1. INTRODUCTION
⌅Nowadays,
society is focused on the risks associated with climate change, leading
to serious, pervasive, and irreversible changes in economies and
ecosystems worldwide (11. European Commission. (2021) Climate change and environmental degradation, European Commission.
).
One of the most frequent effects of climate change is the severe
rainfall that produces overflows in rivers and consequent floods in
landfills, streets, and highways. The consequences of these phenomena
are serious, both for populations and infrastructure, so solutions that
minimize them have been implemented. One of these is the application of
pervious surfaces to drain water which accumulates on pavement surfaces.
Pavement design in general, consists of determining the thickness of the pavement or of the several layers of which it is composed in order to resist the wheel loads of traffic and transmit them safely into the foundation soil. The present study was conducted considering pervious concrete in the wearing course.
Water in pavements is removed through a transverse slope and recently by providing pervious surfaces. In asphalt pavements, this technology is almost completely consolidated and successfully provides safe pavements during rainy conditions. However, for cement concrete pavements, more research is needed to consolidate efficient solutions.
Pervious concretes are materials
of growing importance in building technologies and are currently used to
build sustainable infrastructures and reduce the risks of urban floods.
Pervious concrete can be found worldwide in the paving of roads,
parking lots, sidewalks, viaducts, bike paths, or roadside drainage
channels. It is also used in parks and residential areas, mainly due to
its high permeability (22.
Huang, J.; Luo, Z.; Khan, M.B.E. (2020) Impact of aggregate type and
size and mineral admixtures on the properties of pervious concrete: An
experimental investigation. Constr. Build. Mater. 265, 120759. https://doi.org/10.1016/j.conbuildmat.2020.120759.
).
Pervious concrete is a sustainable building material which is made of widely available and easily handled materials (33. Tennis, P. D.; Leming, M. L.; Akers, J. D. (2004. Pervious concrete pavements, Portland Cement Association, (2004).
).
It contributes positively to environmental sustainability by removing
the first flush pollutants from superficial water (e.g., engine oil or
antifreeze liquids), reducing contamination hazards of underground water
and nearby lakes.
Tennis et al. (44.
Goede, W.G. (2009). Pervious concrete: investigation into structural
performance and evaluation of the aplicability of existing thickness
design methods, Washington State University, (2009).
)
show that pervious concrete presents high performance in removing
pollutants from water. Portland cements achieved removal rates of 82%
and 95% of suspended solids pollutants (55. Wanielista, M.; Chopra, M. (2007) Performance assessment of Portland cement pervious pavement, University of Central Florida.
).
Pervious pavements can increase circulation safety by preventing water
build-up and reduce risks from hydroplaning, reduced surface ponding,
glare, or even slipping (66. Hendrickx, I.L. (1998). Noiseless concrete pavements, European Concrete Paving Association.
).
According to Hendrickx (77.
Bittencourt, S.V.; Magalhães, M.S.; Tavares, M.E.N. (2021) Mechanical
behavior and water infiltration of pervious concrete incorporating
recycled asphalt pavement aggregate, Case Stud. Constr. Mater 14, e00473. https://doi.org/10.1016/j.cscm.2020.e00473.
),
pervious concrete also reduces road noise. The pervious material
structure allows easy escape for air trapped between tire and pavement,
causing lower frequency road noise.
Pervious concrete provides
several important environmental benefits, such as reducing the effect of
urban heat islands, refilling of aquifers, and the effective reduction
of rainwater runoff (88. Montes, F.; Haselbach, L. (2006) Measuring hydraulic conductivity in pervious concrete. Environ. Eng. Sci. 23 [6], 960-969. https://doi.org/10.1089/ees.2006.23.960.
).
By allowing infiltration of rainwater into soils with vegetation,
pervious concrete reduces the need for water irrigation in urban
landscapes (33. Tennis, P. D.; Leming, M. L.; Akers, J. D. (2004. Pervious concrete pavements, Portland Cement Association, (2004).
). Therefore, pervious concrete can play an important role in achieving greener buildings and sustainable infrastructures (33. Tennis, P. D.; Leming, M. L.; Akers, J. D. (2004. Pervious concrete pavements, Portland Cement Association, (2004).
).
Due
to their great importance and growing use in different construction
types, pervious concretes are referenced in the Storm Water Technologies
Fact Sheet Bioretention by the United States Environmental Protection
Agency for rainwater runoff management (99. Envionmental Protection Agency (EPA) (1999) Storm water technology fact sheet. Washington, DC.
