Kevern and Sparks 1. Low Cost Techniques for Improving the Surface Durability of Pervious Concrete

Transcription

1 Kevern and Sparks 1 Low Cost Techniques for Improving the Surface Durability of Pervious Concrete John T. Kevern Assistant Professor of Civil Engineering University of Missouri-Kansas City 370H Flarsheim Hall 5100 Rockhill Rd. Kansas City, MO Office: Fax: kevernj@umkc.edu Joseph Dan Sparks Testing Project Manager INTEC 1910 Merrill Creek Parkway Everett, WA Phone: Fax: dsparks@in-tec.com Abstract 198 (250) Text 3761 Tables 6(250) = 1500 Figures 5(250) = 1250 Total 6511 (7500 max) November 15, 2012

2 Kevern and Sparks 2 ABSTRACT This paper presents the results of a laboratory study to improve the durability of pervious concrete using readily available and low cost techniques. Surface raveling of pervious concrete is a concern for long term use and remediation techniques have not previously been investigated. In this study a high void content pervious concrete was cured in worst case hot and dry conditions to produce poor surface durability. Pervious concrete mixtures included a traditional binder and one which included a super absorbent polymer for internal curing. Various remediation methods including overlaying with fresh pervious concrete, latex paint, epoxy, and a surface densifier were applied to the pavement before abrasion testing using the ASTM C944 rotary cutter method. Results showed that the internally-cured mixture had superior durability to the traditional mixture. Of the surface applied techniques, epoxy provided the best improvement in durability followed by latex paint and the densifier. Material analysis showed that modifying a mixture to include super absorbent polymer was the lowest cost option. On a poorly performing pervious pavement, latex paint or a thin overlay both had low material costs. The summary concludes that low-cost methods are effective techniques to improve the surface durability of pervious concrete.

3 Kevern and Sparks 3 INTRODUCTION Pervious concrete has become more popular in recent years primarily due to its environmental benefits. When compared to traditional pavements, pervious concrete pavements provide superior stormwater runoff control, reductions in potential for hydroplaning, and reduced glare among other benefits (1). Until recently pervious concrete has been utilized only for parking areas, primarily on private projects. Various state environmental departments and the Environmental Protection Agency (EPA) have been considering including roadways in required stormwater management areas. The limited available space within typical roadway right of ways does not accommodate many traditional stormwater best management practices (BMPs) such is detention/retention areas. Pervious concrete shoulders are an attractive option for near-roadway stormwater management because no additional land area is required. The current national interest in pervious concrete shoulders has resulted in the first pervious concrete shoulders installed on a 50,000 average annual daily traffic (AADT) highway in St. Louis, MO, pervious concrete shoulders under construction on several state-owned roads in Nevada, and a NCHRP project investigation design options for permeable shoulders with stone reservoirs (25/25: Task 82). Although much progress has been made towards turning pervious concrete into a widely used pavement material, durability concerns are still present. One of the most prevalent durability distresses presently encountered when using pervious concrete is raveling. Raveling is surface abrasion caused by the separation of individual cement-coated aggregate pieces from pavement surface (2, 3). Pervious concrete is characterized by minimal fine aggregate and a near zero slump (1). The open structure of pervious concrete requires specific construction techniques to ensure adequate permeability and strength performance. As with any concrete, proper curing is the most important step towards achieving the optimum durability of the pavement. Previous studies have shown that covering pervious concrete with plastic sheeting immediately after placement for a minimum of 7 days, is the best means of ensuring proper curing and reducing abrasion (4). Many pervious concretes experience some raveling in the early weeks after removal of curing plastic, especially around sawed joints (5). However, severe raveling typically results from poor curing practices. The main causes for surface raveling are improper curing, low strength mixtures from poor compaction, and heavy early loading (2). Figure 1 shows a pervious concrete where poor workability resulted in a high void content of 40% and isolated locations of excessive raveling when poorly cured. The opposing sections with good durability were cured under plastic. The darker sections had poor durability and excessive raveling caused when the fresh paste on the surface dried when not covered with plastic. The only solution presently available when severe raveling occurs is to remove and reinstall the pervious concrete pavement. Remediation options for less severe raveling include continued vacuuming over time to remove loose particles. Removal and replacement or continuous vacuuming are undesirable from time, financial, and sustainability perspectives.

