Pervious Concrete – Effect of Material Proportions on Porosity

By
Dhawal Desai

ABSTRACT
This paper describes the effect of size of aggregates and proportions of cement, aggregates, admixture and water on porosity of Pervious concrete which is the main feature of pervious concrete. Different sample blocks were made in lab with variations in mixture to see the porosity for final conclusion

INTRODUCTION
Pervious concrete is a type of concrete with high porosity. It is used for concrete flatwork applications that allow water to pass directly through it, thereby reducing the runoff from a site and allowing groundwater recharge. The high porosity is attained by a highly interconnected void content. Typically pervious concrete has water to cementitious materials ratio (w/cm) of 0.28 to 0.40 with a void content of 18 to 35%.

The mixture is composed of cementitious materials, coarse aggregate and water with little to no fine aggregates. Addition of a small amount of fine aggregate will generally reduce the void content and increase the strength, which may be desirable in certain situations. This material is sensitive to changes in water content, so field adjustment of the fresh mixture is usually necessary. Too much water will cause paste drain down, and too little water can hinder adequate curing of the concrete and lead to surface failure. A properly proportioned mixture gives the mixture a wet-metallic appearance.

Pervious concrete is used in parking areas, areas with light traffic, residential streets, pedestrian walkways, and greenhouses. It is an important application for sustainable construction and is one of the techniques used for ground water recharge.

General Properties:
Void content : 18-35%
Strength : 28-281 kg/cm²
Infiltration rate : 80-720 liter per min per sqm
Cement : 267-415 kg/m³
w/cm ratio : 0.26 – 0.40
Coarse aggregate : 9.5 – 19mm
Little to no fine aggregate (less than 10% of wt. of total aggregate)
Just enough cementitious paste to coat the coarse aggregate

WHY DO WE NEED PERVIOUS CONCRETE?
A larger amount of rainwater ends up falling on impervious surfaces such as parking lots, driveways, sidewalks, and streets rather than soaking into the soil. This creates an imbalance in the natural ecosystem and leads to a host of problems including erosion, floods, ground water level depletion and pollution of rivers, lakes, and coastal waters as rainwater rushing across pavement surfaces picks up everything from oil and grease spills to de-icing salts and chemical fertilizers.

A simple solution to avoid these problems is to stop constructing impervious surfaces that block natural water infiltration into the soil. Rather than building them with conventional concrete or asphalt, we should be switching 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. 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. In many cases, pervious concrete roadways and parking lots can double as water retention structures, reducing or eliminating the need for traditional storm water management systems such as retention ponds and sewer tie-ins.

Pervious concrete also naturally filters water from rainfall or storm and can reduce pollutant loads entering into streams, ponds and rivers. So in this way it helps in ground water recharge.

It also reduces the bad impact of urbanization on trees. A pervious concrete ground surface allows the transfer of water and air to root systems allowing trees to flourish. For a given rainfall intensity, the amount of runoff from a pervious concrete pavement system is controlled by the soil infiltration rate and the water storage capacity available in the pervious concrete and aggregate sub base under the pervious concrete. Generally for a given set of materials, the strength and infiltration rate of pervious concrete are a function of concrete density. Greater the density, higher is the strength and lower the infiltration rate.

EXPERIMENTS:
Different sample blocks were made by using different proportions of cement,
aggregates, admixture and water. In all of the tests I have not used sand at all.

Sample no. 1: Mix design for 10 cubes:
TYPE A – Cement (PPC): 10 kg
Fly Ash (P-63): 0 kg
Coarse aggregate: 52 kg (10 – 40 mm)
Water: 3 kg
Admixture: 1% by weight of [cement + fly ash] = 100 gm

TYPE B – Cement (PPC): 11.25 kg
Fly Ash (P-63): 0.75 kg
Coarse aggregate: 52 kg (10 – 40 mm)
Water: 3.33 kg
Admixture: 1% by weight of [cement + fly ash] = 120 gm

The admixture used was ‘Sika Viscocrete 5001’. This made water release from cement particles. From this mix design we filled 8 cubes each of type A and B since wastage was also there in filling the cubes. The top surface of cubes was closed to prevent fast evaporation of water since it is porous.

The cubes were opened the next day and put in water for proper hydration of cement. These cubes were not perfectly pervious. Its base and sides (up to some height) were flat and smooth. The cement-water slurry settled down and it might have happened due to high quantity of admixture.

