Durability is the ability to last a long time without significant deterioration. A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement. Construction and demolition waste contribute to solid waste going to landfills. The production of new building materials depletes natural resources and can produce air and water pollution.
The heavily traveled Wacker Drive replacement in downtown Chicago was designed for a 75 to 100-year life. (PCA No. 16263)
The design service life of most buildings is often 30 years, although buildings often last 50 to 100 years or longer. Most concrete and masonry buildings are demolished due to obsolescence rather than deterioration. A concrete shell can be left in place if a building use or function changes or when a building interior is renovated. Concrete, as a structural material and as the building exterior skin, has the ability to withstand nature’s normal deteriorating mechanisms as well as natural disasters.
Durability of concrete may be defined as the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different concretes require different degrees of durability depending on the exposure environment and properties desired. For example, concrete exposed to tidal seawater will have different requirements than an indoor concrete floor. Concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment determine the ultimate durability and life of concrete.
These 3x5-ft concrete panels with decorative finishes were displayed outdoors in the relatively severe weather in the Skokie, Illinois, area (near Chicago). With only a few exceptions, their appearance changed very little after more than 40 years of exposure to bright sun-light, wind, snow, acid rain, freezing and thawing, hot summers, and cold winters (PCA No. 2101)
High Humidity and Wind-Driven Rain: Concrete is resistant to wind-driven rain and moist outdoor air in hot and humid climates because it is impermeable to air infiltration and wind-driven rain. Moisture that enters a building must come through joints between concrete elements. Annual inspection and repair of joints will minimize this potential. More importantly, if moisture does enter through joints, it will not damage the concrete. Good practice for all types of wall construction is to have permeable materials that breathe (are allowed to dry) on at least one surface and to not encapsulate concrete between two impermeable surfaces. Concrete will dry out if not covered by impermeable treatments.
Portland cement plaster (stucco) should not be confused with the exterior insulation finish systems (EIFS) or synthetic stucco systems that have become popular but may have performance problems, including moisture damage and low impact-resistance. Synthetic stucco is generally a fraction of the thickness of portland cement stucco, offering less impact resistance. Due to its composition, it does not allow the inside of a wall to dry when moisture gets trapped inside. Trapped moisture eventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metal attachments. There have been fewer problems with EIFS used over solid bases such as concrete or masonry because these substrates are very stable and are not subject to rot or corrosion.
Ultraviolet Resistance: The ultraviolet portion of solar radiation does not harm concrete. Using colored pigments in concrete retains the color in concrete long after paints have faded due to the sun’s effects.
Inedible: Vermin and insects cannot destroy concrete because it is inedible. Some softer materials are inedible but still provide pathways for insects. Due to its hardness, vermin and insects will not bore through concrete. Gaps in exterior insulation to expose the concrete can provide access for termite inspectors.
Moderate to Severe Exposure Conditions for Concrete: The following are important exposure conditions and deterioration mechanisms in concrete. Concrete can withstand these effects when properly designed. The Specifier’s Guide for Durable Concrete is intended to provide sufficient information to allow the practitioner to select materials and mix design parameters to achieve durable concrete in a variety of environments.
Resistance to weathering, including freezing and thawing
Resistance to Freezing and Thawing: The most potentially destructive weathering factor is freezing and thawing while the concrete is wet, particularly in the presence of deicing chemicals. Deterioration is caused by the freezing of water and subsequent expansion in the paste, the aggregate particles, or both.
With the addition of an air entrainment admixture, concrete is highly resistant to freezing and thawing. During freezing, the water displaced by ice formation in the paste is accommodated so that it is not disruptive; the microscopic air bubbles in the paste provide chambers for the water to enter and thus relieve the hydraullic pressure generated. Concrete with a low water-cementitious ratio (0.40 or lower) is more durable than concrete with a high water-cementitious ratio (0.50 or higher). Air-entrained concrete with a low water-cementitious ratio and an air content of 5 to 8% will withstand a great number of cycles of freezing and thawing without distress.
Chemical resistance
Chemical Resistance: Concrete is resistant to most natural environments and many chemicals. Concrete is virtually the only material used for the construction of wastewater transportation and treatment facilities because of its ability to resist corrosion caused by the highly aggressive contaminants in the wastewater stream as well as the chemicals added to treat these waste products.
However concrete is sometimes exposed to substances that can attack and cause deterioration. Concrete in chemical manufacturing and storage facilities is specially prone to chemical attack. The effect of sulfates and chlorides is discussed below. Acids attack concrete by dissolving the cement paste and calcareous aggregates. In addition to using concrete with a low permeability, surface treatments can be used to keep aggressive substances from coming in contact with concrete. Effects of Substances on Concrete and Guide to Protective Treatments discusses the effects of hundreds of chemicals on concrete and provides a list of treatments to help control chemical attack.
Resistance to Sulfate Attack: Excessive amounts of sulfates in soil or water can attack and destroy a concrete that is not properly designed. Sulfates (for example calcium sulfate, sodium sulfate, and magnesium sulfate) can attack concrete by reacting with hydrated compounds in the hardened cement paste. These reactions can induce sufficient pressure to cause disintegration of the concrete.
Like natural rock such as limestone, porous concrete (generally with a high water-cementitious ratio) is susceptible to weathering caused by salt crystallization. Examples of salts known to cause weathering of concrete include sodium carbonate and sodium sulfate.
Confederation Bridge, spanning the Northumberland Strait between Prince Edward Island and New Brunswick, was specifically designed for high durability in a severe environment and a 100-year life. The bridge has to resist freezing and thawing, seawater exposure, and abrasion from floating ice. (PCA No. IMG15714)
Sulfate attack and salt crystallization are more severe at locations where the concrete is exposed to wetting and drying cycles, than continuously wet cycles. For the best defense against external sulfate attack, design concrete with a low water to cementitious material ratio (around 0.40) and use cements specially formulated for sulfate environments.
Seawater Exposure: Concrete has been used in seawater exposures for decades with excellent performance. However, special care in mix design and material selection is necessary for these severe environments. A structure exposed to seawater or seawater spray is most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying and/or freezing and thawing. Sulfates and chlorides in seawater require the use of low permeability concrete to minimize steel corrosion and sulfate attack. A cement resistant to sulfate exposure is helpful. Proper concrete cover over reinforcing steel must be provided, and the water-cementitious ratio should not exceed 0.40.
Corrosion resistance
Chloride Resistance and Steel Corrosion: Chloride present in plain concrete that does not contain steel is generally not a durability concern. Concrete protects embedded steel from corrosion through its highly alkaline nature. The high pH environment in concrete (usually greater than 12.5) causes a passive and noncorroding protective oxide film to form on steel. However, the presence of chloride ions from deicers or seawater can destroy or penetrate the film. Once the chloride corrosion threshold is reached, an electric cell is formed along the steel or between steel bars and the electrochemical process of carrions begins.
The resistance of concrete to chloride is good; however, for severe environments such as bridge decks, it can be increase by using a low water-cementitious ratio (about 0.40), at least seven days of moist curing, and supplementary cementitious materials such as silica fume, to reduce permeability. Increasing the concrete cover over the steel also helps slow down the migration of chlorides. Other methods of reducing steel corrosion include the use of corrosion inhibiting admixtures, epoxy-coated reinforcing steel, surface treatments, concrete overlays, and cathodic protection.