).
However, the use of pervious concrete requires an appropriate mix
design of concrete specimens. This affects the physical and mechanical
properties such as permeability, porosity, and strength of the concrete.
The mechanical strength of pervious concrete is inversely proportional to porosity (1010.
Deo, O.; Neithalath. N. (2010) Compressive behavior of pervious
concretes and a quantification of the influence of random pore structure
features. Mater. Sci. Eng. A. 528 [1], 402-412. https://doi.org/10.1016/j.msea.2010.09.024.
, 1111.
Hooton, R.; Crouch, L.; Cates, M.; Dotson, V.; Honeycutt, K.; Badoe, D.
(2003) Measuring the effective air void content of Portland cement
pervious pavements. J. ASTM Int. 25 [1], 1-5. https://doi.org/10.1520/cca10516j.
). Typically, porosity ranges from 15% to 35%, which affects the permeability, mechanical strength, and durability (1212. Zhang, W.; Honghe, L.; Zhang, Y. (2018) Effect of porosity on frost resistance of Portland cement pervious concrete. Adv. Concr. Constr. 6 [4], 363-373. https://doi.org/10.12989/acc.2018.6.4.363.
). Solid advances in knowledge can be found in the literature on this topic (1313.
Montes, F.; Valavala, S.; Haselbach, L. (2005) A new test method for
porosity measurements of Portland cement pervious concrete. J. ASTM Int. 2 [1]. https://doi.org/10.1520/JAI12931.
), in which tests were carried out to determine porosity, whereas (1414.
Zouaghi, A.; Kumagai, M.; Nakazawa. T. (2000) Fundamental study on some
properties of pervious concrete and its applicability to control
stormwater run-off. 22 [2], 43-50.
) and (1515.
Olek, J.; Weiss, W.J.; Neithalath, N.; Marolf, A.; Sell, E.; Thornton,
W.D. (2003) Development of quiet and durable porous Portland cement
concrete paving Mmterials.
) analyzed mechanical parameters. (1515.
Olek, J.; Weiss, W.J.; Neithalath, N.; Marolf, A.; Sell, E.; Thornton,
W.D. (2003) Development of quiet and durable porous Portland cement
concrete paving Mmterials.
) and (1010.
Deo, O.; Neithalath. N. (2010) Compressive behavior of pervious
concretes and a quantification of the influence of random pore structure
features. Mater. Sci. Eng. A. 528 [1], 402-412. https://doi.org/10.1016/j.msea.2010.09.024.
) examined and characterized the material’s acoustic parameters.
Pervious concrete porosity varies from 15% to 25% (1616.
National Ready Mixed Concrete Association (NRMCA). (2004) CIP 38 -
Pervious concrete. Technical Information prepared by NRMCA.
). The National Ready-Mix Concrete Association (NRMCA) indicates a minimum recommended value of 15% (1717. Debnath, B.; Sarkar, P.P. (2018) Pervious concrete as an alternative pavement strategy: A state-of-the-art review. Int. J. Pavement Eng. 21 [12], 1516-1531. https://doi.org/10.1080/10298436.2018.1554217.
). Although porosity is widely accepted as an indicator of pavement permeability (1818.
Martin III, W.D.; Kaye, N.B.; Putman, B.J. (2014) Impact of vertical
porosity distribution on the permeability of pervious concrete. Constr. Build. Mater. 59, 78-84. https://doi.org/10.1016/j.conbuildmat.2014.02.034.
),
it has been demonstrated that other parameters must be analyzed for an
accurate prediction of permeability. Therefore, pervious concrete’s
porosity depends on both the W/C [Water/Cement] ratio and compaction
during the making (1919. Taheri, B.M.; Ramezanianpour, A.M. (2021) Optimizing the mix design of pervious concrete based on properties and unit cost. Adv. Concr. Constr. 11 [4], 285-298. https://doi.org/10.12989/acc.2021.11.4.285.
)
studied the unit cost of ingredients for this type of mixture and
concluded that the W/C had no significant effect on the unit cost of
porous concrete, although it has a considerable effect on strength.
Achieving
a suitable ratio between pores and mechanical strength will lead to
optimized pervious concrete that is designed to meet specific
performance requirements. Thus, the aim of this work is to develop a
pervious concrete that is can be applied in concrete road pavements. The
development of this concrete will be carried out by testing three W/C
ratios and three vibration times. In the study, an appropriate aggregate
was chosen in terms of size. According to (2020.