3 days cube testing:

TYPE A
S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	6.15	77.1	3.43
2.	6.64	192.4	8.55
3.	7.04	425.4	18.90

TYPE B
S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	6.12	127.1	5.65
2.	6.35	58.2	2.60
3.	7.61	562.9	25.01

7 days cube testing:

TYPE A
S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	6.52	142.7	6.34
2.	6.62	177.2	7.87
3.	7.97	729.5	32.42

TYPE B
S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	6.74	123.9	5.51
2.	7.64	431.2	19.16
3.	7.72	755.6	33.58

14 days cube testing:

TYPE A
S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	6.40	41.6	1.85
2.	6.64	165.2	7.34

TYPE B
S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	7.47	531.1	23.60
2.	7.70	930.2	41.34

According to these results it seems that as the density of cube increases the strength also increases.
Since these cubes were not perfectly pervious, I made another sample of 3 cubes with 3 different proportions of admixture. Mix design used was of TYPE B since its strength was comparatively higher than that of TYPE A.

Sample no. 2: Mix design for 1 cube as per TYPE B:
Cement: 1323 gm
Fly Ash: 88.23 gm
Coarse aggregate: 6117 gm

TYPE B1:
Admixture: 0.2% = 2.82 gm
Water: 510 gm

TYPE B2:
Admixture: 0.3% = 4.23 gm
Water: 480 gm

TYPE B3:
Admixture: 0.4% = 5.64 gm
Water: 460 gm

After opening the cubes on the next day, TYPE B2 (with 0.3% admixture) cube was found to be the most pervious than the rest but it was also not perfectly porous. Although I used low % of admixture then too it was not perfectly pervious. It might have happened due to large size of some aggregates.

3 days cube testing:

TYPE B1
S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	7.02	247.2	10.99

TYPE B3
S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	6.60	191.8	8.52

For the next sample we sieved the aggregates and then used aggregates in the size
range of 10 –20 mm. Also I further reduced the amount of admixture in the next
sample.

Sample no. 3: Sample of 3 cubes:
Cement: 3.75 kg
Fly Ash: 255 gm
Coarse aggregate: 17.31 kg
Admixture: 7 gm
Water: 1.36 kg

But this time also when I opened the cubes I found out that its base was flat and smooth. It was porous from the sides but not from the base.

7 days cube testing:

S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	7.23	179.4	7.97
2.	7.38	234.0	10.40
3.	7.44	230.7	10.25

Now since after so many trials and reducing water & admixture quantity I was not getting perfectly pervious concrete (due to settlement of cement-water slurry), in the next sample (sample no. 4) I neither did compaction nor used vibrator while filling the cube. Fortunately this time the cube made was perfectly pervious. Water was flowing from the base also and its finish was also good. The cube had shining appearance.

Sample no. 4: Mix design for 1 cube: (w/o compaction)
Cement: 1 kg
Fly Ash: 0 kg
Coarse aggregate: 5.2 kg
Admixture: 2.33 gm
Water: 380 gm

Calculation for % voids:
Total volume of the moulds: (volume of 15x15x15 cm3 mould) + (volume of 7x7x7 cm³ mould)
3.375 lit + 0.343 lit = 3.718 lit
Volume of the mix: (volume of cement) + (volume of coarse aggregate)
Volume of cement: 0.3175 lit
Volume of coarse aggregate: 3.398 lit
So, volume of the mix: 3.715 lit
Since wastage was also there while filling the cubes and if we assume 10% wastage, the % voids came out to be 10.07%

3 days cube testing:

S.no	Weight of cube (Kg)	Load (kN)	Strength (MPa)
1.	5.75	42.4	1.88

Sample no. 5: mix design for 2 cubes. (w/o compaction)
Cement (PPC): 2 kg
Fly Ash: 0 kg
Coarse aggregate: 10.4 kg
Admixture: 4.66 gm
Water: 760 gm

So in all, sample number 4 and 5 were the successful ones. These were made with low w/cm ratio and w/o compaction.

CONCLUSION
The Pervious concrete allows water to pass through it. It is not composed of fine aggregates since they will fill the voids between the coarse ones. The samples in which aggregates above 20 mm were used were not porous from the base because of larger voids the cement slurry settled down. Also in all those cubes in which compaction was done the cement slurry settled down and thus made a flat bottom surface. So finally the conclusion is to use aggregates in the range 10 – 19 mm and not to compact it while filling. Also the density of this concrete is less than the normal one because fine aggregates were not used. Its strength is lower than the normal concrete.

ACKOWLEDGEMENTS
I, Dhawal Desai, 2nd year undergraduate student of Civil Engineering department, Indian Institute of Technology Bombay, gratefully acknowledge the support and lab facility provided by M/s Ambuja Cements Ltd. Mumbai to carryout this project work on Pervious Concrete during my internship in Dec.2011 winter vacation.

REFERENCES
1) Ambuja Knowledge Centre Library. Ambuja Cements Ltd
2) Handbook for Pervious concrete certification in greater Kansas city by CPG
3) Karthik H. Obla. Pervious concrete – An overview (2010)
4) Pervious concrete mix proportioning by Grace construction products
5) William Gunter Goede. Investigation into structural performance and evaluation of the applicability of existing thickness design methods (2009)

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