pervious concrete

[Tom Servo]

what can you concrete experts tell me about it? what's its purpose? advantages? etc...

[scalziand]

Well, for uses, it's intended as pavement for reducing rain runoff. Its pervious nature make hydroplaning nearly impossible as standing water is drastically reduced. However, since it is very porous it has low durability and is limited to low traffic areas like shoulders and driveways-maybe parking lots also.

[dchan]

Regular concrete is composed of the following materials: cement, water, fine aggregates, and coarse aggregates. There are also several other ingredients added for various advantages, such as admixtures and pozzolans, but let's concentrate on the big picture here. Basically, pervious concrete doesn't have any fine aggregates (ie. sand), leaving only the coarse aggregates, water, and cement. Thus, the resulting pores are large enough to allow water to run through unimpeded.

Its main use and advantage is to provide proper drainage. Normally, lawns provide this function, but there are obviously many places require a hard surface and structure. As scalziand references, parking lots are one such place. Thus, pervious concrete provides the strength and ability to drain water. Its compressive strength is actually comparable to normal concrete, but its durability is obviously lower, given its more porous composition and larger overall surface area.

From what I've been taught, one of its key advantages is in areas that use underground aquifers as their main source of drinking water. In places that are heavily developed, normal concrete and asphalt surfaces sends rainwater runoff into nearby streams and rivers, preventing the local aquifers from recharging. By building parking lots and sidewalks with pervious concrete, much of the rainwater flows back into the ground and helps recharge the aquifer.

[Robert Pence]

How does the freeze-thaw cycle in northern climates affect it? Can it impound water under certain conditions and then spall and crack when temperatures drop below freezing?

Also I'm curious about the effects of heavy salt use on sidewalks and parking lots; sealer can mitigate salt's erosive effect on conventional concrete surfaces, but what's to prevent its incursion deep beneath the surface of pervious concrete, were it could compromise structural integrity.

[dchan]

Originally Posted by Robert Pence

How does the freeze-thaw cycle in northern climates affect it? Can it impound water under certain conditions and then spall and crack when temperatures drop below freezing?

Also I'm curious about the effects of heavy salt use on sidewalks and parking lots; sealer can mitigate salt's erosive effect on conventional concrete surfaces, but what's to prevent its incursion deep beneath the surface of pervious concrete, were it could compromise structural integrity.

Freeze-thaw cycles can affect heavily saturated pervious concrete, since the water remains essentially stuck inside the concrete until it drains away. Saturated conditions can occur if the pores are clogged with debris - the trick, apparently, is to flush out and/or vacuum the debris every now and then in order to prevent it from accumulating.

As for salt - if it dissolves, it will probably drain away with the other water and not stay confined within the concrete. Salt damages when it stays confined in one spot, which shouldn't happen in pervious concrete. Sealers are required for normal concrete surfaces because all concrete has small pores within, usually irregularly spaced and partially isolated from one another. These pores allow for the salt water to be confined within, allowing enough time for the salt to corrode the concrete.

In addition, there should be a reduction of salt usage, anyway. When the snow melts, it will simply flow through the porous concrete instead of pooling on the surface and forming ice.

One important design consideration is the sub-base soils underlying pervious concrete - it needs to be pervious as well. Pervious concrete works best with gravels and larger-grained soil layers. On the other hand, they work poorly with impermeable soils that have high clay and silt contents, since the water simply becomes confined on top of the impermeable sub-base soil.

[dchan]

Originally Posted by scalziand

Well, for uses, it's intended as pavement for reducing rain runoff. Its pervious nature make hydroplaning nearly impossible as standing water is drastically reduced. However, since it is very porous it has low durability and is limited to low traffic areas like shoulders and driveways-maybe parking lots also.

To be fair, those are also about the only places you would use pervious concrete for anyway, regardless of strength and durability. You certainly wouldn't use them as a structural material or facades - you'd want to keep water out of buildings and other structures, not let it in.