Pervious Concrete Construction: Methods and Quality Control

Transcription

1 Pervious Concrete Construction: Methods and Quality Control Kevern, J. 1, Wang, K. 2, Suleiman, M.T. 3, and Schaefer, V.R. 4 Abstract The beneficial properties of pervious concrete on stormwater control are well understood. As the use of pervious concrete becomes more prevalent throughout the United States, the issue of constructability will become more of a concern. A number of practices exist to place pervious concrete, without any theoretical underpinnings or correlation to laboratory scale studies. This paper describes the current state of practice in Portland Cement Pervious Concrete (PCPC) placement and also presents results from a study performed at Iowa State University to determine a field level QC/QA check for fresh PCPC. Test slabs were placed using a variety of techniques currently employed for field placement of PCPC. Results show that PCPC samples with void ratios ranging from 15% to 20% have 7-day compressive strengths of about 3,000 psi and permeabilities of about 300 in./hr., both values have been shown suitable for pervious concrete applications. Our studies show that samples with 15% to 20% voids have unit weights around 129 pcf, which suggests the development of a unit weight QC/QA check to be promising. The construction technology of PCPC is evolving, but the correlation between laboratory and field placement will allow standard QC/QA checks to be developed for producing permeable, strong, durable, and long-lasting pervious concrete. 1 Research Assistant, Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, kevernj@iastate.edu 2 Assistant Professor, Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA,kejinw@iastate.edu 3 Research Assistant Professor, Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, suleiman@iastate.edu 4 Professor, Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, vern@iastate.edu 1

2 Introduction This paper is divided into two sections. The first section presents the state of practice in PCPC construction as determined from the literature and the authors experience, as well as some discussion on the evolution of PCPC construction. Many questions have been raised about the effect of placing methods on the resulting pavement properties and durability of PCPC. The second section presents data from a study at Iowa State University (ISU) with the objective of determining a relationship between current placing and finishing techniques with hardened PCPC properties. The calibration of lab scale results with realistic placing procedures will better help produce QC/QA criteria and procedures to produce more consistent and durable PCPC. Literature Review General. Pervious concrete pavement has been in use for over 30 years in Florida and an experimental road was constructed in England in the 1960 s (Youngs 2005, Maynard 1970). PCPC has seen widespread use in Europe and Japan, although not for stormwater improvements. Many highways use a surface course of PCPC to improve skid resistance and reduce traffic noise (Beeldens 2001). The primary utilization for PCPC in the United States (U.S.) is for stormwater benefits. Current PCPC is most often used in the U.S. for parking lots and recreational pathways and, in some cases, low-volume roads (Tennis et al. 2004). Parking lot applications allow stormwater to infiltrate, and reduce or eliminate the need for additional control structures, such as retention ponds. The large internal surface area of the pervious concrete system catches a majority of the pollutants in the stormwater and allows microbes to naturally reduce the concentration. Instead of accumulating in nearby surface waters, the pollutants are degraded or trapped in the pavement system thereby increasing overall water quality. Other uses include tree grates in sidewalks and hydraulic erosion control structures. Construction Materials. The porosity in PCPC is created by the reduction or elimination of fine aggregate from the concrete mix design. Standard pervious concrete in the U.S. is a mixture of a single-size coarse aggregate and cement combined at low water to cement ratios (Florida Concrete and Products Association, Inc. 2000, Tennis et al. 2004). Table 1 shows typical PCPC mix proportions used in the US as reported by the National Ready Mix Association (2004). 2

