What Class 5 Aggregate Actually Is

Class 5 is a specified gravel base material, a blend of crushed rock and sand sized so it compacts into a dense, stable, well-draining layer. The word that matters is crushed. Crushed aggregate has angular, sharp-edged stone faces. When you compact angular stone, the faces lock against each other and the material stays put under load. Rounded gravel, the kind that looks like river rock, does the opposite. Round stones roll past each other and never truly lock, so a base built on rounded aggregate keeps shifting under the slab no matter how hard you pack it. The gradation matters too. Good Class 5 has a mix of stone sizes down to fine material, so the small pieces fill the voids between the large ones and the whole layer compacts tight with very little air left in it. That density is what makes it stable and what makes it drain.

How Deep the Base Needs to Be

Base depth depends on what the slab has to carry. For a residential driveway, four inches of compacted Class 5 over solid subgrade is a reasonable minimum, and we often go deeper when the native soil is soft or holds water. For a slab that takes heavier or repeated loads, like a commercial apron, a shop floor, or anywhere trucks turn and park, six inches or more is the right call. Garage floors and patios usually land in the four-inch range. The number is not arbitrary. A thicker base spreads load over a wider footprint of subgrade and gives frost more room to act without putting that movement directly into the slab. Skimping on base depth to save a few yards of gravel is one of the most common ways a slab gets set up to fail, and it is invisible the day it is poured.

Frost Depth in Minnesota Changes the Math

Frost in this part of Minnesota drives down several feet in a hard winter. When water in the soil freezes, it expands and heaves whatever is above it. When it thaws, that support drops back out. A slab sitting on soil that heaves and settles every year is a slab that flexes every year, and concrete that flexes enough, often enough, cracks. A well-built, well-drained Class 5 base fights frost two ways. First, it is free-draining, so it does not hold the water that freezes and heaves. Second, its depth and stability spread seasonal movement out instead of concentrating it. The base will not stop frost, nothing on a slab on grade does, but it keeps frost movement from translating straight into the slab. This is why base prep in Olmsted County is not the same conversation as base prep in a climate that never freezes. The base here is doing freeze-thaw defense work, not just providing a flat surface to pour on.

What Happens When Someone Pours on Topsoil or Sand

The fastest way to a cracked slab is to skip the Class 5 and pour on whatever is already there. Topsoil is the worst case. It is full of organic material that breaks down and compresses over time, and it holds water like a sponge, so it heaves hard in frost. A slab on topsoil is a slab on a surface that is actively settling and swelling under it from day one. Plain sand is a more common shortcut and a quieter problem. Sand drains, which sounds good, but uniform sand does not lock together the way graded crushed aggregate does, so it migrates and shifts under load and washes out at the edges where water runs through. We have been called to slabs that cracked and dropped within a few years where the homeowner was told the base was handled, and the base turned out to be a couple inches of sand over native clay. The concrete took the blame. The base was the failure.

How to Tell If a Base Is Actually Compacted

Compaction is not optional and it is easy to check before the pour. The right tool is a plate compactor, and the right method is compacting in lifts, meaning the base goes down in layers of a few inches each, with each layer compacted before the next is added. You cannot dump eight inches of gravel and pack the top and call it compacted, because the bottom stays loose. When a base is properly compacted, the plate compactor stops sinking in and starts to bounce slightly as it rides across the surface. That bounce is the signal that the material has reached density and will not pack down further. You can feel it underfoot too. A finished base should feel like walking on pavement, firm with no give, no soft spots, no footprints pressing in. If you can scuff a heel into it or it shifts under your weight, it is not ready and no concrete should be going over it yet.

Vapor Barriers: When You Need One and When You Do Not

A vapor barrier is a sheet of poly laid over the base, under interior slabs, to stop ground moisture from wicking up through the concrete. For any slab that will have a finished floor over it, a heated space above it, or anything moisture-sensitive nearby, the barrier matters. Without it, water vapor moves up through the slab continuously and can ruin floor coverings, feed mold, and keep a space damp. For an interior garage or basement slab, we run the barrier. For most exterior slabs, a vapor barrier is not needed and can actually work against you by trapping bleed water under the slab during curing, which affects the finish. The rule we follow is simple: interior and conditioned, use a barrier; exterior and open, usually skip it. It is a real decision per slab, not a default we stamp on everything.

Drainage Grade: The Failure That Looks Like a Concrete Problem

The last piece of base prep is grade, and it is the one homeowners notice last and pay for first. The base, and the slab on top of it, has to be graded to send water away from structures and off the slab. When grade is wrong and water pools against a foundation or sits on a slab, that standing water finds its way into the base and into the concrete surface. In Minnesota, standing water plus freeze-thaw is the combination that scales surfaces, undermines bases, and heaves slabs. A slab that was poured flat or, worse, sloped back toward the house, is going to channel meltwater exactly where you do not want it every spring. We grade for positive drainage as part of base prep because a perfect slab on a base that holds water is still going to fail. Drainage is not a finishing touch. It is built into the dirt before any concrete is ordered.