A lot of cracked patios and heaving retaining walls take the blame elsewhere—bad weather, bad pavers, maybe even an inattentive installer—when in reality the real villain was buried long ago in a pile of untested sand. Fine aggregate is the Cinderella of the concrete world, an ingredient that rarely grabs the spotlight, but dictates the long term durability of nearly any hardscape project.
Sand and the science of concrete durability
Sand in a concrete mix serves to fill the voids of the coarse aggregate, and to a lesser extent, the voids in the cement paste. It accomplishes the former task exceptionally well when it is well graded and flows up against the largest particles to fill the gaps between them. In fact, sand achieves a critical function by adhering to these coarse particles in the cement paste, which allows the cured concrete to resist water penetration and therefore improve its durability. Poorly graded sand, on the other hand, causes the cement paste to fill the gaps between sand particles, requiring extra cement and water to achieve a workable mix. The resulting concrete is weaker and more porous than it needs to be, and while it may serve its purpose as a slab, it will likely be compromised long term.
Standards such as ASTM C33 establish aggregate grading requirements, and the reason these specs are so important is because particle distribution has a direct effect on workability, and indirectly, a large number of other properties. Fineness modulus is the metric that quantifies the gradation of a sand, and it helps determine the water demand of a mix more accurately than most other factors. Fineness has a major effect on both the workability and mechanical properties of concrete, as overly coarse sand will cause a harsh mix that tends to bleed and segregate, while overly fine sand will increase water demand and weaken the mix by diluting the cement paste. Both issues drastically reduce durability by either creating larger pores in the concrete matrix (in the case of coarse sand) or reducing the strength of the cement paste itself (in the case of fine sand). A shift in fineness is usually quite apparent in a sieve analysis, and changes to the gradation have serious implications on the performance of a concrete mix. A change in fineness modulus of just 1 or 2 points can reduce compressive strength and increase permeability significantly.
Once the concrete is cured, it’s too late to adjust things. Once it’s cured, it’s done. Getting a consistent, smooth gradation that fills the gaps in the coarse aggregate without creating pockets of air is one of those rare opportunities in concrete design where you get to spend more money without sacrificing durability. It reduces the amount of cement paste needed to make the mix workable, which in turn reduces permeability at a given water-cement ratio.
Interface transition zone: the weakest link
Most cracks that form in hardened concrete originate at the interface transition zone, the line between aggregate and cement paste. This weakness is unavoidable, but it doesn’t have to be as severe as it is. Interface transition zones that formed around clean sand are still weaker than the cement paste that surrounds them, but they’re much stronger than those formed around dirty sand.
The fines that often come with sand reduce both the cohesiveness and adhesion of the ITZ, while also increasing the water demand of the mix. This is because silt and clay act as surfactants, both preventing optimal cohesion between the cement paste and aggregate, and holding water in solution longer than the cement alone would. In short, dirty sand produces ITZ’s that are weaker and more porous than those formed around clean sand. The effects of this are detrimental to the long term durability, as weak zones in the concrete matrix invite cracking under stress. Testing for silt content is one of the cheapest and most overlooked jobs on a construction site, and a seemingly clean sand can have an uncharacteristically high amount of fines that bring down the whole mix.
Reactivity and porosity in aggregates
Some sands are outright dangerous. Siliceous aggregates that contain reactive silica can cause alkali-silica reaction in concrete, where the alkalis in the cement solution react with the silica in the aggregate to form a gel like substance that swells up inside the concrete. This leads to cracking in slabs, footings, and other concrete structures months or even years later, and there’s little that can be done to remedy it once it starts. Petrographic testing on sand from an unfamiliar sand supplier is often more important than most people realize. On the other hand, reactive aggregates can usually be mitigated by replacing some of the binder in the cement mix with supplementary cementitious materials, which lowers the amount of free alkalis in solution.
Porosity in fine aggregate can also lead to problems, particularly in areas of the country where freezing weather is common. Water that gets drawn into the pores of the sand will expand upon freezing, putting the aggregate itself under tension and the surrounding cement paste at risk of cracking. This puts a stress on the pavement around the aggregate as well, eventually leading to scaling on the surface of the concrete. In areas where freezing weather is common, a patio or retaining wall made with marginally clean sand could very well be subjected to several cycles of freezing and thawing, and the damage may become visibly apparent after only a couple of years.
Efflorescence, the white film that sometimes appears on pavers and blocks, is rarely a problem for the paver itself. It usually means that salts and clay fines have made their way up to the surface of the paver and crystallized there, but this is frequently the result of the sand bed underneath the block. Salts and fines make their way up the joints in the paver, and can appear on the surface of the block or on the ground itself in the form of a whitish powdery residue. The paver is merely the medium through which the salts have travelled, and the real source is the sand bed beneath the paver. This is the sort of information that comes in handy to a contractor dealing with a warranty claim, since a second round of sealing is unlikely to resolve the issue.
Why landscape architects fail from the ground up
Big-ticket hardscape projects are often concerned with drainage patterns, sight lines, and the quality and variety of materials used on the job. However, most settlement issues and cracked pavers stem from the installation of the sub-base, underneath the paver surface. Compacting a proper sub-base of well graded aggregate is the key to ensuring that the paver will remain stable for years to come, and skipping this step or using the wrong materials will compromise the paver in short order.
Drainage is another concern that ties in with the sub-base, as a proper drainage solution using aggregate is required to direct water away from the sub-base and the paver itself. Water that accumulates around the sub-base can cause serious heaving issues, particularly in the winter when the frozen ground expands around the paver and lifts it at an angle. Percolation systems and French drains often have the same issue, except that water accumulation causes the aggregate around the drain to freeze in place, hold the surrounding ground in place, and prevent it from draining properly. If a sub-base is installed improperly or with the wrong materials, then there are few opportunities later on to correct the issue.
This is also one of the few areas where the choice of the contractor has the most impact on the finished project. Asking the contractor to specify a reliable source of sand is a good start, but verifying that they do in fact have the required documentation for every load delivered is an even better one. This is one of those few opportunities where you get to pay extra money and get something that will prolong the lifespan of the hardscape, as opposed to a higher end paver that fails within a couple of years of installation.
Sand is not a generic material
There are few decisions on a construction job as expensive as assuming that sand is sand. ASTM C33 concrete sand is a specific product that has been graded and washed to meet certain specifications, while ASTM C144 paver sand has been manufactured to hold pavers together without letting them slide apart. Clean drainage sand has been sorted to carry water away with minimal resistance. Using the wrong type of sand in your paver joints will doom them within a few seasons at best, and completely ruin them in the short term.
What you should ask for when buying sand
A few basic requirements can keep good sand from being replaced by bad sand:
A current sieve analysis on all loads as opposed to a representative one
A silt content test that shows that the sand is within acceptable limits
Source consistency between loads, as silt and gradation can change between stockpiles at the same quarry
Test reports that verify that the sand meets the relevant ASTM standards for the application (structural, paver, or drainage)
Sieve analysis is a requirement wherever sand is used in construction, and the same goes for silt content testing. These are both fairly simple and cheap tests that are usually skipped, presumably because sand looks like sand. This is exactly the kind of attitude that leads to pavers delaminating, retaining walls heaving, and slabs cracking after only a few years of service.
Good landscape architects design what people see, and what most people want to see. But the decisions they make below ground level dictate what the finished product will be able to withstand, and how long it will be able to withstand it.
