Pervious Concrete Construction: Methods and Quality

Kevern et al, submitted to NRMCA Concrete Technology Forum: Focus on Pervious Concrete, May 24-25, 2006, Nashville, TN

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, [email protected] 2 Assistant Professor, Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA,[email protected] 3 Research Assistant Professor, Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, [email protected] 4 Professor, Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA, [email protected] 1


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).

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Table 1. Typical mix design of existing PCPC (NRMCA 2004) Property

Amount / yd3

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

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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

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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.

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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 (