By Claude Goguen, P.E., LEED AP
If someone asked you to name the greatest duos of all time, you might say Penn and Teller, Simon and Garfunkel or Ben and Jerry. But what about hydrogen and oxygen? Water is the result of the bonding of one oxygen atom to two hydrogen atoms, and none of us would be here without it.
Another duo that has had a significant impact on modern history is cement and water. Combining the two creates a chemical reaction known as hydration, resulting in cementitious paste. Add in aggregates and we get concrete, the second most used material in the world, right after… you guessed it, water.
Although concrete needs water, the two have a bit of a love-hate relationship. Water is a main ingredient, but once concrete has hardened, water can also lead to its destruction. This is a challenge faced since the Romans used concrete to build the Pantheon. So how do we make concrete watertight?
Watertightness, permeability and porosity
Watertightness is the ability of concrete to keep water out or in. In other words, the goal is to make concrete virtually impermeable. Precast concrete is used for many purposes – one of the most common is to hold or convey fluids. For structures to fulfill this purpose, they must be watertight.
At the same time, in order for damaging chloride ions, sulfate ions and other aggressive chemicals to make it into concrete, water is required for transport. So while watertightness is important to keep fluids in, it is also important to keep harmful chemicals and ions out.
Concrete is a porous material. Manufacturers employ many methods to control the amount of water and air that remains in hardened concrete, but pores will always be present. Precasters are always trying to minimize porosity, mainly by reducing the water content in the mix. The water to cementitious materials ratio (w/c) is the weight of water divided by the weight of cement. Ideally, precasters want exactly the right amount of water to react with a given amount of cement. However, the mix must be placed in formwork and consolidated around reinforcing. This requires the mix to have a certain measure of workability. The use of water and admixtures enhances the ability for concrete to flow and be placed properly. Any water that is left over after cement is hydrated ends up forming pores in the concrete.
Permeability is the ability of a given concrete to allow liquids or gases to pass through. Permeability is influenced by porosity, but the two are not directionally proportional. We can have a porous material that is also impermeable. Permeability depends not only on the number of pores, but also their size, orientation and connectivity. Despite the presence of tiny pores, however, it would take water approximately 4,800 years to travel through a 6-in. concrete wall if the concrete is of good quality.
Pathways for entry
In terms of the ability for liquids to pass through concrete, there are three roads to take. Liquids can go through the paste (cement and water), the aggregate or the paste to aggregate transition zone, which is commonly referred to as the ITZ (Figure 1). We will look at these three phases separately.
Aggregates. In general, aggregates may have a lower porosity than the surrounding paste. However, depending on the type, aggregates can be more permeable due to the increased size of the pores. The permeability of mature, hardened paste kept continuously moist ranges from 0.1 x 10-12 to 120 x 10-12 cm/s for w/c ratios ranging from 0.3 to 0.7. The permeability of commonly used concrete aggregate varies from about 1.7 x 10-9 to 3.5 x 10-13 cm/s. Selecting an aggregate that has low porosity, low absorption and low permeability is important to aid in the watertightness of the concrete. The aggregates also need to be clean and free of deleterious substances.
ITZ. This zone, surrounding the aggregate particles, is very thin. The shape, size and orientation of the aggregates have an influence on the thickness of the ITZ. The ITZ has a tendency to contain fewer cement particles, and consequently, more water. This can lead to higher porosity in these areas and if the ITZs are linked, can also 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.
Making watertight tighter
Precast concrete manufacturers are in the business of making strong, durable products. Durability implies low permeability, so what are manufacturers doing to ensure their precast concrete structures are virtually impermeable?
W/C ratio. The w/c ratio is the most important factor in making a watertight precast concrete structure. Producers work tenaciously to find the absolute lowest w/c ratio that will enable proper casting. Quality control technicians precisely calculate the moisture content of the aggregates and factor in water from admixtures to make the necessary modifications to maintain the optimum w/c ratio.
Concrete’s strength has a direct relationship with the w/c ratio. By specifying high-strength concrete, the w/c ratio will be low, and, as a result, there will be reduced porosity and permeability. Low w/c ratios produce a strong concrete that can far exceed specified 28-day strengths, and very low porosity that will make it impenetrable to harmful elements.
