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Applying a polyurea coating to high-performance organic cementitious materials
Construction and Building Materials 38 (2013) 1170–1179
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Applying a polyurea coating to high-performance organic cementitious materials H.A. Toutanji a,⇑, H. Choi a, D. Wong a, J.A. Gilbert b, D.J. Alldredge b a b
Dept. of Civil and Environmental Engineering, University of Alabama in Huntsville, Huntsville 35899, USA Dept. of Mechanical and Aerospace Engineering, University of Alabama in Huntsville, Huntsville 35899, USA
h i g h l i g h t s " A low modulus polyurea coating was sprayed under field conditions. " Lightweight concrete and high-performance cementitious composite materials were used. " Flexure tests were conducted on plates constructed with three different mixes. " The addition of PVA fibers and the polyurea coating increased the flexural strength. " The polyurea coating allows the plates to sustain higher strains.
a r t i c l e
i n f o
Article history: Received 2 May 2012 Received in revised form 3 September 2012 Accepted 22 September 2012 Available online 11 November 2012 Keywords: Polyurea coatings Poly(vinyl butyral) (PVB) Poly(vinyl alcohol) (PVA) fiber Lightweight concrete High performance cementitious materials Wet/dry behavior Flexural testing
a b s t r a c t Polyurea is a polymeric material that can be used to provide environmental protection and structural enhancement. In this study, a low modulus polyurea coating, having high elongation and energy absorption capacities and a fast gel time, was sprayed under field conditions onto the surfaces of cementitious materials. Compression tests were conducted to establish the wet–dry performance of uncoated and circumferentially coated cylinders fabricated from a high-performance matrix constructed with Poly(vinyl butyral) (PVB) as the only aggregate. Results showed that the coating increased the compressive strength of specimens exposed to both fresh and sea water environments. Similar results were obtained when a lightweight matrix containing sand was subjected to the same sea water environment. Flexure tests were conducted on uncoated and fully encapsulated plates kept under normal operating conditions to establish the stress–strain behavior of these two matrices, as well as a PVB matrix with Poly(vinyl alcohol) (PVA) fibers added as reinforcement. Results showed that the addition of the fibers and the coating increased the ultimate flexural strength, decreased the stiffness, allowed the structure to sustain higher strains prior to failure, and increased the fracture toughness. Comparisons are made between the performance of lightweight and high-performance concretes. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cementitious-matrix composites are often plagued in applications such as piers, docks, and bridge decks by the extensive cracking that occurs under load. While such structures can be designed to be relatively mechanically insensitive to matrix cracking per se, the cracking can introduce extensive and unwanted internal contamination by the environment resulting in direct corrosion, stress corrosion cracking, etc. of the reinforcing fibers and internal microstructure. As a result, steps have been taken to reduce corrosive attack and chemical wear. One approach to moisture proofing and preventing corrosive attacks on concrete structures is to coat them with polyurea [1]. The coatings have been shown to reduce water absorption and improve ⇑ Corresponding author. E-mail address: [email protected] (H.A. Toutanji). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.09.041
chemical wear and frost resistance [2,3]. The material lends itself to construction because of its flexibility, excellent elongation characteristics, rapid-cure rate, and wide service temperature range ( 50 °C to 150 °C) [4]. Because of its many unique physical and chemical properties, polyurea is widely used for moisture and chemical proof protection of pipelines, bridges, tanks, and roofs [5]. Polyurea also offers unique advantages for structural enhancement due to its excellent flexibility and elongation characteristics [6,7]. The polymer adheres well to concrete, metal and to wood and it has been applied to light-frame rafter to top plate connections for strengthening the building envelope in costal construction [8]. Polyurea is also used for truck bed liners and for explosive blast resistant walls. Thin interlayers of polyurea have been shown to increase blast resistance of carbon fiber foam composites [9]. Researchers have demonstrated that the impact performance of sandwich composites can be improved by strategically positioning
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a thin interlayer of polyurea relative to the impact source [10]. The U.S. Army uses polyurea to coat and harden field buildings against explosive blast because polyurea strain hardens under load [11]; and, polyurea is likely used to retrofit Government offices. Significantly, the addition of polyurea to cement-based materials led to various structural responses due to nonlinear material behavior and dispersive wave propagation, making it possible to contain spall and reduce fragmentation [12,13]. As mentioned below, researchers recently employed an approach, originally developed by material scientists to produce novel nanocomposites, to fabricate cementitious materials with enhanced properties [14,15]. The application of a polyurea coating to the surfaces of these high performance cementitious materials offers the potential to further improve their performance. Thus, the goals of the present study are to see what effects a polyurea coating has on structural integrity when it is used to coat cylinders and plates made with such materials and to discover how this performance compares with that of a lightweight concrete of equal density. The study may help designers fabricate concrete structures having special requirements, work crews to repair or retrofit existing structures, and builders to produce new structures that involve mass concrete and/or skin applications. The results may be especially helpful in costal construction where structures are exposed to sea water from storm surge or high winds from hurricanes. The facts are that fifty percent of the US population lives within 50 miles of the coast in trillions of dollars of insured property [16]. It is evident that significant problems exist in coastal buildings and there is a need for new water resistant building materials and techniques that can reinforce new and existing structures while providing safety for building occupants.
2. Polyurea Polyurea is a high strength polymer with scalable and predictable material characteristics that can be sprayed onto a substrate to make it waterproof or chemically resistant. The material has become widespread in the coating industry because of its quick-cure properties and great tolerance to extreme temperatures. Polyurea has a variable gel time (the period of time that it takes the resin to change from a liquid to a non-flowing gel); tensile strengths in commercially available materials vary from 13.8 to 34.5 MPa with inversely related elongation rates. It can be applied under field conditions by using a brush, a high temperature pump applicator, or a low temperature low pressure dispenser. From a chemical standpoint, polyurea is similar to polyureathane in its chemical makeup. The main components of polyurethane are di- or polyisocyanate molecules (cyanate functional group –NCO) and polyols (hydroxyl functional group –OH). Through an exothermic reaction process, the two components form extended chains and networks bonded by urethane groups –O(CO)(NH)–. As for its mechanical properties, polyurea displays a nearly elastic response to volumetric deformations; while above the glass transition temperature, Tg, its shearing response at moderate pressure and strain rate is soft and viscoelastic, so that its laterally unconfined deformation is nearly incompressible [17]. In the present study, specimens were coated at room temperature by spraying them with a white polyurea called Dyna-Pur 8817 which was manufactured by Creative Material Technologies, Ltd. According to the manufacturer, in this specially designed aliphatic compound, carbon atoms are joined together to produce extensive intermolecular hydrogen bonding which results in ‘‘tough’’ mechanical properties: an elongation of greater than 100%, an elastic modulus of 483 MPa, and a tensile strength of 43 MPa. The polyurea was sprayed onto test specimens at a pressure of 414 kPa with a ‘‘Voyager’’ cold spray system also manufactured by
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Creative Material Technologies, Ltd. The coating was deposited at room temperature and sprayed to a thickness of approximately 0.76 mm. The coating thickness was adjusted by controlling the pressure, offset distance, and exposure time. The compound had a gel time of about 30 s and a full curing time of 30–60 min depending on humidity and temperature.
3. High performance cementitious materials In civil infrastructure and building construction, cement-based materials have been extensively used as the most common and important material but many studies have shown disadvantages. Traditional cement-based concrete consists of two parts: a cement paste matrix and the aggregate. The properties of these constituents and the interactions that take place between them determine the behavior of the material [18]. Prior research has shown that the interfacial transition zone (ITZ) is the weakest region in a concrete structure. It is characterized by the prevalence of calcium hydroxide and higher porosity and interactions that take place there drive many important macroscopic properties, such as strength, permeability, and durability [19–21]. Researchers studying the microstructure of the ITZ and the hydration progression into it have confirmed a wall effect [21– 29]. They noted that ions have a tendency to flow slightly faster near the wall because of the decreased permeability in this zone [30]. As a result, the space around the aggregates is less effectively filled by hydration products. At the same time, there is greater tendency for calcium hydroxide [CH (Ca(OH)2)] and ettringite to develop in this space. As a result, methods have been studied to improve the aggregate/ matrix bonding in the ITZ by reducing the size of the aggregates [31,32], using basalt and quartzite as aggregates [33], and replacing the cement with ultrafine additions of constituents, such as silica fume and metakaolin [34–37]. However, these methods are limited in scope since they are siliceous and do not significantly alter the nature of the interaction between the matrix and the aggregate. A viable alternative is to adjust phenomena associated with atomic and molecular interactions that strongly influence macroscopic material properties [38]. Materials having the potential to form strong interactions at the molecular level, for example, have been developed and utilized to produce novel nanocomposites with enhanced properties [39–43]. This approach was applied to produce high-performance cementitious composites when researchers employed Poly(vinyl butyral) powder as a non-siliceous organic aggregate [14]. Steel and glass fibers are typically added to reinforce the matrix because of their high tensile strength. However, the bond strength between these traditional materials and the matrix is often limited. As a result, in addition to the interactions that take place within the ITZ, fiber/matrix debonding may occur due to mechanisms such as shear type deformation and fiber sliding [44]. This problem can be solved to some degree by adding Poly(vinyl alcohol) fibers to reinforce the cementitious matrix [15]. The concretes studied herein contain Poly(vinyl butyral) and Poly(vinyl alcohol), also known as PVB and PVA, respectively. Prior research has shown that the use of these materials in cementitious composites results in improved ductility, impact resistance, and fracture toughness [15]. PVB is a resin material which is usually used in applications that require strong binding, optical clarity, surface adhesion, toughness, and flexibility [45,46]. As illustrated in Fig. 1, the compound is produced by the well-known reaction between aldehydes and alcohols. ButvarÒ B-79 is one of a number of commercially available PVB products. As illustrated in Fig. 2, it is sold as a white and
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Fig. 1. Chemical reaction for forming PVB [46].