Resistance to alkali-silica reaction (ASR)
Resistance to Alkali-Silica Reaction (ASR): ASR is an expansive reaction between reactive forms of silica in aggregates and potassium and sodium alkalis, mostly from cement, but also from aggregates, pozzolans, admixtures, and mixing water. The reactivity is potentially harmful only when it produces significant expansion. Indications of the presence of alkali-aggregate reactivity may be a network of cracks, closed or spalling joints, or movement of portions of a structure. ASR can be controlled through proper aggregate selection and/or the use of supplementary cementitious materials (such as fly ash or slag cement) or blended cements proven by testing to control the reaction.
Abrasion resistance
Abrasion Resistance: Concrete is resistant to the abrasive affects of ordinary weather. Examples of severe abrasion and erosion are particles in rapidly moving water, floating ice, or areas where steel studs are allowed on tires. Abrasion resistance is directly related to the strength of the concrete. For areas with severe abrasion, studies show that concrete with compressive strengths of 12,000 to 19,000 psi work well.
Why does concrete crack?
Concrete, like most materials, will shrink slightly when it dries out. Common shrinkage is about 1/16th of an inch in a 10-foot length of concrete. The reason contractors place joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint, where concrete cracks due to shrinkage are expected to occur. Control or construction joints are also placed in concrete walls and other structures.
Why do concrete surfaces spall?
Concrete spalling (or flaking) can be prevented. It occurs due to one or more of the following reasons.
1.) In cold climates subjected to freezing and thawing, concrete surfaces have the potential to spall if the concrete is not air-entrained.
2.)Too much water in the concrete mix will produce a weaker, more permeable and less durable concrete. The water-cementitious ratio should be as low as possible (0.45 or less).
3.) Concrete finishing operations should not begin until the water sheen on the surface is gone and the excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because finishing operations have begun too soon, the concrete on the surface will have too high of a water content and this surface will be weaker and less durable.
Kamis, 25 Februari 2010
High Performance Concrete
High-Performance Concrete
2.1 Introduction
At least one type of HPC, that is, High-Strength Concrete (HSC), has been used in many locations for well over a decade. Since the completion of the Strategic Highway Research Program, other types of HPC have moved more and more from the research or limited field trial stage into full scale commercial use. However, significant new developments in the materials or production of HPC have been few since the publication of the SHRP C-205 State-of-the-Art Report (SAR) [Zia et al. 1991].
Most of the research in HPC since the publication of the State-of-the-Art Report in 1990 has concentrated on increasing basic knowledge regarding HPC performance and properties rather than developing new types of HPC. HPC research and utilization continues to be concentrated in HSC or in improved durability, primarily using the materials and methods developed prior to the publication of the SAR. Changes have come primarily in modification of existing mix components and proportions.
While additional research on the mechanical properties of HSC has been conducted, much of the research of the last five years has concentrated on particular applications in buildings and bridges rather than the properties of the concrete itself. With time, data regarding the long term properties of HSC have been reported. Mechanical properties and durability of lightweight HSC have been reported, in many cases for use in extreme environments where concerns include abrasion resistance of the concrete to sea ice, as well as frost durability.
In the area of durability, substantial research has been conducted on the frost resistance of HPC. Much of the research has investigated the behavior of non-air entrained concretes with very low (<0.35) water to cementitious material ratio (W/CM), concentrating on HSC containing silica fume and high range water reducers. Research has also been conducted on the permeability, passage of chloride ions and carbonation of various concretes, many of which are HSC by virtue of their low W/CM ratios. The impact of high cement contents on heat of hydration, and the consequent effects on strength and durability in place have also been investigated. Some work on the abrasion resistance behavior of HPC concrete and its use to improve pavement durability have been reported, as has work on the fire endurance of HSC.
An important improvement in constructibility has been the further development of super-workable concrete. This has added another important, practical dimension to the concept of HPC. These concretes with improved constructibility have been developed using materials which, while not exotic, have not been routinely used, in some cases, and have not been used in the combinations and proportions reported. Other investigations concerned with constructibility issues have involved the relative sensitivity of HPC to curing conditions, including external temperature.
In this chapter, materials and methods used to produce HPC are reviewed which were not addressed previously or in which there has been a shift in focus in the last five years.
2.2 Raw Materials and Proportions
2.2.1 New Materials
Few new raw materials have been introduced into the technology in the last five years which promise to make a substantial difference in HPC production or use. However, some existing chemical admixtures have evolved. Modifications of high range water reducers to reduce slump loss have produced additional alternatives for the engineer, however, these HRWR's typically involve extended set times which may not be advantageous for early strength applications.
Another change in composition has been noted with air entraining admixtures. Air entraining agents are now frequently based on compounds other than "neutralized vinsol resin", in many cases producing finer and more uniform air void systems than were common earlier. Neeley et al. [1992] presented results of a new air entraining admixture intended to provide adequate frost durability at low air contents. Their research, not based on HPC, indicated that low air contents provided insufficient durability to rapid freezing and thawing.
Cements and combinations of cementitious materials have been the subject of continuing research but there have been few new materials used outside the laboratory. Some blended cements, specifically developed for niche markets, have been investigated, but their use in practice has been somewhat limited. Neeley [1995] reports promising results for one product based primarily on Class C fly ash. A key ingredient of the product was three admixtures used in various quantities, added at various times and in different sequences to regulate setting characteristics. Concrete made with this product had very low w/c ratios and setting times which varied from approximately a half hour to over three hous.
In preliminary testing at the U.S. Army Engineer Waterways Experiment Station, Ash Bonding Chemicals Corporation Cement was found to provide relatively high strengths at early ages with good frost resistance, low to very low permeability as measured by Chloride Ion Permeability tests and reduced shrinkage when compared to a conventional Type III portland cement based concrete containing water reducing and high range water reducing admixtures. However, the concrete was more variable and admixture effects are still neither completely understood nor completely predictable. In addition, costs and control issues due to the addition of the admixtures on the job site, possibly including air entraining agents, have apparently not been fully investigated.
2.2.2 Review of Other Selected Raw Materials
2.2.2.1 Cementitious MaterialsRenewed interest in shrinkage compensating cements was generated several years ago in jointless slabs, including bridge decks. While concretes made with these cements have good strength characteristics and exceptional sulfate durability, there have been serious concerns with the durability of the concrete exposed to deicer salts. Reports presented in sessions at the 1995 Transportation Research Board indicated very different experiences in Ohio and New York with shrinkage compensating cement. Bridge members produced with shrinkage compensating cement in Ohio were sealed and have apparently performed well to date. Structures in New York which were not sealed exhibited some premature scaling and deterioration. There was speculation that differences in performance may be due to sealing of the concrete.
The blended cement Pyrament was investigated as part of the SHRP C-205 research. This material could be used to produce concretes with exceptional early strength characteristics and very good later age properties. Further research by Husbands et al. [1994] found that performance and durability were generally good. Concerns with alkali-silica reactivity have not been completely resolved but appear to be manageable. Unfortunately, Pyrament never gained broad market acceptance due to availability and cost, and its production has been suspended. Super-fine cements continue to be unavailable on a commercial basis. Regulated set cements are acceptable for early strength applications except where sulfate exposure is likely.
2.2.2.2 Mineral AdmixturesWhile additional research continues with mineral admixtures, especially silica fume, most of the new research involves different proportions rather than new materials. However, a few other mineral admixtures have also received attention. The use of zeolitic admixtures, a natural pozzolan, were examined by Feng et al. [1990] as was metakaolin, a reactive alumino-silicate pozzolan by Walters and Jones [1991]. The use of 5% and 10% metakaolin was found to be very similar to the use of similar percentages of silica fume in terms of permeability, frost durability and mechanical properties. The major differences noted were in color (the metakaolin was much lighter in color) and in HRWR dosages (concrete containing silica fume had a much higher demand).
