforcement (1).
Figure 3 shows the absorbed energy until a 25mm deformation according to the square slab test. If the load capacity is compared, a better performance is observed in the reinforcement (1) although with a smaller difference which denotes a ductile material.
Figure 1. Comparison of load capacity according EN 14651.
20.000
18.000
16.000 Unreinforced
14.000
12.000 Reinforcement (1) 10.000
8.000
Load(N) Steel fiber 1/60 25 kg/m3 6.000
4.000
2.000 Synthetic fiber PP 4 kg/m3
0 0 0,5 1 1,5 2 2,5 3 3,5 4 CMOD (Crack mouth opening displacement (mm)
7
Figure 2. Comparison of load capacity according EN 14488.
140.000
120.000 Unreinforced
100.000
Reinforcement (1) 80.000
60.000 steel fiber 1/60 25 Load(N) kg/m3 40.000
20.000 Synthetic fiber PP 4 kg/m3
0 0 0,005 0,01 0,015 0,02 0,025 Deformation (m)
Figure 3. Absorbed energy (EN 14488) by typology.
EN14488 ENERGY 1200
1000 Unreinforced
800
Reinforcement (1) 600
Energy(Joules) 400 Steel fiber 1/60 25 kg/m3
200 Synthetic fiber PP 4 kg/m3 0 0 0,005 0,01 0,015 0,02 0,025 Deformation (m)
8
Figure 4 shows the comparative between the beam test reinforcement (2) versus combined reinforcement and fiber concrete solutions. Results are slightly better with combined solutions.
Figure 4. Comparison between continuous reinforcement and combined solution (EN 14651).
45.000
40.000 Reinforcement (2) 35.000
30.000
25.000 Combined 20.000 (reinforcement (2) + steel fiber)
15.000 Load(N)
10.000 Combined 5.000 (reinforcement (2) + synthetic fiber) 0 0 0,5 1 1,5 2 2,5 3 3,5 4 CMOD (Crack mouth opening displacement (mm)
Regarding the solution with shrinkage compensating concrete, specimens were kept in water 28 days in order to avoid shrinkage and thus observe the expansion effect at the moment of the test. Slightly higher than expected values of 300 microns / meter.
Figure 5. Expansion in the shrinkage-compensating concrete system.
UNI 8148 Expansion 400 350 300 250 200 150
microns/meter 100 50 0 0 5 10 15 20 25 30 Day
9
In Figure 6 it can be observed that concrete crack point for reinforced and fibers concrete happens practically in the same point, meanwhile for shrinkage-compensating concrete and post-tensioned happens in a point with a value, which is equal to the generated compression by the expansion or the post-tensioned. Figure 7 shows the image of a beam test according to EN 14651[3].
Figure 6. Comparison to show concrete crack point in different systems.
30.000 EN 14651 reinforcement (1) 25.000
reinforcement (2) 20.000
Steel fiber1/60 25 15.000
kg/m3 Load(N)
10.000 Shrinkage- compensating concrete 5.000 Post-tensioned
0 0 0,1 0,2 CMOD(mm)
Figure 7. Beam test according to EN 14651.
10
Figure 8 shows the image of a square slab test according to EN 14488-5[4]. Figure 9 shows the real failure patterns (yield lines) in the square slab test.
Figure 8. Square slab test according EN14488.
Figure 9. Typical failure pattern in the square slab test.
11
6. DISCUSSION
Design guidelines, both TR 34.4 [2] and ACI 360.10 [1], establish as structural properties of continuously reinforced solutions the yield strength , which means, it is assumed an elastic model, however for the fibers, its properties are considered by a non-lineal and plastic method. The example shows a comparative between the moment capacity equations for continuously reinforced with bars, and fibers according to TR 34.4 [2]:
For continuously reinforced:
0.95 퐴푠 푓푦푘 푑 푀푟 = Equation 2 훾푚
Where:
Mu Moment capacity of continuously reinforced solution
As Area of steel
fyk Yield strength of reinforcement
d Effective depth
m Partial safety factor
For fibers:
ℎ2(0,29휎푟4 +0,16 휎 푟1) 푀푢 = Equation 3 푚
Where:
Mu Moment capacity
h Thickness
σ r1 =0.45 f r1 f r1= the residual flexural strength at CMOD 0.5
σ r4 =0.37 f r1 f r1= the residual flexural strength at CMOD 3.5
m Partial safety factor
CMOD crack mouth opening displacement
12
The next step is to compare the value according to the design guide against the actual test value. To determine the value of moment capacity according to the design guide, equation (2) is applied; taking into account, that yield strength of reinforcement is 500 Mpa. Equation of simply supported beam with center load to determine the theoretical maximum load was applied and compares against the maximum real load according to the test. The results are indicated in table 5.
