Mechanical Properties of Pervious Concrete

 

Siddhant M. Chopda, Bhavesh M. Chhattani

Under graduate Student, School of Mechanical and Building Sciences, Vellore Institute of Technology, Vellore, India

 

ABSTRACT:

In recent times, major cities around the world have experienced frequent flooding due to the combination of increased rainfall and reduced permeable surface areas. As a consequence, the drainage system gets overloaded and flash flooding becomes inevitable, thus causing disruption to the road transport. In order to overcome such issues, an engineered solution is needed. The solution is "Pervious Concrete", a special type of concrete having a high void content which ranges from 15% to 35% and is instrumental in recharging groundwater and reducing storm water run-off. Also, considering the environmental effects of Portland cement, it is essential in the concrete industry to minimize the use of cement in the production of both structural and non-structural concrete. One way to reduce cement consumption is to use fly ash as a cement replacement material because of the pozzolanic reactivity of fly ash. This paper reports the results of an experimental investigation into the various mechanical properties of pervious concrete. The various mixes containing cement with varying percentages of fly ash as binding material were used. Although it may seem tough to balance the demand for sustainable development with the need to preserve our natural resources, but with the results obtained so far, this balance can be easily achieved.

 

INTRODUCTION:

Concrete is the most commonly used building material in the world. It is estimated that the present consumption of concrete in the world is of the order of 5.5 billion tonnes every year. Humans consume no material except water in such tremendous quantities.

 

Concrete is a multifaceted material that can easily be mixed to meet a variety of special needs and be cast easily and economically. It is a durable, fire resistant and energy efficient material.

Concrete finds applications in foundations and slabs-on-ground, walls, beams, columns, floors, roofs, bridges, dams, swimming pools, homes, streets, patios, basements, balustrades, plain cement tiles, mosaic tiles, pavement blocks, kerbs, lamp-posts, drain covers, benches pavements and other infrastructure.

But, impervious nature of normal weight concrete used for pavement construction increases the water runoff and contributes to over-burdening to the existing drainage system. Also when it rains, large amount of water ends up falling on impervious surfaces such as parking lots, driveways, sidewalks, and streets rather than seeping into the soil. This creates an imbalance in the delicate natural ecosystem and leads to a host of problems namely erosion, water logging, floods, ground water depletion and pollution of rivers, lakes and coastal water. A simple engineered solution to avoid these problems is to switch to pervious concrete or porous pavement, a material that offers the inherent durability and low life-cycle costs of a typical concrete pavement while retaining storm water runoff and replenishing local watershed systems ^[1] . Instead of preventing infiltration of water into the soil, pervious pavement assists the process by capturing rainwater in a network of voids and allowing it to percolate into the underlying soil.

 Pervious concrete is a special type of concrete having high water permeability due to the presence of interconnected pores, ranging from 2 to 8 mm ^[2]. Pervious concrete also naturally filters water from rainfall or storm and thereby, reduces pollutant loads entering into streams, ponds and rivers. In this way, it helps in ground water recharge. Pervious concrete has very high permeability compared to standard concrete and the former drains water quickly. Pervious concrete is also lightweight concrete having weight 1600 to 2000 kg/m^{3 [3]}. Pervious concrete should have a porosity of 15% to 35% but most frequently about 20% ^[4].

In addition, carbon dioxide emission from Portland cement production is significant. It is known that one tonne of Portland cement production releases 0.94 tonne of CO₂  into the atmosphere. This contributes to global warming which leads to undesirable climate change. Therefore, it is essential to minimize the use of Portland cement in pervious concrete mixes by partially replacing the cement with industrial by-product, such as fly ash and enable mankind to take a step towards sustainable development. For instance, In Australia, 9 billion tonnes of cement was produced in 2006. But due to the use of alternative materials like fly ash and slag, the CO₂ emissions reduced to 0.72 tonne per tonne of cement produced ^[5] .

The effects of having varying fine aggregate content, water cement ratio and partial cement replacement with fly ash on the various characteristics of pervious concrete are studied. 

 

AIM AND OBJECTIVE:

The main focus of presented paper was to find the optimum mix design of the pervious concrete mixes and test the specimens for various properties of the pervious concrete on different mixes with varying fly ash content after obtaining the optimum desired mix design for pervious concrete. Mechanical properties were examined. Compressive Strength Test and Rapid Chloride Permeability Test were conducted for optimization of the mix design.

Using different proportion of fly ash as substitute for cement may change concrete’s mechanical properties which may improve some characteristics and behavior and bring about new types of applications which has not been done till now.