A number of researchers have confirmed earlier reports that mineral admixtures typically reduce the permeability of concrete. Detwiler et al. [1994] have reported on this phenomenon in steam cured concrete. Geiker et al. [1991] noted that both silica fume and fly ash reduced the permeability of concrete to the penetration of chloride ion without changing the total porosity greatly. Bijen and van Selst [1991] found higher rates of carbonation in concrete with typical commercial quantities of ground granulated blast furnace slag compared with concretes containing typical quantities of fly ash. However, this study was not conducted with HPC.
Silica fume (also called condensed silica fume or microsilica) continues to be a popular element of high performance concrete, and especially high strength concrete. Not only does it provide an extremely rapid pozzolanic reaction, but researchers including Detwiler and Mehta [1989], and Goldman and Bentur [1993] found that its very fine size also appears to provide a beneficial contribution to concrete. Detwiler and Mehta, and Goldman and Bentur examined the effects of silica fume on mechanical behavior. Luther's [1989] review examined durability effects, while Fidjestol [1993], and Khayat and Aitcin [1993], have provided general reviews of the effects. These reports confirm findings that silica fume tends to improve both mechanical properties and durability.
However, St. John et al. [1994] report that deleterious expansion due to alkali silica reactivity is possible under wetting and drying conditions when particles of the densified form of silica fume admixtures are not sufficiently dispersed during mixing. In addition, a number of research efforts have attempted to answer important and unresolved questions in long term strength gain and frost resistance of silica fume concrete.
Maage et al. [1990] report that silica fume concretes continue to gain strength under a variety of curing conditions, including unfavorable conditions. They further indicate that concretes with silica fume appear to be more robust to early drying than similar concretes which do not contain silica fume.
A number of issues with frost resistance of concrete containing silica fume have been investigated, including the need for any entrained air when working with very low W/CM ratio concretes. Due to the dramatic reduction in permeability which accompanies the use of silica fume, concerns with frost durability in general, and with the usefulness of rapid freezing and thawing tests, have complicated the interpretation of research results. These issues are discussed in more detail in section 2.2.4, however it is useful to note here that ACI 318-95 [1995] limits the quantity of silica fume in concrete exposed to deicing salts to no more than 10 percent.
Attempts to improve the performance of systems of cementitious material in HSC have led researchers to examine mixes with multiple cementitious components. The use of ternary cementitious systems has received attention in recent years. Kashima et al. [1993] report on HSC produced with a blend of cement with large amounts of fly ash and ground granulated blast furnace slag in order to reduce heat of hydration. Their report is important because it reviews both experimental work and construction results.
Sarkar et al. [1991], examining the microstructural development in HSC using both silica fume and fly ash, found that strength at twelve hours was improved over similar mixes with silica fume alone. They state that this phenomenon may be related to the liberation of soluble alkalies from the surface of the fly ash. Baalbaki et al. [1993], reported on the properties of HPC produced with an extremely finely ground Type V cement with various mineral admixtures. Mixes with prolonged working times and very high strengths at ages out to one year were produced.
The use of cementitious systems with very high quantities of fly ash have also been investigated. Carette et al. [1993], and Bilodeau and Malhotra [1994], report that mixes have been developed which provide acceptable plastic and hardened properties, although strengths were not high, especially at early ages. In other studies, Bilodeau et al. [1994], and Malhotra [1990] report that performance in rapid freezing and thawing of concrete with high volumes of class F fly ash was adequate but that the concrete with very high quantities of fly ash performed poorly in deicer scaling tests. However, Nasser and Lai [1993] found that high volume, class C fly ash concrete was not frost durable even with a 6% air and after prolonged moist curing. They found that 20% fly ash mixes showed no difficulties in this respect. Kukko and Matala [1991] noted that frost resistance of non-air entrained concrete was reduced for very low W/CM ratio concrete produced with slowly hardening portland cement or containing slag, compared to rapid hardening portland cement with or without silica fume.
Naik et al. [1994] found that although concrete made with high volumes of class C fly ash passed ASTM C-944 for abrasion resistance, better abrasion resistance was obtained for concrete without the high fly ash content. Gjorv et al. [1990] also found that the abrasion resistance of conventional HSC pavements is exceptionally good. It would appear that high volume fly ash mixes have limited applicability to highway structures, although additional research appears warranted.
2.2.2.3 AggregatesHigh Performance Lightweight Concrete (HPLC) has been extensively investigated for, among other applications, use in oil drilling platforms in severe environments. Hoff [1991], and Tachibana et al. [1990], have presented information on the behavior of HSLC in extreme conditions. Hoff has demonstrated that HSLC containing both lightweight aggregate and conventional weight stone is both frost resistant and resistant to abrasion by ice. Holm and Bremner [1991], have provided additional information on the long term durability of lightweight concrete, indicating that in general when well-known prophylactic measures are taken to insure durability, long term durability is good.
Zhang and Gjorv [1991a, 1991b, 1991c, 1991d] have investigated both the mechanical properties and the permeability of lightweight concretes with strengths ranging from 50 to 100 MPa (about 7,000 to about 14,500 psi). Elastic modulus and the tensile-compressive strength ratio were lower than would be expected with conventional stone concrete at the same strength levels. While permeability of the HSLC's was very low, it was noted that permeability might be higher with lightweight aggregate than with conventional aggregate at the same strength. This, of course, would depend on the porosity and permeability of the aggregate itself.
2.2.3 Proportioning Methods
Modifications to conventional proportioning methods have been proposed by several researchers. Mehta and Aitcin [1990], and ACI Committee 363 report [1990] provide an excellent review of proportioning considerations for HSC. Selection of the proper raw materials and adjustment of proportions based on experience, using mixes conducted both in the laboratory and in the field, have typically proven adequate to achieve the desired concrete characteristics, at least within the limits allowed by the available raw materials. With adequate control of production and placement, routine use of concrete with compressive strengths in excess of 70 MPa (10,000 psi) is practical in many areas.
Several articles have been published with suggestions on methods of optimizing the development of particular mixes by reducing the number of trial mixes necessary. Campbell and Detwiler [1993], for example, have provided guidance for proportioning and producing steam-cured concrete. While de Larrard [1990] has provided suggestions for HSC mixes based on rheological considerations, Domone and Soutsos [1994] have reexamined the maximum density theory for applicability to HSC.
Field trials of High Early Strength (HES), Very Early Strength (VES) and Very High Strength (VHS) concretes in SHRP C-205 and C-206 indicated that existing proportioning methods remain valid, with minor modifications, for these mixes. Routine precautions such as those regarding minimum water contents and appropriate quantities or combinations of chemical admixtures, contained in numerous publications and discussed in the previous State-of-the-Art Report, remain valid. Development or adaptation of new types of high performance concrete or combinations of raw materials are better served by engineering judgement than by more sophisticated proportioning techniques.
2.2.4 Air Entrainment
The need for any air entrainment at all in concrete with very low W/CM ratio has been questioned. This issue has been complicated by the problem of interpreting test results of one of the most commonly used test methods in practice. ASTM C 666 measures the resistance of concrete to rapid freezing and thawing. The rate of freezing is much higher in this test than is found in practice, and C 666 has been criticized in this respect even for conventional concrete.
