By Mallory Westbrook

Introduction:

Epoxy floors are known for their durability, chemical resistance, and easy maintenance. They are a popular choice for industrial and commercial facilities like warehouses, hospitals, and car dealerships. However, epoxy flooring installation requires careful preparation and execution, or else you might end up with an uneven, cracking, or peeling floor. To help you achieve a successful epoxy floor installation, here are the five basic steps you need to follow:

Step 1: Surface Preparation

One of the most critical steps in epoxy flooring installation is surface preparation. You need to ensure that the floor is clean, dry, and free from dirt, dust, oil, and other contaminants that can interfere with the adhesion of the epoxy. The surface should also be smooth and level with no cracks or spalling. The preparation process involves removing any existing coating or sealant, cleaning the floor with a degreaser, patching any defects with a filler, and grinding or shot blasting the surface to create a rough texture (or surface profile) that enhances the epoxy’s grip. The moisture gradient of the floor should be measured by a qualified professional to help determine the appropriate epoxy and primer. 

Step 2: Mixing the Epoxy

Once the floor is ready, you need to mix the epoxy resin and hardener according to the manufacturer’s instructions. The mixing ratio is crucial to achieving the desired strength, curing time, and chemical resistance of the epoxy. You can use a drill mixer or a paddle to ensure that the two components are thoroughly combined. Do not over-mix or whip up the epoxy, the entrapped air you create in the bucket is something you will need to contend with on the floor.

Step 3: Applying the Epoxy

After the epoxy is mixed, you should immediately pour (in ribbons) onto the floor in small sections using a squeegee or roller. You need to work quickly but carefully to ensure that the epoxy is evenly spread, without leaving any puddles or ridges. Most epoxies have a ‘pot-life’ or amount of time that can be kept in a bucket. As you start to approach the end of the ‘pot-life’, the epoxy can start heating up in and in some cases start smoking.  You can also add some decorative flakes or pigments to the epoxy to enhance its appearance. Once the first coat is applied, you need to wait for the recommended hardening time, which can range from a few hours to overnight, depending on the type and thickness of the epoxy and whether an accelerator was used.

Step 4: Applying the Topcoat

After the first coat has cured, you will need to sand the surface lightly to remove any bumps or drips and create a smooth surface for the topcoat. Then, you can apply the second coat of epoxy, which is called the topcoat or clearcoat. The topcoat provides extra protection against wear, stains, and UV damage and enhances the gloss and color of the floor. You need to follow the same application process as the first coat, including waiting for the recommended drying time.

Step 5: Finishing and Maintenance

After the topcoat is dry, you can add some finishing touches like stripe painting or safety markings. You should also avoid exposing the floor to heavy traffic or moisture for at least 24-48 hours to allow the epoxy to cure fully. Once the floor is ready for use, you need to maintain it properly by cleaning it regularly with a neutral detergent and warm water, avoiding harsh chemicals or abrasives, and repairing any damage or wear as soon as possible.

Conclusion:

Placing an epoxy floor requires attention to detail, patience, and skill. By following the five basic steps of surface preparation, mixing the epoxy, applying the epoxy, applying the topcoat, and finishing and maintenance, you can achieve a high-quality and long-lasting epoxy floor that meets your specific needs and preferences. Whether you are an engineer, a contractor, or a DIY enthusiast, mastering these steps can help you offer superior flooring solutions to your clients or elevate your own property’s aesthetics and functionality. Happy epoxy flooring!

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Concrete is a popular building material that has been in use for several decades. It is a versatile material that can be used for various construction projects. However, when it comes to pavement construction, the concrete mix must be formulated carefully. An incorrect mix can lead to cracks, unevenness, and ultimately, expensive repairs.

So, what is the best way to approach concrete mix for pavements? In this article, we will look at some crucial factors that must be considered.