3 Table 1. Typical mix design of existing PCPC (NRMCA 2004) Property Amount / yd 3 Cement 300 to 600 lb Coarse aggregate 2400 to 2700 lb Fine aggregate 0 Water / Cement 0.27 to 0.43 PCPC Material Properties. Strength is often the primary concern for concrete pavement designs. Since PCPC has a high void ratio (15%-35%), often without fine aggregate, compressive, tensile, and flexural strengths tend to be lower than that of traditional concrete. Although the NRMCA gives a 28-day compressive strength range of a typical US mixes from 800 psi to 3000 psi with certain mixes exceeding 3000 psi (Tennis et al. 2004), the NRMCA suggests using a 2500-psi compressive strength and a 500-psi flexural strength for low-volume PCPC pavement for design purposes (NRMCA 2004). Since 3000 psi is less than required for many conventional applications (Kosmatka et al. 2002), PCPC use has been limited primarily to parking lots (Tennis et al. 2004). Early mix designs had flexural strengths ranging from 150 psi to 400 psi (Carolinas Ready Mixed Concrete Association, Inc. 2003), but recently flexural strength values have improved. Mix designs not containing fine aggregate have been shown to achieve 500 psi flexural strength. Smaller aggregate was shown to produce higher flexural strength, probably due to the increased contact area of the aggregate particles (Olek et al. 2003, Yang et al. 2003). Construction Practices Past and Present While not a totally unknown material, pervious concrete still remains a specialty item in most of the United States and consequently, placing and finishing techniques have not yet become standardized. Site Preparation and Placing. Pervious concrete placement in the United States, whether it is a parking lot or a recreational trial, requires the use of forms to provide a surface for finishing and support for the pavement during curing. Base thicknesses vary 3

4 based on the pavement design storm and infiltration rate of the natural soil, although typically six to twelve inches of a highly drainable material is used (Tennis et al. 2004). Figure 1 shows the preparation of a pervious concrete site in Sioux City, Iowa. The base at this particular site was 18 in. thick due to anticipated drainage from surrounding impervious area. It should be noted that ruts in the base course are removed prior to placing, Figure 1 shows an intermediate stage in the construction process. 8 inch forms 18 inch drainable base Figure 1. Site preparation Current PCPC mix designs do not have the flowability of normal concrete and therefore can be more labor intensive to place. Typically, a rear discharge ready mix truck is used with one shoot instead of the normal two attached for placing. The shorter than normal shoot length allows a steeper angle and better discharge of the concrete. Even with the steep angle and shorter shoot, the PCPC often needs to be manually pulled from the shoot. Some mixes containing a viscosity modifying admixture (VMA) have had better flow characteristics and do not require this practice. Figure 2 shows typical placement using a number of workers to help discharge and distribute the material in the forms. Single additional shoot and worker aiding concrete distribution Worker raking PCPC into position Figure 2. PCPC placing 4

5 Figure 3 shows placing PCPC at the Minnesota DOT MnRoad facility, which contains a VMA. Notice the full-length shoot and fewer people required for placement. Full-length shoot Figure 3. Placing PCPC containing a VMA In Belgium, PCPC is used as a thin surface course for skid resistance and noise reduction. To achieve a strong bond between the layers, the PCPC layer is placed using a wet-onwet method with a slipform device, placing the PCPC on the still plastic non-pervious concrete layer (Beeldens et al. 2003). At the time of publication there were plans in Michigan to place a subdivision road using a slipform paver in 2006 (Robert Risser, personal communication). Compaction and Finishing. Since PCPC is an open-graded mix design, finishing takes on a different meaning than standard concrete pavement. Finishing and compaction are the most crucial steps to producing a durable pavement. Properly finishing PCPC provides a uniform and level surface that prevents surface raveling of the aggregate, while remaining aethstetically pleasing to the public. Dry, poorly finished slabs can ravel and appear to have failed even though they are structurally sound. Properly finished PCPC provides a surface suitable for wheelchair and rollerblade use and an ideal surface for recreational trails. Many methods exist to finish and compact PCPC and they range from those that strictly are for finishing, to those that only provide compaction, but most operations provide some degree of both. Historically, PCPC is struck off ¾ in. to 1 in. above the forms using a shim and vibratory screen as shown in Figure 4. Then the shims are removed and the pavement is compacted to final grade using a weighted roller as in Figure 5. 5

6 Vibratory screed Figure 4. Strike off with a vibratory screed ¾ in. shim on top of forms Weighted roller Figure 5. Compacting using a weight roller Most recently, roller screeds have been used to finish many PCPC pavements in the U.S. Roller screeds have been used for many years on traditional concrete, and provide a rapid method to produce smooth PCPC pavements. Roller screeds are hydraulically driven, stainless steel tubes that rotate against the direction of placement. Small width slabs (<15 ft.) can easily be finished with a manual roller screed such as the screed used to finish the pervious test slab at the MnRoad facility (Figure 6). Larger width slabs must be finished with a powerscreed such as the screed demonstrated at the American Society of Concrete Contractors (ASCC) conference in Denver 2005 (Figure 7). 6