Concretes with a low W/CM ratio, such as HPC and HSC, have a lower permeability than conventional concrete. A rapid freezing and thawing rate may induce additional damage to concretes with low W/CM ratio simply due to the lower permeability. On the other hand, the very low w/c ratio, for an adequately cured concrete, can reduce or even eliminate the amount of freezable water in the pores for practical temperature ranges. These mixes will also dramatically reduce the ingress of water during the test, therefore reducing the amount of damage due to physically freezing water in the concrete. The time required to achieve an internal moisture content sufficient to contribute to frost damage is less than the time required for the C 666 test for concretes with very low W/CM ratios. However, since hydraulic pressure due to freezing of water is only one of several mechanisms of frost damage, the use of very low W/CM may not be adequate in all cases. Experimental results have been mixed.
Damage to concrete specimens may be due to thermal shock or to disequilibrium between energy states during cooling rather than expansion associated with the presence of freezing water for the mixes of very low W/CM ratio. Hanson et al. [1993], found that the electrical impedance of air entrained concrete of very low W/CM ratio actually increased during ASTM C 666 testing while the dynamic elastic modulus decreased. The increase in impedance could only have come from internal drying associated with the loss of free moisture during curing. Therefore, the decrease in elastic modulus was not related to the formation of ice.
Many believe that C 666 testing is still a valid discriminant for frost durability even though it is extremely severe for concretes of low W/CM ratio. Others have chosen to rely on deicer scaling as a more useful and informative test, particularly for highway and pavement applications.
Hammer and Sellevold [1990] report that salt scaling resistance is acceptable for HPC with W/CM ratios below 0.37, even without entrained air but that rapid freezing and thawing is accompanied by deterioration for all non-entrained air concrete tested down to a W/CM of 0.25. However, they also note that calorimeter data indicates very little ice formation until -20 c. They state that this indicates that much of the deterioration may be due to thermal incompatibility of the components rather than the formation of ice.
In research conducted by Kashi and Weyers [1989], non-air entrained concrete with a W/CM ratio of less than 0.30 was found to be resistant to rapid freezing and thawing based on ASTM C 666, Method A. For concretes with a W/CM ratio of 0.32, the concrete was frost resistant only if silica fume was not used in the mix. Cohen et al. [1992] similarly found that non-air entrained concrete with a W/CM ratio of 0.35 and containing 10% silica fume were not resistant to rapid freezing and thawing when tested in accordance with ASTM C 666 (A), even when curing had been extended to 56 days. They also noted that although there was a dramatic drop in the elastic modulus, the reduction in compressive strength was much less severe. Li et al. [1994], on the other hand, found that a maximum W/CM ratio of 0.24 was necessary for adequate frost protection of non-air entrained concrete when based on ASTM C 666 (A) testing.
Tests conducted by Pigeon et al. [1991], and by Gagne et al. [1991], using both ASTM C 666 (A) and ASTM C 672 deicer scaling tests, indicated that non-air entrained concrete containing silica fume and good quality coarse aggregate, with a W/CM ratio of 0.30 generally, but not uniformly, had good resistance to deicer scaling. When tested using ASTM C 666 (A), the W/CM ratio required to provide acceptable performance ranged from less than 0.25 to over 0.30. The cement used was found to play a significant role in the performance of otherwise similar mixes. It was also noted that the air void system produced by the use of water reducing or high range water reducing admixtures, commonly used in all concretes with low W/CM ratio, may be contributing in a fashion not yet well documented.
Additional research in these areas, perhaps concentrating more on test methods such as resistance to deicer scaling and critical dilation test concepts rather than ASTM C 666 (A), would appear to be useful. At the present time, the use of at least minimal quantities of entrained air appear prudent for concrete exposed to severe freezing, especially when exposed to deicing salts, unless that concrete has a W/CM ratio less than 0.24.
2.2.5 Other Types of HPC
Another type of HPC has been developed for use in situations where vibration is difficult or impossible and where reinforcing steel is highly congested. "Super-workable" or "flowable" concrete has been developed in Japan and used in both bridge structures and buildings. Although not specifically developed for high strength, low water to "powder" ratios are common. Kuroiwa et al. [1993], report super-workable concrete with strengths in excess of 50 MPa, a marginally high strength concrete.
Low W/CM ratio, flowing concretes utilizing HRWR's are well known. Super-workable mixes are an extension of this concept which have been specifically formulated to resist segregation. Super-workable concretes also derive partly from concrete developed for underwater placement. Research has led in several different directions. Paste and aggregate volumes, and paste composition, admixture type and dosages, and testing methods have been investigated.
A number of different "powder" combinations are reported including portland cement, fly ash, GGBFS and silica fume. In addition, where a low heat of hydration was a concern, finely ground limestone powder was used as a partial replacement for cement. This material was reported [Tanaka et al. 1993] to have both a chemical and physical effect similar to that reported for silica fume. Low heat of hydration mixes will typically exhibit considerably extended set times, with final set at about twenty hours, and low early strengths. This is due both to large quantities of HRWR in conjunction with a blended cement composed of 30% low heat of hydration portland cement and 70% GGBFS. The aggregate paste ratio was somewhat lower compared to conventional concrete.
Cellulose based products have been known to improve the cohesion of concrete and to reduce segregation. Sogo et al. [1987] report that a polymer based on cellulose ether can be used to increase both water reduction and cohesiveness. Many long chain organic molecules have water reducing capability and, if air content and setting time can be controlled, can be successfully used in concrete [Mehta 1975] with various effects on cohesiveness. Kuroiwa et al. [1993] report on findings using a polysaccharide polymer. Ozawa et al. [1990] describe studies to optimize the combination of HRWR and other admixtures affecting the viscosity of this type of mix.
Self compacting capability and resistance to segregation were determined by so-called "slump-flow", by a modified grout cone flow test and by self-leveling flow through reinforcing bars, as well as by mock-ups of particular members. Self-leveling, non-segregating performance was reported with several different mixtures. Details of these tests are discussed in the next chapter.
The addition of materials to compensate for increased drying shrinkage due to the lower aggregate content has been suggested, although Kuroiwa et al. state that drying shrinkage is equal to or better than comparable, conventional concretes. Frost durability was also found to be adequate for super-workable concrete containing at least 4% entrained air. Testing was based on ASTM C 666, although it was not stated whether Procedure A or B was used.
It is significant to note that these types of mixes have been successfully used, without vibration, in field placements. Applications include the Akashi Kaikyo Bridge near Kobe, Japan, and in the heavily reinforced concrete core and shear wall of a 20-story building. Concrete delivery was by pump. The use of super-workable concretes in certain applications appears promising.
2.3 Production Considerations
For high strength concrete where early strength is not a critical consideration, the use of conventional production methods and facilities appears adequate, as long as well recognized practices for the production of good quality concrete are enforced. Howard and Leatham [1989] and Sanchez and Hester [1990] discuss the production and delivery of HSC noting the importance of a team approach. Kakizaki et al. [1993], note that the mixing sequence can affect the slump and compressive strengths of very high strength concretes. Leming et al. [1993], emphasize that pre-pour conferences and field trials are necessary with any HPC prior to actual use.
In situations where early strength is critical, particular care must be taken to insure that temperature is closely controlled and that high dosages of water reducing or high range water reducing admixtures are avoided, since these can extend the time of set. While strength after one day may not be significantly affected, strengths at less than twenty-four hours and particularly before twelve hours, can be significantly reduced. If mineral admixtures are included, caution must be exercised in dosing the entire quantity of cementitious material. When significant percentages of mineral admixtures are employed, the result may be to effectively overdose the portland cement, again resulting in extended set times.
Much of the VES and HES concrete used in field trials for SHRP C-205 and C-206 contained a corrosion inhibiting admixture as a non-chloride accelerator, added at the job site to mitigate rapid slump loss [Hanson et al. 1994]. The admixture was added either by hand or by pump typically from trailer-mounted tanks. In either case, provision for adding the admixture, adequate quality control and sufficient remixing time are necessary.