Table 2 Properties of Mowital M-B75H [49].
a b c
Property
Unit
B-75H
Non-volatile content Polyvinyl alcohol contentsa Polyvinyl acetate contentb Viscosityc Glass transition temperature Water absorption Bulk density
wt.% wt.% wt.% mPa s °C wt.% g/l
>97.5 18–21 0–4 60–100 73 4–6 200
Hydroxyl groups, in terms of polyvinyl alcohol. Acetyl groups, in terms of polyvinyl acetate. 10% Solution in ethanol.
Fig. 2. Butvar B-79.
free-flowing powder; and, based on properties associated with Butvar dispersions, the particle size ranges between 0.25 and 1.5 lm [47]. It has a unique combination of properties for coating or adhesive applications and its addition to a system can improve adhesion, toughness, and flexibility [48]. Table 1 shows the properties of ButvarÒ Resin B-79. Mowital M-B75H is another type of Poly(vinyl butyral). It is a thermoplastic material that is soluble in a large number of organic solvents and can be cross-linked with other compounds [49]. Table 2 shows the properties of Mowital M-B75H. Poly(viny alcohol) is a synthetic material formed from the polymerization of vinyl acetate, followed by partial hydrolysis of the acetate in the presence of an alkaline catalyst [50]. The chemical structure of PVA is shown in Fig. 3.
Fig. 3. Chemical structure of PVA.
The compound is a white powder with a specific gravity in the range of 1.2–1.3 and a glass transition temperature of approximately 80 °C. The powder can be formed and extruded into PVA fibers [51]. PVA fibers typically have a tensile strength between 1600 and 2500 MPa. PVA fibers also have alkaline resistance, a high tenacity, and a high modulus [52]. Because of the high strength and alkaline resistance, PVA fibers are considered to be one of the most suitable
Table 1 Properties of Butvar Resin B-79 (white, free-flowing powder) [46]. Property
Units
ASTM methods
B-79
Molecular weight (weight average in thousands) Specific gravity 23°/23° (±0.002) Water absorption (24 h) Hydroxyl contents expressed as% polyvinyl alcohol Acetate contents expressed as% polyvinyl acetate Butyral contents expressed as% polyvinyl butyral, approx.
– cp. % – – –
– D792-50 D570-59aT – – –
50–80 1.083 0.3 11.0–13.5 0–2.5 88
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H.A. Toutanji et al. / Construction and Building Materials 38 (2013) 1170–1179 Table 3 Properties of Kuraray PVA fiber (RECS7) [56]. Properties Units
Diameter (mm)
Thickness dtex
Cut length (mm)
Young’s modulus (kN/mm2)
Density (g/cm3)
Specific gravity –
RECS7
0.027
7
6
39
1.19–1.31
1.3
Table 4 Mix proportions for PVB, PVA fiber, and lightweight concretes (kg/m3).
PVB PVA fiber Lightweight
Cement
MK
833.0 833.0 508.9
79.3 79.3
Beach sand
147.0
Lightweight sand
B-79
B-75
Water
Sika
PVA fiber
W/C
182.4 182.4
119.0 119.0
364.8 364.8 246.8
26.7 26.7 23.0
– 7.9 (Vf = 0.6%)
0.438 0.438 0.530
728.0
polymeric fibers used in the reinforcement of concrete [53]. They firmly bond to the surrounding cementitious matrix which causes the fibers to fail by rupture rather than pull out. This hydrophilic nature of PVA fiber tends to limit the multiple cracking effect and results in lower strain hardening for the composite [53–55]. Table 3 lists the properties of Kuraray PVA fiber (RECS7) used in the present study [56]. 4. Experimental test program 4.1. Mix proportions Compression and bending specimens (cylinders and plates, respectively) were fabricated from two different high-performance concrete mixes, and a lightweight concrete mix, having the mix proportions described in Table 4. The PVB mix contains Poly(vinyl butyral) as the only aggregate while the PVA fiber mix includes Poly(vinyl alcohol) fibers added for reinforcement. The lightweight concrete mix has the same density (1500 kg/m3) as the other mixes. It was constructed using sand as the aggregate for comparative purposes. The mixes have comparable slumps which rage between 25 and 35 mm. Referring to the constituents listed in Table 4, the cement was a Ò SAKRETE Portland Cement Type I-II which conformed to ASTM C150 [57]. Metakaolin [58], conforming to ASTM C618 [59], was added at approximately ten percent by mass to take advantage of the ‘‘filler’’ effect and narrow the interfacial transaction zone (ITZ). This reduces the amount of bleeding and helps yield a more homogenous material [32]. The SikaÒ ViscoCreteÒ 2100 high range water reducing admixture conformed to ASTM C494 [60] and acts as a superplasticizer to reduce the amount of water. The PVB added to each mix consisted of a combination of Butvar B-79 and Mowital B75H. These constituents were selected and blended based on their particle sizes, densities, and tensile strengths (40–47 MPa) to provide a unique combination of aggregates [14,15]. Butvar B-79 is manufactured by Solutia Inc.; Mowital is produced by Kuraray Specialties Europe (KSE). As mentioned earlier, the PVA fibers used were manufactured by Kuraray Co. Ltd of Japan and are classified by the manufacturer as RECS7. They were added to the PVA Fiber mix at a fiber volume fraction, Vf, equal to 0.6% as reinforcement. Their high tensile strength (1.6 GPa) helps bridge cracks which may form in the matrix. Both PVB and PVA contain hydroxyl groups that have the potential to form a hydrogen bond between molecules, or within different parts of a single molecule. This unique feature provides remarkable changes in the surface bond strength, not only between the aggregate and the matrix, but also between the fiber reinforcement and the matrix. Additionally the ether oxygen functional groups act as a weak base and could potentially interact with Lewis acids and electropositive materials such as CSH [14].
Interaction may also occur between the high performance cementitious matrix and the aliphatic polyurea used to coat it. The reactions between Butvar and isocynates, for example, are shown in Fig. 4 [46]. 4.2. Compression tests Table 5 summarizes the game plan developed for wet–dry testing. A total of 30 cylinders (7.62 cm diameter by 15.24 cm long) were prepared; 18 from the PVB mix and 12 from the lightweight concrete mix (see Table 4). As can be seen from Table 5, the game plan called for coating one half of the cylinders (15 of them) around their circumferences with polyurea. Of the nine PVB cylinders of each type (uncoated and coated), three remained unexposed while the remaining six were subjected to wet/dry conditions; three cylinders of each type were exposed to sea water while the remaining three were exposed to fresh water. Of the six lightweight cylinders of each type, three remained unexposed while the remaining three were exposed to sea water. No exposure to fresh water was done for the lightweight concrete cylinders primarily because the research effort was geared toward costal construction. Photographs of typical uncoated specimens are shown in Fig. 5 while Fig. 6 shows a photograph taken during the coating process. Fig. 7a shows a photograph of the exterior of the wet–dry environmental chamber that was built to perform the wet/dry study while Fig. 7b shows a schematic of its interior. The device utilizes two electronically timed pumps and an industrial dryer to allow specimens to be subjected to an aqueous solution, with alternating wet and dry cycles (hot air at 35 °C averages and 90% humidity). The test method involved a cyclic regime developed based on prior research [61,62] to simulate site conditions. To establish what would happen to a structure located on the sea coast, for example, the concentration of the saline solution was made 35 ppt (35 g of salt per 1000 g of water), which corresponded to average ocean salinity. Specimens were subjected to a total of 100 cycles. The duration of the wet cycle was 4 h and that of the dry cycle, 8 h; thus, specimens were exposed to two cycles per day for a total of 50 days. All specimens were tested to determine the ultimate strength in accordance with ASTM C39/C39M-11a [63]. Table 6 includes the results obtained for the compressive strength while Figs. 8 and 9 illustrate how the compressive strength varies with different coating and curing conditions for PVB and lightweight concretes, respectively. Each of the vertical range bars shown on the plots in Figs. 8 and 9 correspond to the standard deviation. Referring to Fig. 8, the addition of the polyurea coating to the high-performance concrete led to a slight decrease in the compressive strength of cylinders cured at room temperature. However, the
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Fig. 4. Reactions between Butvar and isocynates [46]. Fig. 6. Half of the cylinders were coated with polyurea.
