The Effects of Water/Cement Ratio and Air Entraining on Portland Cement Concrete Freeze/Thaw Durability

Transcription

1 The Effects of Water/Cement Ratio and Air Entraining on Portland Cement Concrete Freeze/Thaw Durability by Shell S. Hodgson Civil and Environmental Engineering Department University of Wisconsin-Madison May 10, 2000

2 The Effects of Water/Cement Ratio and Air Entraining on Portland Cement Concrete Freeze/Thaw Durability to the Editors of Undergraduate Engineering Review by Shell S. Hodgson Civil and Environmental Engineering Department University of Wisconsin-Madison May 10, 2000 Abstract This paper deals with the issues of air entraining and water/cement ratio in Portland Cement Concrete as related to its freeze thaw durability. Practices have been in place for nearly thirty years which demanded high early strengths in Portland Cement Concrete pavements. This paper looks at those practices, investigates the correlation between water/cement ratio and air entraining, and presents conclusions and recommendations based on available data.

3 Table of Contents Introduction p. 1 Background p. 2 Methods of Testing PCC Freeze-Thaw Durability p. 5 Quantifying W/C and AE Effects p. 6 Preventitive Measures and Good Practice p. 9 Conclusion p. 10 Recommendations p. 11 Glossary p. 12 Bibliography p. 13

4 Introduction Portland Cement Concrete (PCC) plays an important role in everyone s life as the most largely used construction material in the world. It is used aesthetically and structurally all around us. It is strong when loaded in compression, and relatively inexpensive and easy to produce. PCC is used to support most of the homes we live in, makes up a large portion of the buildings we study in, and serves as a quality paving material for the sidewalks we walk on and many of the roads we travel, as well as many other applications in the construction industry. However, in the seasonally variable climate of Wisconsin, exposed PCC surfaces are subjected to stresses due to repetitive freezing and thawing. These stresses cause many of the ugly, rough surfaces we see and drive on which, are an economic burden and social inconvenience. Pavements constructed using PCC are an important part of our nation s infrastructure. It is used extensively in our interstate highway system, highway bridges, and in a large number of urban streets. The physical characteristics of PCC provide for greater weight transfer in loading, which extends longevity of the road and its lighter color reflects light, providing much greater visibility at night. These benefits, however, may be outweighed by high life cycle costs due to premature failure from poor freeze-thaw cycle durability when poor quality control exists and unsubstantiated mixed design criteria are followed. PCC is a mix of many different materials and is inherently porous, not solid and impervious like a material such as steel. In hardened concrete, ice will form from free water in the larger pores. The ice crystals will grow by drawing water from the walls of the additional voids. This then causes water to be drawn from the smallest pores until eventually, as the ice crystals grow large enough to where the pressure build-up in dense pastes will result in rupture.

5 There are two main factors which have variability in production, as well as in the field that greatly affect the freeze thaw durability of PCC. The effect of these two highly variable factors, Water-Cement (W/C) ratio and Air-Entraining (AE) are the basis of this paper. This paper will present pertinent background information for PCC, an explanation of freeze-thaw testing procedures, and a summary of research data, conclusions and recommendations. Background Water/Cement Ratio. W/C ratio is a factor that affects freezethaw durability but also PCC s. Freeze-thaw durability for PCC is the concrete s ability to resist degradation due to exposure to repeated freeze-thaw cycles. This is the ratio of the amount of water to the amount of cement used (by weight) in the cement paste. Concrete achieves its strength through a chemical process called hydration. Hydration is a complex process but in simple terms, is the reaction between water and the cement dust in the mix. As in any chemical reaction, only a certain amount of water is needed for the reaction to reach completion. In PCC a water cement ratio of.22 to.25 will satisfy the chemical need. However, low W/C ratio mixes are very difficult to place, finish and work with. Higher W/C rations provide for more fluidity, which allows it to flow into desired, formed geometries more readily. In order for the mixed PCC to reach a manageable level of workability, water cement ratios will be in the.35 to.40 range or higher. The excess water used in the mix migrates to the surface of the concrete and evaporates. This leaves a network of flow channels called capillaries and as a result, a hardened finished product, which is less dense, and porous. The capillaries also provide the pathway for water to be absorbed.