Aggregates. Producers are judicious in their selection of aggregates to ensure they are as impermeable as possible. For that reason, most issues with permeability will occur in the paste. Still, quality control managers will select an aggregate gradation that will maximize aggregate volume while ensuring that enough paste is present to fully envelop the aggregates. The aggregates are also closely inspected and tested to ensure they do not contain any deleterious substances or excess dirt.
SCMs. Producers use SCMs for their many benefits, including permeability reduction. According to PCA’s Design and Control of Concrete Mixtures, “These materials may or may not reduce the total porosity to any great extent, but instead, act to refine and subdivide the capillary pores so that they become less continuous.” SCMs also have a binding effect that can inhibit ingress of chloride ions into the concrete. Silica fume and metakaolin are particularly effective in reducing concrete’s permeability.
Admixtures. Producers use many different admixtures to modify fresh or hardened concrete properties.
Air-entraining admixtures can be beneficial to reduce permeability due to their ability to enhance workability and cohesiveness, therefore reducing water demand.
Water-reducing admixtures maintain or even increase workability while lowering mix water content. The advent of water reducers and the ongoing technological advances in the development of these products have led to overall reductions in w/c ratios and more watertight precast concrete structures.
Admixtures such as accelerators and retarders are used to control concrete’s rate of set, especially during extreme weather conditions. This set control fosters proper hydration and results in decreased permeability.
Permeability-reducing admixtures are gaining popularity in concrete manufacturing. They can be divided into two categories: non-hydrostatic (PRAN) and hydrostatic (PRAH).
Due to their hydrophobic nature, PRANs act as a water repellent and are traditionally used to reduce the absorptive tendency of concrete. This is commonly referred to as damp proofing. These products are generally used where there is little or no hydrostatic head.
PRAHs, on the other hand, are hydrophilic, meaning they react with water. Crystalline waterproofing additives exemplify this technology. The active ingredient in these products reacts with byproducts of cement hydration to form additional CSH gel and/or precipitates that block microcracks and capillaries. This blockage will resist water ingress under hydrostatic pressure.
Consolidation. Consolidation serves many purposes. One of the most important is to ensure that air does not remain trapped in the concrete after it has been placed. Improper consolidation can lead to honeycombing and aggregate segregation, making it much easier for water to enter the concrete.
Curing. The curing period of a precast concrete structure is critical in determining how durable it will be. Proper curing requires three things: time, temperature and moisture. Producers start with a low w/c ratio and then must work to keep that moisture in the concrete to enhance proper hydration.
Sealers, Penetrants and Coatings. Properly designed, mixed, placed and cured precast concrete can stand up to most conditions and serve as a long-lasting product. However, conditions do exist when additional products may be needed. These are products applied to the surface of hardened precast concrete to reduce the penetration of water and harmful ions into the concrete.
Generally classified as film-forming, sealers block penetration of water by forming a clear protective barrier on the concrete surface. Some are water- or solvent-based while others are epoxy or urethane sealers.
Penetrants, sometimes called penetrating sealers, actually soak into the concrete and enter the voids and capillaries at the surface to form a water-repellent layer. Water will actually bead on these treated surfaces.
Coatings are engineered products designed to form a protective barrier, shielding the concrete from chemical attack. Coatings are typically epoxy, urethane or acrylic, but also include asphalt coatings, polyureas, polyaspartic, and poly and vinyl esters.
How to test for permeability
Many tests exist to determine the permeability of concrete. One such test measures the electrical conductivity of the precast concrete structure to get an indication of permeability. ASTM C1202, “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration,” is also called the Coulomb or rapid chloride permeability test. Other tests are indicated in Table 1.
Lewis and Clark, peanut butter and jelly, cement and water… all are influential duos. Unless specifically called for by design, though, water getting into concrete serves no good purpose. Thankfully, precast concrete producers are diligent in finding the best materials and production methods to deliver a product that will keep harmful elements at bay and provide an exceptionally long service life.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Sustainability and Technical Education.
PCA Design and Control of Concrete Mixtures – 15th Edition
ACI 212.3R-10 – Report on Chemical Admixtures for Concrete
ACI 318 – 11 – Building Code Requirements for Structural Concrete
ACI 515.1 defines waterproofing as a treatment of a surface or structure to resist the passage of water under hydrostatic pressure, whereas damp proofing is defined as a treatment of a surface or structure to resist the passage of water in the absence of hydrostatic pressure.