Proportion of materials:

The proportion of materials used in the mix is one of the essential factors that must be considered. The correct combination of cement, sand, aggregates, and water must be used to ensure that the concrete is strong and durable. The ideal proportion of materials is:

Cement: 10-15%

Aggregates: 60-75%

Water: 15-20%

Admixtures: 0.1-0.5%

Quality of materials:

The quality of materials used in the concrete mix is also crucial. To ensure that the mix is of high quality, use high-quality cement, sand, and aggregates. Using substandard materials can lead to cracks, unevenness, and low durability.

Mixing process:

The mixing process must be done correctly and thoroughly. Mixing times are critical. Overmixing or under-mixing the concrete can lead to inconsistencies in the mix, making it vulnerable to cracking and breaking. The recommended time for mixing is 3-5 minutes, or until an even consistency is reached.

Ambient temperature:

The ambient temperature must also be considered when making concrete for pavements. When the temperature is high, the water in the mix evaporates much faster, making the mix dry too quickly and leading to cracks. Low temperatures can also cause the concrete to set too slowly, also leading to cracks. The ideal temperature for preparing concrete mix falls between 50°F (10°C) and 77°F (25°C).

Curing:

Curing is a process that allows the concrete to dry and harden. Adequate curing is critical to the concrete’s durability and preventing cracks from developing. Curing lasts for 7-28 days, depending on the conditions.

In conclusion, approaching concrete mix for pavements must be done with careful consideration. The right proportion of materials, good quality ingredients, proper mixing, and curing all play a vital role in ensuring that the pavement is strong, durable, and long-lasting. Remember: a well-formulated concrete mix will save you time and money in the long run.

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As a concrete enthusiast, I have always asked myself, “Why does moss grow on concrete?” Through web-based research, I have discovered more about the biological structure of moss and how it interacts with physical and chemical structure of concrete.

The Structure of Moss

As a non-vascular plant, the body of moss has no roots; rather, it uses tiny threads (rhizoids) to anchor itself to the stones, trees, or ground where it grows. These rhizoids allow the moss to firmly attach and grow on the surface, while absorbing water.

Why Concrete?

Most moss requires damp and sheltered areas to absorb water through the rhizoids to prevent the moss from drying out. Concrete is a porous material which allows the rhizoids to attach to the sheltered areas of the concrete.

In addition to the beneficial physical structure of concrete, different species of moss grow based on the acidity of the structure. For example, some species of moss thrive on base-rich carboniferous limestone while others thrive on more acidic rocks. Limestone is often used as an additive to cement which may explain why certain species of moss are attracted to concrete structures.

Why should you care ?

Moss and lichen growth on concrete creates the environment for slipping and tripping hazards.

Our next article will cover means and methods for creating a concrete that is resistant to moss and lichen growth.

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At the beginning of this journey I knew little of natural composite materials, much less the applications of these types of resources. At first glance, this small project seemed quite demanding. Therefore, I attacked it the only way I saw fit, I referenced the infinite glossary of information, the World Wide Web.

From a Composite Material discussion, I learned that one of the most durable, tough, and overall lightweight natural composites is wood. This is where my search began. I started with looking up the different types of wood for construction. From my research, I found an average of 17 different kinds of wood used to construct everything from houses to tables. I even found a paper that studied the, “Nanotechnology for Forest Products.” This paper gave insight on how Research and Development teams were, “manipulating the cell with nanostructure of woody plants in order to…enhance their physical properties…” (Wegner 1). But my research would not be grounded to what seemed like an over analyzed topic.

While surfing the WWW, I found a small article about the microstructure of spider silk. Spider silk, while extremely strong, is also fairly elastic. This makes the material extremely tough. With an average diameter of 0.25 µm, the spider web can stretch to 30–40% of its size before reaching ultimate failure. Please refer to Figure 1, for an illustration of the elastic property of spider silk.

What is a spider’s web made of?

It is a protein with a molecular mass of 30.000 Dalton in the gland. Outside the gland, it polymerizes to a molecule named fibroin with a molecular mass of around 300.000 Dalton. It is still not clear what activates the polymerization process. (Nieuwenhuys).