Maximum theoretical load TR34.4 [2] in test Actual maximum test value CMOD 2,5 for 14651 for reinforcement (1) reinforcement (1)
12, 57 KN 18,37 KN
Table 5. Comparison between design value and test value.
Observed difference is justified first by the partial safety coefficient (1,15), and second, by the difference between the considered Yield strength (500Mpa) and the ultimate tensile strength of steel which is greater than 575 Mpa (minimum value guaranteed by the manufacturer). Real value was 690 Mpa.
Based on the results of the tests and having analyzed the design guides to determine the moment capacity of fiber-reinforced concrete and continuously reinforced, it is considered that in the case of continuously reinforced the guides should incorporate a nonlinear method with plasticity considering higher tensile strength of continuous reinforcement. In this way, the comparison between both systems would be closer to reality.
The concrete pavements design in ultimate limit state (ULS) should consider a method with plasticity for continuously reinforced, (as it already does for fiber reinforced pavement) and consequently, the value to be considered of strength of reinforcement should be higher than yield strength, the same situation occurs in the case of combined solutions.
This study was performed with a focus on the industrial concrete pavements.
Pavement design guidelines used nowadays do not show a clear comparative between the real capacity of continuously reinforced and fiber systems, underestimating the continuously reinforced real capacity. It seems appropriate in view of the considerations made to propose that in the current formulation a higher reinforcement strength value should be considered.
13
It is important to point out that this comparative is based on trials, where the thicknesses of the specimens are smaller than those usually used in real structures, and that the difference the mechanical capacity between reinforced continuously and fibers is still higher when the thickness is increased due to that the mechanical lever arm increases approximately along with the thickness in the reinforced continuously, however in the case of the fiber, the mechanical lever arm increases approximately with half the thickness. Figure 10 shows a graph where it can be observed how the difference between mechanical lever arm, and in consequence mechanical capacity, increase as the thickness increases.
Figure 10. Comparison mechanical lever arm vs thickness.
40
35
30
25 FIBERS 20
15 CONTINUOUSLY REINFORCED 10
5 MECHANICAL MECHANICAL LEVERARM 0 0 10 20 30 40 50 THICKNESS
14
7. CONCLUSIONS
Based on the trials a comparative between the different types of pavement was established. As a main conclusion, the results confirm that with the same steel quantity, mechanical capacity of continuously reinforced is higher than with steel fiber. Also, with the same volume of steel fiber and polymeric macrofibers, steel fiber is more effective than polymeric macrofibers.
The difference of the mechanical capacity between reinforced continuously and fibers is still higher when the thickness is increased due to the mechanical lever arm increases approximately along with the thickness in the continuously reinforced, and in the case of the fiber, the mechanical lever arm increases approximately with half the thickness.
Guides TR 34.4 [2] and ACI 360.10 [1] reference for industrial concrete pavement design, establish the yield strength value to determine the moment capacity of continuously reinforced pavements, that is to say, to assume a behavior according to an elastic model. Nevertheless, in the case of fibers, it is established a moment capacity taking into account a non-lineal and plastic method.
According to the test results, it is concluded that in order to compare the maximum load between both systems, a plastic behavior in continuously reinforced pavements should be considered.
Based on the tests and having analyzed the design methods included in the guides ACI 360[1] and TR34.4 [2] to determine the moment capacity of fiber-reinforced concrete and continuously reinforced. It is considered that in the case of continuously reinforced pavements the guides should incorporate a nonlinear method with plasticity considering higher tensile strength of steel instead of the yield strength.