Hosseini, S.A.; Toghroli, A. (2021) Effect of mixing nano-silica and
perlite with pervious concrete for nitrate removal from the contaminated
water. Adv. Concr. Constr. 11 [6], 531-544. https://doi.org/10.12989/acc.2021.11.6.531.
),
porosity is the main factor for estimating the efficiency of pervious
concrete, with performance being more affected by aggregate type than
size.
Because of their various advantages, concrete pavements are widely used worldwide (2121.
Abut, Y.; Yildirim, S.T.; Ozturk, O.; Ozyurt, N. (2020) A comparative
study on the performance of RCC for pavements casted in laboratory and
field. Int. J. Pavement Eng. 23 [6], 1777-1790. https://doi.org/10.1080/10298436.2020.1823391.
). To design the concrete pavements, various design methods (22-2522. Packard, R.G. (1984) Thickness design for concrete highway and street pavements. Skokie, IL: Portland Cement Association.
23.
American Association of State Highway and Transportation Officials
(AASHTO) (1993) Guide for design of pavement structures. Washington, DC.
24 CROW (2004) VERCON 2.0. Design software for concrete roads. CD-ROM D925. CROW, Ede, the Netherlands.
25.
American Association of State Highway and Transportation Officials
(AASHTO). (2008) Mechanistic-empirical pavement design guide - A manual
of practice, Interim Edition, Washington.
) are used in different countries. In most design guides (2222. Packard, R.G. (1984) Thickness design for concrete highway and street pavements. Skokie, IL: Portland Cement Association.
, 2424 CROW (2004) VERCON 2.0. Design software for concrete roads. CD-ROM D925. CROW, Ede, the Netherlands.
),
resistance of concrete slabs and erosion of slab supports are two
common cases of design failure. The purpose of the design is to
determine the slab thickness required to resist the repeated application
of axle loads during the life span of the pavement. In addition,
flexural strength, modulus of elasticity and density are the important
properties of concrete which are used to determine the required pavement
thickness. Since the properties of the aggregates are used in preparing
the concrete mixtures, it thus has an important impact on the
properties of the concrete used in the design and its required thickness
values. This may significantly alter based on the type of aggregate
used.
The performance of the pervious concrete will be evaluated by assessing: i) fresh concrete workability by means of the slump, vee bee, density and air-void content tests; ii) hardened concrete with the following tests: immersion porosity, permeability, compressive strength and abrasion resistance.
At the end of the study, several recommendations are given for future research and design works. The authors believe that the results, discussions and recommendations of this study that are presented in the scope of the laboratory study will be highly beneficial for future studies and design applications, especially including the use of recycled aggregates.
2. MATERIALS AND MANUFACTURE OF PERVIOUS CONCRETE
⌅2.1 Materials
⌅Pervious
concretes were produced from cement, water, and fine and coarse
aggregates. Aggregates were produced by a specific process to obtain
spherical or rounded-aggregate-shaped grains [empirical-based process],
providing a uniform distribution of the different particle sizes in the
concrete. Portland cement (CEM I 42.5 R) was used (2626. SECIL, Supratek Cimento Portland. (2022) Retrieved from https://www.secil.pt/pt/produtos/cimento/cimento-cinzento/supratek-cem-i-42-5r. (Accessed 28 February 2022).
) with the properties shown in Tables 1 and 2.
).
Properties | Values |
---|---|
Accumulated diameter for 10% [µm] | 1.33 |
Accumulated diameter for 50% [µm] | 9.3 |
Accumulated diameter for 90% [µm] | 24.2 |
Ignition loss [%] | 2.33 |
Specific weight [kg/m3] | 3120 |
Blaine fineness [cm2/g] | 4072 |
Age | Strength [MPa] | |
---|---|---|
Flexural | Compressive | |
2 | 5.6 | 31.8 |
7 | 7.5 | 47.0 |
28 | 8.5 | 55.5 |
It is assumed that the materials used, other than cement, are local materials and coarse and fine aggregates, natural sand and other materials available locally in all countries.
The use of water in concrete has two functions: to activate the binder and give the concrete workability. From the point of view of hydraulic activation of the binder material, the literature generally states that 23% of water in the binder is necessary to activate it, with the remaining amount destined for workability.