4 Kevern and Sparks 4 Poor Durability Good Durability FIGURE 1 Excessive Raveling of Poor Durability Surface PROJECT SIGNIFICANCE AND OBJECTIVE When poor surface durability results in excessive raveling of pervious concrete the remediation option is removal and replacement. The objective was to determine what the effect of pervious concrete mixture, curing conditions, and surface applied remediation techniques provided the best reduction in surface raveling. The pervious concrete mixtures investigated included a conventional low water-to-cement mixture and one internally cured with a super absorbent polymer. The curing conditions included optimum curing in lime water, curing under plastic, and curing in a hot and dry environmental chamber. The surface-applied techniques included densifier, epoxy, cement slurry, latex paint, and thin overlay. Results indicated that all techniques provided significant improvement in surface raveling with the internally cured mixture having the best raveling resistance. MATERIALS AND METHODS Mixture Proportions A typical pervious concrete design void content of 20%-25% produces acceptable strength and surface durability; however impacts from long haul times, hot weather, slow placement from difficult site geometry, improper compaction, or admixture ineffectiveness can cause the in-place void content to be significantly higher than the design, as shown in Figure 1. For this study a typical mixture was selected and cylinder samples were placed at three densities as shown in Figure 2. All pervious concrete mixtures follow a linear compaction density relationship which allows prediction of any in-place void content using the unit weight measurement (6). For this

5 Kevern and Sparks 5 study the highest void content (37%) was selected to ensure excessive raveling and the ability to measure improvements to surface durability. The selected mixture would have unacceptable performance related to raveling in the field and be subject to removal and replacement. FIGURE 2 Compaction density relationship of the selected mixture Mixture proportions for the two pervious concretes tested are shown in Table 1. One was a typical pervious concrete control (PC) mixture and the second pervious concrete mixture (SAP) contained a super absorbent polymer used for internal curing. The SAP consisted of crushed crystalline partial sodium salts of cross-linked polypromancic acids rated at 2000 times absorption in plain water. The selected SAP has previously been shown to improve the properties of pervious concrete and reduce moisture loss (7). Additional water was added to satisfy SAP absorption which raised the dosed water-to-cement ratio from 0.34 to Void content was maintained by reducing the overall cement paste content. Other admixtures included a vinsol resin air entraining agent, a polycarboxylate water reducer, and a hydration stabilizing admixture. The extra water present in the SAP mixture also allows a reduction in required admixtures as shown in Table 1 for the water reducer and hydration stabilizer. The coarse aggregate was ASTM C33 size 8 limestone (8) with a specific gravity of 2.59 and absorption of 1.8%. The fine aggregate was Missouri river sand conforming to ASTM C33 with specific gravity of 2.62 and absorption of 0.4%. ASTM C150 cement marketed as Type I/II was utilized (9).

6 Kevern and Sparks 6 TABLE 1 Mixture Proportions PC SAP Material kg/m 3 (lb/yd 3 ) kg/m 3 (lb/yd 3 ) Cement 280(470) 260(430) Coarse Agg. 1050(1770) 1050(1770) Fine Agg. 80(130) 80(130) Water 90(160) 100(170) Air Entraining Agent 1.3 ml/kg (2 oz/cwt) HR Water Reducer 3 ml/kg (4 oz/cwt) 2 ml/kg (2 oz/cwt) Hydration Stabilizer 4 ml/kg (6 oz/cwt) 2 ml/kg (3oz/cwt) SAP 1.3 g/kg of cement (2 oz/cwt) Mixing and Testing Methods Concrete was mixed according to ASTM C192 (10). Fresh concrete was preweighed for all specimens before placing to ensure all samples had the same unit weight and voids. The high void content of 37% did not require any compaction techniques and represented the loosest state possible for the selected mixtures. Triplicate 100mm by 200 mm (4 in by 8in) cylinders were cast for unit weight and void content testing determined according to ASTM C1754 (11). In addition to the cylinders, both the PC and SAP mixtures were used to cast slab samples for curing method investigation on abrasion. Fresh unit weight of the slab samples was also controlled and the same as the cylinders. Slab samples had surface area of 750 cm 2 (120 in 2 ) and were 50mm (2 in.) thick. Samples were cured for 7 days 1) in a standard lime-water bath per ASTM C192 (10), 2) under plastic sheeting commonly used for pervious concrete, and 3) cured in a climate controlled cabinet at 38 C (100 F) and 32% relative humidity. ASTM has a standard for potential raveling of pervious concrete ASTM C1747 (3). The test abrades cast specimens in the LA abrasion apparatus and is suitable for comparing overall durability between mixtures, but is not appropriate for measuring changes to surface-applied materials. Rotary cutter surface abrasion was selected for its ability to physically measure differences between surface treatments. Testing was performed according to ASTM C944 (12) which uses a weighted rotary cutter to induce surface wear. Figure 3 shows the rotary cutter device which is used to apply a 98N (22lbf) force during a 2 minute cycle. Pervious concrete specimen weight was recorded before and after testing. A stiff-bristled broom and shop-type vacuum were used to remove loose particles from the surface pores before weighing.