There is a plethora of uses for spider silk:

• Polynesian fishermen use the thread of the golden orb web weaver, Nephilaas, as fishing line
• The New-Hebrides spider web was used to make nets for the transportation of arrow points, tobacco, and for dried poison for the arrow points
• During World War II, the threads of Araneus diadematus, Zilla atrica, Argiope aurantia,and other orb weavers were used as hairs in measuring equipment
• Americans used the threads of the black widow (Latrodectus) in their telescopic gun sights

All in all, my short stint in the research of natural composite materials yielded a wide variety of information on spider silk on the nano-scale, its durability, and most importantly, the uses of this substance.

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In Colorado and surrounding states, the availability of quality virgin coarse aggregate that is resistant to chemical and physical degradation has decreased significantly in past years and will continue to decrease over the next decade (Hurcomb 2009). This reduction in availability is due to a greater amount of construction that has taken place over the last several decades. Finding a suitable replacement for virgin coarse aggregate is imperative for both the concrete industry and the construction industry.

This reduction in locally available coarse aggregate has forced ready-mixed concrete suppliers to purchase aggregates from outside their local area causing an increase in concrete cost. At the same time, the waste concrete from civil structures and infrastructure that have been demolished are filling landfills all around the world. Often times this landfilled concrete is made from the same high-quality aggregate that is in low supply.

Using these demolished concrete structures as recycled concrete aggregate (RCA) in concrete could provide a sustainable solution to the coarse aggregate shortage.

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Cement Paste Design, Casting, and Curing

Two grout mixes were designed, batched, and tested for the project. A Type I/II Portland Cement was used in combination with an ASTM C 33 concrete sand. One mix contained a colloidal silica dispersion with a particle size distribution between 3 to 100 nanometers (Mix ID 9011), while the second mix contained Portland cement without colloidal silica (Mix ID 9016). Both mixes included a high-range water reducer at 4.0 fluid oz. per cement hundred weight and a water-to-cement ration of 0.35.

Failure Stress Determination

Both sample sets were cured according to ASTM C protocol in a temperature-controlled lime-water bath for 28-day before breaking.

By magnifying the failed specimens and identifying the striation distances, a CGR (da/dN) could be determined. Refer to Figure 1 and Figure 2 for illustrations of the striation separations. The points between the striations were used to determine a trend fit using a 4th order polynomial equation. This derivative of this equation was taken to determine the CGR. Refer to Figure 3 for the trend equations for the various mixes.

Figure 1 —SEM Image of Striations After Compressive Strength Failure, No Colloidal Silica

Figure 2— SEM Image of Striations After Compressive Strength Failure, With Colloidal Silica

Figure 3 — Crack Growth Rate

Using the Paris Equation (Eqn. 1) the stress at which the specimen failed could be calculated and compared to the actual failure stress. From the analysis conducted by Ciavarella et al., the properties for C and m were determined(8). The value for KIC was determined from experimental testing conducted by Shah(9).

EQUATION 1 — — — da /dN = C∆K^m

From the retrieved data and the given equations, it was found that both samples were calculated to be approximately 6600 PSI.

Mix ID 9011 and 9016 broke with an average compressive strength of 11980 psi and 12570 psi, respectively.

While the project was a failure it was interesting to identify the striations in the failed specimens and research the possibility that fatigue analysis could be extended to cement composites.

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The hydrated cement matrix surrounds a wide variety of colorful aggregates.

The hydration of cement in concrete
Concrete has been considered a multi-scale material dating back to its first use and documentation in the catalogs of Egyptian and Roman histories. The strength and durability of concrete is reliant not only on the heterogeneous nature of the macro-composite, but more importantly, on micro-composite gels and (recently discovered) nanostructures. The interest in the latter grew within the last two decades, due to advances in manufacturing reactive nano particles, specifically nano silica.

The larger concrete composite, a heterogeneous material, can be considered a four-component material, both in a fluid (plastic) and hardened (cured) states. The plastic state consists of 1) rock and sand, which make up the granular skeleton; 2) cement powder; 3) H2O and admixtures; and finally 4) the entrapped and entrained air. As the concrete cures from plastic to hardened states, the cement paste (components 2 to 4) morphs from the fluid-like state to a rock-like state. In the hardened state, concrete is a composite material made up of stiff aggregate (rock and sand) embedded in a softer matrix (hydrated cement paste).