Table 5 Game plan for testing concrete cylinders under wet–dry conditions. Environment
Room temperature Wet/Dry (Sea water) Wet/Dry (Fresh water) Total
Uncoated
Coated with Polyurea
PVB
Lightweight
PVB
Lightweight
Total
3 3 3 9
3 3 – 6
3 3 3 9
3 3 – 6
12 12 6 30
Fig. 7. Wet–dry environmental chamber: (a) exterior and (b) interior.
Table 6 Results obtained from compressive strength tests of concrete cylinders. Coated condition
Uncoated
Coated
Curing condition/Type of concrete
PVB*
Lightweight*
PVB*
Lightweight*
Room temperature Wet/dry (sea water) Wet/dry (fresh water)
32.81 31.11 33.64
38.06 35.15 –
32.01 31.89 38.01
37.53 38.09 –
Fig. 5. Uncoated (a) PVB cylinders and (b) lightweight cylinders were prepared for compression tests.
*
process did improve the strength of the PVB concrete when it was subjected to a wet/dry environment, especially in the case of fresh water. Referring to Fig. 9, similar to the trend seen in Fig 8, the addition of the polyurea coating to the lightweight concrete led to a slight decrease in the compressive strength of cylinders cured at room temperature. Although no tests were conducted using fresh water, the coating process resulted in an increase in strength when the lightweight concrete was subjected to sea water. It should be noted that the sample sets used to generate these figures were small. Moreover, the standard deviations were relatively large in some cases, especially when the lightweight concrete was subjected to sea water.
In general, maintaining constant environmental conditions leads to increased strength because of better cement hydration. The trends seen in Figs. 8 and 9 suggest that the application of a polyurea coating helped in this regard when the specimens were subjected to wet–dry cycling. It should also be noted that the curing and coating conditions influenced the manner in which different concretes failed. This is evident in Fig. 10. In the case of the lightweight concrete, for example, failure took place when cracks developed at the end of the cylinder. When the PVB cylinders were tested, the polyurea coating contained the concrete until the coating ruptured.
All units are in MPa (MN/m2).
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Fig. 8. Wet/dry behavior of uncoated and polyurea coated samples for PVB concrete.
Fig. 9. Wet/dry behavior of uncoated and polyurea coated samples for lightweight concrete.
Fig. 10. Failure modes for cylinders cured (a) at room temperature and (b) underwater.
4.3. Flexure tests Table 7 summarizes the game plan developed to evaluate the flexural strength and toughness of uncoated and fully encapsulated
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plates. Eighteen plates were fabricated from the mixtures listed in Table 4. One half of the plates (nine total; three from each mix) were coated with polyurea. Six, relatively large cementitious panels (three for each mix) were fabricated to produce the plates. Molds were constructed by nailing 1.27 cm thick pine rails to plywood that was covered with PVC plastic film. The final dimensions of each mold were 61 cm 30.5 cm 1.27 cm. The mix was placed in each mold and placed on a vibration table to help fill the mold evenly. After placing the panels, the molds were loosely covered with plastic and allowed to cure for 3 days. After 3 days, the panels were carefully removed and placed in water to finish curing. The panels were water cured for 25 days, for a total cure cycle of 28 days. The panels were removed and allowed to dry before cutting. Each plate was cut using a table saw into the final dimensions. Fig. 11 shows the eighteen, 61 cm long 10.2 cm wide 1.27 cm thick, plates cut from the panels. Flexure tests were conducted in accordance with ASTM C78/ C78-10 [64]. As illustrated in the photos shown in Fig. 12, each specimen was tested to failure over a 45.7 cm span with supports placed at 15.24 cm apart. Strain was measured in the central span using strain gages. Two gages were initially used on the uncoated samples to check for consistency. This number was reduced to one on the coated samples after tests revealed that the readings from the gage pairs were nearly equal. A load cell was used to measure the applied force which was used to compute the bending moment by multiplying half the load by the distance between the outer and inner supports. Stresses were computed based on the standard flexure formula and the dimensions of the cross section at the gage location. Fig. 13 shows six, stress versus strain curves generated for the six different specimen categories found by averaging the results obtained for the three specimens in each. For comparison and clarity purposes, Fig. 14 shows the same curves plotted for strains up to 1000 le. Each of the vertical range bars shown on the plots in Fig. 14 correspond to the standard deviation computed for the stresses measured in three different specimens at the strain level where the bar is located. Table 8 lists the average maximum flexural strength and the elastic modulus for each category. The strain energy density, found by computing the total area under the stress versus strain curve, is shown for the uncoated samples. The latter, referred to as the toughness, indicates how much energy a material can absorb before rupturing. Fig. 15 illustrates how the flexural strength of each mix varies with different coating conditions. Each of the vertical range bars shown on the plots in Fig. 15 correspond to the standard deviation. The stress–strain results for all of the uncoated and coated samples were fairly consistent when the materials remained in the elastic range. The peak stresses determined for the coated samples agreed fairly well but strain results varied dramatically at high strain levels. Consequently, the curves drawn past the point at which the coated specimens begin to strain harden represent only behavioral trends drawn to the point at which maximum load was sustained. In these regions, the underlying substrate is sustaining progressively more damage at a critical location which, in most cases, is not located where strain is actually being measured. As cracks develop in tension beneath the polyurea coating in the critical section, the centroid begins to shift toward the compressive side of the beam. The moment of inertia decreases as stress is progressively transferred to the coating where bonding, thickness, and strength considerations come into play as the coating reaches its breaking strength. In general, the addition of PVA fibers created more dispersion in the data as compared to that collected for plates placed with the lightweight concrete and PVB alone because of the random orientation and distribution of the free fibers [38,65].
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Table 7 Game plan for testing unreinforced plates under normal operating conditions. Coated condition
Uncoated
Coated with Polyurea
Curing condition/type of concrete
PVB
PVB fiber
Lightweight
PVB
PVB fiber
Lightweight
Total
Room temperature
3
3
3
3
3
3
18
Fig. 11. Plate specimens: (a) uncoated and (b) coated.
Fig. 12. Flexure tests were conducted on (a) uncoated and (b) coated plates.
Fig. 13. Stress versus strain curves for uncoated and coated specimens (full range).
Fig. 14. Stress versus strain curves for uncoated and coated specimens (limited range). See legend in Fig. 13.
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Table 8 Data compiled based on averaging results for three specimens. Mix
Surface condition
Flexural strength (MPa)
Elastic modulus (GPa)
Strain energy density (MN/m2 or MJ/m3)
Lightweight PVB PVA fiber Lightweight PVB PVA fiber
Uncoated Uncoated Uncoated Coated Coated Coated
3.31 4.39 4.71 4.48 7.37 7.53
15.11 9.44 7.49 10.23 8.22 6.87
535.54 1110.91 1719.62 – – –
Fig. 16. Compressive versus flexural strengths for the lightweight and PVB mixes.
Fig. 15. Flexural strengths for uncoated and coated plates.
Referring to the curves in Figs. 13 and 14, the stress–strain responses of the uncoated plates were all fairly linear to failure. The plate that contained PVB showed a slight hint of strain hardening while the one with the PVA fibers showed slightly more. The addition of the polyurea coating allowed the plates to sustain dramatically higher strains especially when PVA was added. Strain hardening is prevalent in the plots corresponding to the coated specimens, much more so in the PVA specimens. Referring to Table 8, the addition of PVA fibers to the PVB mix increases the flexural strength, lowers the elastic modulus, and increases the toughness. In general, the addition of the polyurea coating increases the maximum flexural strength and decreases the global stiffness. 5. Lightweight versus PVB concrete Fig. 16 shows a comparison between the compressive and flexural strengths for the lightweight and PVB mixes. Each of the vertical range bars shown on the plots correspond to the standard deviation. Although the compressive strength of the lightweight mix is higher than that of the PVB mix, the flexural strength is lower. This trend was seen elsewhere where it was observed that the addition of PVB to a baseline mix led to an increase in the tensile-tocompressive strength ratio [65]. The further addition of PVA increased this ratio even further which accounts for the higher flexural strength seen here when PVA fibers are added. The lower modulus and higher flexural strength of PVB concretes make them more attractive for applications in which load reversals take place. The lower modulus leads to a greater stress transfer from the matrix to the reinforcement and the higher tensile-to-compressive strength ratio increases the potential to store and release energy. Thus, PVB concretes show great potential for applications ranging from the construction of seismic structures to energy harvesting devices. The greater toughness of PVB concretes also makes them more attractive for creating impact resistant structures in applications ranging from blast resistant walls to rocket casings. Finally, the
cross linking which takes place between the polyurea and the constituents used to produce high-performance concretes (i.e., PVB and PVA) helps to make structures fabricated with them more efficient. It is important to mention that although various comparisons were made between lightweight and PVB concretes, it is evident from Table 4 that the cement content of the latter are much higher and that the water/cement ratio is smaller. Both of these parameters, as well as the size and shape of the aggregates, strongly influence the properties of concrete; and, in this case, the differences are both in favor of the PVB concretes. 6. Discussion The results of the tests depend drastically on the scale of the specimens and structural elements tested. For example, in terms of compressive or flexural strength, the improvements obtained due to the coating will tend to be negligible in larger specimens. These factors should be taken into consideration during the design process, specifically while evaluating the feasibility of the solution. As mentioned previously, it is fundamental to guarantee that excess water in cementitious materials is allowed to be released as water vapor through its skin. Although the water vapor permeability of the Dyna-Pur 8817 polyurea used in this study was not measured, a value of 10 mg/m2/day was reported for a similar product [66]. In general, the key constituents in high performance concrete cost far more than those in typical normal weight concrete. There is also a sizable cost associated with applying a polyurea coating to the substrates made from them. Based on a study done in 2010, typical normal weight concrete mixes used for civil engineering structures cost about $103/m3 [67]; albeit, this price is for large construction projects. For small batch sizes batches similar to those used in the present study, the cost for procuring the cement and aggregates used for normal weight concrete is estimated to be twice as much as the figure quoted above, or about $206/m3. By comparison, cost estimates for the PVB and PVA Fiber mixes used in the present study are shown in Tables 9 and 10, respectively. The estimates were prepared based on the mix proportions listed in Table 4. It is evident that for small batch sizes, the cost of high performance concrete is about ten times that of normal weight concrete. However, for specialized applications where impact resistance is of paramount importance, the additional performance may justify the cost. As far as the polyurea coating is concerned, the installed cost for depositing a 0.75 mm thick coating like the one used in the present study is estimated to be on the order of $100/m2. But this cost
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– The addition of PVA fibers to the PVB mix increased the flexural strength, decreased the stiffness, and increased the toughness.