Considering only moment capacity and punching shear capacity, it is concluded that solutions with continuous reinforcement are more effective for high loads than fibers.
In shrinkage-compensating concrete an expansion effect is observed. This creates a tension in interior reinforcement, generating a compression in the concrete that will compensate the shrinkage in equal proportion to the expansion generated. It is concluded that it helps in the early stages avoiding stresses in the concrete generated by shrinkage and temperature effects, but should be studied the adequate reinforcement to undertake jointless pavements.
A compression of “2 Mpa” was generated in the concrete in the post-tensioned solution, which is reflected in the test. Post-tensioned solution is considered an effective solution for jointless pavements.
Square slab test has allowed to observe the real failure patterns (yield lines) in the case of industrial pavements.
Conclusions are indicated in order to facilitate the design decisions on the type of pavement to be used depending on the requirements.
15
8. ACKNOWLEDGEMENTS
The authors acknowledge the support from University of Vigo. The authors would also like to thank Enmacosa Laboratory and Prosistemas for their technical assistance. The contents of this paper reflect views of the authors, who are responsible for the accuracy of the data and facts presented herein.
REFERENCES
[1] ACI Committee 360, “360R-10 Guide to Design of Slabs-on-Ground,” p. 72, 2010.
[2] Concrete Society, “TR34 4th Edition - Concrete industrial ground floors a guide to design and construction,” 2014.
[3] A. 83, “EN 14651.Test method for metallic fibre concrete - Measuring the flexural tensile strength (limit of proportionality (LOP), residual.” 2008.
[4] AENOR/CTN83, “EN 14488-5.Testing sprayed concrete - Part 5: Determination of energy absorption capacity of fibre reinforced slab specimens,” 2008.
[5] AENOR, “UNE-EN 1992-1-1.Eurocódigo 2 Proyecto de estructuras de hormigón,” p. 300, 2013.
[6] A. C. I. Committee, ACI 544.5R-10.Report on the Physical Properties and Durability of Fiber-Reinforced Concrete. 2010.
[7] P.-T. Institute, “Pti dc10.1-08 Design of Post-Tensioned Slabs-on-Ground,” 2008.
[8] A. C. I. Committee, “ACI 223R-10. Guide for the Use of Shrinkage-Compensating Concrete,” 2010.
[9] AENOR/CTN83, “EN 206:2013+A1 Concrete. Specification, performance, production and conformity.” 2013.
[10] AENOR/CTN146, “EN 12620:2003+A1. Aggregates for concrete.,” 2009.
[11] AENOR/CTN 80, “EN 197-1:2011.Cement. Part 1: Composition, specifications and conformity criteria for common cements.,” 2011.
[12] AENOR/CTN83, “EN 14889-2. Fibres for concrete. Part 2: Polymer fibres. Definitions, specifications and conformity.,” pp. 16–18, 2008.
16
[13] AENOR/CTN83, “EN 934-1. Admixture s for concrete, mortar and grout. Part 1: Common requirements.” 2009.
[14] AENOR/CTN83, “EN 12350-2. Testing f resh concrete. Part 2: Slump-test.,” 2009.
[15] AENOR/CTN83, “EN 12350-7. Testing f resh concrete. Part 7: Air content. Pressure methods.,” 2010.
[16] AENOR/CTN83, “EN 12350-6.Testing fresh concrete. Part 6: Density. Essai,” 2009.
[17] AENOR/CTN83, “EN 12390-3. Testing hardened concrete. Part 3: Compressive strength of test specimens.,” 2009.
[18] E. N. I. di Unificazione, “UNI 8148. Determinazione dell ’ espansione contrastata del calcestruzzo Not metallic expansive agents for cement mixings Determination of restrained expansion of concrete.”
[19] AENOR/CTN36, “EN 10080. Steel for the reinforcement of concrete. Weldable reinforcing steel. General,” 2007.
[20] E. Standard, “prEN 10138-1. Prestressing steels,” 2000.
17