Testing of concrete is vital to ensuring the strength and resilience of built structures. Testing of concrete materials can be divided into two primary categories: field testing and laboratory testing.
Field Testing of Concrete
Field testing of concrete can occur during concrete installation or during investigative evaluations of installed concrete to determine strength qualities.
Concrete Slump Tests
Concrete slump testing is used to evaluate the flow characteristics of freshly mixed concrete. To conduct a slump test, concrete is placed into an inverted cone in three stages, using a metal rod to tamp down the concrete after every stage. Once the cone is full, it is lifted off the working surface using handles on both sides of the cone, at which point the concrete subsides or “slumps” towards the ground due to gravity. The distance between the original height and slumped height is measured, and this is recorded as the slump.
Typically, slumps in the range of 4 to 5 inches are considered an ideal balance between workability and consistency. Anything less than this range is hard to work, while anything greater tends to segregate during placement. This video shows a slump test demonstration.
Air Content Testing
Air is entrained within concrete to provide for expansion and contraction capability, particularly in areas that experience significant swings in outside temperature, such as the northern US and Canada. Field air content testing of concrete is conducted to determine if delivered concrete is within the air content specifications established by the engineer.
To conduct an air content test, the field technician fills a circular metal base with three lifts of concrete, which are tamped using a metal rod similar to the technique used for concrete slump testing. Once the base is full of concrete, a metal lid with a pressure gauge attached is placed on top and the two parts are locked together. A hand pump is used to pressurize the device to a calibration point and then it is allowed to stabilize. After stabilization, the pressure is released, and the technician can read the concrete air content from a dial attached to the device.
Unit weight determination of concrete is relatively easy. Fresh concrete is placed inside a container of known volume and weighed to provide the unit weight or density of the concrete. This is typically reported in pounds per cubic foot in the United States.
Field Testing of Existing Concrete
Forensic investigations and retrofits of existing structures sometimes require knowledge of the strength of existing concrete. There are two primary means of establishing the strength: nondestructive and destructive.
Schmidt Rebound Hammer
The primary means of nondestructive testing concrete is through the use of a Schmidt Rebound Hammer. The hammer works through firing a spring-loaded mass at the concrete and measuring the value of the rebound of the mass off the concrete. This value can be compared against a conversion chart to provide a rough estimate of the compressive strength of the concrete. Typically, the hammer must be calibrated before use and the test applied to several points in the testing area to establish an average value.
Destructive Concrete Testing
To obtain a much more accurate value of the concrete compressive strength, destructive testing can be used. In this method the existing concrete is cored, and a cylinder removed, which is then delivered to a laboratory for testing using the same method to that applied to cylinders from recently poured concrete. We cover laboratory compressive tests below.
Lab Testing of Concrete
Tests for compressive strength and flexural strength are best done through destructive testing in a laboratory.
Compressive Strength Lab Test
In order to determine if the concrete that was delivered and placed at a construction site is meeting the strength requirements as established by the engineer, laboratory testing is conducted on representative cylinders of concrete from that project. The cylinders are created in the field during concrete placement and are chosen at random points during the concrete pouring process. To create the cylinders, concrete is placed in plastic molds in three lifts, which are compacted with a metal rod during the placement of each lift. Cylinders are typically allowed to cure in the field for a few days before being picked up and delivered to a concrete testing laboratory. An image of some concrete cylinders on a job site is shown below.
The laboratory will test the concrete cylinders at 7, 14, 28, and occasionally 56 days after field installation to determine compressive strength at those curing intervals. This is accomplished using a device that applies force to the ends of the concrete cylinder until it breaks under the load. The value of the force at breakage is divided by the cross-sectional area of the cylinder to provide a strength in pounds per square inch. The following video shows the compressive test for a concrete cylinder.
Tensile or Flexural Testing of Concrete
In addition to a compressive strength test, concrete installed in aircraft runway and highway applications often undergoes a flexural strength or a tensile strength test. The testing specimens are created in the shape of a rectangular beam, which when cured is subject to a load on both ends until is snaps in the center, providing engineers with a measure of the concrete’s ability to withstand bending forces.There are many ways to finish concrete to provide a desired aesthetic. This article covers many options for finishing concrete.
Concrete is well-known as a versatile construction material, used worldwide a myriad number of residential commercial and industrial applications. Concrete carries its strength on the inside, but its beauty on the outside: this exterior appearance can be modified by use of various materials and techniques which will be described below.
The most basic type of concrete finish is a smooth surface created through the use of screeds and trowels. Immediately after concrete has been placed in forms, concrete finishers utilize a screed to level out the concrete surface. Screeds often consist of long pieces of metal or wood that are pulled and pushed across the concrete surface to remove excess concrete and fill in gaps in the concrete surface.