An important factor is the origin of the water used. NP EN 1008 (2727. Portuguese Institute for Quality (IPQ). (2003) NP EN 1008, Water for construction use. (in Portuguese).
) regulate the application of water in concrete and makes considerations about the source of the water, e.g., potable water.
Aggregates
used in the concrete production were a mix of fine (0/4 mm aggregate),
and coarse (4/10 mm and 10/14 mm aggregate) that allowed the particle
size distribution presented in Table 3 to be obtained, following Portuguese construction specifications (2828. Estradas de Portugal. (2014) Pavement - Characteristics of materials - Specification type of work 03: 125. (in Portuguese).
). Table 3 shows a synthesis of the particle size distribution of the aggregates. Table 4 shows the physical properties of the aggregate.
Sieve opening size [mm] | 0.063 | 4 | 10 | 14 | 16 |
Cumulative passing [%] | 0.0 | 13.5 | 49.1 | 37.4 | 100 |
Properties | Units | Standard | Aggregates | ||
---|---|---|---|---|---|
0/4 | 4/10 | 10/14 | |||
Density | kg/m3 | EN 1097-6 | 2670 | 2660 | 2620 |
Resistance to fragmentation [Los Angeles method] | % | EN 1097-2 | - | 20.64 | 23.17 |
Pervious concretes were prepared with different water/cement (W/C) ratios, 0.30, 0.35, and 0.40, and submitted to different vibration times (3000 vibrations/min at 50 Hz) during 20, 40, and 60 seconds. As shown in Table 5, nine concretes were made. For each concrete, three cubic specimens (150×150×150 mm) were made, making a total of 27 specimens. The aspect of the specimens can be seen in Figure 1.
Water Cement | Vibration (s) | Quantities of materials [kg/m3] | |||
---|---|---|---|---|---|
CEM | Agg. 0/4 | Agg. 4/10 | Agg. 10/14 | ||
0.30 | 20 | 300 | 25 | 1391 | 324 |
40 | |||||
60 | |||||
0.35 | 20 | 22 | 1364 | 314 | |
40 | |||||
60 | |||||
0.40 | 20 | 19 | 1337 | 305 | |
40 | |||||
60 |
In
the production of the concrete specimens, care was taken to adjust the
manufacturing process. After 24 hours, the specimens are removed from
the mold and marked for curing in the chamber for up to 28 days at a
temperature of 20ºC as per standard EN-12390-7 (2929. European Committee for Standardization (CEN). (2019) EN-12390-7, Testing Hardened Concrete - Density of Hardened Concrete.
).
2.2 Manufacture of pervious concrete
⌅Rigid pavements are made up of a cement concrete slab, which can rest directly on the foundation (in the case of roads subject to low traffic) or contain other layers between the foundation and the concrete slab, namely, layers of lean concrete.
For the pervious concrete design, the nine pervious concretes produced in the laboratory were tested for assessing:
2.2.1 Fresh concrete tests
⌅Workability
was tested by means of the slump and vee bee tests. The conventional
slump test and the vee bee test were used to assess the concrete’s
rheological behavior. The slump test followed European standard EN
12350-2 (3030. European Committee for Standardization (CEN). (2019) EN 12350-2, Testing fresh concrete - Slump test.
). The Vee bee test followed European standard EN 12350-3 (3131. European Committee for Standardization (CEN). (2019) EN 12350-3, Testing fresh concrete - Vebe test.
). A slump cone was placed into the cylinder container of the vee bee test equipment.
Next,
fresh concrete density and air-void content were analyzed. Fresh
concrete was tested by analyzing and correlating fresh density EN
12350-6 (3232. European Committee for Standardization (CEN). (2019) EN 12350-6, Testing fresh concrete - Density.
) with the air-void content EN 12350-7 (3333. European Committee for Standardization (CEN). (2019) EN 12350-7, Testing fresh concrete - Air content. Pressure methods.
).
2.2.2 Hardened concrete tests
⌅After
seven days the specimens are considered to be hardened concrete. Before
the test, the dimensions of the specimen were measured in different
positions and the average values of the areas and volume were calculated
with calipers or rulers. The specimens were tested in accordance with
the standard for testing hardened concrete EN-12390-7 (2929. European Committee for Standardization (CEN). (2019) EN-12390-7, Testing Hardened Concrete - Density of Hardened Concrete.
).