6 Excessive water added to the paste is problematic by reducing ultimate strengths achieved in the finished product. This is because of the reduced density and increased porosity, similar sandstone compared to limestone. Low W/C ratios have been considered to provide greater freeze-thaw durability properties due to high ultimate strengths, and by reducing porosity, limiting the ability of free water to penetrate. This assumption has little supporting data. Air Entraining. Air content quantities in PCC are controlled by the use of an air-entraining agent. An air-entraining agent is an organic material which, added as an admixture to concrete in very small dosages (about 1 per cent or less by weight of the cement), will generate air bubbles through a foaming action during mixing. This is much like the development of bubbles in water when dish soap is mixed in vigorously. Under optimum mixing conditions, the bubbles are spaced about 0.01 inch apart or less and are of about the same order of magnitude in maximum diameter. It has been stated that PCC exposed to severe freeze-thaw cycles should have an air content of 5.5 to 6.0 percent by volume [Mamlouk & Zaniewski, 1999]. This incorporation of unfilled, well-distributed voids in the cement paste prevents excessive pressures from developing, due to water crystal formation during freezing, by providing spaces for these developing pressures to disperse, reducing disruptive damage. This accounts for the remarkable effect that AE has on improving the freeze-thaw damage resistance of PCC. Air entraining provides an additional benefit to concrete mixes. This is due to the fact that air entraining improves workability. This phenomenon is caused by the micro-bubbles acting as a kind of lubricant, better described as behaving like miniature ball bearings where these well-dispersed bubbles act as air cushions between aggregate particles, reducing friction and interlocking. This allows for a

7 slight reduction in water content for concrete with an air-entraining admixture. Concrete Performance. While many older concrete pavements are still performing well [Shilstone, 1999], the repair and restoration of new concrete pavements are becoming the fastest growing segment of the concrete industry. These repairs and restorations to newer pavements are not only costly in terms of direct costs, but also in terms of user s lost time due to delays in construction zones, and in terms of resources lost that could have been used in other projects. In the 1970s, soaring interest rates controlled many construction techniques and practices. At that time, the focus on concrete paving was to produce early high strengths. These early strengths are usually achieved by the use of larger quantities of cement dust. They provide for a faster paving process allowing for greater production and an earlier opening to traffic. This practice was done to cut high costs, due to finance charges, by accelerating construction, which reduced project duration. The focus on long term performance has been limited to the thought that high strengths along with moderate AE would provide durability. For those of us who live in the Midwest and drive on the highways within, obvious flaws in this thinking become apparent. Rough riding pavements and aggravating delays due to pavement repairs and renovations impact us daily. Obvious to owners, constructors, and users is the fact that many new concrete surfaces do not last. I do not feel that many current PCC pavements reach their designed life and that deterioration is often evident after only one season of freeze-thaw exposure. Premature deterioration of concrete pavements is being researched by many public and private organizations. The Minnesota Department of Transportation

8 has identified types of issues contributing to premature deterioration, including high water/cement ratios and low air contents. The W/C ratio and air content standards in the concrete paving industry are two main factors that affect concrete freeze/thaw durability. Long lasting concrete surfaces are a benefit economically. Included in this paper is data supporting and/or contradicting standard practices concerning freeze-thaw durability and suggested alternatives to today s practices that may result in extended concrete pavement surfaces life span. Reduced W/C ratios and adequate AE both contribute to better freeze thaw durability in concrete, and there are potential benefits of each. The benefits of air entraining have been relatively well researched and documented, however, it had been much more difficult to qualify and quantify the benefits of reduced water-cement ratio with the lack of research material in this area. As stated above, there has been limited research as to the correlation of freeze-thaw durability as it relates to water-cement ratio. In fact, it has been suggested that water-cement ratio has less to do with durability than the amount of water alone. That is reducing the amount of cement dust reduces the amount of water needed which could be the reason behind older pavements that are still functional [Shilstone, 1999]. The recently finished research by Walls and Cramer is what I believe, to be the first attempt at correlating air content and watercement ratio to freeze thaw durability. Methods of Testing PCC Freeze-Thaw Durability There are several methods of laboratory testing of concrete specimen s freeze- thaw durability. One is the Rate and degree of absorption. This has been used for many years as a means of predicting durability of concrete to frost damage. The test is relatively simple and