Cement strength is based on reactions between H2O and the several successive cementitious phases that occur during hydration. To understand the importance of these reactions, it is first important to identify the stages at which these reactions take place in the cement hydration process. The time to reach those stages and the length of time in those stages of cement hydration varies between cement compositions. The development of the crystal structures and gels correlates closely with the changes in temperature that occur over time during the cement hydration process, so temperature is a useful tool in tracking the process. The temperature of cement hydration process is illustrated in Figure 1 [4].

Figure 1 — The five stages of the cement hydration process: Stage 1, Mixing; Stage 2, Dormancy; Stage 3, Acceleration; Stage 4, Deceleration; Stage 5, Densification [4] The abbreviated compounds commonly found in hydrated cement are defined in Table 1. The main chemical components from Portland cement that are of concern for cement hydration are the tri-calcium silicates (C3S) and di-calcium silicates (C2S). The concentration of the C3S exceeds that of C2S, making up approximately 50–70% and 15–30%, respectively, of total Portland cement chemical composition [4, 5]. In conjunction with a smaller amount distribution, the C2S can be found in the form of nests or clusters included in the C3S. The greater availability of C3S over C2S ensures an earlier reaction time for the C3S in the presence of H2O. Finally, due to its hexagonal-like geometry, the C3S will have more surfaces to facilitate initial reaction with H2O when compared to the rounded structure of the C2S [5]. When H2O is mixed with cement there is an initial production of ettringite and precipitation of CH. These two crystalline compounds, in combination, form (over time) a surface layer covering the cement particles, which acts as a diffusion barrier to H2O [5]. The initial release of energy from these reactions causes a spike in the temperature of the mix during Stage 1 shown in Figure 1. The formation of these early products contributes little to concrete early or later strength development, or durability against ASR. In Stage 1, tri-calcium aluminate (C3A) reacts with H2O to form calcium aluminate hydrate (ettringite) in the cubic (C3AH6) or the hexagonal (C4AH12) crystalline structures shown in Figure 2 and described by

Equation 1

Figure 2 — Hydrated cement composite under scanning electron microscopy [6]
Table 1 — Abbreviated cementitious compounds [4]

In Stage 2, the surface layer described in Stage 1 germinates and begins to coat the cement particles. As the surface layer grows in thickness, it slows the access of H2O into the inner layers
of the anhydrous cement particle, thereby decelerating the hydration reactions. Within the surface layer, the reaction rate is maintained until the free H2O approaches exhaustion. Gradually, the coating over the cement particle becomes thicker, and the rate of reaction decreases to a point where the cement hydration approaches a period of dormancy. Preservation of the dormancy stage is a necessity for the commercial concrete industry, as this is the stage during which the contractor places the fluid concrete into forms. As the dormancy stages continues, residual water within the surface layer increases the amount of hydrated product but the surface layer contains the anhydrous product, keeping the concrete in a fluid state. As the cement hydration process continues, more H2O reacts with more of the cementitious components, the C3S and C2S, to form calcium hydroxide (CH) and calcium-silicate-hydrates (C-S-Hs). The calcium silicates do not hydrate in the solid state, but the anhydrous silicates probably first release a CH solution and then react to form less soluble hydrated silicates, which separate out of the supersaturated solution [5]. Richardson supported this, showing that the C-S-H forms from solution after the CH separates out [7]. The chemical reaction below shows the principal reaction of H2O and C3S in Stage 2 in Equation 2:

Equation 2

Within the hydrated and cured cement matrix, CH has a greater potential for solubility and growth as compared to the other hydration products in the presence of H2O, [5]. The CH crystal can form into a myriad of shapes that include plate-like, hooded, and fractured. An example of a plate-like CH crystal is shown in Figure 2. During Stage 2, CH and C-S-Hs grow within the initial hardened surface layer to the point where they break through the germinated surface layer over the anhydrous cement. Once the surface layer is broken, H2O can react with the remaining anhydrous cement and more crystals develop. The reaction products in Equation 2 release heat to the surroundings to increase the temperature, and initiate transition from Stage 2 (Dormancy) to Stage 3 (Hardening/Acceleration).