7 a) b) Figure 6. a) Finishing using a manual roller screed, b) Finished surface a) b) Figure 7. Finishing using a powerscreed The Iowa Concrete Paving Association (ICPA) and ISU have identified freeze-thaw durable mix designs, but initial field implementation has been limited due to the lack of placing equipment, such as roller screeds. Consequently, some unique finishing methods have been used both in Iowa and throughout the country to produce PCPC. The sidewalks at a hospital expansion in Davenport, Iowa were bid as PCPC, but limited space and experience presented a problem for the contractor. A test slab was placed to determine the best method using equipment available to the contractor. Figure 8 shows striking a slab off ¾ in. above the final thickness using a hand-held vibrating screed. The shims were then removed and compaction was achieved using a plate compactor on top of a sheet of plywood (Figure 9). The entire sidewalk at the hospital site was finished using this method and has weathered its first winter without any raveling. 7

8 ¾ inch shims Figure 8. Striking off using a vibrating screed Figure 9. Compaction using plywood and a plate compactor Other unique compaction and finishing methods included that used at the University of Tennessee-Chattanooga, where an asphalt paver was used to place a pervious parking lot at Finely Stadium (Sparkman 2005) as shown in Figure 10. PCPC edge raveling can be minimized by using a standard edging tool around the forms. The edging also adds to the aesthetics of the finished PCPC. Figure 11 shows use of an edging tool at the MnRoad placement. 8

9 Figure 10. Pervious placement using an asphalt paver Figure 11. Finishing a PCPC slab Jointing and Curing. Similar to normal concrete, joints are used in PCPC to control and prevent random cracking; however, due to the rougher texture of PCPC control joints are not always required. While most PCPC applications contain joints, a 9

10 few parking lots in California have been placed without the use of control joints (Youngs 2005). Due to the open structure, PCPC shrinks less than standard concrete, and if joints are installed, the spacing can be increased from the standard 12 ft to 15 ft spacing (Tennis et al. 2004). The NRMCA recommends slab lengths not exceeding 20 ft, although spacings of up to 45 ft have been reported without shrinkage cracking (Paine 1992, Tennis et al. 2004). Joints can either be cut or formed, with formed joints being the preferred method. A joint roller, often called a pizza cutter, quickly and easily forms PCPC joints in the plastic concrete as shown in Figure 12. Joints can be saw cut, but experience has shown that saw cut joints have more potential for raveling than formed joints. Due to the large amount of exposed surface area, fresh PCPC must by sprayed with curing compound and covered with clear plastic sheeting soon after placement to prevent drying (Carolinas Ready Mixed Concrete Association, Inc. 2003). Since normal curing compounds only coat the top layer of PCPC, often the surface is misted with water and then covered with plastic. Figure 13 shows uneven coverage of curing compound caused by the pervious surface. Covered curing is recommended until the pavement is opened for operation, which should be a minimum of seven days after placing (Tennis et al. 2004). Figure 12. Forming a joint using a pizza cutter Figure 13. Uneven distribution of curing compound 10

11 Iowa State University Compaction Study As previously mentioned, there are many methods to finish and compact PCPC but little or no information is known about the effect of various placing methods on the final pavement properties. A study is currently underway at Iowa State University to determine the effects of placing/finishing methods on PCPC properties and ultimately freeze-thaw durability. Some initial results are presented herein and additional information can be found in Kevern (2006) and Schaefer et al. (2006). Slabs were prepared in the lab using a mix design comprised of locally available materials and proportions that have proven to be freeze-thaw durable (Kevern et al. 2005). Cores were taken to determine void ratio, permeability, and strength, while beams were taken for freeze-thaw testing. Previously presented data has shown that PCPC samples with unit weight around 129 pcf can achieve greater than 3,000 psi compressive strength and permeabilities of about 900 in./hr. This unit weight was used as a reference value (Kevern et al. 2005). Compaction methods were compared using a nuclear gauge to determine density of plastic concrete and void ratio analysis on cores taken from the hardened slabs. Compaction was achieved using a roller screed as a baseline and a plf weighted roller and varying the number of passes. Initial results continue to follow a linear trend: unit weight of the hardened concrete decreases with increased voids, as reported by Kevern et al. (2005) and Kevern (2006). Figure 14 shows the linear relationship between unit weight and void ratio as determined from core samples. The core unit weights for both the 8 pass and 16 pass samples were similar at 24% voids, on the following graph these data points overlap y = x R 2 = Hardened Unit Weight (pcf) Void Ratio (%) Figure 14. Relationship between unit weight and voids 11