9 inexpensive but, unfortunately, yields very poor correlation with field performance [Swenson, 1999]. The generally accepted method of testing is the direct freeze-thaw cycling of concrete specimens. This is a simulation of natural freeze-thaw cycling, except that it is more rapid and gives results in reasonable time. Rate of deterioration is usually determined by measuring resonant frequencies from which the dynamic modulus can be calculated. Weight loss is another but less reliable measure. This method is the method used by Walls and Cramer, whose paper provides the bulk of the evidence used in this paper. The freeze-thaw cycling test gives results that permit reasonably good assessment of the field performance that can be expected of a particular concrete mix. It has, however, several drawbacks. Equipment is fairly expensive and there are not many agencies or institutions with such facilities. Fortunately, the University of Wisconsin has the facilities needed for Professors Walls and Cramer to conduct their research. Another valuable test, known as the linear traverse method, consists of determining by optical microscope the air bubble content and spacing in hardened cement paste or concrete. This test readily discovers whether the entrained air will provide adequate protection. Cramer and Walls make use of this test to verify air contents found by testing the freshly mixed concrete with the simple air-meter measurement. This test indicates whether or not enough air has been added, but it does not indicate the size and distribution of bubbles in the set concrete. Quantifying W/C and AE Effects The preliminary results obtained from research conducted by Steven M. Cramer and Richard A. Walls from the Civil and Environmental Engineering Department at the University of Wisconsin- Madison has offered some very interesting and important data.

10 It has been suggested that some evidence has been presented that durable PCC mixes can be obtained with minimal AE provided that low W/C are used. Sufficient AE does greatly improve freeze-thaw durability and this is well documented in a variety of sources. However, the correlation between the controlling W/C and AE together for improved freeze-thaw durability is not well documented [Walls and Cramer, 2000]. Walls and Cramer s research focus was to examine the interrelationship between W/C ratio and AE freeze-thaw durability benefits. Using the testing techniques and procedures described in the previous section, Walls and Cramer evaluated performances of various PCC mix designs. These designs are variations of mix designs used by the Wisconsin Department of Transportation in paving applications. The variations were in the partial replacement of Portland cement with either fly ash or ground blast furnace slag. These variations are not important for the purposes of this paper. Specimens were subjected to strength, shrinkage, permeability and accelerated freeze-thaw testing. All testing was conducted according guidelines set forth by the American Society for Testing and Materials (ASTM) [Walls and Cramer, 2000]. The procedure for freeze-thaw testing of PCC in ASTM C666 does not explicitly define a point of failure. Walls and Cramer have reported in their paper that a relative modulus of 60% or less after exposure to 300 freeze-thaw cycles is considered failure in other literature and is implied in ASTM C666. Studying the plotted data of their research on the pages to follow show that two mixes, FA-2 and S-2, clearly fail this criteria. Figure 1 on the following page shows the graphical relation relative modulus, strength, of nine specimens with respect to the number of freeze-thaw cycles each were subjected to. The samples, as stated in the legend, had w/c ratios from.3 to.45 and volumes of air entrainment ranging from 1.8 percent to 8.3 percent. Figure 2, immediately following portrays the same data with four individual specimen s data removed to

11 more clearly show the trend in the research results. Figure 2 also depicts a range of one standard deviation for each specimen s trend line. It should be noted that even considering the possible deviations that the results are unmistakable. Figure 1. Plotted results for nine specimen s relative strength through an exaggerated number of freeze-thaw cycles (Walls and Cramer, 2000).