2.2.2.1 Porosity by immersion
⌅Considering that the porosity increase is matched with a decrease in strength on pervious concrete, the selection of the binder is of vital importance.
The porosity of pervious concrete mixtures has been calculated based on the following European standards, EN-12390-7 (2929. European Committee for Standardization (CEN). (2019) EN-12390-7, Testing Hardened Concrete - Density of Hardened Concrete.
) and EN 1097-6 (3434.
European Committee for Standardization (CEN). (2000) EN 1097-6, Test
for mechanical and physical properties of aggregate part 6:
Determination of particles density and water absorption.
).
Three specimens of each type of mixture with dimensions 100x100x100 mm
were used to study the porosity in order to obtain an adequate mean of
values. Each specimen was initially saturated. Then, lateral faces were
sealed with an extendable sleeve. Finally, the bottom face was placed
over an acrylic plate, and the resulting joints were sealed with
silicone.
2.2.2.2 Permeability
⌅The weight of each saturated specimen with the complete setup was recorded. Afterwards, the membrane was filled with water and weighed. The weight of the added water is the difference between the two weights. The value obtained is quantified as the percentage of pores.
The concrete
permeability was characterized and assessed by determining the kf
permeability [coefficient falling head], according to standard NLT327/00
(3535.
Centro de Estudios y Experimentación de Obras Públicas (CEDEX). (2000)
NLT-327/00, Permeabilidad in situ de pavimentos drenantes mediante
permeámetro. (in Spanish).
). Following (3636.
Neithalath, N.; Weiss, W.J.; Olek, J. (2006) Predicting the
permeability of pervious concrete (enhanced porosity concrete) from
non-destructive electrical measurements. Retreived from https://www.researchgate.net/publication/228788494_Predicting_the_Permeability_of_Pervious_Concrete_Enhanced_Porosity_Concrete_from_Non-Destructive_Electrical_Measurements. (Accessed 15 February 2022).
) and the report standard 522R-06 (3737. American Concrete Institute (ACI). (2010) Report on pervious concrete 522R-10.
), the falling head permeability test was used for this purpose, as depicted in Figure 2a.
Cylindrical specimens (100 mm in diameter and 200 mm in height) were
used to test each concrete. Three specimens were used for each concrete.
Each specimen (with a cross-section of AC) was connected to a graduated cylinder (with a cross-section of AP), filled with de-aired water. Side leakage was prevented by sealing the specimen inside an extendable sleeve (Figure 2b).
The water flows through the specimens, where kf is the permeability coefficient (mm/s). The test tube is filled with water to a given height h0 (mm), and after the valve of the test tube is opened, the water flows until it reaches h1. The test time is recorded between h0 and h1. The permeability coefficient was calculated using the Darcy equation (Equation [1]):
where:
Kf- permeability coefficient (mm/s)
Ap- transparent pipe of cross-sectional area (m2)
Ac- cross sections area (in m2)
L- water traveled a distance (in m)
t- time (in seconds)
h0- level of water dropped zero (in m)
h1- level of water dropped from height (in m)
Flow
and permeability were determined using the LCS (laboratory la Caminos
Santander) permeameter test, following the Spanish standard NLT-327/00 (3535.
Centro de Estudios y Experimentación de Obras Públicas (CEDEX). (2000)
NLT-327/00, Permeabilidad in situ de pavimentos drenantes mediante
permeámetro. (in Spanish).
) as shown in Figure 3,
using 150×150×150 mm cubic specimens. Three specimens per concrete were
tested. Time was recorded for the flow of a given volume of water
through a hole 3 cm in diameter. The flow resulting from the loss of
water load was quantified by the permeability coefficient KLCS.
) and b) image of the permeameter used to quantify flow.
The permeability coefficient (KLCS) was calculated based on the flow time T (in seconds), according to Equation [2]:
where:
KLCS- permeability coefficient (mm/s)
T- time (s)
Equation [2]
allows the characterization of pervious concrete permeability and
validates the calculated coefficients in compliance with the technical
requirements for pervious pavements defined in European standard EN
13108-7 (3838.
European Committee for Standardization (CEN). (2013) EN 13108-7,
Bituminous mixtures - Material specifications - Part 7: Porous asphalt
mélanges.
).
2.2.3 The mechanical tests
⌅The different
concretes were tested for mechanical characterization, using specimens
aged for 28 days. Strength was assessed by compression following EN
12390-3 (3939.