Troweling or Floating
Once the concrete has been tooled with a screed, concrete finishers utilize trowels to smooth and fine-level the surface of the concrete. This can be accomplished through manual or mechanical means. To smooth concrete manually a hand trowel, which is typically composed of a flat steel blade with attached handle, is pushed and pulled across the concrete surface. Power trowels are available and are typically used on large commercial and industrial projects where using hand trowels is not feasible. Power trowels resemble large fans with the blades sitting directly against the concrete. These power trowels are available in both walk behind and riding versions.
The image below shows an operator riding a power trowel, which is working to smooth the concrete floor surface.
Edging of the concrete is conducted to provide rounded or beveled edges on the finished concrete as well as to create joints where needed in the surface to help minimize cracking. A specific edging tool is used to accomplish this task, and requires quite a bit of practice to master.
In order to make concrete surfaces slip resistant, a broom finish can be applied. This is done after placement, leveling, and troweling of concrete. Once a smooth surface has been created, a broom is dragged across the surface of the concrete to create small ridges that provide for traction control, particularly when the concrete surface is wet. Concrete surfaces without a broom finish tend to be slippery and dangerous when liquids are present on the surface.
Concrete TextureAside from broom finishing, there are several other means of creating textures on the surface of concrete, some of which are listed below.
Exposed Aggregate FinishAn exposed finish, once commonly found in sidewalks of old cities, is created by washing the top layer of concrete away, which exposes the edges of the natural stone aggregates that are mixed into the concrete. This provides an attractive and slip resistant finish.
In addition to the use of the normal concrete materials (cement, sand, gravel and water), other materials may be added into the mix to provide exposed finishes with unique looks. Examples are rose quartz, limestone, dark gray or black basalt, red or blue granite and even colored glass or seashells. The key with any of these additives is to avoid materials containing iron, which can stain the concrete. Also, it’s important to provide a high-quality seal after concrete curing in order to protect the surface.
A salt finish is a type of finish used mainly for swimming pool decks. Salt finishes are created by applying rock salt to the top of the wet concrete and then washing it away, which leaves small pits in the finished surface.
A common method of texturing is to use concrete stamps. Concrete stamps are comprised of panels with inlaid designs, which are placed on concrete while it is still curing. Designs may consist of brick, stone or other decorative patterns to provide the desired look, sometimes mimicking other common building materials, but retaining the strength and durability of concrete. Once the forms are removed, the concrete surface may have color applied via staining, as described below.
Concrete have color added to provide a look that fits with the architecture of the associated structure. This can be accomplished through mix-added pigments or post-cure staining, both of which are discussed below.
Concrete coloring using pigments is a simple process, accomplished by adding the pigments directly to the concrete mix prior to pouring. Pigments are available in liquid form or in “mix-ready” dissolvable bags. In both cases, the pigments are placed in the mixer with the other concrete ingredients. The range of colors available is typically confined to “earthy” variants of browns and tans, although greens, blues and grays can also be purchased. It is important to keep pigmented concrete well sealed throughout its lifetime in order to prevent water infiltration, which may cause the pigment to fade.
The color of concrete can also be manipulated through the use of various staining products. One common method of staining concrete is through the use of acid. Similar to concrete pigments, the range colors is typically confined to non-bright, relatively subtle tones. Water-based (acrylic) staining provides for a much larger number of colors, including black and white. Stains can be applied to concrete of any age, though the colors are typically more vibrant if the stain is applied relatively soon after the concrete has been placed. Application of stain is typically followed up with installation of a seal over the concrete to protect the surface.
Cured concrete, whether freshly-placed or well-aged, can be provided with a polished surface for a clean and glossy look, ease of maintenance and a surface that provides additional slip resistance over that of non-polished concrete.
The polishing process is typically accomplished using concrete floor grinders that are outfitted with diamond abrasives. The grade of the abrasives, from coarse to fine, will determine the final smoothness of the concrete surface at the completion of the polishing process. First, concrete is stripped of any existing sealer or coatings and any visible cracks are repaired. This is followed by the polishing process using the floor grinders mentioned above. Part way through the polishing process, chemical hardeners are often added to the concrete to provide future protection against water infiltration. Finer and finer abrasives are used until the desired surface finish is achieved. If desired, the final step involves application of a sealing product to protect the concrete from oil, chemicals, staining and moisture.Concrete admixtures allow architects and engineers to alter the concrete recipe for various circumstances. This article covers a number of common concrete additives including superplasticizers, fly ash, corrosion inhibitors, and many others.
Concrete is a remarkable product, having been used for thousands of years as a reliable and durable building material for a countless amount of structures. In its most basic form, concrete is a simple combination of cement, water and granular material (sand and/or rock). However, to maximize the potential of concrete and provide flexibility for its use in a variety of environments and situations, additional materials can be added to the concrete during the mixing process to change its characteristics. These materials are known as admixtures, and their impact on concrete properties will be discussed further below.
Superplasticizers are polymer admixtures that are primarily used to provide a fast-flowing concrete at standard water content. This is useful in applications where there are narrow forms, tight working spaces, or in the event the concrete needs to be pumped a long distance or dropped from a significant height in order to reach its final placement position without significant separation of materials. Superplasticizers act to disperse particles within a concrete mix, improving the flow of the mix while maintaining overall cohesiveness.