Table 9 Cost estimate for PVB concrete ($/m3). Constituent
Quantity (kg/m3)
Unit cost ($/kg)
Price ($/m3)
Cement Metakaolin B-79 (Butvar-PVB) B-75 (Butvar-PVB) Water Sika
832.96 79.30 182.39 118.95 364.78 26.68
$0.13/kg $1.65/kg $5.50/kg $5.50/kg $0.005/gal $3.72/kg
$108/m3 $131/m3 $1003/m3 $654/m3 $0/m3 $99/m3 $1995/m3
Total cost
Table 10 Cost estimate for PVA fiber concrete ($/m3). Constituent
Quantity (kg/m3)
Unit cost ($/kg)
Price ($/m3)
Cement Metakaolin B-79 (Butvar-PVB) B-75 (Butvar-PVB) Water Sika PVA fiber (RECS7) Total cost
832.96 79.30 182.39 118.95 364.78 26.68 7.93
$0.13/kg $1.65/kg $5.50/kg $5.50/kg $0.005/gal $3.72/kg $27.43/kg
$108/m3 $131/m3 $1003/m3 $654/m3 $0/m3 $99/m3 $218/m3 $2213/m3
could be easily offset by safety considerations in some applications and/or reductions in insurance premiums in others. Additionally, the polyurea family is a specialty market now. The applied costs could decrease toward the $40/m2–$60/m2 range when and if this technology is commercialized. Despite the fact that polyurea coatings enhance the structural performance of PVB concretes, additional research is needed to fully explore this potential in specific applications. In cases where a structure would likely be subjected to fire, for example, it would be important to evaluate the fire behavior of PVB concrete, PVA fibers, and the polyurea coating relative to that of conventional concrete and aggregates. In environmentally sensitive applications, it would be beneficial to know whether there was leaching of deleterious substances from the PVB concretes or the polyurea coating. And, in cases where long term performance was important, assessing the durability of the polyurea coating and concrete constituents would be essential. 7. Conclusions In this study, a low modulus polyurea coating was sprayed under field conditions onto the surface of a lightweight concrete and two high-performance cementitious composite materials: one containing Poly(vinyl butyral) (PVB) as the only aggregate, the other with Poly(vinyl alcohol) (PVA) fibers added for reinforcement. Compression tests were conducted to establish the wet–dry performance of uncoated and circumferentially coated cylinders made from the PVB and lightweight concretes; and, flexure tests were conducted on plates constructed with all three mixes to measure the flexural strength and toughness of uncoated and coated samples. As a result, the following conclusions were reached: Regarding wet/dry behavior: – The circumferential polyurea coating improved the strength of the PVB concrete cylinders when they were subjected to a wet/dry environment, especially in the case of fresh water. An increase in strength was seen when the lightweight concrete cylinders were coated and exposed to sea water. Regarding flexure: – The stress–strain plots of uncoated plates made from the materials were all fairly linear to failure.
Regarding coating flexure specimens with polyurea: – The addition of the polyurea coating allowed the plates to sustain higher strains especially when PVA was added. The strain hardening was prevalent in the stress–strain plots; much more so when PVA fibers were added. – The addition of the coating increased the flexural strength, decreased the stiffness, and increased the toughness. Regarding lightweight versus high-performance concretes: – Although the compressive strength of the lightweight concrete was greater than that of the PVB concrete, the flexural strength was not. This trend was attributed to the higher tensile-to-compressive strength ratio associated with high-performance concretes.
Acknowledgements The authors would like to thank the US Department of Commerce for supporting this research under NOAA SBIR Contract No. WC133R-09-CN-0108. They would also like to thank Dr. Kirk Biszick, Mr. Ravi Bomu, and Mr. Shigeyuki Ueno for their help with specimen preparation, running the tests, and acquiring data; and, John Becker for designing and producing the polyurea used to coat the specimens. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Department of Commerce. References [1] Delucchi M, Barbucci A, Cerisola G. Crack-bridging ability and liquid water permeability of protective coatings for concrete. Progr Org Coat 1998;33:76–82. [2] Eliezer A. Polyurea – the new generation of lining and coating. Adv Mater Res 2010;95:85–6. [3] Awad WH, Wilkie CA. Investigation of the thermal degradation of polyurea: the effect of ammonium polyphosphate and expandable graphite. Polymer 2010;51(11):2277–85. [4] Broekaert M. Coating solution for concrete applications, Concrete. ABI/Inform Trade Industry 2007;41:20. [5] Polyurea.com. Markets and applications for spray polyurea. 2012.spgid=18; [last date of access: August 29, 2012]. [6] Morrison S. Tough at the top: Heavy duty protective coating. 2006. id=4023&q=polyurea [last date of access: August 29, 2012]. [7] Futura Tech Company, Truth and mith of polyurea. 2011. http://www.futuratech.com/news/200512041620_2.pdf [last date of access: August 29, 2012]. [8] Alldredge DJ, Gilbert JA, Toutanji HA, Lavin T, Balasubramanyam MS. Uplift capacity of polyurea-coated light-frame rafter to top plate connections. J Mater Civil Eng 2012;24(9):1–10. [9] Bahei-El-Din YA, Dvorak GJ. Behavior of sandwich plates reinforced with polyurethane/polyurea interlayers under blast loads. J Sandwich Struct Mater 2007;9:261–81. [10] Gardner N, Wang E, Kumar P, Shukla A. Blast mitigation in a sandwich composite using graded core and polyurea interlayer. Exp Mech 2012;52(2):119–34. [11] Roland CM, Twigg JN, Vu Y, Mott PH. High strain rate mechanical behavior of polyurea. Polymer 2007;48:574–8. [12] Carey NL, Myers JJ. Impact testing of polyurea coated reinforced concrete and hybrid panels, FRPRCS-9, Sydney, Australia, 2009. [last date of access: August 29, 2012]. [13] Tekalur SA, Shukla A, Shivakumar K. Blast resistance of polyurea based layered composite materials. Compos Struct 2008;84:271–81. [14] Lavin T, Toutanji HA, Xu B, Ooi TK, Biszick KR, Gilbert JA. Matrix design for strategically tuned absolutely resilient structures (STARS). Proc. of SEM XI international congress on experimental and applied mechanics, Orlando, Florida, June 2–5, 2008. Paper no. 71. 12 pages. [15] Xu B, Toutanji HA, Lavin T, Gilbert JA. Characterization of poly(vinyl alcohol) fiber reinforced organic aggregate cementitious materials. Poly Concr Key Eng Mater 2011;666:73–83.