Centro de Estudios y Experimentación de Obras Públicas (CEDEX). (1992)
NLT - 362/92, Efecto del agua sobre la cohesión de mezclas bituminosas
de granulometría abierta, mediante el ensayo cántabro de perdida por
desgaste. (in Spanish).
). In the case of this test, a
speed of 13.5 kN/s was applied according to the dimension of the
specimen and perpendicularly to the faces of the specimen.
Abrasion resistance was evaluated using the abrasion test following the standard NLT-362/92 (4040.
European Committee for Standardization (CEN). (2020) EN 1097-2, Tests
for mechanical and physical properties of aggregates - Methods for the
determination of resistance to fragmentation.
), in
which the mass lost after 300 rotations at a speed of 33 rpm in the Los
Angeles drum is measured. The smaller the test result, the higher the
abrasion resistance.
In Spain, the method of formulating bituminous mixtures uses the Cantabro test, which was developed with the aim of evaluating the wear loss of the wear layer. This test consists of determining the mass loss due to wear of cylinder-type draining concrete specimens (Ø100 mm and 150 mm in height), subjected to 300 cycles in the Los Angeles wear machine, without balls.
The test
consists of molding six specimens, of which three are placed in a humid
chamber (CH) at 25ºC for 672 hours and the rest are initially immersed
in water at 25ºC, where they remain for 28 days and are then removed
from the water and kept in a free environment at 25ºC for 24 hours.
After this time, the specimens were then measured at Ø 100 mm and 63 mm
thick and weighed (4040.
European Committee for Standardization (CEN). (2020) EN 1097-2, Tests
for mechanical and physical properties of aggregates - Methods for the
determination of resistance to fragmentation.
).
At
the end of the test, the specimen is cleaned with a compressor and then
the specimen is weighed and the Los Angeles coefficient (LA) (4040.
European Committee for Standardization (CEN). (2020) EN 1097-2, Tests
for mechanical and physical properties of aggregates - Methods for the
determination of resistance to fragmentation.
) is calculated, based on Equation [3]:
Where:
P= resistance of wear (%),
P1= initial mass of the specimen (g),
P2= retained mass (g).
3. ANALYSIS AND DISCUSSION OF THE EXPERIMENTAL RESULTS
⌅The workability of the pervious concrete was assessed by the slump and vee bee tests (see Figures 4a and 4b, respectively), and the results are indicated in Table 5. The slump test results show a constant value of 0, consistent with typical workability behavior for pervious concretes.
The results of the Vee bee test vary from 6.17 to 9.67 s, indicating that as the W/C ratio of the mixtures increases, the value of the vee bee test decreases, meaning that the concretes have higher workability, as expected.
3.1 Fresh concrete density and air content
⌅ Figure 5 presents the results of the fresh concrete density and air content. Density varies between 1886 kg/m3 and 2088 kg/m3, while air-void content varies between 4% and 7%. These values are consistent with the literature for pervious concrete (1717. Debnath, B.; Sarkar, P.P. (2018) Pervious concrete as an alternative pavement strategy: A state-of-the-art review. Int. J. Pavement Eng. 21 [12], 1516-1531. https://doi.org/10.1080/10298436.2018.1554217.
).
Concrete | Slump Test [mm] EN 12350-2:2016 | Vee bee Test [s] EN 12350-3:2011 |
---|---|---|
C0.30-20 | 0 | 9.67 |
C0.30-40 | 0 | |
C0.30-60 | 0 | |
C0.35-20 | 0 | 8.00 |
C0.35-40 | 0 | |
C0.35-60 | 0 | |
C0.40-20 | 0 | 6.17 |
C0.40-40 | 0 | |
C0.40-60 | 0 |
The values presented in Figure 5 also show that air-void content is similar for the C0.30 and C0.35 concretes. C0.40 presents higher air-void content due to the higher water content. It seems that a correlation exists between air-void content and density.
3.2 Porosity
⌅The porosity results, presented in Figure 6, are in the range between 20.1% and 24.6%. The lowest values were achieved by C0.40-40 and the highest by C0.30-40 concrete.
Porosity presents a linear trend according to the values observed in Figure 6. The trend is related to a relationship between the times of vibration and the water/binder ratios of each mixture. However, the porosity is significantly reduced as the W/C ratio increases, as seen in the trend in Figure 6. This is due to the better workability of the concretes with a higher W/C ratio. There is also a tendency to decrease the porosity with the increase of the vibration time. This is due to some segregation of the concrete with higher vibration times.