During Stage 3, CH grows within the cement paste, giving the cement paste its initial, strength. After the CH precipitates, the remaining solution is supersaturated with calcium oxide, CaO (Ca) and silica, SiO2 (Si). As the Ca/Si ratio climbs between 0.7 and 3.0, the H2O in this supersaturated solution is consumed, see Appendix B for more detail [8]. C-S-Hs polymerize and harden into globular masses with successive layers developing into the backbone of concrete strength and durability.

The C-S-H gel has two distinct forms that are categorized by development either early (outer- product, OP) or late (inner-product, IP) [7]. The early C-S-Hs are the impure form of the gel with a high density of pores [8]. The residual H2O bound in these pores accounts for a large portion of concrete later strength reduction and chemical degradation. The proportion of this bulk (liquid) H2O is decreased if it is incorporated into the reaction of excess cementitious and pozzolanic reactions.

The successive layers of the C-S-H are saturated with H2O at early stages of hydration as shown in Figure 3. Once the surface area of OP C-S-H starts increasing, free H2O not bonded to the cement particle migrates through the porous HCM to the residual anhydrous cement. While the anhydrous cement absorbs H2O, the OP C-S-H gel cures and hardens and the C-S-H arranges itself into clay-like sheets. Figure 3 illustrates the calcium oxide (CaO) layer enclosed by pairing silicon tetrahedra. Each of these C-S-H sheets is connected over inter-layer space by bridging silicon tetrahedra, which establishes the framework for the C-S-H gelled network. The orientation of the C-S-H structure (interlayer space and pore structure) determines the structural integrity of the HCM and the final concrete [8, 7]. Throughout the OP C-S-H structural web, there are a myriad of pore types and sizes. A listing of pore types and sizes is shown in Table 2. These pores, if filled with nano silica particles (unreacted) or with the reaction products of nano silica treatment, could offer more resistance to the strength and durability of concrete.

Figure 3 — Schematic of the C-S-H layers
Table 2 — Classification of pore size in hydrated cement composite [4]

The other type of C-S-H formed is the late product or IP C-S-H. This product will be a more pure and denser version of the previous OP C-S-H [7]. The density, and therefore the eventual ability
of the cured concrete to resist compressive loads, increases as the hydration products increase for IP C-S-H. In Stage 3, the majority of the OP C-S-H is formed from the C3S, while only a small
trace of C2S reacts [5]. In other words, Stage 3 will primarily have early OP C-S-H and trace amount of late IP C-S-H.

When the C3S reacts with H2O and starts to produce the C-S-H structures, the C-S-H lamellae and microstructure grow out into the space between adjacent particles, as well as into the cement
particle. During the fourth stage of cement hydration, reaction of C3S slows the reaction of C2S increases.

The reaction of the C2S develops slowly due to lower abundance, lower surface area, and the fact that it is often embedded in the C3S. By way of its slower reaction, due to a lower percent composition in cement and a lower surface area than the C3S, the C2S releases less heat to the surroundings, compared to the C3S [5]. As previously mentioned, C2S yields growth of late IP C-S-H. Therefore, as the propensity for C2S to react increases, so does the concentration of late IP C-S-H. For this reason, heat from C2S formation is difficult to measure, as the heat balance of the whole process is dominated by the reaction of C3S.