 

MATERIALS AND METHODOLOGY:

Materials

53 Grade Ordinary Portland Cement (OPC) as per clause (IS12269 1987) and low calcium fly ash (IS3812-1 2003) were used as binder materials in the concrete mixes. Coarse aggregate (10 mm to 20 mm) Crushed River Gravel (specific gravity of 2.70) was used as coarse aggregate (IS383 1970). Fine aggregate (specific gravity of 2.60) as per clause (IS383 1970) (River Sand) of Zone II was used. The water to be used for casting should be free from organic matter. Potable water is generally considered satisfactory as per clause no. (5.4 of IS456-2000). Tap water available in the laboratory was used for mixing the ingredients of concrete and curing of the specimens. No chemical admixture was used in the concrete mixes.

 

Mixture Proportions

Mix proportion by weight for pervious concrete throughout the work carried out was 1 : 4.1^[6]  (Binder material, Coarse aggregate), respectively. The overall aggregate to binder ratio for concrete mix was 4.4 to 4.7, by weight. Water to Binder ratio is important in the production of pervious concrete since it has an influence of the workability of pervious concrete. Various trial mixes were made with varying W/C ratios (0.30, 0.33, 0.36) and each W/C ratio was used for varying fine aggregate (0.3, 0.45, 0.6). Fine aggregate was deliberately reduced in proportion in the mixes to create large open textured porous concrete.

After coming to a conclusion about the optimum mix design, Cement was replaced partially with fly ash by 10%, 20% and 30%, by weight and various results were investigated.

 

Table 1 : Mix proportions for pervious concrete by weight

W-Water; C.A.-Coarse Aggregate; F.A.-Fine Aggregate

 

Methodology

Casting

Fresh pervious concrete mixes were produced in a pan-type of mixer. For each concrete mix, a  number of 100mm length by 100mm breadth by 100mm high cubes and 100mm by 200mm cylindrical specimens were cast ^[7].

 

Curing

All the test specimens were demoulded after 24 hours and stored in water at 20⁰ C until the ages of testing ^[7] .

 

Testing Of Concrete

Compressive Strength, Porosity and Permeability of hardened concrete was determined using the cube and cylindrical specimens.

 

Compressive Strength ^[8]: Compressive strength test was performed according to (IS:516-1959). For the pervious Concrete, nine cube specimens of varying mix design were used. The specimens were cured in water (20°C) until the testing. The compressive strength of each and every cube was reported for pervious concrete mixes without fly ash after 7 and 28 days. After finding the optimum pervious concrete mix design, the compressive strength of each and every cube was reported for pervious concrete mixes with fly ash after 7 and 28 days.

 

Porosity (ASTM C1754/C1754M-12): The void content for hardened concrete was determined using oven-dry and saturated weights , using the following equation ^[9]:

V_r = 100 * [1 – {(W₂ – W₁) / (p_w * V)}]

Where:

W₂   - Oven dry weight (g)

W₁  -  Weight under water for 24 hours (g)

 V_r  -  Porosity (%)

 p_w  -  Density of water (10^-3 g/m³)

 V   -  Volume of the sample (m³)

 

Permeability (ASTM C 1202 -12): The specimen's permeability was evaluated on the basis of the Rapid Chloride Permeability Test set up ^[10].        

 

RESULT AND DISCUSSION:

Effect of Shape on Strength of Test Specimens

Compressive strength of concrete can be determined by using either cubes or cylinders. In this study, both cubes (100 mm) and cylinders (100 mm diameter by 200 mm high) were used. Cubes were easy to handle and 100 mm cubes were appropriate. Further research is needed to establish the appropriate test specimen size and shape for the evaluation of compressive strength for pervious concrete. Both cube and cylinder strength results were seen to be nearly the same. So, it became evident that the compressive strength is not significantly affected by the specimen size. This implies that 100 mm cubes could be used for strength testing and capping of the specimens may be needed, considering the uneven surface profile even of the cast surface.

 

Table 2 Compressive Strength of Cubes and Cylinder Specimens for 7 and 28 days

   

Fig. 1 Comparison between Compressive Strength                               Fig. 2 Comparison between Compressive Strength

          of  Cubes and Cylinder Specimens for 7 days                             of Cubes and Cylinder Specimens for 28 days

 

Porosity of Pervious Concrete Cube Specimens

The porosities of  pervious concrete mixes are shown in Table 3. The porosity of the conventional concrete decreases with the increase of age, due to continuous hydration of the cement. In contrast, the porosity of the pervious concrete was found to be  independent on the age of concrete. However, the porosity in pervious concrete is mainly due to large size air voids which are bigger than the pores in cement paste. The porosity of pervious concrete is influenced by aggregate grading and compaction. Hence, the porosity of pervious concrete is not noticeably changed with an increase in the age of concrete.

