Concrete
3.3 Concrete Strength
The water–cement ratio, w/c, (or more properly the water–cementitious materials ratio) is one of the most important parameters affecting concrete strength (Figure 3.2). For high w/c ratios, there is more water in the mixture than required for hydration, and this increases porosity of the matrix and reduces strength. For w/c ratios below about 0.3, there is a significant increase in the strength of the interfacial transition zone leading to rapid increase in concrete strength.
FIGURE 3.2 Compressive strength of 6 × 12 in (150 × 300 mm) cylinders as function of water–
cement ratio. (After PCA, 1988.)
Aggregate strength usually does not determine concrete strength.
However, size, shape, surface texture, grading, and mineralogy can affect concrete strength. In general, larger aggregates and poorer grading lead to reduced strength.
Air entrainment can improve concrete workability, which in turn can lead to improved compaction. The increased porosity generally leads to reduced strength, but this can be compensated by positive effects of improved compaction.
Other chemical admixtures are classed according to function, and include (1) water-reducing admixtures that improve workability with less water, leading to reduced w/c ratio; (2) retarding admixtures that slow the setting rate of concrete; (3) accelerating admixtures to increase the rate of early strength development; (4) superplasticizers for high-slump concrete with reduced w/c ratio; and (5) corrosion-inhibiting admixtures to slow
corrosion of reinforcing steel in extreme environments. In addition, air- entraining admixtures are used to increase concrete resistance against frost.
Mineral admixtures such as fly ash, blast-furnace slag, and silica fume are used as a partial replacement for portland cement. In addition to the ecological advantages of reduced cement usage, these can lead to reduced cost, reduced permeability, and increased strength.
3.3.2 Curing Time and Conditions
Concrete curing describes the process by which concrete matures and develops its mechanical and durability properties over time as a result of continued hydration of the cement. Concrete curing is affected by the availability of water for hydration and by temperature. In the absence of deleterious effects, curing and associated concrete strength gain can continue indefinitely.
Figure 3.3 shows the influence of curing humidity on strength gain of concrete with time. If curing occurs in a relatively dry environment, it effectively ceases once the free water in the mixture is used up or evaporates from the capillaries. Because the amount of free water in a concrete mixture usually is more than sufficient for complete hydration, application of an impermeable membrane to seal in mixture water can be an effective way to maintain hydration in dry environments.
FIGURE 3.3 Compressive strength of 6 × 12 in (150 × 300 mm) cylinders as a function of age for a variety of curing conditions. (After PCA, 1988.)
Curing temperature also affects the rate of strength gain. Concrete cast and cured in the normal temperature range [40°F to 100°F (5°C to 35°C)]
shows slower strength gain at colder temperatures within the range, but eventually reaches its strength potential. For higher temperatures, the rate of hydration is increased, producing a higher early strength, at the cost of some reduction in the final strength. The same applies to large structural members with high-strength concrete for which temperature rise associated with heat of hydration can be high. Cold weather concreting also can cause problems.
See ACI 308R-01 (2001).
Cement fineness also affects rate of strength gain. The properties of portland cements have varied with changes in governing specifications over time, such that more recent cements gain early strength faster than older cements. Figure 3.4 presents a composite of test results from various sources including early-age strengths from more recent cements and long- term strengths for older cements. The data refer to concrete with continuous supply of moisture for curing. Long-term compressive strengths for specimens stored outdoors may develop lower strength depending on the
curing environment (Wood, 1991).
FIGURE 3.4 Concrete strength gain with time under moist curing for different types of portland cements: data up to 28 days based on mortar tests using U.S. cements from the 1990s (Mindess et al., 2003, p. 29); data for 90 days and 1 year based on concrete tests from the 1970s (Mindess et al., 2003, p. 28); data for 30 years based on concrete tests for Type I (Wood, 1991) and Type II (Monteiro and Moehle, 1995) cements.
3.3.3 In-Place Concrete
Acceptance testing of concrete for new structures generally is based on standardized tests of concrete that has been placed in a standard mold (usually a cylinder but in some countries a cube), compacted in a specified way, stored in a controlled curing environment, and tested under idealized boundary conditions. Concrete in actual structures has form, compaction, curing, and boundary conditions that differ from the standardized test conditions. Given the dependence of concrete strength on these variables, it should not be surprising that in-place strength differs from the strength measured in the standardized test.
Specific causes for in-place concrete strength variations include:
• Consolidation: In normal placement operations, good construction
practice uses vibration to expel entrapped air from plastic concrete. In deep members, higher static pressure at the bottom increases
consolidation and may improve strength relative to the top (ACI 214.4R-10, 2010).
• Bleeding and segregation: During placing and consolidation
activities, segregation of concrete constituents and bleeding of water toward the top surface can result in localized differences in concrete mixtures throughout a structure. In deep members such as columns, the increased w/c ratio near the top of the member can result in reduced strength. It is common to estimate the strength of concrete at the top of a column as in which C = 0.85. [CSA (2004) uses
in which λc = 0.00001 (psi) or 0.0015 (MPa).]2
• Curing: Curing may vary throughout the structure and generally
deviates from the ideal conditions specified for control test specimens.
In large sections (e.g., large columns), heat of hydration effects can cause high temperatures that permanently reduce concrete strength potential. Slabs and beams have exposed surfaces, making them sensitive to moisture loss that can result in reduced strength.
• Micro-cracks: Drying shrinkage, temperature change, and applied loads cause internal stresses and strains that produce micro-cracks and reduce stress-resisting capacity of in-place concrete.