7 Kevern and Sparks 7 FIGURE 3 Surface Abrasion Testing Using ASTM C944, a) Rotary Cutter Head Surface Treatments Table 2 shows the curing regimes and surface treatments performed on the two mixtures. The curing regime was selected to represent a worst-case scenario for pervious concrete placement in addition to the high void content. All specimens for investigating the surface treatments were first cured in an environmental cabinet at 38 C (100 F) and 32% relative humidity for 7 days before surface treatments were applied. Previous research has shown that the use of recycled latex paint in concrete mixtures improves surface durability (13). Additionally, research has shown that the use of latex polymers in pervious concrete improves the resistance to abrasion when compared to other pervious concrete mixtures (14). The use of latex admixtures has also been recommended as a means of improving freeze-thaw durability of pervious concrete (15). Research shows that the use of colloidal silica as a densifier in concrete mixtures improves the strength of the concrete (16). In order to test the beneficial use of densifier as a repair for raveling of pervious concrete, colloidal silica was applied directly to the surface of a pervious concrete so that the silica reacted with the calcium hydroxide present in the concrete to create additional calcium silicate hydroxide gel (CSH). The additional CSH present in the concrete surface was expected to improve surface durability. Previous research has shown that the use of epoxy in concrete mixtures can improve the compressive strength and freeze-thaw durability of the concrete (17). Pervious concrete used as an overlay on traditional, non-pervious pavements is known to reduce both the risk for hydroplaning and noise pollution (18). A high solids latex paint was applied at 7400 m 2 /m 3 (300 ft 2 /gal.). The paint was thinned with equal parts water before application to prevent clogging of the surface pores. Colloidal silica densifier was applied at 9800 m 2 /m 3 (400 ft 2 /gal.). The colloidal silica densifier had a consistency similar to water and was applied at twice the dosage as recommended for a flat, impervious application. A cementitious slurry was created using 1:3 Portland cement to latex admixture ratio. The latex admixture is marketed as a mortar bonding agent and contains 7% solids. The cement slurry was applied at a rate of 580 m 2 /m 3 (177 ft 2 / ft 3 ). The application rate was selected by visually balancing a sufficient surface coating of the slurry while maintaining surface voids. A low viscosity marine grade cycloaliphatic clear epoxy was used at 4900 m 2 /m 3 (200 ft 2 /gal.). The low epoxy viscosity allowed coating of the surface particles without reducing surface porosity. After abrasion testing on a set of pervious concrete control samples cured in the environmental cabinet (PC-C), the SAP mixture was applied as an overlay at a thickness of 12.5 mm (1/2 in.) directly on the abraded surface. The existing surface was cleaned and dried before