The heat exchange for both C3S and C2S are listed in Table 3; a lower heat of hydration is shown for the C2S when compared to C3S [4]. Even though the reaction of the C2S is slower and releases less energy, the reaction yields less CH and a denser C-S-H denser meaning that there is less CH and fewer/smaller pores intermingled in the C-S-H. This denser C-S-H crystal can resist more compressive loads than the CH [9, 10, 11]. The reaction for the C2S and H2O in Equation 3 is:

Equation 3
Table 3 — Isothermal heat of hydration of common cementitious reactions [4]

In Stage 4, there is a significant reduction in heat due to the reduction of the primary cementitious (C3S) reactions. There is a transition period from Stage 3 where the majority of the dissolving components were C3S to Stage 4 where the majority is C2S. The second layer (diffusion layer) that has developed at the outer surface of the hydrating cement particles decreases the amount of H2O that can migrate to the inner anhydrous cement. The residual water within the hydrating cement matrix reacts with residual C3S, but at a certain time the majority of the constituents left to react comes from the C2S.

In Stage 5, Densification, there is a greater decrease in the C3S reactions and therefore, a measurable decrease (based on the equipment used in this PhD dissertation) in temperature that leads to a temperature plateau. The heat of hydration curve, shown in Figure 1, illustrates this heat plateau. Stage 5 of the cement hydration process continues as long as H2O and sufficient reacting compounds are present. Therefore, the longer a sample is allowed to cure with silica and H2O present, the higher the concentration and density of C-S-H. This same phenomenon is found in commercial concrete applications where the original concrete design strength at 28 days is far exceeded by the strengths measured years after the structure has cured under favorable conditions.

In addition to the cementitious reactions in Equation 2 and Equation 3, a pozzolanic reaction occurs with the addition of silica into the pore solution. A pozzolanic material, Class F fly ash, served as a basis of comparison for laboratory and field trials during the research for this PhD dissertation. The Silica Fume Association stipulates that a pozzolanic material will not gain strength when mixed with H2O, while a cementitious material will hydrate and gain strength [11]. The pozzolanic material assists in the development of C-S-H through a pozzolanic reaction with the CH, such that:

Equation 4

While commonly used and current pozzolanic materials have a tendency to increase later strengths in concrete, they have also been shown to significantly reduce early strengths. In performance-based concretes (with secondary cementitious and pozzolanic materials) this reduction in early strength can be attributed to a dilution effect where the Class F fly ash has replaced a portion (by mass) of the OPC in performance-based concrete, thus decreasing the cementitious reactions, heat and early strength.

Bentz found that the significant benefits gained by using micrometer-sized pozzolanic materials were from the densification of the C-S-H gel and the HCM of concrete [9, 10]. Bentz showed that, in addition to densifying the body of concrete within the HCM, silica in the interfacial transition zone (ITZ) packs more efficiently than cement particles. The ITZ is the region where the aggregate surface meets the cement matrix. Normally, the amount of cement paste at the ITZ is less than that in the body of the specimen. This is known as the wall effect, a thinning of the cement paste at this ITZ (that is, on the surface of the aggregate) due to the higher surface area of the aggregate. Because of this limited availability of paste, it is desirable to ensure that the paste that is available contains a more homogenous mixture of silica and calcium, which will increase the strength of the cured cement matrix [12]. This homogeneity contributes to a tougher and more durable C-S-H gel. Silica-enriched cement at the ITZ region is denser then the cement paste without the addition of silica. Hydration of the densified paste at the ITZ improves the bonding of the aggregate into the HCM. Furthermore, as with the ITZ, within the body of the HCM and concrete composite, excess free silica contributes to the increased density through pore packing. All of these phenomena combined lead to a concrete composite that is stronger and more durable. Ultimately this PhD research was to study the impact of a newer pozzolanic material, nano silica, which may offer better performance and/or enhance the performance of industry standard pozzolanic materials.

