Washing off the paste, not adding anything on top
Every batch of ready mix carries stone as part of the mix itself. Ordinarily that stone stays buried under a layer of cement paste and fine sand, the part a float or a broom works on. Exposed aggregate skips smoothing that top layer away and instead washes it off, on a timed window while the slab is green, meaning set enough to hold together but soft enough that the cream rinses free.
The stone that shows afterward is whatever was already in that particular load. It is not a topping applied afterward, and it is not a separate material trucked in for the look.
The size and look of the stone that shows through depends entirely on what the ready mix truck delivered that day, since nothing is sorted or added afterward to change the effect.
Exposed aggregate against a plain broom finish
Broom finish is one pass and it is done: drag a broom across the floated surface and the slab is finished. Exposed aggregate adds a real extra step, and a finicky one. Wash too early and the stone comes loose with the paste. Wash too late and nothing exposes at all.
What that extra step buys is more visual depth and texture than broom lines give you, and dirt and everyday wear tend to disappear into that texture rather than show on top of it.
Exposed aggregate against stamped concrete
Stamped work carries color and a pressed pattern in the top layer, and that top layer is what wears, so a stamped slab needs sealer on a recurring schedule to keep the color looking right. Exposed aggregate has no surface color layer to protect. The look comes from the stone itself, which does not fade or wear off the way a pigmented top coat does.
That is most of why exposed aggregate lands between broom and stamped rather than beside either one. It costs more than broom because of the extra finishing step, and it asks less of you afterward than stamped does, because there is no color riding on the surface.
Where it does the most good
Patios, walkways, and pool deck margins are where we see exposed aggregate used most, since the grippy, slightly rough texture underfoot works well anywhere bare feet and wet concrete meet. It also holds up visually on a slab that sees regular foot and lawn traffic, since the texture hides everyday grime better than a smooth or broomed surface does.
The timing window is the hard part
Exposed aggregate lives or dies on timing more than any other finish we pour. The slab has to set up enough that washing the top will not undermine the stone underneath, and it has to happen before the surface hardens past the point where a hose and a light brush can still move the paste. That window is a matter of an hour or two, not a full day, and it shifts with the weather and the mix.
That is why it is a hand finish done by an experienced crew rather than something rushed at the end of a long pour day.