8 Kevern and Sparks 8 overlaying. The paint, densifier, and epoxy were allowed to cure at ambient conditions for 7 days after application. The slurry and overlay samples were cured under plastic for 7 days. TABLE 2 Mixtures and Surface Treatments Specimen ID Surface Treatment Curing Technique PC - Lime Water Bath PC-P - Under Plastic PC-C - 32 C, 32% RH SAP - Lime Water Bath SAP-P - Under Plastic SAP-C - 32 C, 32% RH PC-Paint Latex Paint Ambient PC-Dense Silicate Densifier Ambient PC-Slurry Polymer-modified cement slurry Under Plastic PC-Epoxy Marine Epoxy Ambient PC-Over SAP Overlay Under Plastic RESULTS AND DISCUSSION Testing on hardened cylinders verified that both mixtures achieved an average of 37% voids at a unit weight of 1600 kg/m 3 (100 pcf). The only treatment that caused a noticeable visual decrease in surface voids, and presumably permeability, was the cementitious slurry. All other treatments only provided a thin coating to the surface particles. Figure 4 shows the visual results of abrasion testing on the sample surfaces. On samples with higher mass loss values the abrasion resulted in loss of entire aggregate particles and aggregate wearing. Figure 4a shows the control mixture before testing and Figure 4b shows the control after testing where a high mass loss was observed from aggregate raveling. Figure 4c shows the SAP mixture before testing; the picture is of fresh concrete. Figure 4d shows the SAP mixture after testing where all of the mass loss was aggregate wear. Figure 4e shows a latex paint sample after testing and Figure 4f shows an epoxy sample after testing. The other treatments not shown in Figure 4 had no distinguishable visual difference from the control before or after testing. Results of the curing regime trials are shown in Table 3 and represent an average of six trials. Previous studies using ASTM C944 to determine abrasion on pervious concrete have produced similar variability (4). Surface abrasion testing on pervious concrete has a coefficient of variation (COV) higher than observed for other tests such as compressive strength. Because of the high variability, six tests were required for statistical analysis to show differences between groups. Statistical significance was determined using an analysis of variance (ANOVA) with α=0.05. In both mixtures the hot and dry conditions in the environmental cabinet produced higher abrasion from surface raveling (PC-C, SAP-C). There was statistically no difference in abrasion for either mixture between samples cured in the lime-water tank (PC, SAP) and those cured under plastic for 7-days (PC-P, SAP-P). The SAP mixture had significantly less raveling than the control mixture for all conditions, but similar standard deviation which resulted in SAP samples having higher COV. Raveling of the control mixture increased by 110% when cured in the environmental cabinet, however the SAP mixture only increased by 41%. The included SAP has been previously shown to reduce moisture loss from pervious concrete specimens, providing increased cement hydration and strength (7).

9 Kevern and Sparks 9 a b c d e f FIGURE 4 Abrasion Test Results (a) Control Before Testing, (b) Control After Testing, (c) SAP Before Testing, (d) SAP After Testing, (e) Latex Paint After Testing, and (f) Epoxy After Testing

10 Kevern and Sparks 10 TABLE 3 Curing Regime Results Specimen Avg. Mass Loss (g) Std. Dev. (g) COV (%) PC PC-P PC-C SAP SAP-P SAP-C The abrasion results for the various surface treatments are shown in Figure 5 and represent an average of six trials. The error bars represent one standard deviation. All surface treatments provided a significant reduction in raveling when compared to the control samples cured in the environmental cabinet (PC-C), which had the greatest mass loss. The epoxy coating (PC-Epoxy), slurry (PC-Slurry), and thin overlay (PC-Over) had similar performance. The thin overlay had similar performance to the full-depth specimen cured under plastic (SAP-P). FIGURE 5 Abrasion Results A full statistical comparison of results is shown in Table 4. Where two treatments intersect an O represents no difference between groups and X shows a statistically significant difference. There was no difference in durability for either mixture cured in lime water or cured under plastic. Generally there were three levels of performance with no difference between the high durability samples of SAP, SAP-P, PC-Slurry, PC-Epoxy, and PC-Over. The moderate durability specimens of PC, PC-P, SAP-C, PC-Paint, and PC-Dense had statistically less raveling than the cabinet-cured control, but more raveling than the higher durability samples. All treatments had statistically less raveling than the control mixture cured in the environmental cabinet.

