MIX-DESIGN TIPS FOR PERVIOUS CONCRETE

Typically, concrete is supposed to be difficult for water to penetrate, despite its porosity. Still, there is a type of concrete that has been developed exactly for the opposite reason; to literally allow water to go through like a screen. This concrete is called pervious and in my opinion is one of the most brilliant ideas for managing storm water in an urban environment.

Almost all cities use concrete as their basic construction material because it’s strong and durable. They also use asphalt tarmac for paving the streets, to stabilize soil and allow easy mobility of vehicles. But the combination of these materials renders urban environment almost impervious to rainwater, one of the biggest problems big cities face today, especially due to extreme storms that are becoming more frequent and intense, amplified by climate change. Pervious concrete can be an effective and smart solution for this problem, as it could replace existing pavements and tarmac, making water drainage more efficient and sustainable.

So, is pervious concrete mix-design straightforward, just another concrete mix? I would say not. There are many details involved in the mix itself, but a lot more depends on the preparation of subbase, compaction method and curing, so I will try to highlight as much as I can.

1.????? Aggregates: in order to achieve the required porosity, usually there is very limited use or complete omission of sand in the mix. Also, only one size of gravel is used, either 10-20 mm for the standard, coarse version, or 5-10 mm for the fine version.

2.????? Cement: cement content is usually low, about 200 kg for the coarse mix, even lower for the fine mix, as the target is to bind the gravel grains together without closing the pores. Generally, the target is to generate a thick, sticky cement paste that will cover each single grain, joining them together, leaving the voids between them intact.

3.????? Water: The mix has a very low W/C ratio, at about 0.3, which needs to be defined with a lot of lab testing. The margin of error is very narrow, so it’s best to go initially for a little drier mix and add a few liters at the jobsite. The target is to achieve the thick cement paste attaching firmly on each grain, looking a little shiny with humidity. A loose, watery cement paste is to be avoided at all cost.

4.????? Superplasticizer: Although it looks controversial, use of a significant superplasticizer dosage, typically close to 1%, helps create a stickier mix, easier to apply and compact. The optimum combination between water and superplasticizer must be determined with lab and field trials.

5.????? Workability: the mix is expected to behave like roller compacted concrete, so there is no slump to measure, ease of compaction is the target.

6.????? Subbase: the role of well compacted subbase, made of medium to large size stones (10-30 cm) is essential for the good performance of pervious concrete. Placement of geotextile between subbase and pervious concrete helps stabilize both layers. The subbase layer performs as temporary storage tank for the gradual dissipation of rain water to the ground. Alternatively, pervious concrete can be placed on top of an underwater tank or perforated pipelines that can store or distribute rainwater.

7.????? Delivery to jobsite: due to the nature of pervious concrete and the relatively fast setting of the mix in the truck, it is advised to limit the load size below 5 m3 and keep application time below 2 hours. Small amounts of water can be added to the truck from time to time, to reduce the chance of early setting and good, constant mixing is required until all quantity of the mix is emptied from the truck.

8.????? Compaction: since pervious concrete behaves like RCC (roller compacted concrete), placement requires tools suitable for such application and the most common tool is of course a manually operated roller. Total thickness of pervious layer ranges from 15 to 30 cm, so it’s better to apply and compact it gradually, in 2-3 slimmer intermediate layers, to optimize compaction and achieve better binding of grains. Attention must be paid to the consistency of the mix, to avoid unnecessary clogging of pores through loose cement paste, or dry grains that don’t bind together.

9.????? Curing: it is essential for the successful binding and later performance of pervious concrete. Since water easily goes through the concrete layer, to achieve the required curing, it is important to cover the pervious layer with plastic sheet, to minimize water evaporation and spray water under the sheet as frequently as possible, until the concrete layer has fully set. It is advisable to continue curing after removing the sheet for 3-7 days, preferably by using a sprinkler.