Due to the high water content of this admixture and the low W/CM ratio of the paving mixes investigated, it was necessary to employ a HRWR in the initial batching. Inadvertent use of large dosages of HRWR caused low strengths at ages up to and including one day. The low water content and rich paste of these mixtures required strict control of the batching sequence. Although only minor adjustments to conventional practice were required, some adjustment must be anticipated. Exact procedures will vary from one production facility to another, but there are several keys to a successful placement. The plant itself should be equipped with an automatic moisture indicator for the aggregate.
A pre-placement conference, including all parties who will be involved in the slab-on-grade placement, is required. A practice placement to adjust operations, if necessary, to develop estimates for slump and air loss in transit, and to acquaint the crews involved in the placement is highly recommended. Inspection of trucks, especially in a dry batch operation, is necessary to insure that only trucks with clean fins and adequate mixing speed are used. Trucks should carry no more than two-thirds of their rated mixing capacity to insure adequate mixing on the job site if there is to be any addition of admixtures on the job site. Trucks should discharge their entire load as soon as possible. A time limit of ten minutes after arrival on the job site should be used for planning purposes but may vary depending on the type and composition of the concrete being used. Special ready-mixed concrete trucks, intended for paving operations, should be used if discharge of very low slump is anticipated.
Since most HPC's are paste rich with low water content, bleeding is typically very low. This can potentially create difficulties with plastic shrinkage cracking. Therefore, it is necessary to apply curing compound or take other precautionary measures to reduce evaporation as soon as possible for slabs or members with large exposed surfaces. However, field trials for SHRP 205 [Leming et al. 1993] of HES and VES concretes in slabs found that cracks due to plastic shrinkage were rare. The concrete was apparently gaining strength faster than it shrank. Another consequence of the rapid setting and strength gain was that time prior to sawing the slabs was reduced. It was critical that joints be sawed as early as possible. Delays past eight hours were found to cause cracking of 20 cm depth (8 in.) pavements at approximately 7.5 m (approximately 25 ft) intervals.
2.4 Fiber Reinforced Concrete
Although there has been continued interest and research in the use of fiber-reinforced concrete (FRC), there have been few major innovations in proportioning or production of high performance fiber reinforced concrete (HPFRC) since the last State-of-the-Art Report. In addition, while research in FRC has examined the influence of modifications of existing fibers, fibers with larger aspect ratios, and higher fiber volumes, and there continues to be interest in non-metallic fibers or combinations of fibers, these researches were based on existing fiber materials.
One of the few new approaches in this area has been the development of SIMCON, or Slurry Infiltrated Mat Concrete, described by Hackman et al. [1992] and Krstulovic-Opara et al. [1994]. They noted that SIMCON is a different material from SIFCON, or Slurry Infiltrated Fiber-Reinforced Concrete, which is based on the use of prepacked discontinuous steel fibers. SIMCON, on the other hand, uses a manufactured continuous mat of interlocking discontinuous steel fibers, placed in a form, and then infiltrated with a flowable cement-based slurry. The use of continuous mats, typically made with stainless steel to control corrosion in very thin members, permits development of high flexural strengths and very high ductility with a reduced volume of fibers than SIFCON.
The use of SIMCON appears to be very promising for at least two reasons. First, the very high volume of fibers required to provide significant increases in mechanical properties such as SIFCON can create a problem with economic justification in a large number of practical applications. However, with SIMCON, direct tensile strengths of 15.9 MPa at 1.1% strain have been reported with only a 5% volume fraction of fibers. Secondly, in situations where normal FRC may be economically justified, such as in pavements, the addition of fibers to the mix and the placement of the fiber-reinforced mix required special care, and considerable extra time and expense. SIMCON overcomes many of these limitations since the fiber mat, normally delivered in large rolls, can be laid out by hand and the slurry simply pumped into place. The use of SIMCON permits fabrication of thin, complex shapes with very high ductility and flexural strength.
Another interesting and useful development in FRC construction has been to provide non-metallic fibers in small, cylindrical bundles, approximately 50 mm high (the length of the fiber) and 55 mm in diameter, wrapped in a water soluble compound. This permits the easy addition of the fibers, by hand, into the mixing drum of a truck mixer, either during charging or at the job site. The wrapper disintegrates, allowing the fibers to disperse into the concrete mixture with little balling or segregation. Quality control is improved by making the quantity of fibers added easy to determine and easy to check, and by minimizing problems in dispersion in the mixer. Further, production rates are maintained with little additional effort. Successful field applications in a full-depth pavement, a thin bridge-deck overlay, a Jersey barrier, and white-topping on scarified asphalt pavement have been reported from South Dakota
2.1 Introduction
At least one type of HPC, that is, High-Strength Concrete (HSC), has been used in many locations for well over a decade. Since the completion of the Strategic Highway Research Program, other types of HPC have moved more and more from the research or limited field trial stage into full scale commercial use. However, significant new developments in the materials or production of HPC have been few since the publication of the SHRP C-205 State-of-the-Art Report (SAR) [Zia et al. 1991].
Most of the research in HPC since the publication of the State-of-the-Art Report in 1990 has concentrated on increasing basic knowledge regarding HPC performance and properties rather than developing new types of HPC. HPC research and utilization continues to be concentrated in HSC or in improved durability, primarily using the materials and methods developed prior to the publication of the SAR. Changes have come primarily in modification of existing mix components and proportions.
While additional research on the mechanical properties of HSC has been conducted, much of the research of the last five years has concentrated on particular applications in buildings and bridges rather than the properties of the concrete itself. With time, data regarding the long term properties of HSC have been reported. Mechanical properties and durability of lightweight HSC have been reported, in many cases for use in extreme environments where concerns include abrasion resistance of the concrete to sea ice, as well as frost durability.
In the area of durability, substantial research has been conducted on the frost resistance of HPC. Much of the research has investigated the behavior of non-air entrained concretes with very low (<0.35) water to cementitious material ratio (W/CM), concentrating on HSC containing silica fume and high range water reducers. Research has also been conducted on the permeability, passage of chloride ions and carbonation of various concretes, many of which are HSC by virtue of their low W/CM ratios. The impact of high cement contents on heat of hydration, and the consequent effects on strength and durability in place have also been investigated. Some work on the abrasion resistance behavior of HPC concrete and its use to improve pavement durability have been reported, as has work on the fire endurance of HSC.
An important improvement in constructibility has been the further development of super-workable concrete. This has added another important, practical dimension to the concept of HPC. These concretes with improved constructibility have been developed using materials which, while not exotic, have not been routinely used, in some cases, and have not been used in the combinations and proportions reported. Other investigations concerned with constructibility issues have involved the relative sensitivity of HPC to curing conditions, including external temperature.
In this chapter, materials and methods used to produce HPC are reviewed which were not addressed previously or in which there has been a shift in focus in the last five years.
2.2 Raw Materials and Proportions
2.2.1 New Materials
Few new raw materials have been introduced into the technology in the last five years which promise to make a substantial difference in HPC production or use. However, some existing chemical admixtures have evolved. Modifications of high range water reducers to reduce slump loss have produced additional alternatives for the engineer, however, these HRWR's typically involve extended set times which may not be advantageous for early strength applications.
Another change in composition has been noted with air entraining admixtures. Air entraining agents are now frequently based on compounds other than "neutralized vinsol resin", in many cases producing finer and more uniform air void systems than were common earlier. Neeley et al. [1992] presented results of a new air entraining admixture intended to provide adequate frost durability at low air contents. Their research, not based on HPC, indicated that low air contents provided insufficient durability to rapid freezing and thawing.