These effects have been demonstrated by various types of tests on in- place concrete. Nondestructive methods (e.g., ultrasonic pulse velocity) determine hardened concrete properties in ways that cause no noticeable damage to concrete (ACI 228.2R-98, 1998). In-place methods include nondestructive methods plus other methods conducted in place that may cause observable but insignificant damage (e.g., rebound hammer) (ACI 228.1R-03, 2003). Destructive methods usually refer to removing cores from an existing structure for testing in a laboratory (ACI 214.4R-10, 2010).
Section 3.3.4 includes additional discussion on evaluation of test results from concrete cores.
3.3.4 Test Specimen Parameters
In the United States, the standard test specimen for determining compressive strength is a 6 × 12 in (150 × 300 mm) cylinder cast in a mold and tested in a moist condition. If any of these parameters is changed, the apparent compressive strength obtained from the test may change.
• Moisture content: Air-dried cylinders are on average 10% to 14%
stronger than soaked cylinders, though values differ for different concretes (ACI 214.4R-10, 2010). The effect may be associated with effects of internal hydraulic pressure.
• Length-to-diameter ratio: During testing (Figure 3.5a), steel platens at the loaded ends of a compression test specimen restrain lateral expansion, confining the concrete and increasing the apparent strength.
A length-to-diameter ratio of 2.0 is recommended so that the
confinement effect near mid-length is reduced. When a shorter length- to-diameter ratio is used, as sometimes occurs when cores are taken from an existing structure, correction factors are required (ACI 214.4R-10, 2010). Many countries use a cube for concrete
compressive strength tests. Cube strength is generally larger than cylinder strength, in part because of the confinement effect. The ratio of cube to cylinder strength is commonly assumed to be 1.25 for normal-strength concrete, decreasing to 1.0 for high compressive strength (around 14 ksi or 100 MPa).
• Size: Cylinders (or cubes) smaller than the standard sizes may be required under special circumstances such as coring between closely spaced reinforcement. For cylinders between 4 and 6 in (100 and 150 mm) there is little size effect. For smaller cylinders there is conflicting information on size effect. Minimum dimensions should not approach the size of the largest aggregate.
• Cores taken from existing structures: Existing structures have strength variations that have been discussed in Section 3.3.3. Cores also may have sizes and length-to-diameter ratios that differ from the standards.
In addition to these aspects, drilled cores may have reduced strength because of micro-cracking that occurs during drilling. Core drilling may or may not coincide with the direction of concrete placement depending on the member being cored, so bleeding effects may
manifest themselves differently. Defects such as pre-existing cracks or inclusions (including unintended reinforcing steel) also affect core test results. See ACI 214.4R-10 (2010) for detailed discussion of
planning, conducting, and interpreting results of a coring program.
FIGURE 3.5 (a) Uniaxial compression test on 6 × 12 in (150 × 300 mm) cylinder at UC Berkeley laboratories. (Photo courtesy of L. Stepanov.) (b) Stress–strain relations of
normalweight concretes under uniaxial compressive loading. (After Wischers, 1979, as reported by ACI 363R-92, 1992.)
3.3.5 Expected Strength in Structures
Quality control requirements of building codes are written so that there is only a small chance that under-strength concrete will compromise the safety of a building. ACI 318 specifies both the frequency of concrete sampling and testing, and the strength required for acceptance. The strength requirement is as follows: (1) no individual strength test result [the average of two 6 by 12 in (150 by 300 mm) or of three 4 by 8 in (100 by 200 mm) cylinder tests] shall fall below by more than 500 psi (3.4 MPa) if ≤ 5000 psi (34 MPa) or by more than 0.1 if > 5000 psi (34 MPa), and (2) no average of three consecutive strength tests shall be less than . To satisfy these requirements, the concrete supplier must design the concrete mixture for an average compressive strength that exceeds these minimum acceptance values.
The target value of will necessarily depend on the variability of the concrete strength and the accepted probability of a test not satisfying the acceptance requirement. In general, the value of should be established such that nonconformance is anticipated no more than 1 in 100 times (ACI 214R-11, 2011). Therefore, assuming a normal distribution of measured compressive strengths, the following must be satisfied:
where σ = standard deviation of compressive strengths. Equation (3.1) defines the required target strength so there is no more than 0.01 probability that averages of three consecutive tests will be below the specified value of . Equation (3.2) defines the target strength so there is no more than 0.01 probability of an individual test falling more than 500 psi (3.5 MPa) below the specified value of . For substitute 0.1 for 500 psi (3.4 MPa) in Eq. (3.2).
Studies in North America have defined expected dispersions for typical concretes (ACI 214R-11, 2011). For average quality control, a coefficient of variation of 0.15 is to be expected. Excellent quality control (the upper 10% of projects studied) resulted in coefficient of variation of about 0.10.
Poor quality control (the lower tenth percentile) resulted in coefficient of variation of about 0.20.3
Example 3.1. Concrete is specified to have = 4000 psi (28 MPa). The supplier uses Type I cement. What is the expected compressive strength after several years?
Solution
Assuming average quality control, with coefficient of variation of 0.15, Eq.
(3.1) requires target compressive strength = 5000 psi (34 MPa), and Eq.
(3.2) requires = 5400 psi (37 MPa). Assume the supplier aims to provide 5400 psi (37 MPa) concrete. Assuming the average long-term strength gain of 32% relative to the 28-day strength for Type I cement (Section 3.3.2), the expected strength at advanced age is 1.32 × 5400 psi = 7100 psi (49 MPa) or 1.8 .
The preceding example illustrates that expected compressive strength at advanced age can exceed the specified strength by a significant margin. The California Department of Transportation (Caltrans), based on limited field testing, commonly assumes compressive strength of concrete in older construction is 1.5 . ASCE 41 (2013) uses the same multiplier to obtain expected strength. Common practice for performance-based seismic design of new buildings is to estimate expected compressive strength as 1.3 (TBI, 2010). Of course, these expectations assume that the concrete is not overloaded and does not sustain durability problems.