References

1. US-DOI. Materials in Use in the U.S. Interstate Highways. USGS Department of the Interior, 2006.
2. Thomas, M., B. Fournier, and K. Folliard. Report on Determining Reactivity of Concrete Aggregates and Selecting Appropriate Measures for Preventing Deleterious Expansion in Concrete Construction. Departmant of Transportation – FHWA, 2009. p. 1–22.
3. Kuennen, T. FHWA Research Zeroes in on Concrete Nanotechnology. Concrete Products, 2010. www.concreteproducts.com/mag/concrete_fhwa_research_zero.
4. Mindess, S., J. Young, and D. Darwin. Concrete. Prentice Hall, 2003. 2: p. 57–120.
5. Taylor, H.F.W. Cement Chemistry. Thomas Telford Books, 1997. 2: p. 89–226.
6. Belkowitz, J., and D. Armentrout. An Analysis on Nano Silica in Cement Hydration. National Ready-Mixed Concrete Association Conference Proceedings on Concrete Sustainability, 2010: p.1–15.
7. Richardson, I. The Calcium Silicate Hydrates. Cement and Concrete Research, 2007. 38: p.137–158.
8. Allen, A., J. Thomas, and H. Hennings. Composition and Density of Nanoscale Calcium-Silicate-Hydrate in Cement.Nature Materials, 2007. 6: p. 311–316.
9. Bentz, D. and Garboczi, E. Simulation Studies of the Effects of Mineral Admixtures on the Cement Paste-Aggregate Interfacial Zone. ACI Materials Journal, 1991: p. 518–529.
10. Bentz, D. and Stutzman, P. Evolution of Porosity and Caclium Hydroxide in Laboratory Concretes Containing Silica Fume. Cement and Concrete Research, 1994. 24: p. 1044–1050.
11. Holland, T. Silica Fume User’s Manual. Department of Transpotation — FHWA, 2005: p.8–13.
12. Manzano, H., J. Dolado, A. Guerrero, and A. Ayuela. Mechanical Properties of Crystalline Calcium-Silicate-Hydrates: Comparison with Cementitious C-S-H Gels. Physical State Solutions, 2007. 204: p. 1775–1780.

Copyright Intelligent Concrete, LLC 2017.

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Let’s take a few minutes to break down both sides of the concrete slump test controversy and how it can impact a project.

What is the cause of the concrete slump test controversy?

• Ordering the incorrect slump when ordering the concrete: Oftentimes what we see on a concrete job site is that a contractor will order a slump that is lower than what they actually need. 

For example, a concrete contractor would want a 6 to 8 inch slump; however, they’ll order somewhere between a four to a seven inch slump from the provider. 

As it turns out with transit (especially during the warmer times of the construction season), the concrete slump will be reduced due to one of the following reasons:

• Ambient temperature
• Batch temp
• Haul time
• Concrete Admixtures (like Accelerators and Crack Compensators)

At this point, the contractor and the Ready Mix truck driver must add water or a water reducer to get to the slump that is actually needed on the jobsite.

1. Misuse and incorrect testing procedures when employing the slump test: This is probably the biggest aspect of the slump test controversy. The concrete slump test is an unforgiving test. Two components buried within the standard operating procedures include:

• The instructions on what speed to pull the concrete slump cone
• The need to be cognizant of no lateral deformation during the twisting or pulling of the cone

The concrete slump test is difficult to do right even in the perfect conditions of the laboratory. Because of this, the concrete slump test becomes ever less forgiving when added to the complexities of the job site and the changing conditions of the ambient environment. 

Slump testing requires skilled labor (or even better stated, an artisan), and as it turns out, there has been a major exodus of skilled labor in the concrete technician industry. 

1. Misinterpretation of the slump data: Misinterpretation of the slump data is often easily overcome through education. At one time, the water/cement ratio slump, the slump, and the strength could all be tied together. However, with the advent of noon emerging water reducers over the last 30 years, slump and workability cannot be directly related to water/cement ratio and strength. 

The slump test for concrete is a quality control test that is used or can be used by the purchaser to determine if the material that they bought is the material that they purchased. What does this mean? The slant of the swamp concrete that is received and measured on the job site should have a uniformity (or a similar slump) to what was ordered from the concrete provider and what is on the batch ticket. Slump can also be used to and for the workability and placeability of the concrete mixtures for specific job sites.

Who does the slump controversy impact? 

The slump test controversy impacts the concrete provider, purchaser, concrete owner, and concrete contractor. 