Cements and combinations of cementitious materials have been the subject of continuing research but there have been few new materials used outside the laboratory. Some blended cements, specifically developed for niche markets, have been investigated, but their use in practice has been somewhat limited. Neeley [1995] reports promising results for one product based primarily on Class C fly ash. A key ingredient of the product was three admixtures used in various quantities, added at various times and in different sequences to regulate setting characteristics. Concrete made with this product had very low w/c ratios and setting times which varied from approximately a half hour to over three hous.
In preliminary testing at the U.S. Army Engineer Waterways Experiment Station, Ash Bonding Chemicals Corporation Cement was found to provide relatively high strengths at early ages with good frost resistance, low to very low permeability as measured by Chloride Ion Permeability tests and reduced shrinkage when compared to a conventional Type III portland cement based concrete containing water reducing and high range water reducing admixtures. However, the concrete was more variable and admixture effects are still neither completely understood nor completely predictable. In addition, costs and control issues due to the addition of the admixtures on the job site, possibly including air entraining agents, have apparently not been fully investigated.
2.2.2 Review of Other Selected Raw Materials
2.2.2.1 Cementitious MaterialsRenewed interest in shrinkage compensating cements was generated several years ago in jointless slabs, including bridge decks. While concretes made with these cements have good strength characteristics and exceptional sulfate durability, there have been serious concerns with the durability of the concrete exposed to deicer salts. Reports presented in sessions at the 1995 Transportation Research Board indicated very different experiences in Ohio and New York with shrinkage compensating cement. Bridge members produced with shrinkage compensating cement in Ohio were sealed and have apparently performed well to date. Structures in New York which were not sealed exhibited some premature scaling and deterioration. There was speculation that differences in performance may be due to sealing of the concrete.
The blended cement Pyrament was investigated as part of the SHRP C-205 research. This material could be used to produce concretes with exceptional early strength characteristics and very good later age properties. Further research by Husbands et al. [1994] found that performance and durability were generally good. Concerns with alkali-silica reactivity have not been completely resolved but appear to be manageable. Unfortunately, Pyrament never gained broad market acceptance due to availability and cost, and its production has been suspended. Super-fine cements continue to be unavailable on a commercial basis. Regulated set cements are acceptable for early strength applications except where sulfate exposure is likely.
2.2.2.2 Mineral AdmixturesWhile additional research continues with mineral admixtures, especially silica fume, most of the new research involves different proportions rather than new materials. However, a few other mineral admixtures have also received attention. The use of zeolitic admixtures, a natural pozzolan, were examined by Feng et al. [1990] as was metakaolin, a reactive alumino-silicate pozzolan by Walters and Jones [1991]. The use of 5% and 10% metakaolin was found to be very similar to the use of similar percentages of silica fume in terms of permeability, frost durability and mechanical properties. The major differences noted were in color (the metakaolin was much lighter in color) and in HRWR dosages (concrete containing silica fume had a much higher demand).
A number of researchers have confirmed earlier reports that mineral admixtures typically reduce the permeability of concrete. Detwiler et al. [1994] have reported on this phenomenon in steam cured concrete. Geiker et al. [1991] noted that both silica fume and fly ash reduced the permeability of concrete to the penetration of chloride ion without changing the total porosity greatly. Bijen and van Selst [1991] found higher rates of carbonation in concrete with typical commercial quantities of ground granulated blast furnace slag compared with concretes containing typical quantities of fly ash. However, this study was not conducted with HPC.
Silica fume (also called condensed silica fume or microsilica) continues to be a popular element of high performance concrete, and especially high strength concrete. Not only does it provide an extremely rapid pozzolanic reaction, but researchers including Detwiler and Mehta [1989], and Goldman and Bentur [1993] found that its very fine size also appears to provide a beneficial contribution to concrete. Detwiler and Mehta, and Goldman and Bentur examined the effects of silica fume on mechanical behavior. Luther's [1989] review examined durability effects, while Fidjestol [1993], and Khayat and Aitcin [1993], have provided general reviews of the effects. These reports confirm findings that silica fume tends to improve both mechanical properties and durability.
However, St. John et al. [1994] report that deleterious expansion due to alkali silica reactivity is possible under wetting and drying conditions when particles of the densified form of silica fume admixtures are not sufficiently dispersed during mixing. In addition, a number of research efforts have attempted to answer important and unresolved questions in long term strength gain and frost resistance of silica fume concrete.
Maage et al. [1990] report that silica fume concretes continue to gain strength under a variety of curing conditions, including unfavorable conditions. They further indicate that concretes with silica fume appear to be more robust to early drying than similar concretes which do not contain silica fume.
A number of issues with frost resistance of concrete containing silica fume have been investigated, including the need for any entrained air when working with very low W/CM ratio concretes. Due to the dramatic reduction in permeability which accompanies the use of silica fume, concerns with frost durability in general, and with the usefulness of rapid freezing and thawing tests, have complicated the interpretation of research results. These issues are discussed in more detail in section 2.2.4, however it is useful to note here that ACI 318-95 [1995] limits the quantity of silica fume in concrete exposed to deicing salts to no more than 10 percent.
Attempts to improve the performance of systems of cementitious material in HSC have led researchers to examine mixes with multiple cementitious components. The use of ternary cementitious systems has received attention in recent years. Kashima et al. [1993] report on HSC produced with a blend of cement with large amounts of fly ash and ground granulated blast furnace slag in order to reduce heat of hydration. Their report is important because it reviews both experimental work and construction results.
Sarkar et al. [1991], examining the microstructural development in HSC using both silica fume and fly ash, found that strength at twelve hours was improved over similar mixes with silica fume alone. They state that this phenomenon may be related to the liberation of soluble alkalies from the surface of the fly ash. Baalbaki et al. [1993], reported on the properties of HPC produced with an extremely finely ground Type V cement with various mineral admixtures. Mixes with prolonged working times and very high strengths at ages out to one year were produced.
The use of cementitious systems with very high quantities of fly ash have also been investigated. Carette et al. [1993], and Bilodeau and Malhotra [1994], report that mixes have been developed which provide acceptable plastic and hardened properties, although strengths were not high, especially at early ages. In other studies, Bilodeau et al. [1994], and Malhotra [1990] report that performance in rapid freezing and thawing of concrete with high volumes of class F fly ash was adequate but that the concrete with very high quantities of fly ash performed poorly in deicer scaling tests. However, Nasser and Lai [1993] found that high volume, class C fly ash concrete was not frost durable even with a 6% air and after prolonged moist curing. They found that 20% fly ash mixes showed no difficulties in this respect. Kukko and Matala [1991] noted that frost resistance of non-air entrained concrete was reduced for very low W/CM ratio concrete produced with slowly hardening portland cement or containing slag, compared to rapid hardening portland cement with or without silica fume.
Naik et al. [1994] found that although concrete made with high volumes of class C fly ash passed ASTM C-944 for abrasion resistance, better abrasion resistance was obtained for concrete without the high fly ash content. Gjorv et al. [1990] also found that the abrasion resistance of conventional HSC pavements is exceptionally good. It would appear that high volume fly ash mixes have limited applicability to highway structures, although additional research appears warranted.
2.2.2.3 AggregatesHigh Performance Lightweight Concrete (HPLC) has been extensively investigated for, among other applications, use in oil drilling platforms in severe environments. Hoff [1991], and Tachibana et al. [1990], have presented information on the behavior of HSLC in extreme conditions. Hoff has demonstrated that HSLC containing both lightweight aggregate and conventional weight stone is both frost resistant and resistant to abrasion by ice. Holm and Bremner [1991], have provided additional information on the long term durability of lightweight concrete, indicating that in general when well-known prophylactic measures are taken to insure durability, long term durability is good.
