“The objective in designing concrete mixtures is to determine the most economical and practical combination of readily available materials to produce a concrete that will satisfy the performance requirements under particular conditions of use.”
PCC mix design has evolved chiefly through experience and well-documented empirical relationships. Normally, the mix design procedure involves two basic steps:
- Mix proportioning. This step uses the desired PCC properties as inputs then determines the required materials and proportions based on a combination of empirical relationships and local experience. There are many different PCC proportioning methods of varying complexity that work reasonably well.
- Mix testing. Trial mixes are then evaluated and characterized by subjecting them to several laboratory tests. Although these characterizations are not comprehensive, they can give the mix designer a good understanding of how a particular mix will perform in the field during construction and under subsequent traffic loading.
This section covers mix design fundamentals common to all PCC mix design methods. First, two basic concepts (mix design as a simulation and weight-volume terms and relationships) are discussed to set a framework for subsequent discussion. Second, the variables that mix design may manipulate are presented. Third, the fundamental objectives of mix design are presented. Finally, a generic mix design procedure is presented.
Before discussing any mix design specifics, it is important to understand a couple of basic mix design concepts:
- Mix design is a simulation
- Weight-volume terms and relationships
Mix Design is a Simulation
First, and foremost, mix design is a laboratory simulation. Mix design is meant to simulate actual PCC manufacturing, construction and performance. Then, from this simulation we can predict (with reasonable certainty) what type of mix design is best for the particular application in question and how it will perform.
Being a simulation, mix design has its limitations. Specifically, there are substantial differences between laboratory and field conditions. For instance, mix testing is generally done on small samples that are cured in carefully controlled conditions. These values are then used to draw conclusions about how a mix will behave under field conditions. Despite such limitations mix design procedures can provide a cost effective and reasonably accurate simulation that is useful in making mix design decisions.
Weight-Volume Terms and Relationships
The more accurate mix design methods are volumetric in nature. That is, they seek to combine the PCC constituents on a volume basis (as opposed to a weight basis). Volume measurements are usually made indirectly by determining a material’s weight and specific gravity and then calculating its volume. Therefore, mix design involves several key aggregate specific gravity measurements.
PCC is a complex material formed from some very basic ingredients. When used in pavement, this material has several desired performance characteristics – some of which are in direct conflict with one another. PCC pavements must resist deformation, crack in a controlled manner, be durable over time, resist water damage, provide a good tractive surface, and yet be inexpensive, readily made and easily placed. In order to meet these demands, mix design can manipulate the following variables:
- Aggregate. Items such as type (source), amount, gradation and size, toughness and abrasion resistance, durability and soundness, shape and texture as well as cleanliness can be measured, judged and altered to some degree.
- Portland cement. Items such as type, amount, fineness, soundness, hydration rate and additives can be measured, judged and altered to some degree.
- Water. Typically the volume and cleanliness of water are of concern. Specifically, the volume of water in relation to the volume of portland cement, called the water-cement ratio, is of primary concern. Usually expressed as a decimal (e.g., 0.35), the water-cement ratio has a major effect on PCC strength and durability.
- Admixtures. Items added to PCC other than portland cement, water and aggregate. Admixtures can be added before, during or after mixing and are used to alter basic PCC properties such as air content, water-cement ratio, workability, set time, bonding ability, coloring and strength.
By manipulating the mixture variables of aggregate, portland cement, water and admixtures, mix design seeks to achieve the following qualities in the final PCC product (Mindess and Young, 1981):
- Strength. PCC should be strong enough to support expected traffic loading. In pavement applications, flexural strength is typically more important than compressive strength (although both are important) since the controlling PCC slab stresses are caused by bending and not compression. In its most basic sense, strength is related to the degree to which the portland cement has hydrated. This degree of hydration is, in turn, related to one or more of the following:
- Water-cement ratio. The strength of PCC is most directly related to its capillary porosity. The capillary porosity of a properly compacted PCC is determined by its water-cement ratio (Mindess and Young, 1981). Thus, the water-cement ratio is an easily measurable PCC property that gives a good estimate of capillary porosity and thus, strength. The lower the water-cement ratio, the fewer capillary pores and thus, the higher the strength. Specifications typically include a maximum water-cement ratio as a strength control measure.
- Entrained air (air voids). At a constant water-cement ratio, as the amount of entrained air (by volume of the total mixture) increases, the voids-cement ratio (voids = air + water) decreases. This generally results in a strength reduction. However, air-entrained PCC can have a lower water-cement ratio than non-air-entrained PCC and still provide adequate workability. Thus, the strength reduction associated with a higher air content can be offset by using a lower water-cement ratio. For moderate-strength concrete (as is used in rigid pavements) each percentile of entrained air can reduce the compressive strength by about 2 – 6 percent (PCA, 1988).
- Cement properties. Properties of the portland cement such as fineness and chemical composition can affect strength and the rate of strength gain. Typically, the type of portland cement is specified in order to control its properties.
- Controlled shrinkage cracking. Shrinkage cracking should occur in a controlled manner. Although construction techniques such as joints and reinforcing steel help control shrinkage cracking, some mix design elements influence the amount of PCC shrinkage. Chiefly, the amount of moisture and the rate of its use/loss will affect shrinkage and shrinkage cracking. Therefore, factors such as high water-cement ratios and the use of high early strength portland cement types and admixtures can result in excessive and/or uncontrolled shrinkage cracking.