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[45] Xu J, Li Y, Ge D. Experimental investigation on constitutive behavior of PVB under impact loading. J Impact Eng 2011;38(2–3):106–14. [46] Solutia, Inc., Butvar polyvinyl butyral resin, properties and uses, Publication no. 2008084E Technical Brochure, Solutia Inc., St. Louis, Missouri, 2008.; [last date of access: August 29, 2012]. p. 1–30. [47] Blomstrom TP. Polyvinyl acetal resins, Chapter 60, Coatings Technology Handbook. 3rd ed. In: Tracton AA, editor. CRC Press; 2006. Table 60.7. [p. 60–10]. [48] Solutia, Inc., Butvar B-79, Publication no. 2008160A Data Sheet, Solutia, Inc., St. Louis, Missouri, 2008. ; [last date of access: August 29, 2012]. [p. 1–2]. [49] KSE Specialties Europe, Mowital: Polyvinyl butyral of superior quality: specialized in specialties brochure, Kuraray Specialties Europe (KSE) Gesellschaft mit beschränkter Haftung (GmbH), Frankfurt am Main, Germany, 2003. ; [last date of access: August 29, 2012]. p. 1–36. [50] Zheng ZH, Feldman D. Synthetic fiber-reinforced concrete. Progr Polym Sci 1995;20(2):185–210. [51] Feldman D, Barbalata A. Synthetic polymers: technology, properties, applications. London: Chapman and Hall; 1996. p. 101. [52] Hoshiro H, Ogawa A, Hitomi Y. Super ductile PVA-fiber reinforced cement board. In: 11th Int. inorganic-bonded fiber composites conference Madrid, Spain. November 5–7; 2008. [53] Wang S, Li VC. Polyvinyl alcohol fiber reinforced engineered cementitious composites: material design and performances. Department of Civil and Environmental Engineering. University of Michigan. p. 1–8. doi 10.1.1.67.4523. [54] Davidson JS, et al. PVA fiber reinforced shotcrete for rehabilitation and preventative maintenance of aging culverts, Final Report submitted to the Alabama Department of Transportation, Department of Civil Engineering Auburn University. ; [last date of access: August 29, 2012]. p. 1–67; 2008, [55] Redon C, Li VC, Cynthia Wu. Measuring and modifying interface properties of PVA fibers in ECC matrix. J Mater Civil Eng 2001;13(6):399–406. [56] Kurray. Worldwide, PVA fiber & industrial materials: Kurray PVA fiber for cement, mortar and concrete. ; [last date of access: August 29, 2012]. [57] ASTM C150/C150M-11, Standard specification for Portland cement. American Society for Testing and Materials; 2011. ; [last date of access: August 29, 2012]. [58] Metakaolin description: PowerPozz™ high reactivity Metakaolin (HRM): Engineered mineral admixture for use with Portland cement bulletin, Advanced Cement Technologies Limited Liability Company (LLC), Blaine, Washington, 1–4, 2010, . [59] ASTM C618-08a, Standard specification for coal fly ash and raw or calcined natural pozzolan for use in concrete. American Society for Testing and Materials; 2008. ; [last date of access: August 29, 2012]. [60] ASTM C494/C494-11, Standard specification for chemical admixtures for concrete. American Society for Testing and Materials; 2011. ; [last date of access: August 29, 2012]. [61] Toutanji H, Gomez W. Durability of concrete beams externally bonded with FRP sheets in aggressive environments. Cem Concr Compos J 1997;19a(4):351–8. [62] Toutanji H. Durability characteristics of concrete columns confined with advanced composite materials. J Compos Struct 1999;44(2–3):155–61. [63] ASTM C39/C39M-11a. Test method for compressive strength of cylindrical concrete specimens. American Society for Testing and Materials; 2011. ; [last date of access: August 29, 2012]. [64] ASTM C78/C78M-10. Standard test method for flexural strength of concrete (using simple beam with third-point loading. American Society for Testing and Materials; 2010. ; [last date of access: August 29, 2012]. [65] Pinkston M. Quantative evaluation of polymer enhanced cementitious materials, MS Thesis. Department of Mechanical and Aerospace Engineering, University of Alabama in Huntsville; 2011. [p. 105–11]. [66] Broekaert M, Van Buyten T. A polyurea waterproofing membrane, the ideal surface preparation for Huntsman’s innovative green roof VYDROÒ substrate foam, PDA Europe 2010 Annual Conference; 2010. ; [last date of access: August 29, 2012]. [67] King T. The average cost of a cubic yard of concrete, Ehow.com article, Bellevue, Washington; 2012. ; [last date of access: August 29, 2012]. [p. 1–2].
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Applying a polyurea coating to high-performance organic cementitious materials H.A. Toutanji a,⇑, H. Choi a, D. Wong a, J.A. Gilbert b, D.J. Alldredge b a b
Dept. of Civil and Environmental Engineering, University of Alabama in Huntsville, Huntsville 35899, USA Dept. of Mechanical and Aerospace Engineering, University of Alabama in Huntsville, Huntsville 35899, USA
h i g h l i g h t s " A low modulus polyurea coating was sprayed under field conditions. " Lightweight concrete and high-performance cementitious composite materials were used. " Flexure tests were conducted on plates constructed with three different mixes. " The addition of PVA fibers and the polyurea coating increased the flexural strength. " The polyurea coating allows the plates to sustain higher strains.
a r t i c l e
i n f o
Article history: Received 2 May 2012 Received in revised form 3 September 2012 Accepted 22 September 2012 Available online 11 November 2012 Keywords: Polyurea coatings Poly(vinyl butyral) (PVB) Poly(vinyl alcohol) (PVA) fiber Lightweight concrete High performance cementitious materials Wet/dry behavior Flexural testing
a b s t r a c t Polyurea is a polymeric material that can be used to provide environmental protection and structural enhancement. In this study, a low modulus polyurea coating, having high elongation and energy absorption capacities and a fast gel time, was sprayed under field conditions onto the surfaces of cementitious materials. Compression tests were conducted to establish the wet–dry performance of uncoated and circumferentially coated cylinders fabricated from a high-performance matrix constructed with Poly(vinyl butyral) (PVB) as the only aggregate. Results showed that the coating increased the compressive strength of specimens exposed to both fresh and sea water environments. Similar results were obtained when a lightweight matrix containing sand was subjected to the same sea water environment. Flexure tests were conducted on uncoated and fully encapsulated plates kept under normal operating conditions to establish the stress–strain behavior of these two matrices, as well as a PVB matrix with Poly(vinyl alcohol) (PVA) fibers added as reinforcement. Results showed that the addition of the fibers and the coating increased the ultimate flexural strength, decreased the stiffness, allowed the structure to sustain higher strains prior to failure, and increased the fracture toughness. Comparisons are made between the performance of lightweight and high-performance concretes. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cementitious-matrix composites are often plagued in applications such as piers, docks, and bridge decks by the extensive cracking that occurs under load. While such structures can be designed to be relatively mechanically insensitive to matrix cracking per se, the cracking can introduce extensive and unwanted internal contamination by the environment resulting in direct corrosion, stress corrosion cracking, etc. of the reinforcing fibers and internal microstructure. As a result, steps have been taken to reduce corrosive attack and chemical wear. One approach to moisture proofing and preventing corrosive attacks on concrete structures is to coat them with polyurea [1]. The coatings have been shown to reduce water absorption and improve ⇑ Corresponding author. E-mail address: [email protected] (H.A. Toutanji). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.09.041
chemical wear and frost resistance [2,3]. The material lends itself to construction because of its flexibility, excellent elongation characteristics, rapid-cure rate, and wide service temperature range ( 50 °C to 150 °C) [4]. Because of its many unique physical and chemical properties, polyurea is widely used for moisture and chemical proof protection of pipelines, bridges, tanks, and roofs [5]. Polyurea also offers unique advantages for structural enhancement due to its excellent flexibility and elongation characteristics [6,7]. The polymer adheres well to concrete, metal and to wood and it has been applied to light-frame rafter to top plate connections for strengthening the building envelope in costal construction [8]. Polyurea is also used for truck bed liners and for explosive blast resistant walls. Thin interlayers of polyurea have been shown to increase blast resistance of carbon fiber foam composites [9]. Researchers have demonstrated that the impact performance of sandwich composites can be improved by strategically positioning
H.A. Toutanji et al. / Construction and Building Materials 38 (2013) 1170–1179
a thin interlayer of polyurea relative to the impact source [10]. The U.S. Army uses polyurea to coat and harden field buildings against explosive blast because polyurea strain hardens under load [11]; and, polyurea is likely used to retrofit Government offices. Significantly, the addition of polyurea to cement-based materials led to various structural responses due to nonlinear material behavior and dispersive wave propagation, making it possible to contain spall and reduce fragmentation [12,13]. As mentioned below, researchers recently employed an approach, originally developed by material scientists to produce novel nanocomposites, to fabricate cementitious materials with enhanced properties [14,15]. The application of a polyurea coating to the surfaces of these high performance cementitious materials offers the potential to further improve their performance. Thus, the goals of the present study are to see what effects a polyurea coating has on structural integrity when it is used to coat cylinders and plates made with such materials and to discover how this performance compares with that of a lightweight concrete of equal density. The study may help designers fabricate concrete structures having special requirements, work crews to repair or retrofit existing structures, and builders to produce new structures that involve mass concrete and/or skin applications. The results may be especially helpful in costal construction where structures are exposed to sea water from storm surge or high winds from hurricanes. The facts are that fifty percent of the US population lives within 50 miles of the coast in trillions of dollars of insured property [16]. It is evident that significant problems exist in coastal buildings and there is a need for new water resistant building materials and techniques that can reinforce new and existing structures while providing safety for building occupants.
