By Claude Goguen, P.E., LEED AP
This is an article about nothing. To be precise, it is about the little volumes of nothing that exists in concrete, otherwise known as pores.
Concrete is a porous material, but few often give interior pores much thought. Pores tend to get more attention when they are on a structure’s outside in the form of bug holes.
Concrete producers can employ strategies to reduce those unsightly surface imperfections, but the pores inside remain. Nobody can eliminate them all, but we always should endeavor to reduce them.
HOW DO PORES FORM?
Swiss cheese manufacturing relies on carbon dioxide. Since the cheese is made at a warm temperature, it is soft and malleable, and those gases end up creating the round pores that we see in the end product.
In concrete, the pores come from different sources, mainly water and air. Even aggregates have pores. In this article, we will focus on pores in the paste that encase those aggregates.
Approximately 25 pounds of water is required to fully hydrate 100 pounds of portland cement. This means a water-to-cement ratio of 0.25 is roughly needed for complete hydration.
But this is for paste, not mortar (includes fine aggregates) or concrete (includes fine and coarse aggregates). A fresh concrete that can be placed in forms, flow around reinforcing and block outs, completely fill the formwork\ and maintain consistent mix properties throughout most likely needs a higher w/c ratio than 0.25.
That is why extra water serves the purpose of concrete placement and workability. But once the concrete sets, that extra water remains and occupies space in the form of pores.
A 0.40 w/c ratio mix volumetrically would produce around 55% water and 45% cement when added together (0% hydration). Once hydration theoretically has completed, roughly 5-10% of the hardened concrete volume consists of that leftover water.
Chemical shrinkage is a relatively small volume lost during the chemical reaction and is more pronounced at lower w/c ratio concretes. The remaining water volume occupies pores distributed throughout the hydrated product. (Figure 1)
Water-filled pores usually are smaller pores in concrete and typically are classified as capillary or gel pores. Capillary pores are the larger of the water-filled pores. They form as water in the mix is no longer exposed to cement grains. Gel pores are much smaller.
As cement grains are exposed to water, hydrated product composed mostly of calcium silicate hydrate (CSH) and calcium hydroxide (CH) start to envelop the cement. The grain gets smaller as water keeps reaching it and creating more hydrated product. As the hydrated product grows, water sometimes gets trapped within the CSH. These are gel pores. (Figure 2)
Eventually, outside water may no longer be able to reach the remaining unhydrated cement encased in hydrated product. That water will seek other unhydrated cement until it has no access to any. That water is now in the form of capillary pores. (Figure 3)
Air gets trapped in fresh concrete as it moves in the mixer, down the chute, into the bucket into the form and around the embedded items.
The air bubbles that remain after consolidation are called “entrapped air.” They typically are larger voids and have no contribution to concrete quality or durability.
Entrained air are smaller spherical distributed air bubbles created through the use of an air entraining admixture. Unlike entrapped air, entrained air is intentional and most often used for freeze-thaw durability.
Both types of air bubbles in concrete also are pores.
Aggregate paste interface
Aggregates in concrete are surrounded by a very thin zone known as the interfacial transition zone (ITZ), which typically has a higher porosity than the bulk paste. (Figure 4)
The ITZ tends to contain fewer cement particles and, consequently, more water. This can lead to higher porosity in these areas, and if the ITZs are linked, it also can provide a high permeability area through the concrete.
Proper curing, low w/c ratio and use of supplementary cementitious materials (SCMs), chemical admixtures and additives can strengthen this zone and make it more watertight. Avoiding the use of dirty aggregates also will help make the ITZ stronger and less porous.
PORE SIZE, SHAPE AND DISTRIBUTION
Concrete pores exist in a vast range of sizes and shapes. The largest generally range between 1-10 mm (0.04 to 0.4 inches) and typically are entrapped air.
Entrained air pores trend much smaller, closer to 100 micrometers (0.004 inches). With smaller water-filled pores, capillary pores can range in sizes around 0.1 nanometers (0.000000004 inches) and gel pores that are closer to 0.001 nanometers (0.00000000004 inches) in size. (Figure 5)
That is 2,500 times smaller than a human hair.
The size range from the smallest to the largest concrete pore is comparable to the range in size between an orange and planet Earth.
Pores come in many different shapes. Some are more spherical while others are elongated and erratic. The distribution depends on the properties of the mixture as a whole.
Most pores are fairly evenly distributed except for entrapped air, which can be concentrated in areas depending on production practices.
HOW DO PORES RELATE TO STRENGTH AND DURABILITY?