12 Figure 2. Five specimen s results form Figure 1. The range for a standard deviation of one is represented on each trend line (Walls and Cramer, 2000). As seen in the above data, mixes containing little AE, regardless of W/C ratios, performed unsatisfactorily. It can be seen that mixes possessing W/C ratios as low as 0.30 performed poorly. Walls and Cramer report that By 400 cycles, the specimens with little AE had disintegrated to pieces of paste and aggregate. Preventive Measures and Good Practice The moisture content of concrete is the most critical factor of frost damage for those elements subject to freeze-thaw cycling. Adequate drainage to provide rapid water run-off is thus a most important design feature. Air entrainment should be a requirement for all exposed concrete.

13 It should be noted that air entrainment is achieved only in a limited range of plastic consistency of the concrete, which is dependent on the water content. Very wet or very dry mixes do not entrain air in proper size and distribution. Over-vibration and excessive trowelling also affect the entrained air adversely. Vibration is a technique used to consolidate low water cement ratio mixes with low workability and trowelling is the method of finishing the concrete surface to a desired smoothness. Over finishing or vibrating can eliminate the durability that was intended in the mix design. Conclusions I have more than fifteen years of experience in the construction industry, eight of which were directly involved in concrete production, testing, or placement and finishing. The quality of work and durability of the finished products of projects I have been involved with in any capacity has been my primary focus. Poor quality control issues in the concrete industry have always been apparent. I feel that this research raises questions as to the effectiveness of the practices in the PCC paving industry today. With the limited research material available, it seems evident that high early strengths alone do not provide for long term durability. Evidence that many older concrete pavements are still performing well while newer surfaces are not, leads to the question of whether or not the high early strengths are even necessary. It has been shown in this study that low water/cement ratios are no substitute for adequate air entraining concerning PCC freeze/thaw durability. Ensuring that adequate durability is provided in northern climates would benefit all of us who use and depend on an efficient infrastructure system. The elimination or reduction of preventable premature failures would greatly reduce the amount of lost time of users

14 caused by construction zone delays. The tax dollars saved would be available for infrastructure expansion projects like the much needed north corridor beltline needed around Madison. Recommendations It is evident that more research must be done in the correlation between air-entraining and water/cement ratio. Walls and Cramer s research has exemplified the importance of air entraining. I feel that the industry must look at the possibility of increasing air contents in PCC pavements, sacrificing high early strengths to a small degree, and delaying opening of pavements to traffic in order to reach the desired strengths. As Shilstone had pointed out, many older pavements have outperformed many newer pavements. For this reason the needed ultimate strength of concrete used in pavements should also be investigated.

15 Glossary Admixture A chemical additive that enhances the properties of PCC. Air entraining The introduction of micro air bubbles into a PCC mix. A method of increasing freeze-thaw durability. ASTM American Society of Testing and Materials. Blast furnace slag Residual mineral waste from coal power generation. Fly Ash Ash material removed from emmissions of coal burning power plants. Enhances hydration process of Portland Cement. Hydration The chemical reaction between water and Portland Cement. PCC Portland Cement Concrete. Trowelling The method of finishing the surface of freshly placed concrete. W/C Water/cement ratio. The ratio of weight of water to the weight of cement dust in a given PCC mix.

16 Bibliography Cohen, Non-Air-Entrained High Strength Concrete-Is it Frost Resistant? A.C.I. Materials Journal, Vol. 89, No. 2, pp , July-August, Li, Y, Freezing and Thawing: Comparison Between Non Air-Entrained and Air-Entrained High Strength Concrete, Proceedings, ACI International Conference, Singapore, SP-149, pp , Mamlouk, Michael S. and Zaniewski, John P. Materials for Civil and Construction Engineers. Addison Wesley Longman, Inc. Menlo Park, CA Shilstone, J.M., Sr., Go Back to Old Ways of Mixing, Engineering News Record, Vol. 242, No. 17, pp. 71, May 3,1999. State of Wisconsin Department of Transportation. Standard Specifications for Highway and Structure Construction. Wisconsin Department of Transportation, Walls, R.A. Jr. and Cramer, S.M., Effects of Water-Cementitious Materials Ratio and Air Content on the Freeze-Thaw Durability of Portland Cement Concrete-Preliminary Findings, Transportation Research Board (Washington, D.C.: Transportation Research Board, 2000)..