2. Polyurea Polyurea is a high strength polymer with scalable and predictable material characteristics that can be sprayed onto a substrate to make it waterproof or chemically resistant. The material has become widespread in the coating industry because of its quick-cure properties and great tolerance to extreme temperatures. Polyurea has a variable gel time (the period of time that it takes the resin to change from a liquid to a non-flowing gel); tensile strengths in commercially available materials vary from 13.8 to 34.5 MPa with inversely related elongation rates. It can be applied under field conditions by using a brush, a high temperature pump applicator, or a low temperature low pressure dispenser. From a chemical standpoint, polyurea is similar to polyureathane in its chemical makeup. The main components of polyurethane are di- or polyisocyanate molecules (cyanate functional group –NCO) and polyols (hydroxyl functional group –OH). Through an exothermic reaction process, the two components form extended chains and networks bonded by urethane groups –O(CO)(NH)–. As for its mechanical properties, polyurea displays a nearly elastic response to volumetric deformations; while above the glass transition temperature, Tg, its shearing response at moderate pressure and strain rate is soft and viscoelastic, so that its laterally unconfined deformation is nearly incompressible [17]. In the present study, specimens were coated at room temperature by spraying them with a white polyurea called Dyna-Pur 8817 which was manufactured by Creative Material Technologies, Ltd. According to the manufacturer, in this specially designed aliphatic compound, carbon atoms are joined together to produce extensive intermolecular hydrogen bonding which results in ‘‘tough’’ mechanical properties: an elongation of greater than 100%, an elastic modulus of 483 MPa, and a tensile strength of 43 MPa. The polyurea was sprayed onto test specimens at a pressure of 414 kPa with a ‘‘Voyager’’ cold spray system also manufactured by
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Creative Material Technologies, Ltd. The coating was deposited at room temperature and sprayed to a thickness of approximately 0.76 mm. The coating thickness was adjusted by controlling the pressure, offset distance, and exposure time. The compound had a gel time of about 30 s and a full curing time of 30–60 min depending on humidity and temperature.
3. High performance cementitious materials In civil infrastructure and building construction, cement-based materials have been extensively used as the most common and important material but many studies have shown disadvantages. Traditional cement-based concrete consists of two parts: a cement paste matrix and the aggregate. The properties of these constituents and the interactions that take place between them determine the behavior of the material [18]. Prior research has shown that the interfacial transition zone (ITZ) is the weakest region in a concrete structure. It is characterized by the prevalence of calcium hydroxide and higher porosity and interactions that take place there drive many important macroscopic properties, such as strength, permeability, and durability [19–21]. Researchers studying the microstructure of the ITZ and the hydration progression into it have confirmed a wall effect [21– 29]. They noted that ions have a tendency to flow slightly faster near the wall because of the decreased permeability in this zone [30]. As a result, the space around the aggregates is less effectively filled by hydration products. At the same time, there is greater tendency for calcium hydroxide [CH (Ca(OH)2)] and ettringite to develop in this space. As a result, methods have been studied to improve the aggregate/ matrix bonding in the ITZ by reducing the size of the aggregates [31,32], using basalt and quartzite as aggregates [33], and replacing the cement with ultrafine additions of constituents, such as silica fume and metakaolin [34–37]. However, these methods are limited in scope since they are siliceous and do not significantly alter the nature of the interaction between the matrix and the aggregate. A viable alternative is to adjust phenomena associated with atomic and molecular interactions that strongly influence macroscopic material properties [38]. Materials having the potential to form strong interactions at the molecular level, for example, have been developed and utilized to produce novel nanocomposites with enhanced properties [39–43]. This approach was applied to produce high-performance cementitious composites when researchers employed Poly(vinyl butyral) powder as a non-siliceous organic aggregate [14]. Steel and glass fibers are typically added to reinforce the matrix because of their high tensile strength. However, the bond strength between these traditional materials and the matrix is often limited. As a result, in addition to the interactions that take place within the ITZ, fiber/matrix debonding may occur due to mechanisms such as shear type deformation and fiber sliding [44]. This problem can be solved to some degree by adding Poly(vinyl alcohol) fibers to reinforce the cementitious matrix [15]. The concretes studied herein contain Poly(vinyl butyral) and Poly(vinyl alcohol), also known as PVB and PVA, respectively. Prior research has shown that the use of these materials in cementitious composites results in improved ductility, impact resistance, and fracture toughness [15]. PVB is a resin material which is usually used in applications that require strong binding, optical clarity, surface adhesion, toughness, and flexibility [45,46]. As illustrated in Fig. 1, the compound is produced by the well-known reaction between aldehydes and alcohols. ButvarÒ B-79 is one of a number of commercially available PVB products. As illustrated in Fig. 2, it is sold as a white and
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Fig. 1. Chemical reaction for forming PVB [46].
Table 2 Properties of Mowital M-B75H [49].
a b c
Property
Unit
B-75H
Non-volatile content Polyvinyl alcohol contentsa Polyvinyl acetate contentb Viscosityc Glass transition temperature Water absorption Bulk density
wt.% wt.% wt.% mPa s °C wt.% g/l
>97.5 18–21 0–4 60–100 73 4–6 200
Hydroxyl groups, in terms of polyvinyl alcohol. Acetyl groups, in terms of polyvinyl acetate. 10% Solution in ethanol.
Fig. 2. Butvar B-79.
free-flowing powder; and, based on properties associated with Butvar dispersions, the particle size ranges between 0.25 and 1.5 lm [47]. It has a unique combination of properties for coating or adhesive applications and its addition to a system can improve adhesion, toughness, and flexibility [48]. Table 1 shows the properties of ButvarÒ Resin B-79. Mowital M-B75H is another type of Poly(vinyl butyral). It is a thermoplastic material that is soluble in a large number of organic solvents and can be cross-linked with other compounds [49]. Table 2 shows the properties of Mowital M-B75H. Poly(viny alcohol) is a synthetic material formed from the polymerization of vinyl acetate, followed by partial hydrolysis of the acetate in the presence of an alkaline catalyst [50]. The chemical structure of PVA is shown in Fig. 3.
Fig. 3. Chemical structure of PVA.
The compound is a white powder with a specific gravity in the range of 1.2–1.3 and a glass transition temperature of approximately 80 °C. The powder can be formed and extruded into PVA fibers [51]. PVA fibers typically have a tensile strength between 1600 and 2500 MPa. PVA fibers also have alkaline resistance, a high tenacity, and a high modulus [52]. Because of the high strength and alkaline resistance, PVA fibers are considered to be one of the most suitable
Table 1 Properties of Butvar Resin B-79 (white, free-flowing powder) [46]. Property
Units
ASTM methods
B-79
Molecular weight (weight average in thousands) Specific gravity 23°/23° (±0.002) Water absorption (24 h) Hydroxyl contents expressed as% polyvinyl alcohol Acetate contents expressed as% polyvinyl acetate Butyral contents expressed as% polyvinyl butyral, approx.
– cp. % – – –
– D792-50 D570-59aT – – –
50–80 1.083 0.3 11.0–13.5 0–2.5 88
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H.A. Toutanji et al. / Construction and Building Materials 38 (2013) 1170–1179 Table 3 Properties of Kuraray PVA fiber (RECS7) [56]. Properties Units
Diameter (mm)
Thickness dtex
Cut length (mm)
Young’s modulus (kN/mm2)
Density (g/cm3)
Specific gravity –
RECS7
0.027
7
6
39
1.19–1.31
1.3
Table 4 Mix proportions for PVB, PVA fiber, and lightweight concretes (kg/m3).
PVB PVA fiber Lightweight
Cement
MK
833.0 833.0 508.9
79.3 79.3
Beach sand
147.0
Lightweight sand
B-79
B-75
Water
Sika
PVA fiber
W/C
182.4 182.4
119.0 119.0
364.8 364.8 246.8
26.7 26.7 23.0
– 7.9 (Vf = 0.6%)
0.438 0.438 0.530
728.0
polymeric fibers used in the reinforcement of concrete [53]. They firmly bond to the surrounding cementitious matrix which causes the fibers to fail by rupture rather than pull out. This hydrophilic nature of PVA fiber tends to limit the multiple cracking effect and results in lower strain hardening for the composite [53–55]. Table 3 lists the properties of Kuraray PVA fiber (RECS7) used in the present study [56]. 4. Experimental test program 4.1. Mix proportions Compression and bending specimens (cylinders and plates, respectively) were fabricated from two different high-performance concrete mixes, and a lightweight concrete mix, having the mix proportions described in Table 4. The PVB mix contains Poly(vinyl butyral) as the only aggregate while the PVA fiber mix includes Poly(vinyl alcohol) fibers added for reinforcement. The lightweight concrete mix has the same density (1500 kg/m3) as the other mixes. It was constructed using sand as the aggregate for comparative purposes. The mixes have comparable slumps which rage between 25 and 35 mm. Referring to the constituents listed in Table 4, the cement was a Ò SAKRETE Portland Cement Type I-II which conformed to ASTM C150 [57]. Metakaolin [58], conforming to ASTM C618 [59], was added at approximately ten percent by mass to take advantage of the ‘‘filler’’ effect and narrow the interfacial transaction zone (ITZ). This reduces the amount of bleeding and helps yield a more homogenous material [32]. The SikaÒ ViscoCreteÒ 2100 high range water reducing admixture conformed to ASTM C494 [60] and acts as a superplasticizer to reduce the amount of water. The PVB added to each mix consisted of a combination of Butvar B-79 and Mowital B75H. These constituents were selected and blended based on their particle sizes, densities, and tensile strengths (40–47 MPa) to provide a unique combination of aggregates [14,15]. Butvar B-79 is manufactured by Solutia Inc.; Mowital is produced by Kuraray Specialties Europe (KSE). As mentioned earlier, the PVA fibers used were manufactured by Kuraray Co. Ltd of Japan and are classified by the manufacturer as RECS7. They were added to the PVA Fiber mix at a fiber volume fraction, Vf, equal to 0.6% as reinforcement. Their high tensile strength (1.6 GPa) helps bridge cracks which may form in the matrix. Both PVB and PVA contain hydroxyl groups that have the potential to form a hydrogen bond between molecules, or within different parts of a single molecule. This unique feature provides remarkable changes in the surface bond strength, not only between the aggregate and the matrix, but also between the fiber reinforcement and the matrix. Additionally the ether oxygen functional groups act as a weak base and could potentially interact with Lewis acids and electropositive materials such as CSH [14].