- Durability. PCC should not suffer excessive damage due to chemical or physical attacks during its service life. As opposed to HMA durability, which is mainly concerned with aging effects, PCC durability is mainly concerned with specific chemical and environmental conditions that can potentially degrade PCC performance. Durability is related to:
- Porosity (water-cement ratio). As the porosity of PCC decreases it becomes more impermeable. Permeability determines a PCC’s susceptibility to any number of durability problems because it controls the rate and entry of moisture that may contain aggressive chemicals and the movement of water during heating or freezing (Mindess and Young, 1981). The water-cement ratio is the single most determining factor in a PCC’s porosity. The higher the water-cement ratio, the higher the porosity. In order to limit PCC porosity, many agencies specify a maximum allowable water-cement ratio.
- Entrained Air (Air voids). Related to porosity, entrained air is important in controlling the effects of freeze-thaw cycles. Upon freezing, water expands by about 9 percent. Therefore, if the small capillaries within PCC are more than 91 percent filled with water, freezing will cause hydraulic pressures that may rupture the surrounding PCC. Additionally, freezing water will attract other unfrozen water through osmosis (PCA, 1988). Entrained air voids act as expansion chambers for freezing and migrating water and thus, specifying a minimum entrained air content can minimize freeze-thaw damage.
- Chemical environment. Certain chemicals such as sulfates, acids, bases and chloride salts are especially damaging to PCC. Mix design can mitigate their damaging effects through such things as choosing a more resistant cement type.
- Skid resistance. PCC placed as a surface course should provide sufficient friction when in contact with a vehicle’s tire. In mix design, low skid resistance is generally related to aggregate characteristics such as texture, shape, size and resistance to polish. Smooth, rounded or polish-susceptible aggregates are less skid resistant. Tests for particle shape and texture can identify problem aggregate sources. These sources can be avoided, or at a minimum, aggregate with good surface and abrasion characteristics can be blended in to provide better overall characteristics.
- Workability. PCC must be capable of being placed, compacted and finished with reasonable effort. The slump test, a relative measurement of concrete consistency, is the most common method used to quantify workability. Workability is generally related to one or more of the following:
- Water content. Water works as a lubricant between the particles within PCC. Therefore, low water content reduces this lubrication and makes for a less workable mix. Note that a higher water content is generally good for workability but generally bad for strength and durability, and may cause segregation and bleeding. Where necessary, workability should be improved by redesigning the mix to increase the paste content (water + portland cement) rather than by simply adding more water or fine material (Mindess and Young, 1981).
- Aggregate proportion. Large amounts of aggregate in relation to the cement paste will decrease workability. Essentially, if the aggregate portion is large then the corresponding water and cement portions must be small. Thus, the same problems and remedies for “water content” above apply.
- Aggregate texture, shape and size. Flat, elongated or angular particles tend to interlock rather than slip by one another making placement and compaction more difficult. Tests for particle shape and texture can identify possible workability problems.
- Aggregate gradation. Gradations deficient in fines make for less workable mixes. In general, fine aggregates act as lubricating “ball bearings” in the mix. Gradation specifications are used to ensure acceptable aggregate gradation.
- Aggregate porosity. Highly porous aggregate will absorb a high amount of water leaving less available for lubrication. Thus, mix design usually corrects for the anticipated amount of absorbed water by the aggregate.
- Air content. Air also works as a lubricant between aggregate particles. Therefore, low air content reduces this lubrication and makes for a less workable mix. A volume of air-entrained PCC requires less water than an equal volume of non-air-entrained PCC of the same slump and maximum aggregate size (PCA, 1988).
- Cement properties. Portland cements with higher amounts of C3S and C3A will hydrate quicker and lose workability faster.
Knowing these objectives, the challenge in mix design is then to develop a relatively simple procedure with a minimal amount of tests and samples that will produce a mix with all the qualities discussed above.
In order to meet the requirements established by the preceding desirable PCC properties, all mix design processes involve four basic processes:
- Aggregate selection. No matter the specific method, the overall mix design procedure begins with evaluation and selection of aggregate and asphalt binder sources. Different authorities specify different methods of aggregate acceptance. Typically, a battery of aggregate physical tests is run periodically on each particular aggregate source. Then, for each mix design, gradation and size requirements are checked. Normally, aggregate from more than one source is required to meet gradation requirements.
- Portland cement selection. Typically, a type and amount of portland cement is selected based on past experience and empirical relationships with such factors as compressive strength (at a given age), water-cement ratio and chemical susceptibility.
- Mix proportioning. A PCC mixture can be proportioned using experience or a generic procedure (such as ACI 211.1).
- Testing. Run laboratory tests on properly prepared samples to determine key mixture characteristics. It is important to understand that these tests are not comprehensive nor are they exact reproductions of actual field conditions.
The selected PCC mixture should be the one that, based on test results, best satisfies the mix design objectives.
PCC mix design is a laboratory process used to determine appropriate proportions and types of aggregate, portland cement, water and admixtures that will produce desired PCC properties. Typical desired properties in PCC for pavement are adequate strength, controlled shrinkage, durability, skid resistance and workability. Although mix design has many limitations it had proven to be a cost-effective simulation that is able to provide crucial information that can be used to formulate a high-performance PCC.