Porosity is a measure expressed in percentage of concrete pores volume compared to the overall structure volume. Whether air-filled, water-filled or somewhere in between, pores have no load carrying capacity. Therefore, the higher the porosity of a concrete, the lower the strength.
Permeability is the speed at which liquids and gases can pass through a material. It is expressed as a distance over time such as inches or millimeters per year.
Concrete permeability is significantly important as the majority of threats to durability come from outside of concrete. The ability for something to get from outside of concrete to the inside depends on its permeability.
Permeability and porosity become related when a fluid is trying to get through the hydrated product that holds the aggregate together. It will seek out and find the ways to facilitate its transport. Pores provide clearings in the dense forest that is concrete to speed up permeability.
Figure 6 shows two concrete samples. The one on the left has lower porosity, and the one on the right has higher porosity. The fluid trying to get from top to bottom of both samples find it easier in the more porous sample.
As porosity increases, strength decreases, and permeability increases as show in the figure below (Figure 7).
Excess water within the w/c ratio is the primarily influencer in porosity. As a result, there are lower strengths and higher permeability with higher w/c ratios. Permeability, meanwhile, affects concrete durability and serves as the link between w/c ratio and durability.
CONTROLLING POROSITY IN CONCRETE
While no current processes can eliminate pores in concrete, they certainly can be reduced. The most efficient way to do that is by reducing the amount of excess water in the mix.
Less water brings the cement grains closer together. A properly hydrated product takes up more space, therefore less leftover water means fewer pores (Figure 8)
Keep in mind there are two primary sources of mix water: water added through the batching system and water carried in by aggregates. There may be an acceptable w/c ratio on paper, but if no adjustment is made to the batch water once wet aggregates are added, the actual w/c ratio jumps up – along with porosity.
Reducing the larger entrapped air pores comes down to minimizing drop heights for fresh concrete into forms (reduce splashing) and proper consolidation.
Proper curing also plays an important role in reducing porosity. Despite minimizing excess water, if hydration is not allowed to progress because of improper curing, the water intended for hydration is left over as pores.
Concrete needs high humidity and temperature to cure. Help maintain these conditions by keeping the product covered or sealed to maintain this crucial environment.
Optimizing mix proportions and utilizing certain admixtures also can help reduce porosity. Optimizing aggregate gradation is one example of reducing porosity through mix design. Since most pores exist in the paste, reducing the amount of paste will reduce pores.
Reduce paste content by optimizing aggregate gradation. By addressing the voids in some gap-graded aggregate blends, concrete still can possess the required workability and require less paste.
Supplementary cementitious materials (SCMs) such as fly ash, slag and silica fume help reduce porosity through pozzolanic reactions and resulting enhanced hydrated product quality. Some of these SCMs also can reduce water demand, reducing excess water and porosity.
Water reducing admixtures also can maintain the workability needed while reducing excess water, which in turn reduces porosity.
MEASURING POROSITY IN CONCRETE
There are few tests available to accurately measure porosity in hardened concrete. The test method outlined in ASTM C642-21 – “Standard Test Method for Density, Absorption, and Voids in Hardened Concrete 2” involves drying samples of hardened concrete in an oven and letting them cool down before weighing. The sample then is immersed in water for 48 hours, surface dried and weighed again.
Samples also are immersed in water, the water is boiled for five hours, the water is cooled naturally, and the sample is surface dried and weighed. The different weights are used in a formula to determine the volume of permeable pore space.
Work is ongoing to develop a porosity test based on vacuum saturation.
FEWER PORES, BETTER CONCRETE
Non-porous concrete simply doesn’t exist.
Some concretes are designed to be very porous in order to allow stormwater to flow through it. When precast concrete structures are intended to convey or store stormwater and wastewater, protect valuable equipment and provide exceptional durability while exposed to harsh conditions, its ability to resist the penetration of fluids is consequential.
Controlling and minimizing porosity increases strength and reduces permeability. Implementing measures to control excess water in the mix, properly place and consolidate the fresh concrete and enabling it to hydrate under ideal curing conditions are the most efficient ways to control porosity.
You could say that when it comes to quality durable concrete, less nothing is everything. PI
Claude Goguen, P.E., is the director of outreach and technical education at NPCA.
1 Mehta, P. K. and Monteiro, P. J. Concrete: Microstructure, Properties and Materials. 3rd ed. 1994. London: McGraw-Hill; 2006. DOI: 10.1036/0071462899\
2 ASTM C642-21 – Standard Test Method for Density, Absorption, and Voids in Hardened Concrete