Laboratory evaluation of permeability and strength of polymer-modified pervious concrete
Introduction
Portland cement pervious concrete (PCPC), also referred to as porous concrete or permeable concrete, is a mixture of portland cement, uniform coarse aggregate, with either a small amount of or without fine aggregate, and water. Appropriate amounts of water and cementitious material are employed to create a paste that forms a thin coat around aggregate particles but leaves free spaces between them. Thus, pores are formed in the pervious materials [1], [2]. PCPC has been used for over 30 years in many countries, especially in the United States and Japan. It is increasingly used in the United States because of its various environmental benefits such as controlling storm water runoff, restoring groundwater supplies, and reducing water and soil pollution [3], [4], [5]. In the meantime, it has the potential to reduce urban heat island effects and can be used to reduce acoustic noise in roads [5], [6].
PCPC contains little or no fine aggregate, using an adequate amount of cement paste to coat and bind the aggregate particles together to create a system of high porosity and interconnected voids that can drain off water quickly. Generally, the void content of PCPC is between 15% and 25%, and the water permeability is typically about 2–6 mm/s [5], [7]. However, relatively low strength is usually associated with the high porosity in PCPC. The low strength of conventional pervious concrete not only limits its application in heavy traffic highways but also influences the stability and durability of the structures, because of, for example, susceptibility to frost damage and low resistance to chemicals. Therefore, PCPC with low strength can only be utilized in some applications, such as sidewalks, parking lots, recreation squares and subbases for conventional pavement [8], [9], [10]. And with some effective improvement in strength and using smaller size aggregate, PMPC could be applied in pavement shoulder and local roads.
However, by using appropriately-selected aggregates, fine aggregates mixtures, and organic intensifiers and by adjusting the concrete mix proportion, strength and abrasion resistance of PCPC can be improved greatly [11]. Previous studies show that gradation, particle size of aggregate, and mass ratio of aggregate to cement are the primary factors affecting porosity, permeability and compressive strength of PCPC. Water cement ratio has a minor effect on properties of PCPC [12]. Using smaller size aggregate can increase the number of aggregate particles per unit volume of concrete, the specific surface of aggregate, and the binding area, which eventually results in an improvement in the strength of pervious concrete. Wang [10] used river sand to replace approximate 7% (by weight) coarse aggregate to improve the concrete strength. Their results indicated that the 7-day compressive strength increases from 9.6–14.5 MPa to 22.2–22.7 MPa. Although the void content is reduced due to the fine sand in the mixtures, all void content values are still within an acceptable range (>15%) for PCPC applications, and the permeability value is still higher than the minimum requirement to drain [10]. Yang and Jiang [11] showed that use of silica fume (SF) and superplasticizer (SP) in pervious concrete can enhance its strength significantly. The results also indicated that SF had a better effect for improving the properties of pervious concrete than polymer when used with SP. Their results indicated that the compressive strength of PCPC can reach 50 MPa and the flexural strength 6 MPa. At the same time, the requirements of water penetration, abrasion resistance can also be satisfied. Some fibers are helpful in improving the tensile strength and permeability of pervious concrete. Generally, the fibers in PCPC slightly increase the void content, significantly increase the permeability, and more significantly improve the splitting tensile strength of PCPC [10], [13]. The addition of polypropylene fiber at 0.56% by volume of the concrete causes a 90% increase in the indirect tensile strength and a 20% increase in the flexural strength. Polypropylene fiber does not significantly affect the other mechanical properties [12]. Another effective method to improve strength is to use some chemical additives, such as polymer. Kevern [13] also presented that the addition of polymer (styrene butadiene rubber, SBR) significantly improves workability, strength, permeability, and freeze–thaw resistance, which makes pervious concrete obtain higher strength at relatively lower cement contents and results in relative higher porosity.
Section snippets
Research objective and scope
The objective of the present study is to evaluate the effect of polymer modification on the mechanical and physical properties of PCPC. The research efforts were made to balance the permeability and strength of the polymer-modified pervious concrete (PMPC) so that the mixtures are permeable and also strong enough to support traffic loading.
In this study, three types of single-sized limestone aggregates (12.5 mm, 9.5 mm, and 4.75 mm) were used, and one type of polymer (SBS latex) was considered to
Materials
Ordinary Type I portland cement was selected in the experiments. Three gradations of single-sized sieved limestone were considered as coarse aggregate: 12.5 mm, 9.5 mm, and 4.75 mm. The properties of coarse aggregate were measured according to ASTM specifications and listed in Table 1. The grain-size distribution of the river sand from the Tennessee River used in this study is shown in Fig. 1.
Latex polymer, styrene butadiene rubber (SBR), was selected and incorporated into the mixtures in order to
Porosity
Fig. 5 presents the porosity results for all pervious concrete mixtures and the effect of latex on porosity. It is seen that most of the mixtures had porosities within the range from 20% to 30%, which is acceptable. The three coarse aggregates with different sizes exhibited similar porosity, indicating that aggregate gradation did not have a significant effect on the porosity results.
It also can be seen form Fig. 5 that the addition of latex and sand resulted in a slight decrease in porosity.
Summary and conclusions
A laboratory experiment was conducted to investigate the permeability and strength characteristics of polymer-modified pervious concrete. The effects of latex, natural sand, and fiber were evaluated based on the laboratory test results. Based on this study, the following conclusions can be drawn:
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Use of the combination of latex, natural sand, and fiber could produce acceptable pervious concrete with both enough drainage and strength properties.
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Latex and sand could both decrease the porosity and
Future research
This is a preliminary laboratory study on the effect of polymer modification on the performance of pervious concrete with the emphasis on the permeability and strength properties. The durability of polymer-modified pervious concrete should be included in future studies to evaluate the abrasion resistance of PMPC.
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