12 Figure 15 shows the relationship between plastic unit weight and the number of passes with a weighted roller. The lowest unit weight represents finishing using only the roller screed. The three subsequent samples were struck off ¾ inch above the forms with a roller screed and roller with the corresponding number of passes. Figure 16 shows a finished slab after compaction, the laboratory slab specimens had texture similar to that observed in field placement Plastic density (pcf) Number of Passes Figure 15. Relationship between unit weight and compaction Weighted Roller Figure 16. Test slab after finishing and compaction 12

13 The plastic unit weight as determined by a nuclear density gauge in backscatter mode (surface density) increased linearly from 0 to 8 passes and then began to asymptotically approach a maximum density. Freeze-thaw beams have yet to complete testing, but the initial trend is better performance with increased density. Conclusions Many methods exist to place and finish PCPC, but little is known about the effect of construction method on long-term durability. Compaction energy as related to PCPC properties through density can aid in achieving pavements which balance strength and required permeability. Initial freeze-thaw results from the ISU slab study have shown that compaction energy plays an important role in the pavement performance. Therefore, additional research is needed to further understand the effect of construction and compaction on freeze-thaw durability of a variety of mix designs. References Beeldens, A,. Van Gemert, D., and Caestecker, C. (2003). Porous Concrete: Laboratory Versus Field Experience. Proceedings 9th International Symposium on Concrete Roads, Istanbul, Turkey. Beeldens, A. (2001). Behavior of Porous PCC Under Freeze-Thaw Cycling. Paper presented at the Tenth International Congress on Polymers in Concrete, Honolulu. Carolinas Ready Mixed Concrete Association, Inc. (2003). Pervious Concrete Installation Course. Florida Concrete and Products Association, Inc. (2000). Portland Cement Pervious Pavement Manual. Kevern, J. (2006). Mix Design Determination for Freeze-Thaw Resistant Portland Cement Pervious Concrete, Master s Thesis, Iowa State University, Ames, IA. Kevern, J., Wang, K., Suleiman, M., and Schaefer, V. (2005). Mix Design Development for Pervious Concrete in Cold Weather. Proceeding of the 2005 Mid-Continent Transportation Research Symposium, Ames, IA. Kosmatka, S., Kerkhoff, B, and Panarese, W. (2002). Design and control of Concrete Mixes. 14 th edition, Portland Cement Association. Maynard, D.P. (1970). A Fine No-Fines Road, Concrete Construction, p National Ready Mixed Concrete Association (NRMCA). (2004). Freeze-Thaw Resistance of Pervious Concrete. 13

14 Olek, J., and Weiss, W. J., (2003). Development of Quiet and Durable Porous Portland Cement Concrete Paving Materials. Final Report SQDH Center for Advanced Cement Based Materials, Purdue University, West Lafayette, IN. Paine, J. (1992) Portland Cement Pervious Pavement Construction, Concrete Construction, p C Schaefer, V.R., Wang, K., Kevern, J., Suleiman, M.T. (2006). Mix Design Development for Pervious Concrete In Cold Weather Climates. Research Report, Center for Transportation Research and Education, Iowa State University, Ames, Iowa. Sparkman, A. (2005). Presentation on Pervious Construction by the Tennessee Concrete Association at Pervious Concrete and Parking Area Design Workshop, Omaha. Tennis, P.D., Leming, M.L., and Akers, D.J. (2004). Pervious Concrete Pavements, EB302, Portland Cement Association, Skokie, Illinois, and National Ready Mixed Concrete Association, Silver Spring, Maryland. Yang, J., and Jiang, G. (2003). Experimental Study on Properties of Pervious Concrete Pavement Materials. Cement and Concrete Research, V. 33, p Youngs, Andy. (2005). Pervious Concrete It s for Real. Presentation at Pervious Concrete and Parking Area Design Workshop, Omaha, NE. 14