A Look Inside the Concrete Slab

There is a classic urban legend—a scary one—where a young woman is either home alone or babysitting when she gets a creepy phone call. She calls the police. They trace it and tell her, “The calls are coming from inside the house.” It’s shocking and goosebump-inducing every time because the bad guy has already breached the perimeter. In architecture, there’s a similar story and, although fewer movies have been made about it, it’s even more terrifying because it’s true. The villain, in this story, is moisture, and, in most commercial structures, it’s already inside. That’s right. Traditional concrete is a trojan horse for moisture.
The Ingredients in Concrete
Concrete is a composite material composed of fine and coarse aggregates bonded together with a fluid cement (cement paste) that hardens over time. Supplemental cementitious materials such as fly ash, slag, and silica fume are also regularly present, in varying amounts, in a concrete mix composition.
One of the main ingredients necessary for the creation of concrete is water. Water is mixed with the cement and aggregate to form a cementitious liquid slurry, which can be easily poured, placed, and molded. The slurry wraps around the ingredients to form a hard matrix that creates the durable, structural material known as concrete. To demonstrate how essential water is in the production of concrete, concrete mixes are often described by their water-to-cementitious-material ratio or their w/cm. It is calculated by dividing the pounds of water by the pounds of total cementitious material. A 0.45 w/cm mix refers to a concrete mix where the weight of water is 45% of the weight of the total cementitious material. To put that in context, a full truckload of concrete mix could easily contain more than 320 gallons of water.
Introducing Admixtures
Almost all concrete mix designs also contain at least one chemical or mineral admixture. An admixture is defined as any material other than water, aggregates and hydraulic cement that is used as an ingredient in concrete or mortar and is added to the batch immediately before or during batching. Batching refers to the part of the process where the different concrete materials—cement, aggregates, admixture, etc.—are measured and combined. Once the materials are batched, they are placed in a concrete mixture machine that produces a uniform and homogeneous concrete mix.
So why use an admixture? They can reduce the cost of concrete construction; modify the properties of the hardened concrete; help ensure the quality of the concrete throughout the mixing, transporting, placing, and curing process; and overcome certain emergencies during concrete operations.
In short, there are several reasons why admixtures are used and, so, it would follow that there are several types of admixtures. Each is designed to make a specific improvement in the concrete. There are air entrainers, water reducers, set retarders, set accelerators, superplasticizers, and specialty admixtures that can include corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring. Below is a quick overview of how some of the different types of chemical admixtures can improve the performance of concrete.
Air Entrainers
This admixture causes the development of microscopic air bubbles in the concrete, mortar, or cement paste during mixing. This type of admixture is usually used to improve the workability of the material and increase its resistance to freezing and thawing.
Water Reducers
Water reducing admixtures are designed to make a fresh cementitious mixture easier to place without increasing water content or maintaining the slump of a certain mix with a reduced amount of water. In the concrete world, slump is the term that is used to describe the consistency, fluidity, and overall workability of freshly mixed concrete, so water-reducing admixtures basically make the cementitious material easier to work with, without using more water.
Superplasticizers, also known as high range water reducers, can reduce the water content of a concrete mix by up to 30 percent or more, or create great flowability in the mixture, or both, without causing undue set retardation or air entrainment in the cementitious paste.
Set Accelerators
This admixture causes an increase in the rate of hydration of the hydraulic cement. It achieves this by shortening the time required for setting or increasing the rate of strength development or both.
Set Retarders
The opposite of a set accelerator, this admixture decreases the rate of hydration of the hydraulic cement and lengthens the time of setting.
Alkali-Silica Reactivity Inhibitors
In cement, when siliceous aggregates are in the presence of alkali hydroxides, the reaction can cause abnormal expansion and cracking of the concrete. Admixtures that are alkali-silica reactivity inhibitors can hinder this chemical reaction and prevent the expansion and cracking of the concrete.
It should be noted that this is not a comprehensive list of admixtures. Others exist, are currently in development, or will be in development, because concrete is one of the most popular construction materials in the world and making it better is a good business opportunity. Moisture management is one area where concrete continues to need improvement.
Managing Moisture in Concrete
Water and concrete have a complicated relationship. On one hand, they need each other. Water is one of the most important ingredients necessary to transform the disparate cementitious materials and aggregates into the matrix material known as concrete. Water may also intentionally be added to the concrete slab surface to keep it hydrated during the curing phase and there is often ample environmental exposure to moisture that occurs during project construction. On the other hand, the presence of moisture in the finished concrete slab can cause some significant problems during construction and over the operational life of the building, especially when the moisture-ridden concrete slab is covered by an impermeable floor covering, coating, or roofing membrane.
So, the question becomes: “What happens to all the water used to transform the cementitious material into concrete? And how does it stick around long enough to overstay its welcome, migrate out whenever it wants, and become an issue?”
There are basically three destinies for the water that is incorporated into the concrete-creating process.
  1. It becomes a component of the paste—which eventually becomes the physical concrete.
  2. It evaporates from the slab as it forms.
  3. It is captured as residual in the pores, voids, and capillary structure within the concrete slab.
With a traditional concrete mixture, an elaborate structure of capillaries and voids are formed in the material while the cementitious part of the concrete is hardening. The open capillary network and void spaces within the concrete can hold residual mix water from the time of placement and allow the concrete to take on more moisture when the concrete is exposed to liquid water or a higher concentration of humidity than it possesses. This free water in the concrete mix, referred to as water of convenience, can travel to the surface of the concrete and evaporate or it can remain trapped under aggregates and reinforcement. It’s a journey, but the capillary network essentially creates a two-way street for water to regularly enter and leave the slab over the life of the building.
Allowing free water to migrate to a surface and evaporate decreases that water/cement ratio in the top surface. It can also result in premature delaminating, blistering, or cracking if the surface is exposed to high levels of traffic or aggressive environments. There are several ways to reduce bleed water rates in concrete. The use of supplementary cementitious materials—fly ash and silica fumes—can decrease bleed rates. Increasing the fines in the mix can also be effective. Air entraining and water-reducing admixtures can also help to decrease bleeding, but in some cases, they may actually end up increasing it depending upon their chemical composition. One of the biggest factors in determining bleed water rates is the water-to-cementitious-materials ratio of the mix. A higher ratio can lead to excessive bleeding.
When a project is using traditional concrete slabs and the moisture test reveals elevated levels of moisture within the material, project teams may decide to wait it out for a little while and retest, ad dehumidification, or take more aggressive moisture mitigation actions such as topical treatments to the concrete surface. While these solutions may vary in the amount of time they require or the cost they incur, they are similar in that all these actions are executed after the concrete mixture has been batched and after the slab set. They are all a response to an existing moisture problem—which is becoming more and more common on construction sites today, especially as ever-shrinking timelines collapse the amount of time that concrete slabs are allowed to dry.