Interaction may also occur between the high performance cementitious matrix and the aliphatic polyurea used to coat it. The reactions between Butvar and isocynates, for example, are shown in Fig. 4 [46]. 4.2. Compression tests Table 5 summarizes the game plan developed for wet–dry testing. A total of 30 cylinders (7.62 cm diameter by 15.24 cm long) were prepared; 18 from the PVB mix and 12 from the lightweight concrete mix (see Table 4). As can be seen from Table 5, the game plan called for coating one half of the cylinders (15 of them) around their circumferences with polyurea. Of the nine PVB cylinders of each type (uncoated and coated), three remained unexposed while the remaining six were subjected to wet/dry conditions; three cylinders of each type were exposed to sea water while the remaining three were exposed to fresh water. Of the six lightweight cylinders of each type, three remained unexposed while the remaining three were exposed to sea water. No exposure to fresh water was done for the lightweight concrete cylinders primarily because the research effort was geared toward costal construction. Photographs of typical uncoated specimens are shown in Fig. 5 while Fig. 6 shows a photograph taken during the coating process. Fig. 7a shows a photograph of the exterior of the wet–dry environmental chamber that was built to perform the wet/dry study while Fig. 7b shows a schematic of its interior. The device utilizes two electronically timed pumps and an industrial dryer to allow specimens to be subjected to an aqueous solution, with alternating wet and dry cycles (hot air at 35 °C averages and 90% humidity). The test method involved a cyclic regime developed based on prior research [61,62] to simulate site conditions. To establish what would happen to a structure located on the sea coast, for example, the concentration of the saline solution was made 35 ppt (35 g of salt per 1000 g of water), which corresponded to average ocean salinity. Specimens were subjected to a total of 100 cycles. The duration of the wet cycle was 4 h and that of the dry cycle, 8 h; thus, specimens were exposed to two cycles per day for a total of 50 days. All specimens were tested to determine the ultimate strength in accordance with ASTM C39/C39M-11a [63]. Table 6 includes the results obtained for the compressive strength while Figs. 8 and 9 illustrate how the compressive strength varies with different coating and curing conditions for PVB and lightweight concretes, respectively. Each of the vertical range bars shown on the plots in Figs. 8 and 9 correspond to the standard deviation. Referring to Fig. 8, the addition of the polyurea coating to the high-performance concrete led to a slight decrease in the compressive strength of cylinders cured at room temperature. However, the
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Fig. 4. Reactions between Butvar and isocynates [46]. Fig. 6. Half of the cylinders were coated with polyurea.
Table 5 Game plan for testing concrete cylinders under wet–dry conditions. Environment
Room temperature Wet/Dry (Sea water) Wet/Dry (Fresh water) Total
Uncoated
Coated with Polyurea
PVB
Lightweight
PVB
Lightweight
Total
3 3 3 9
3 3 – 6
3 3 3 9
3 3 – 6
12 12 6 30
Fig. 7. Wet–dry environmental chamber: (a) exterior and (b) interior.
Table 6 Results obtained from compressive strength tests of concrete cylinders. Coated condition
Uncoated
Coated
Curing condition/Type of concrete
PVB*
Lightweight*
PVB*
Lightweight*
Room temperature Wet/dry (sea water) Wet/dry (fresh water)
32.81 31.11 33.64
38.06 35.15 –
32.01 31.89 38.01
37.53 38.09 –
Fig. 5. Uncoated (a) PVB cylinders and (b) lightweight cylinders were prepared for compression tests.
*
process did improve the strength of the PVB concrete when it was subjected to a wet/dry environment, especially in the case of fresh water. Referring to Fig. 9, similar to the trend seen in Fig 8, the addition of the polyurea coating to the lightweight concrete led to a slight decrease in the compressive strength of cylinders cured at room temperature. Although no tests were conducted using fresh water, the coating process resulted in an increase in strength when the lightweight concrete was subjected to sea water. It should be noted that the sample sets used to generate these figures were small. Moreover, the standard deviations were relatively large in some cases, especially when the lightweight concrete was subjected to sea water.
In general, maintaining constant environmental conditions leads to increased strength because of better cement hydration. The trends seen in Figs. 8 and 9 suggest that the application of a polyurea coating helped in this regard when the specimens were subjected to wet–dry cycling. It should also be noted that the curing and coating conditions influenced the manner in which different concretes failed. This is evident in Fig. 10. In the case of the lightweight concrete, for example, failure took place when cracks developed at the end of the cylinder. When the PVB cylinders were tested, the polyurea coating contained the concrete until the coating ruptured.
All units are in MPa (MN/m2).
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Fig. 8. Wet/dry behavior of uncoated and polyurea coated samples for PVB concrete.
Fig. 9. Wet/dry behavior of uncoated and polyurea coated samples for lightweight concrete.
Fig. 10. Failure modes for cylinders cured (a) at room temperature and (b) underwater.
4.3. Flexure tests Table 7 summarizes the game plan developed to evaluate the flexural strength and toughness of uncoated and fully encapsulated
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plates. Eighteen plates were fabricated from the mixtures listed in Table 4. One half of the plates (nine total; three from each mix) were coated with polyurea. Six, relatively large cementitious panels (three for each mix) were fabricated to produce the plates. Molds were constructed by nailing 1.27 cm thick pine rails to plywood that was covered with PVC plastic film. The final dimensions of each mold were 61 cm 30.5 cm 1.27 cm. The mix was placed in each mold and placed on a vibration table to help fill the mold evenly. After placing the panels, the molds were loosely covered with plastic and allowed to cure for 3 days. After 3 days, the panels were carefully removed and placed in water to finish curing. The panels were water cured for 25 days, for a total cure cycle of 28 days. The panels were removed and allowed to dry before cutting. Each plate was cut using a table saw into the final dimensions. Fig. 11 shows the eighteen, 61 cm long 10.2 cm wide 1.27 cm thick, plates cut from the panels. Flexure tests were conducted in accordance with ASTM C78/ C78-10 [64]. As illustrated in the photos shown in Fig. 12, each specimen was tested to failure over a 45.7 cm span with supports placed at 15.24 cm apart. Strain was measured in the central span using strain gages. Two gages were initially used on the uncoated samples to check for consistency. This number was reduced to one on the coated samples after tests revealed that the readings from the gage pairs were nearly equal. A load cell was used to measure the applied force which was used to compute the bending moment by multiplying half the load by the distance between the outer and inner supports. Stresses were computed based on the standard flexure formula and the dimensions of the cross section at the gage location. Fig. 13 shows six, stress versus strain curves generated for the six different specimen categories found by averaging the results obtained for the three specimens in each. For comparison and clarity purposes, Fig. 14 shows the same curves plotted for strains up to 1000 le. Each of the vertical range bars shown on the plots in Fig. 14 correspond to the standard deviation computed for the stresses measured in three different specimens at the strain level where the bar is located. Table 8 lists the average maximum flexural strength and the elastic modulus for each category. The strain energy density, found by computing the total area under the stress versus strain curve, is shown for the uncoated samples. The latter, referred to as the toughness, indicates how much energy a material can absorb before rupturing. Fig. 15 illustrates how the flexural strength of each mix varies with different coating conditions. Each of the vertical range bars shown on the plots in Fig. 15 correspond to the standard deviation. The stress–strain results for all of the uncoated and coated samples were fairly consistent when the materials remained in the elastic range. The peak stresses determined for the coated samples agreed fairly well but strain results varied dramatically at high strain levels. Consequently, the curves drawn past the point at which the coated specimens begin to strain harden represent only behavioral trends drawn to the point at which maximum load was sustained. In these regions, the underlying substrate is sustaining progressively more damage at a critical location which, in most cases, is not located where strain is actually being measured. As cracks develop in tension beneath the polyurea coating in the critical section, the centroid begins to shift toward the compressive side of the beam. The moment of inertia decreases as stress is progressively transferred to the coating where bonding, thickness, and strength considerations come into play as the coating reaches its breaking strength. In general, the addition of PVA fibers created more dispersion in the data as compared to that collected for plates placed with the lightweight concrete and PVB alone because of the random orientation and distribution of the free fibers [38,65].
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Table 7 Game plan for testing unreinforced plates under normal operating conditions. Coated condition
Uncoated
Coated with Polyurea
Curing condition/type of concrete
PVB
PVB fiber
Lightweight
PVB
PVB fiber
Lightweight
Total
Room temperature
3
3
3
3
3
3
18
Fig. 11. Plate specimens: (a) uncoated and (b) coated.
Fig. 12. Flexure tests were conducted on (a) uncoated and (b) coated plates.
Fig. 13. Stress versus strain curves for uncoated and coated specimens (full range).
Fig. 14. Stress versus strain curves for uncoated and coated specimens (limited range). See legend in Fig. 13.