Zhang and Gjorv [1991a, 1991b, 1991c, 1991d] have investigated both the mechanical properties and the permeability of lightweight concretes with strengths ranging from 50 to 100 MPa (about 7,000 to about 14,500 psi). Elastic modulus and the tensile-compressive strength ratio were lower than would be expected with conventional stone concrete at the same strength levels. While permeability of the HSLC's was very low, it was noted that permeability might be higher with lightweight aggregate than with conventional aggregate at the same strength. This, of course, would depend on the porosity and permeability of the aggregate itself.
2.2.3 Proportioning Methods
Modifications to conventional proportioning methods have been proposed by several researchers. Mehta and Aitcin [1990], and ACI Committee 363 report [1990] provide an excellent review of proportioning considerations for HSC. Selection of the proper raw materials and adjustment of proportions based on experience, using mixes conducted both in the laboratory and in the field, have typically proven adequate to achieve the desired concrete characteristics, at least within the limits allowed by the available raw materials. With adequate control of production and placement, routine use of concrete with compressive strengths in excess of 70 MPa (10,000 psi) is practical in many areas.
Several articles have been published with suggestions on methods of optimizing the development of particular mixes by reducing the number of trial mixes necessary. Campbell and Detwiler [1993], for example, have provided guidance for proportioning and producing steam-cured concrete. While de Larrard [1990] has provided suggestions for HSC mixes based on rheological considerations, Domone and Soutsos [1994] have reexamined the maximum density theory for applicability to HSC.
Field trials of High Early Strength (HES), Very Early Strength (VES) and Very High Strength (VHS) concretes in SHRP C-205 and C-206 indicated that existing proportioning methods remain valid, with minor modifications, for these mixes. Routine precautions such as those regarding minimum water contents and appropriate quantities or combinations of chemical admixtures, contained in numerous publications and discussed in the previous State-of-the-Art Report, remain valid. Development or adaptation of new types of high performance concrete or combinations of raw materials are better served by engineering judgement than by more sophisticated proportioning techniques.
2.2.4 Air Entrainment
The need for any air entrainment at all in concrete with very low W/CM ratio has been questioned. This issue has been complicated by the problem of interpreting test results of one of the most commonly used test methods in practice. ASTM C 666 measures the resistance of concrete to rapid freezing and thawing. The rate of freezing is much higher in this test than is found in practice, and C 666 has been criticized in this respect even for conventional concrete.
Concretes with a low W/CM ratio, such as HPC and HSC, have a lower permeability than conventional concrete. A rapid freezing and thawing rate may induce additional damage to concretes with low W/CM ratio simply due to the lower permeability. On the other hand, the very low w/c ratio, for an adequately cured concrete, can reduce or even eliminate the amount of freezable water in the pores for practical temperature ranges. These mixes will also dramatically reduce the ingress of water during the test, therefore reducing the amount of damage due to physically freezing water in the concrete. The time required to achieve an internal moisture content sufficient to contribute to frost damage is less than the time required for the C 666 test for concretes with very low W/CM ratios. However, since hydraulic pressure due to freezing of water is only one of several mechanisms of frost damage, the use of very low W/CM may not be adequate in all cases. Experimental results have been mixed.
Damage to concrete specimens may be due to thermal shock or to disequilibrium between energy states during cooling rather than expansion associated with the presence of freezing water for the mixes of very low W/CM ratio. Hanson et al. [1993], found that the electrical impedance of air entrained concrete of very low W/CM ratio actually increased during ASTM C 666 testing while the dynamic elastic modulus decreased. The increase in impedance could only have come from internal drying associated with the loss of free moisture during curing. Therefore, the decrease in elastic modulus was not related to the formation of ice.
Many believe that C 666 testing is still a valid discriminant for frost durability even though it is extremely severe for concretes of low W/CM ratio. Others have chosen to rely on deicer scaling as a more useful and informative test, particularly for highway and pavement applications.
Hammer and Sellevold [1990] report that salt scaling resistance is acceptable for HPC with W/CM ratios below 0.37, even without entrained air but that rapid freezing and thawing is accompanied by deterioration for all non-entrained air concrete tested down to a W/CM of 0.25. However, they also note that calorimeter data indicates very little ice formation until -20 c. They state that this indicates that much of the deterioration may be due to thermal incompatibility of the components rather than the formation of ice.
In research conducted by Kashi and Weyers [1989], non-air entrained concrete with a W/CM ratio of less than 0.30 was found to be resistant to rapid freezing and thawing based on ASTM C 666, Method A. For concretes with a W/CM ratio of 0.32, the concrete was frost resistant only if silica fume was not used in the mix. Cohen et al. [1992] similarly found that non-air entrained concrete with a W/CM ratio of 0.35 and containing 10% silica fume were not resistant to rapid freezing and thawing when tested in accordance with ASTM C 666 (A), even when curing had been extended to 56 days. They also noted that although there was a dramatic drop in the elastic modulus, the reduction in compressive strength was much less severe. Li et al. [1994], on the other hand, found that a maximum W/CM ratio of 0.24 was necessary for adequate frost protection of non-air entrained concrete when based on ASTM C 666 (A) testing.
Tests conducted by Pigeon et al. [1991], and by Gagne et al. [1991], using both ASTM C 666 (A) and ASTM C 672 deicer scaling tests, indicated that non-air entrained concrete containing silica fume and good quality coarse aggregate, with a W/CM ratio of 0.30 generally, but not uniformly, had good resistance to deicer scaling. When tested using ASTM C 666 (A), the W/CM ratio required to provide acceptable performance ranged from less than 0.25 to over 0.30. The cement used was found to play a significant role in the performance of otherwise similar mixes. It was also noted that the air void system produced by the use of water reducing or high range water reducing admixtures, commonly used in all concretes with low W/CM ratio, may be contributing in a fashion not yet well documented.
Additional research in these areas, perhaps concentrating more on test methods such as resistance to deicer scaling and critical dilation test concepts rather than ASTM C 666 (A), would appear to be useful. At the present time, the use of at least minimal quantities of entrained air appear prudent for concrete exposed to severe freezing, especially when exposed to deicing salts, unless that concrete has a W/CM ratio less than 0.24.
2.2.5 Other Types of HPC
Another type of HPC has been developed for use in situations where vibration is difficult or impossible and where reinforcing steel is highly congested. "Super-workable" or "flowable" concrete has been developed in Japan and used in both bridge structures and buildings. Although not specifically developed for high strength, low water to "powder" ratios are common. Kuroiwa et al. [1993], report super-workable concrete with strengths in excess of 50 MPa, a marginally high strength concrete.
Low W/CM ratio, flowing concretes utilizing HRWR's are well known. Super-workable mixes are an extension of this concept which have been specifically formulated to resist segregation. Super-workable concretes also derive partly from concrete developed for underwater placement. Research has led in several different directions. Paste and aggregate volumes, and paste composition, admixture type and dosages, and testing methods have been investigated.
A number of different "powder" combinations are reported including portland cement, fly ash, GGBFS and silica fume. In addition, where a low heat of hydration was a concern, finely ground limestone powder was used as a partial replacement for cement. This material was reported [Tanaka et al. 1993] to have both a chemical and physical effect similar to that reported for silica fume. Low heat of hydration mixes will typically exhibit considerably extended set times, with final set at about twenty hours, and low early strengths. This is due both to large quantities of HRWR in conjunction with a blended cement composed of 30% low heat of hydration portland cement and 70% GGBFS. The aggregate paste ratio was somewhat lower compared to conventional concrete.