Admixtures related to moisture control (called water-reducing admixtures) generally consist of additives that allow for concrete to retain its workability given a water content lower than that of typical concrete. Reducing the amount of water in a concrete mix will generally cause an increase in strength; however, in unmodified concrete (no admixtures), the reduction in water creates a reduction in the concrete’s ability to flow, and it becomes more difficult to work with and form. Standard water-reducing admixtures generally allow for a drop in water content of 7 to 10%. It should be noted that using a water–reducing admixture may affect the curing time of concrete, increasing or decreasing it dependent on the type of chemical used. Superplasticizers may be used to reduce the amount of water required even further -- from 12-30% -- and maintain normal workability. However, the effect of these compounds has a limited duration (30-60 minutes) and their addition must be timed carefully to provide the maximum positive effect.
Concrete reinforcing bars (commonly referred to as rebar) typically consist of uncoated carbon steel, and as such have a tendency to corrode over time, particularly when exposed to air, water or chloride ions (salt). In order to counter this tendency, corrosion inhibitors may be added to the concrete.
There are two goals behind the addition of corrosion inhibitors, both with the objective of reducing the sensitivity of the steel to environmental factors. The first goal is to extend the period between the installation of the concrete and the onset of the reinforcement corrosion, and the second goal is to reduce the overall amount of corrosion, if it does occur.
There are several types of corrosion inhibitors, each with a different mechanism of operation. Calcium nitrite has been used extensively for many years to mitigate the impact of chloride ion migration in concrete. Chloride ions are introduced into concrete commonly through exposure to sea water or deicing salts, and may make their way to the reinforcing steel. Normally, the surface of the reinforcing steel inside typical concrete is coated with a compound known as ferrous oxide that results from exposure to air. The ferrous oxide is in an unstable state, and will attract nearby chloride ions, which will lead to steel degradation. The calcium nitrite reacts with the surface layer of the steel and converts it to ferric oxide, which is in a passive or neutral state. Encroachment of chloride ions into the concrete will then have little to no effect on the reinforcing steel as they cannot react with stable molecules
Other types of corrosion inhibitors, such as those comprised of amino alcohols, create a coating over the steel that prevents intrusion of chloride ions, as well as disrupting the effects of oxygen and water on the steel.
Concrete strength is typically enhanced through the use of compounds consisting of microsilica (also known as silica fume), which when added to the mix, create a significant increase in compressive and flexural (bending) strength in concrete, as well as a reduction in its permeability. An example of an application for high strength concrete is in lower-level building columns in tall structures, which are under extremely heavy loads and require concrete compressive strengths and values of up to 20,000 pounds per square inch. For comparison, most standard concrete is mixed to provide compressive strengths of 3,000 to 4,000 pounds per square inch.
The introduction of air into a concrete mix can provide extensive protection against freezing, by creating an internal buffer within the concrete that allows it to contract without immediately resulting in breakage. To create the addition of tiny air bubbles to the concrete, a surfactant (detergent) is incorporated into the concrete prior to mixing. The mixing process generates the bubbles, most of which remain in the concrete as it hardens and become part of the final structure.
Winter weather also has an effect on concrete placement, as low temperatures may cause a notable reduction in concrete cure times. In these cases, an accelerator can be added to allow the concrete to cure within a normal timeframe and help reduce the possibility of frost damage.
Hot weather has significant effects on standard, non-modified concrete. The elevated temperatures cause the concrete to cure at an increased rate and begin to set up relatively quickly, which results in problems in workability and also results in shrinkage cracking. In order to counter the effects of hot weather, set-reducing or retarding admixtures can be incorporated to slow down the curing process and provide a normal period of workability and uniform cohesiveness of the concrete mix. These set-reducing admixtures are also used in the event that the mixing plant is located a significant distance away from the delivery area. The set-reducers help to prevent premature setting of the concrete inside of the mixing truck.
Fly ash has been used successfully as a concrete admixture for decades. Consisting of residue that results from incomplete combustion, typically as a result of coal processing, fly ash is truly a “green” admixture that makes use of a material that would normally be considered waste.
Fly ash can be directly substituted for the cement in a concrete mix in percentages ranging from approximately 20 to 35 percent (by weight). Aside from the reduction in environmental impact, fly ash also provides benefits during the construction process and in the finished concrete product. Generally, mixes with fly ash have improved workability and require less water to maintain a given strength requirement. The cured concrete provides enhanced resistance to corrosion of the internal steel reinforcement and also helps provide a barrier against sulfate intrusion in cases where the concrete is cast directly against soil.
Underwater Concrete Placement
The biggest challenge with placing concrete in an underwater environment is preventing washout of the cement before the concrete has had a chance to set. To maintain the consistency and integrity of the mix during underwater placement, anti-washout admixtures are often used in tandem with superplasticizers to increase the viscosity of the concrete mix and minimize the segregation of the materials.