Whether it is rejected trucks due to incorrectly measured slumps or an incorrect addition of water (to adjust the slump to design values after transportation to the field), the incorrect slump values can cause unpredicted concrete costs on a construction jobsite.  

These costs can include but are not limited to: 

• Concrete waste from ordering new concrete patches
• Delay in the construction schedule
• The need for more raw materials 
• The incurred cost on the environment

What does all of this mean for the industry?

There’s a lot involved with the potential drawbacks of the concrete slump test. But the question is: where do we go from here?

Do we continue to deal with faulty data and the consequences that stem from that faulty data? Or do we demand more of our industry and provide further continued education and support? Or… do we find another way?

Is there an alternative out there that we have yet to uncover or do we simply need to build better, more detailed standards and demand more accountability of our civil engineers on the jobsite?

Let us know what you think.

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The Moisture Vapor Emission test, governed by ASTM F 1869, is a concrete test used to measure the weight of water emitting from concrete. The value is used to determine if a concrete subfloor is in the right state to install a resilient flooring system (epoxy coating, laminate flooring, carpet, …).  The concrete test employs a puck of dry calcium chloride in a sealed environment.  The calcium chloride absorbs water, and the weight gain is used to calculate the weight of water emitted in a 24-hr period over 1000 sq feet. 

When and why would you need a concrete moisture test?

Flooring contractors and concrete testing firms use the MVER test to determine if a concrete subfloor has a moisture emission rate (MVER) that is adequate for the flooring type and adhesive.  If the MVER of the concrete subfloor is too high, the contractor must delay the installation of the resilient floor until the concrete MVER reduces below specified values (generally below 5.0). When the MVER is below the specified value, the moisture-sensitive adhesives can be applied with little concern of failure of the resilient flooring system.

The MVER is used to determine the moisture vapor emission rate of the concrete.  If the MVER is not below the manufacturer’s specified value (generally above 5.0), the adhesive is susceptible to breakdown (hydrolysis), and the following concrete occurs with the resilient flooring system:

 Edge Delamination of VCT

Center Delamination of VCT

Joint/Crack Delamination of VCT

In the case of the resilient flooring system failure, a concrete testing firm can use the MVER test to evaluate the in-place concrete subfloor for MVER.  The value that is calculated from the recorded subfloor measurements is compared to the manufacturer’s specified MVER limits. This comparison is often used in dispute resolution when stores that are servicing customer experience loss in revenue due to resilient floor system failures and repairs.

What equipment is used during concrete moisture testing? 

• • Moisture Vapor Emission Packet
  • Calcium Chloride Puck
  • Plastic Dome with Adhesive
  • Litmus Paper
  • Distilled Water
  • Gram Scale with 0.01 g accuracy
  • Equipment to remove resilient floor (VCT, carpet, …) and adhesive without the use of water
  • Calculator

What’s the concrete moisture test process?

• • Step 1: Secure testing area that will not be disturbed for at least 72-hrs.
  • Step 2: Remove the resilient flooring system down to bare concrete without the use of water.

• • Step 3: Acclimate relative humidity and temperature of the environment (store) to be tested to normal use or 75°F (+/-10°F) and 50% (+/-10%) relative humidity and maintain these conditions 48 hours prior to and during MVER testing.
  • Step 4: Pre weigh the Calcium Chloride Puck from the MVER testing package, time and date stamp the weight and record the temperature and RH of the testing environment.
  • Step 5: Seal the open Calcium Chloride Puck under the Plastic Dome from the MVER package.  Reinforce the sides of the Plastic Dome with duct tape or waterproof tape.
  • Step 6: Protect the test setup from being disturbed for 72-hrs.
  • Step 7: After 72-hrs, remove the plastic dome and weigh the Calcium Chloride Puck, time and date stamp the weight and record the temperature and RH of the testing environment.
  • Step 8: Use the change in weight along with the MVER algorithm provided by the MVER package manufacturer to determine MVER in lbs per 1000 sq feet per 24-hrs.
  • Step 9: Compare the allowable MVER to the measured MVER for installation.

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