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Table 8 Data compiled based on averaging results for three specimens. Mix
Surface condition
Flexural strength (MPa)
Elastic modulus (GPa)
Strain energy density (MN/m2 or MJ/m3)
Lightweight PVB PVA fiber Lightweight PVB PVA fiber
Uncoated Uncoated Uncoated Coated Coated Coated
3.31 4.39 4.71 4.48 7.37 7.53
15.11 9.44 7.49 10.23 8.22 6.87
535.54 1110.91 1719.62 – – –
Fig. 16. Compressive versus flexural strengths for the lightweight and PVB mixes.
Fig. 15. Flexural strengths for uncoated and coated plates.
Referring to the curves in Figs. 13 and 14, the stress–strain responses of the uncoated plates were all fairly linear to failure. The plate that contained PVB showed a slight hint of strain hardening while the one with the PVA fibers showed slightly more. The addition of the polyurea coating allowed the plates to sustain dramatically higher strains especially when PVA was added. Strain hardening is prevalent in the plots corresponding to the coated specimens, much more so in the PVA specimens. Referring to Table 8, the addition of PVA fibers to the PVB mix increases the flexural strength, lowers the elastic modulus, and increases the toughness. In general, the addition of the polyurea coating increases the maximum flexural strength and decreases the global stiffness. 5. Lightweight versus PVB concrete Fig. 16 shows a comparison between the compressive and flexural strengths for the lightweight and PVB mixes. Each of the vertical range bars shown on the plots correspond to the standard deviation. Although the compressive strength of the lightweight mix is higher than that of the PVB mix, the flexural strength is lower. This trend was seen elsewhere where it was observed that the addition of PVB to a baseline mix led to an increase in the tensile-tocompressive strength ratio [65]. The further addition of PVA increased this ratio even further which accounts for the higher flexural strength seen here when PVA fibers are added. The lower modulus and higher flexural strength of PVB concretes make them more attractive for applications in which load reversals take place. The lower modulus leads to a greater stress transfer from the matrix to the reinforcement and the higher tensile-to-compressive strength ratio increases the potential to store and release energy. Thus, PVB concretes show great potential for applications ranging from the construction of seismic structures to energy harvesting devices. The greater toughness of PVB concretes also makes them more attractive for creating impact resistant structures in applications ranging from blast resistant walls to rocket casings. Finally, the
cross linking which takes place between the polyurea and the constituents used to produce high-performance concretes (i.e., PVB and PVA) helps to make structures fabricated with them more efficient. It is important to mention that although various comparisons were made between lightweight and PVB concretes, it is evident from Table 4 that the cement content of the latter are much higher and that the water/cement ratio is smaller. Both of these parameters, as well as the size and shape of the aggregates, strongly influence the properties of concrete; and, in this case, the differences are both in favor of the PVB concretes. 6. Discussion The results of the tests depend drastically on the scale of the specimens and structural elements tested. For example, in terms of compressive or flexural strength, the improvements obtained due to the coating will tend to be negligible in larger specimens. These factors should be taken into consideration during the design process, specifically while evaluating the feasibility of the solution. As mentioned previously, it is fundamental to guarantee that excess water in cementitious materials is allowed to be released as water vapor through its skin. Although the water vapor permeability of the Dyna-Pur 8817 polyurea used in this study was not measured, a value of 10 mg/m2/day was reported for a similar product [66]. In general, the key constituents in high performance concrete cost far more than those in typical normal weight concrete. There is also a sizable cost associated with applying a polyurea coating to the substrates made from them. Based on a study done in 2010, typical normal weight concrete mixes used for civil engineering structures cost about $103/m3 [67]; albeit, this price is for large construction projects. For small batch sizes batches similar to those used in the present study, the cost for procuring the cement and aggregates used for normal weight concrete is estimated to be twice as much as the figure quoted above, or about $206/m3. By comparison, cost estimates for the PVB and PVA Fiber mixes used in the present study are shown in Tables 9 and 10, respectively. The estimates were prepared based on the mix proportions listed in Table 4. It is evident that for small batch sizes, the cost of high performance concrete is about ten times that of normal weight concrete. However, for specialized applications where impact resistance is of paramount importance, the additional performance may justify the cost. As far as the polyurea coating is concerned, the installed cost for depositing a 0.75 mm thick coating like the one used in the present study is estimated to be on the order of $100/m2. But this cost
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– The addition of PVA fibers to the PVB mix increased the flexural strength, decreased the stiffness, and increased the toughness.
Table 9 Cost estimate for PVB concrete ($/m3). Constituent
Quantity (kg/m3)
Unit cost ($/kg)
Price ($/m3)
Cement Metakaolin B-79 (Butvar-PVB) B-75 (Butvar-PVB) Water Sika
832.96 79.30 182.39 118.95 364.78 26.68
$0.13/kg $1.65/kg $5.50/kg $5.50/kg $0.005/gal $3.72/kg
$108/m3 $131/m3 $1003/m3 $654/m3 $0/m3 $99/m3 $1995/m3
Total cost
Table 10 Cost estimate for PVA fiber concrete ($/m3). Constituent
Quantity (kg/m3)
Unit cost ($/kg)
Price ($/m3)
Cement Metakaolin B-79 (Butvar-PVB) B-75 (Butvar-PVB) Water Sika PVA fiber (RECS7) Total cost
832.96 79.30 182.39 118.95 364.78 26.68 7.93
$0.13/kg $1.65/kg $5.50/kg $5.50/kg $0.005/gal $3.72/kg $27.43/kg
$108/m3 $131/m3 $1003/m3 $654/m3 $0/m3 $99/m3 $218/m3 $2213/m3
could be easily offset by safety considerations in some applications and/or reductions in insurance premiums in others. Additionally, the polyurea family is a specialty market now. The applied costs could decrease toward the $40/m2–$60/m2 range when and if this technology is commercialized. Despite the fact that polyurea coatings enhance the structural performance of PVB concretes, additional research is needed to fully explore this potential in specific applications. In cases where a structure would likely be subjected to fire, for example, it would be important to evaluate the fire behavior of PVB concrete, PVA fibers, and the polyurea coating relative to that of conventional concrete and aggregates. In environmentally sensitive applications, it would be beneficial to know whether there was leaching of deleterious substances from the PVB concretes or the polyurea coating. And, in cases where long term performance was important, assessing the durability of the polyurea coating and concrete constituents would be essential. 7. Conclusions In this study, a low modulus polyurea coating was sprayed under field conditions onto the surface of a lightweight concrete and two high-performance cementitious composite materials: one containing Poly(vinyl butyral) (PVB) as the only aggregate, the other with Poly(vinyl alcohol) (PVA) fibers added for reinforcement. Compression tests were conducted to establish the wet–dry performance of uncoated and circumferentially coated cylinders made from the PVB and lightweight concretes; and, flexure tests were conducted on plates constructed with all three mixes to measure the flexural strength and toughness of uncoated and coated samples. As a result, the following conclusions were reached: Regarding wet/dry behavior: – The circumferential polyurea coating improved the strength of the PVB concrete cylinders when they were subjected to a wet/dry environment, especially in the case of fresh water. An increase in strength was seen when the lightweight concrete cylinders were coated and exposed to sea water. Regarding flexure: – The stress–strain plots of uncoated plates made from the materials were all fairly linear to failure.
Regarding coating flexure specimens with polyurea: – The addition of the polyurea coating allowed the plates to sustain higher strains especially when PVA was added. The strain hardening was prevalent in the stress–strain plots; much more so when PVA fibers were added. – The addition of the coating increased the flexural strength, decreased the stiffness, and increased the toughness. Regarding lightweight versus high-performance concretes: – Although the compressive strength of the lightweight concrete was greater than that of the PVB concrete, the flexural strength was not. This trend was attributed to the higher tensile-to-compressive strength ratio associated with high-performance concretes.
Acknowledgements The authors would like to thank the US Department of Commerce for supporting this research under NOAA SBIR Contract No. WC133R-09-CN-0108. They would also like to thank Dr. Kirk Biszick, Mr. Ravi Bomu, and Mr. Shigeyuki Ueno for their help with specimen preparation, running the tests, and acquiring data; and, John Becker for designing and producing the polyurea used to coat the specimens. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Department of Commerce. References [1] Delucchi M, Barbucci A, Cerisola G. Crack-bridging ability and liquid water permeability of protective coatings for concrete. Progr Org Coat 1998;33:76–82. [2] Eliezer A. Polyurea – the new generation of lining and coating. Adv Mater Res 2010;95:85–6. [3] Awad WH, Wilkie CA. Investigation of the thermal degradation of polyurea: the effect of ammonium polyphosphate and expandable graphite. Polymer 2010;51(11):2277–85. [4] Broekaert M. Coating solution for concrete applications, Concrete. ABI/Inform Trade Industry 2007;41:20. [5] Polyurea.com. Markets and applications for spray polyurea. 2012.
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[45] Xu J, Li Y, Ge D. Experimental investigation on constitutive behavior of PVB under impact loading. J Impact Eng 2011;38(2–3):106–14. [46] Solutia, Inc., Butvar polyvinyl butyral resin, properties and uses, Publication no. 2008084E Technical Brochure, Solutia Inc., St. Louis, Missouri, 2008.