Cellulose based products have been known to improve the cohesion of concrete and to reduce segregation. Sogo et al. [1987] report that a polymer based on cellulose ether can be used to increase both water reduction and cohesiveness. Many long chain organic molecules have water reducing capability and, if air content and setting time can be controlled, can be successfully used in concrete [Mehta 1975] with various effects on cohesiveness. Kuroiwa et al. [1993] report on findings using a polysaccharide polymer. Ozawa et al. [1990] describe studies to optimize the combination of HRWR and other admixtures affecting the viscosity of this type of mix.
Self compacting capability and resistance to segregation were determined by so-called "slump-flow", by a modified grout cone flow test and by self-leveling flow through reinforcing bars, as well as by mock-ups of particular members. Self-leveling, non-segregating performance was reported with several different mixtures. Details of these tests are discussed in the next chapter.
The addition of materials to compensate for increased drying shrinkage due to the lower aggregate content has been suggested, although Kuroiwa et al. state that drying shrinkage is equal to or better than comparable, conventional concretes. Frost durability was also found to be adequate for super-workable concrete containing at least 4% entrained air. Testing was based on ASTM C 666, although it was not stated whether Procedure A or B was used.
It is significant to note that these types of mixes have been successfully used, without vibration, in field placements. Applications include the Akashi Kaikyo Bridge near Kobe, Japan, and in the heavily reinforced concrete core and shear wall of a 20-story building. Concrete delivery was by pump. The use of super-workable concretes in certain applications appears promising.
2.3 Production Considerations
For high strength concrete where early strength is not a critical consideration, the use of conventional production methods and facilities appears adequate, as long as well recognized practices for the production of good quality concrete are enforced. Howard and Leatham [1989] and Sanchez and Hester [1990] discuss the production and delivery of HSC noting the importance of a team approach. Kakizaki et al. [1993], note that the mixing sequence can affect the slump and compressive strengths of very high strength concretes. Leming et al. [1993], emphasize that pre-pour conferences and field trials are necessary with any HPC prior to actual use.
In situations where early strength is critical, particular care must be taken to insure that temperature is closely controlled and that high dosages of water reducing or high range water reducing admixtures are avoided, since these can extend the time of set. While strength after one day may not be significantly affected, strengths at less than twenty-four hours and particularly before twelve hours, can be significantly reduced. If mineral admixtures are included, caution must be exercised in dosing the entire quantity of cementitious material. When significant percentages of mineral admixtures are employed, the result may be to effectively overdose the portland cement, again resulting in extended set times.
Much of the VES and HES concrete used in field trials for SHRP C-205 and C-206 contained a corrosion inhibiting admixture as a non-chloride accelerator, added at the job site to mitigate rapid slump loss [Hanson et al. 1994]. The admixture was added either by hand or by pump typically from trailer-mounted tanks. In either case, provision for adding the admixture, adequate quality control and sufficient remixing time are necessary.
Due to the high water content of this admixture and the low W/CM ratio of the paving mixes investigated, it was necessary to employ a HRWR in the initial batching. Inadvertent use of large dosages of HRWR caused low strengths at ages up to and including one day. The low water content and rich paste of these mixtures required strict control of the batching sequence. Although only minor adjustments to conventional practice were required, some adjustment must be anticipated. Exact procedures will vary from one production facility to another, but there are several keys to a successful placement. The plant itself should be equipped with an automatic moisture indicator for the aggregate.
A pre-placement conference, including all parties who will be involved in the slab-on-grade placement, is required. A practice placement to adjust operations, if necessary, to develop estimates for slump and air loss in transit, and to acquaint the crews involved in the placement is highly recommended. Inspection of trucks, especially in a dry batch operation, is necessary to insure that only trucks with clean fins and adequate mixing speed are used. Trucks should carry no more than two-thirds of their rated mixing capacity to insure adequate mixing on the job site if there is to be any addition of admixtures on the job site. Trucks should discharge their entire load as soon as possible. A time limit of ten minutes after arrival on the job site should be used for planning purposes but may vary depending on the type and composition of the concrete being used. Special ready-mixed concrete trucks, intended for paving operations, should be used if discharge of very low slump is anticipated.
Since most HPC's are paste rich with low water content, bleeding is typically very low. This can potentially create difficulties with plastic shrinkage cracking. Therefore, it is necessary to apply curing compound or take other precautionary measures to reduce evaporation as soon as possible for slabs or members with large exposed surfaces. However, field trials for SHRP 205 [Leming et al. 1993] of HES and VES concretes in slabs found that cracks due to plastic shrinkage were rare. The concrete was apparently gaining strength faster than it shrank. Another consequence of the rapid setting and strength gain was that time prior to sawing the slabs was reduced. It was critical that joints be sawed as early as possible. Delays past eight hours were found to cause cracking of 20 cm depth (8 in.) pavements at approximately 7.5 m (approximately 25 ft) intervals.
2.4 Fiber Reinforced Concrete
Although there has been continued interest and research in the use of fiber-reinforced concrete (FRC), there have been few major innovations in proportioning or production of high performance fiber reinforced concrete (HPFRC) since the last State-of-the-Art Report. In addition, while research in FRC has examined the influence of modifications of existing fibers, fibers with larger aspect ratios, and higher fiber volumes, and there continues to be interest in non-metallic fibers or combinations of fibers, these researches were based on existing fiber materials.
One of the few new approaches in this area has been the development of SIMCON, or Slurry Infiltrated Mat Concrete, described by Hackman et al. [1992] and Krstulovic-Opara et al. [1994]. They noted that SIMCON is a different material from SIFCON, or Slurry Infiltrated Fiber-Reinforced Concrete, which is based on the use of prepacked discontinuous steel fibers. SIMCON, on the other hand, uses a manufactured continuous mat of interlocking discontinuous steel fibers, placed in a form, and then infiltrated with a flowable cement-based slurry. The use of continuous mats, typically made with stainless steel to control corrosion in very thin members, permits development of high flexural strengths and very high ductility with a reduced volume of fibers than SIFCON.
The use of SIMCON appears to be very promising for at least two reasons. First, the very high volume of fibers required to provide significant increases in mechanical properties such as SIFCON can create a problem with economic justification in a large number of practical applications. However, with SIMCON, direct tensile strengths of 15.9 MPa at 1.1% strain have been reported with only a 5% volume fraction of fibers. Secondly, in situations where normal FRC may be economically justified, such as in pavements, the addition of fibers to the mix and the placement of the fiber-reinforced mix required special care, and considerable extra time and expense. SIMCON overcomes many of these limitations since the fiber mat, normally delivered in large rolls, can be laid out by hand and the slurry simply pumped into place. The use of SIMCON permits fabrication of thin, complex shapes with very high ductility and flexural strength.
Another interesting and useful development in FRC construction has been to provide non-metallic fibers in small, cylindrical bundles, approximately 50 mm high (the length of the fiber) and 55 mm in diameter, wrapped in a water soluble compound. This permits the easy addition of the fibers, by hand, into the mixing drum of a truck mixer, either during charging or at the job site. The wrapper disintegrates, allowing the fibers to disperse into the concrete mixture with little balling or segregation. Quality control is improved by making the quantity of fibers added easy to determine and easy to check, and by minimizing problems in dispersion in the mixer. Further, production rates are maintained with little additional effort. Successful field applications in a full-depth pavement, a thin bridge-deck overlay, a Jersey barrier, and white-topping on scarified asphalt pavement have been reported from South Dakota
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