3.3 Permeability
⌅Concrete permeability was evaluated by the falling head permeability test and the LCS test as described in the methodology. The results will be expressed as a function of the water absorption because they are related to the air voids in the concrete.
The permeability obtained with the falling head permeability test is indicated in Figure 7 compared to the permeability results, while Figure 8 presents the LCS permeability. The comparison between both permeabilities is indicated in Figure 9.
Figure 7 shows that an increase in porosity is matched by an increase in falling head values, thus revealing the influence of vibration times over the porous structures of the mixes.
During the study, it became clear that increases in the W/C ratio of the mixes are related to lower porosity and permeability, as observed in both the falling head and LCS permeability tests results (Figure 7 and 8).
Figure 9 shows a linear correlation between the falling head and LCS permeability tests values. Such results confirm that higher W/C ratios are related to lower permeability values in both tests, as previously explained. This is due to the greater workability.
Technical requirements in EP - Estradas de Portugal (2828. Estradas de Portugal. (2014) Pavement - Characteristics of materials - Specification type of work 03: 125. (in Portuguese).
) for pavement surface courses, following the in situ permeability parameters of standard NLT-327 (3535.
Centro de Estudios y Experimentación de Obras Públicas (CEDEX). (2000)
NLT-327/00, Permeabilidad in situ de pavimentos drenantes mediante
permeámetro. (in Spanish).
), require acceptable
drainage values for pervious concretes in a range between 10 and 30
seconds. These correspond to a LCS coefficient of permeability between
20.89 and 91.85 (10-2 cm/s).
The results of the study
show that C0.30 and C0.35 concretes comply with such requirements. C0.30
permeability values fall into the minimum limit value, whereas C0.35
shows acceptable mid-range values. Therefore, C0.35 results comply with
acceptable technical requirements for pervious concrete wear layers for
pavements. The concretes tested were also rated for vertical
permeability according to European Standard EN 13108-7 (3838.
European Committee for Standardization (CEN). (2013) EN 13108-7,
Bituminous mixtures - Material specifications - Part 7: Porous asphalt
mélanges.
), as shown in Table 7.
Min. vertical permeability [10-3 m/s] | Category - kv | Concrete |
---|---|---|
4.0 | kv4.0 | C0.30 |
3.5 | kv3.5 | |
3.0 | kv3.0 | |
2.5 | kv2.5 | C0.35 |
2.0 | kv2.0 | C0.35 |
1.5 | kv1.5 | C0.40 |
1.0 | kv1.0 | C0.40 |
0.5 | kv0.5 | |
0.1 | kv0.1 | |
n.a. | kvNA |
The results of the study show that all tested concretes comply with EN 13108-7 (3838.
European Committee for Standardization (CEN). (2013) EN 13108-7,
Bituminous mixtures - Material specifications - Part 7: Porous asphalt
mélanges.
) standard regarding application of pervious
concretes. C0.30 is rated as superior class (kv4.0), whereas C0.40 is
rated as low class (kv1.0 - kv1.5) but with considerable vertical
permeability values.
3.4 Compressive strength
⌅ Figure 10 presents the compressive strength results for each concrete tested.
With a range of values between 16 and 22 MPa, results show that the
concretes comply with the technical requirements regarding compressive
strength resistance parameter for pervious pavements (3939.
Centro de Estudios y Experimentación de Obras Públicas (CEDEX). (1992)
NLT - 362/92, Efecto del agua sobre la cohesión de mezclas bituminosas
de granulometría abierta, mediante el ensayo cántabro de perdida por
desgaste. (in Spanish).
).
).
These results show that C0.40 concretes achieved higher compressive strength values. Due to greater workability, C0.40 concrete with 60-second vibration has the highest compressive strength value (22.20 MPa). Regarding the porosity results, values are in a range between 20.0% and 24.5%.
The
values obtained, both for compressive strength and for porosity content
of aged specimens, correlate well with reference values (3939.
Centro de Estudios y Experimentación de Obras Públicas (CEDEX). (1992)
NLT - 362/92, Efecto del agua sobre la cohesión de mezclas bituminosas
de granulometría abierta, mediante el ensayo cántabro de perdida por
desgaste. (in Spanish).
). Higher compressive strength was obtained for the concretes with lower porosity, as expected.
