Best Exposed Aggregate Concrete Specification For Driveways
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Exposed Aggregate Concrete Specification: Concrete Mix
The mix ratio for exposed aggregate concrete driveway should be 1:1:3 (5000 psi) or 1:1.5:3 (4500 psi). The exposed aggregate concrete specification must be higher than that of a standard concrete driveway.
Stronger bonding concrete is required as the exposed aggregates are not completely embedded. Stronger bonding will ensure strength & durability of the exposed aggregate concrete driveway.
Exposed aggregate concrete is most often used for patios, pool decks, sidewalks and walkways. These surfaces need to bear the load of household furniture and foot traffic only.
Exposed aggregate driveways are, however, subjected to heavier loads of cars and trucks. The tires exert an abrasive force on the driveway surface. Moreover, the driveway is completely exposed to the elements.
It stands to reason that a higher exposed aggregate concrete specification is necessary. Build it right and your exposed aggregate driveway will last for 25 – 30 years.
Does More Cement Make Concrete Stronger?
There are several factors that affect the compressive strength of concrete. Water to Cement Ratio and Cement to Aggregate Ratio are possibly the two most important factors.
Generally speaking, more cement (and less sand) makes the concrete stronger.
Concrete is essentially a matrix of aggregates embedded in a crystalline web-like structure formed by the reaction (hydration process) between water and cement. Cement is the binder in concrete.
As the cement to aggregate ratio increases the quantity of the binder and the binder strength increases. Of course this is not a purely linear relationship. There is a point beyond which additional binding is not required and will even be detrimental to the concrete strength.
The Table below is indicative ratios for different strengths:
| Compressive Strength (psi) | Cement | Sand | Stone | Water |
| 3000 | 1 | 3 | 3 | 0.5 |
| 3500 | 1 | 2.5 | 3 | 0.5 |
| 4000 | 1 | 2 | 3 | 0.5 |
| 4500 | 1 | 1.5 | 3 | 0.5 |
| 5000 | 1 | 1 | 3 | 0.5 |
As evident from the above table, compressive strength increases as the quantity of aggregates is reduced. More specifically it is the quantity of sand that is reduced while the quantity of stones is kept the same.
What Is the Effect of Water Cement Ratio on Workability and Strength?
A water cement ratio of 0.5 (or slightly less) is ideal for developing maximum compressive strength and yet keep the concrete mix workable. A much lower water cement ratio will increase the strength but will be almost impossible to trowel.
The hydration process is in fact a chemical reaction between water and cement. The reaction results in the water-cement paste to form crystalline structures. The aggregates are enmeshed within this crystalline lattice.
Portland cement used in concrete consists of various natural compounds of calcium. These are mostly silicates with a smaller percentage of aluminates and ferrites. In the hydration process water reacts with these compounds to form hydrates.
An example of the reaction is:
Tricalcium silicate + Water —> Calcium silicate hydrate + Calcium hydroxide + heat
2 Ca3SiO5 + 7 H2O —> 3 CaO.2SiO2.4H2O + 3 Ca(OH)2 + 173.6kJ
Lower water cement ratio means that the cement is more closely packed and the compounds that can be hydrated have more access to water for the hydration reaction.
Other Important Exposed Aggregate Concrete Specifications
The other factors that should form a part of the Exposed Aggregate Concrete Specifications for a driveway are:
Concrete Mix
- Cement used for exposed aggregate concrete driveway must be Portland Cement conforming to ASTM C150, Type I.
- Water used must be potable.
- Aggregates used must be hard, strong, non-flaky, free of fault lines, chemically inert, and non-porous. Aggregate size should be ⅜ “ – ¾ “.
- Slump of 4” – 5” is ideal for pouring concrete mix in exposed aggregate concrete driveway projects.
- Air Entrapment of 6% for cold climates where the driveway will be subject to freeze thaw cycles.
Thickness & Reinforcement
The exposed aggregate concrete specification for a driveway recommended by me is that the concrete slab should be at least 6” thick.
Standard concrete driveway can even be 4” thick (though not recommended by me) for use by standard passenger cars. However, exposed aggregate concrete driveways should be thicker as part of the top cement layer will be removed.
Moreover, you can use steel rebar for reinforcement in a 6” thick slab. You can not use steel rebars in a 4” thick slab. Reinforcement in a 4” thick concrete slab can only be done using welded wire mesh.
Deactivator / Retarder
Once the concrete has been placed you need to spray the surface with a retarder that will slow down the setting process of the top layer.
Sugar solution is an effective retarder and can be used. However, it has two drawbacks.
- It is quite messy and raises a stink
- It is quite difficult to ascertain the time at which you must start the removal of the soft top layer.
It is better to use a “deactivator” from a reputed manufacturer, such as Deco-crete Supply.
The deactivators come with predetermined exposure depths. This ensures that you wash off exactly the right thickness of the top layer. While most of the concrete slab sets in normal time, the top layer sets at a slower predetermined rate.
Thank you very much for reading this post. I do hope you found it informative and helpful.