11 Kevern and Sparks 11 TABLE 4 Statistical Comparison of Results Treatment PC PC-P PC-C SAP SAP-P SAP-C PC-Paint PC-Dense PC-Slurry PC-Epoxy PC-Over PC - O X X X O O O X X X PC-P O - X O X O O O X X X PC-C X X - X X X X X X X X SAP X O X - O X X X O O O SAP-P X X X O - X X X O O O SAP-C O O X X X - O X X X X PC-Paint O O X X X O - O X X X PC-Dense O O X X X X X - X X X PC-Slurry X X X O O X X X - O O PC-Epoxy X X X O O X X X O - O PC-Over X X X O O X X X O O - Remediation costs were determined for a 929 m 2 (10,000 sf) pervious concrete parking lot 150mm (6 inches) thick. Presented costs only represent material costs determined using prevailing local material and admixture costs and do not incorporate labor for the repairs or material delivery costs. The prevailing costs used for the concrete materials are shown in TABLE 5. The materials only remediation costs are shown in TABLE 6. The pervious concrete control mixture cost $60/m 3 ($46/cy), while the SAP mixture cost $63/m 3 ($48/cy). The SAP mixture line represents the cost difference between using the SAP mixture and the control mixture for the project initially. The observed ease of remediation from least to most intensive was: SAP mixture, densifier, paint, epoxy, SAP overlay, and slurry. If the control mixture was changed to the SAP mixture no additional labor would be required. The paint and epoxy both had low viscosity and flowed into the surface pores with relative ease. The slurry also had low viscosity from the water-to-cement ratio of 3, but had to be carefully applied to prevent clogging surface pores. Based only on material cost and durability, the thin SAP overlay was a superior remediation technique, but would require significantly more labor than an option like paint or densifier. The additional height created by an overlay may not be allowable at certain locations. Those locations might necessitate milling the pavement before overlaying or option for a surface-applied strategy. While pervious concrete has been successfully overlaid on conventional concrete, the thin 12.5 mm (1/2 in.) overlay used in this study has not evaluated under traffic. Epoxy was superior for surface-applied strategies, although latex paint provided a 51% reduction in abrasion and had a lower cost. TABLE 5 Prevailing material costs Material Cost Portland Cement $91/mton ($100/ton) Limestone Sand Water HRWR AEA HS SAP $15.4/mton ($17/ton) $10.9/mton ($12/ton) $0.4/10,000l ($1.5/10,000 gal) $4.5/l ($17/gal) $1.00/l ($3.75/gal) $4.23/l ($16/gal) $28.6/kg ($13/lb)

12 Kevern and Sparks 12 TABLE 6 Remediation Costs Material Application Rate Cost Amount Required Additional Cost SAP Mixture 1.3g/kg (2 oz/cwt) $28.5/kg ($0.81/oz) 45kg (100 lb) $310 SAP Overlay 12.5mm (1/2 in.) $63/m 3 ($48/cy) 11.8 m 3 (15.4 cy) $731 Paint 7400 m 2 /m 3 (300 ft 2 /gal.) $10.5/l ($40/gal) 125l (33 gal) $1,333 Epoxy 4900 m 2 /m 3 (200 ft 2 /gal.) $16.1/l ($61/gal) 189l (50 gal) $3,033 Slurry 580 m 2 /m 3 (177 ft 2 / ft 3 ) $3091/m 3 ($2349/cy) 47m 3 (61 cy) $4,995 Densifier 9800 m 2 /m 3 (400 ft 2 /gal.) $14.8/l ($56/gal) 380l (100 gal) $5,600 CONCLUSIONS Surface raveling is the material related distress most common for pervious concrete. Loose particles on the surface are unattractive, create a difficult walking surface, contribute to clogging, cause further abrasion when driven over, and cause vehicle and windshield damage. Current options for poor surface durability are continued vacuuming to remove the loose particles or complete removal and replacement. This study investigated available low cost techniques to reduce surface abrasion. Pervious concrete was created with high voids (37%) and was subsequently cured in worst-case hot and dry conditions. Abrasion was measured before and after applying treatments to the surface. From this study the following conclusions can be drawn: The mixture containing the super absorbent polymer for internal curing had less abrasion in all curing conditions than the control mixture. No difference in abrasion resistance was observed between mixtures cured in a lime water tank or cured under plastic for 7-days. All remediation techniques tested resulted in improved surface durability. Latex paint and epoxy were both effective at improving surface abrasion resistance. The thin overlay had similar surface abrasion resistance to properly cured samples of the same mixture. Results from surface abrasion laboratory testing indicate that low-cost methods are promising methods to improve the surface durability of pervious concrete. ACKNOWLEDGEMENTS The authors would like to thank Geiger Ready Mixed Concrete for providing the aggregate for the study, Lafarge North America for the cement, BASF construction chemicals for the admixtures, Protecrete for the densifier, and ProCure USA for the super absorbent polymer. Findings are those of the authors and do not represent the opinion or position of the material suppliers. REFERENCES 1. American Concrete Institute (ACI) Pervious Concrete. 522-R10: ACI 522 Committee Report, Farmington Hills, MI: ACI, Kevern, J.T. (2011). Operation and Maintenance of Pervious Concrete Pavements, 90 th Annual Transportation Research Board Annual Meeting, CD-ROM, Transportation Research Board of the National Academies, Washington D.C.