3.5 Abrasion test
⌅Figure 11 presents the abrasion test results and their correlation with compressive strength. The results for lower W/C ratios correspond to higher abrasion concrete.
The concrete with the highest W/C ratio (C0.40) presented the lowest abrasion and the highest compressive strength; it can be seen that an increased W/C ratio produces better concretes with high compressive strength and low loss of mass by abrasion. A linear fit can be proposed for this relationship, despite the reduced correlation coefficient of 0.65 obtained for these nine measurements. These results can be also justified by the lower porosity of the concretes with the highest W/C ratio (Figure 11).
3.6 Porosity modeling using Image J software
⌅The
porosity analysis was performed using the same cubic specimens used in
the porosity tests. Specimens were filled with a very fluid cement to
ensure that all existent pores were filled in. After that, specimens
were divided into four slices of 25 mm each. The porosity of each
specimen is calculated as the average value of all slices (3434.
European Committee for Standardization (CEN). (2000) EN 1097-6, Test
for mechanical and physical properties of aggregate part 6:
Determination of particles density and water absorption.
).
By applying an image processing procedure, black and white images are produced for each slice. Porosity is quantified as a percentage by dividing the white by the total area. Using this process, a digital image showing vertical porosity for each specimen face is achieved. Porosity analysis was carried out in the specimens used in the hydraulic conductivity tests, in which vertical permeability was evaluated.
The first stage is to make images of each slice face, using a known scale (see Figure 12a). The scale will be used by the software to relate pore dimensions and to quantify the percentage. In the second stage, each image is formatted to 8-bit black and white images, allowing the software to distinguish pores from the solid areas (Figure12b and Figure 12c).
Figure 13 presents digital images showing the porosity for a single slice of each concrete, in which the porosity variation due to W/C ratio and vibration time can be observed. Figure 14 presents the correlation between porosity and modeling porosity.
Test results for modeling porosity show values in the range between 23.7% and 35.2%, whereas for the immersion porosity test results, values are between 20.0% and 24.5%. Thus, a trend can be established between porosity and water absorption; however, the values of both are different. The difference is mainly due to the calculation of the total porosity.
The method for modeling porosity calculation includes the entire porosity of the area of the specimen under test, including the closed porosity that does not have access to permeability. This factor contributes somehow to a small dispersion of porosity values in both methods. Another reason that is related to the difference in presented values is with the location of the slice in the sample, which probably affects the evaluation of porosity.
In the global analysis of the porosity study carried out for both methods we concluded that the lower the W/C ratio, the greater the total porosity.
4. CONCLUSIONS
⌅In this study, several different permeable concretes were designed and tested to obtain an optimized concrete for road pavements with simple materials. This is aimed at improving safety regarding wet weather disasters e.g., aquaplaning, friction and sticking by quickly removing rainwater from the surface. From the study carried out in this work, some conclusions can be drawn:
-
The slump test results did not allow any correlation to be established between the concretes studied. A constant value of 0 was obtained for all concretes, matching a typical value for pervious concrete. However, from the Vee bee test results, C0.40 concretes present the best workability.
-
The mechanical characterization tests showed that 0.40 W/C ratio concretes achieved higher compressive strength. Due to its higher workability, C0.40-60 presented the highest compressive strength (22.20 MPa).
-
The results show a trend of continuous permeability increase following the decrease in the W/C ratio. Such behavior is due to the concrete’s rheology properties.
-
LCS permeability shows that the C0.30 and C0.35 mixes comply with the technical permeability specifications for pervious concretes.
-
Results also show that these concretes comply with standard EN 13108-7 for application of pervious concretes. C0.30 is rated as the superior class (kv4.0), whereas C0.35 is rated as the mid-range class (kv2.0-kv2.5), but with considerable vertical permeability.
-
The study showed modeling porosity values ranging between 23.7% and 35.2%, whereas for the porosity, these are between 20.0% and 24.5%.
-
A similar trend was observed for strength. An increase in permeability is matched with a compressive decrease in strength resistance.
-
With values ranging from 16 MPa to 22 MPa, it was proven that all tested concretes comply with the pervious concrete requirements for compressive strength resistance. Results from the Cantabria abrasion test also revealed that lower W/C ratios are matched with low abrasion resistance.
-
From the results presented and discussed here, C0.35-40 concrete complied with the technical requirements for pervious concretes and was identified as the most optimized due to its superior mechanical performance, respecting permeability specifications.