 

Table 3 Porosity of pervious concrete mixes                                                       Table 4 Permeability Of Pervious Concrete Specimens

                                                                                                               

Mix	Vr (%) (7 days)	Vr (%) (28 days)
1	30.98	31.12
2	21.25	23.1
3	12.92	13.01
4	29.2	30.1
5	16.87	17.09
6	8.28	8.31
7	30.98	31.23
8	14.69	15.12
9	3.507	3.78

Mix	K (56 days)	Charge Passing (Coulombs)
1	HIGH	4566
2	MODERATE	3980
3	MODERATE	3611
4	HIGH	4983
5	MODERATE	3720
6	LOW	1500
7	HIGH	4683
8	MODERATE	3774
9	LOW	1374

Fig. 3 Relationship between Porosity and Compressive Strength of Cube Specimen

 

Fig. 3 Relationship between Porosity and Strength

 

Optimization of Pervious Concrete Mix Design without Fly ash

Usually, for pavement design using concrete a range of 3.5MPa to 28MPa strength, especially 17MPa is expected. Also, the porosity must vary between 15% to 35%. The permeability can be High or Moderate as per RCPT test. After satisfying all the component values, we finally render 1: 4.1: 0.3 : 0.36 (Binder material, Coarse Aggregate, Fine Aggregate, Water) respectively as an optimum Pervious Concrete Design.

 

Table 5 Compressive Strength of                                                          Table 6 Porosity of Pervious Concrete with Fly ash

Cement	Fly ash	Strength
%	%	MPa
		7 Days	28 Days
100	0	11.2	17.4
90	10	11.6	17.6
80	20	12	18.4
70	30	12.6	19.6

Pervious Concrete with Fly ash

Cement	Fly ash	Porosity
%	%	%
		7 Days	28 Days
100	0	12.92	13.01
90	10	21.12	12.68
80	20	20.56	11.55
70	30	19.71	9.85

     

CONCLUSION:

Pervious concrete is produced with conventional concrete making materials with no-fines and fine aggregate contents of 6.5% and 12.5% of the total aggregate content. Based on the preliminary trial mixes for the balling ability the water to binder ratio of 0.36 by weight was found to be the ideal to produce workable pervious concrete. In order to reduce the cement content in pervious concrete, 10%, 20%, 30% of the cement was replaced with fly ash.  The results showed that the no-fines pervious concrete had the void content of about 15% compared to the negligible void content value for the conventional concrete. 30% cement replacement with fly ash increased the 28-day compressive strength of pervious concrete by about 12.5 %.

 

After successfully executing the experiment and obtaining the output, the results were critically analyzed which brings us the following conclusions:

Based on the experimental investigation on the pervious concrete with Fly ash, the following conclusions could be made:

·        Compressive strength of pervious concrete depends primarily on the porosity of concrete. Compressive strength of pervious concrete varies inversely with the porosity of concrete as observed in the Fig. 4.4.

·        Test specimen's shape had marginal influence on the strength of pervious concrete for a given porosity.

·        For a specific porosity, naturally obtained concrete aggregate increases the compressive strength of pervious concrete as age increases.

·        Permeability of pervious concrete is influenced by the porosity and the use of Fly Ash had no significant effect on the permeability of pervious concrete, whereas the Compressive Strength increased a bit with 30% replacement of cement with Fly ash.

 

REFERENCES:

1.      Meininger, R.C., “No-Fines Pervious Concrete for Paving,” Concrete International, Vol. 10, Issue 8, Aug 1998 pp. 20-27.

2.      ACI Committee 522. (2006). Pervious concrete. Report No. 522R-06, American Concrete Institute. Detroit, USA, 2006, 25p.

3.      Ajamu S.O., Jimoh A.A. “Evaluation of structural Performance of  Previous Concrete in Construction” , International Journal of Engineering and Technology Volume 2 No. 5,  May, 2012

4.      NRMCA, "What, Why, and How? Pervious Concrete," Concrete in Practice Series. CIP 38, 2004.

5.      Australian Cement Industry Sustainable Report, Cement Industry Federation, 2007, 32p

6.      ACI 211.3R, Guide for Selecting Proportions for No-Slump Concrete

7.      Sri Ravindrarajah R. and Aoki Y., “Environmentally friendly porous concrete”, Proceedings of the Second International Conference on Advances in Concrete and Construction, Hyderabad, India, Feb 2008

8.      IS: 516–1956, Indian Standard Methods of Tests for Strength of Concrete (1999)

9.      Park and Tia, 2004. “An experimental study on the water-purification properties of porous concrete,” Cement and Concrete Research, (Vol. 34), pp.177-184.

10.    “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration,” ASTM C 1202-97, Annual Book      of ASTM Standards, Vol. 04.02, pp. 639–644.

 

 

 

 

Received on 14.11.2015            Accepted on 21.12.2015            © EnggResearch.net All Right Reserved Int. J. Tech. 5(2): July-Dec., 2015; Page 113-117 DOI: 10.5958/2231-3915.2015.00006.1