13 Kevern and Sparks ASTM Standard C-1747, Standard Test Method for Determining Potential Resistance to Degradation of Pervious Concrete by Impact and Abrasion, Annual Book of ASTM Standards, West Conshohocken, PA, Vol. 4, No. 2, Kevern, J. T., Schaefer, V. R., and Wang, K. The Effect of Curing Regime on Pervious Concrete Abrasion Resistance, Journal of Testing and Evaluation. Vol. 37, No. 4, JTE101761, Tennis, P.D., Leming, M.L., and Akers, D.J. Pervious Concrete Pavements. EB302, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland, Kevern, J.T. and Montgomery, J. Hitting the Targets: A Case Study of a Pervious Concrete Quality Assurance Program, ACI Concrete International magazine, March Kevern, J.T. and Farney, C. Reducing Curing Requirements for Pervious Concrete Using a Superabsorbent Polymer for Internal Curing. Transportation Research Record: Journal of the Transportation Research Board (TRB), Construction 2012, Transportation Research Board of the National Academies, Washington D.C. (accepted for publication) 8. ASTM, Standard C-33, Standard Specification for Concrete Aggregates, Annual Book of ASTM Standards Vol. 4(2), ASTM International, West Conshohocken, PA: ASTM International, ASTM Standard C-150, Standard Specification for Portland Cement, Annual Book of ASTM Standards, West Conshohocken, PA, Vol. 4, No. 1, ASTM Standard C-192, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory, Annual Book of ASTM Standards, West Conshohocken, PA, Vol. 4, No. 2, ASTM Standard C-1754, Standard Test Method for Density and Void Content of Hardened Pervious Concrete, Annual Book of ASTM Standards, West Conshohocken, PA, Vol. 4, No. 2, ASTM Standard C944, Standard Test Method for Abrasion Resistance of Concrete or Mortar Surfaces by the Rotary-Cutter Method, Annual Book of ASTM Standards, West Conshohocken, PA, Vol. 4, No. 2, Nehdi, M. and Sumner, J. Recycling Waste Latex Paint in Concrete. Cement and Concrete Research, Vol. 33, No. 6, pp , Kevern, J.T., Schaefer, V.R., and Wang, K. Mix Design Development and Performance Evaluation of Pervious Concrete for Overlay Applications, ACI Materials Journal, July- August, Title No. 108-M47, Vol. 108, No. 4, Kevern, J.T., Wang, K., and Schaefer, V. R. Pervious Concrete in Severe Exposures: Development of pollution-reducing pavement for northern cities, ACI Concrete International magazine, pg 43-49, July, Bigley, C. and Greenwood, P. Using Silica to Control Bleed and Segregation in Self- Compacting Concrete, Concrete, Vol. 37, No. 2. Pp , El-Hawary, M. and Abdul-Jaleel, A. Durability Assessment of Epoxy Modified Concrete, Construction and Building Materials, Vol. 24, No. 8, pp , Schaefer, V.R., Kevern, J.T., Izevbekhai, B., Wang, K., Cutler, H., and Wiegand, P. Construction and Performance of the Pervious Concrete Overlay at MnROAD, Transportation Research Record: Journal of the Transportation Research Board (TRB),

14 Kevern and Sparks 14 No. 2164, Transportation Research Board of the National Academies, Washington D.C., pp , DOI / , 2010.