Testing, mix design, and durability

This chapter covers the following:

• Concrete strength testing requirements and sampling criteria
• Definition and evaluation of compressive strength tests
• Acceptable strength criteria and structural adequacy requirements
• Field-cured cylinders and core testing standards
• Concrete mix design parameters, including yield and absolute volume
• Water-cement ratio and cementitious material components
• Aggregate moisture conditions and calculations
• Aggregate properties, including density, absorption, and specific gravity
• Concrete maturity and time-temperature relationships
• Concrete exposure categories and classification systems

Concrete strength testing requirements

Sampling frequency

• Samples for strength tests of each class of concrete placed each day shall be taken not less than:
  • Once per day, and
  • Once for each $150y{d}^3$ of concrete, or
  • Once for each $5000f{t}^2$ of slab or wall surface area (whichever governs).

Small quantities

• When total quantity of a given class of concrete is less than 50 yd³:
  • Strength tests are not required if evidence of satisfactory strength is submitted and approved by the building official.

Strength test definition

• A strength test is the average compressive strength of:
  • At least two 6 in. × 12 in. cylinders, or
  • At least three 4 in. × 8 in. cylinders
• Cylinders are made from the same sample and tested at 28 days, or at the designated test age.

Specified strength notation

• Let:

$$f_c^{\prime }=\text{specified compressive strength of concrete (psi)}$$

Acceptable strength criteria

The strength level of an individual concrete class is satisfactory if:

1. Three-test average requirement:

$$\text{Every arithmetic average of any three consecutive strength tests}\ge f_c^{\prime }$$

2. No single test underperformance:

   • If:

$$f_c^{\prime }\le 5000psi\Rightarrow \text{No test}<f_c^{\prime }-500\text{psi}$$

• If:

$$f_c^{\prime }>5000psi\Rightarrow \text{No test}<0.90\cdot f_c^{\prime }$$

Field-Cured cylinders

• If field-cured cylinder strength at test age is less than 85% of that of companion lab-cured cylinders:

• Exception: This 85% limit does not apply if field-cured strength exceeds:

$$f_c^{\prime }+500\text{psi}$$

Core tests for existing structures

• Concrete cores from structures must be tested within 7 days after coring.

Structural adequacy criteria:

Let the average of 3 core strengths be denoted as ${\bar{f}}_{core}$.

Concrete is structurally adequate if:

1. Average strength:

$${\bar{f}}_{core}\ge 0.85\cdot f_c^{\prime }$$

2. Minimum single core strength:

$$f_{core,\text{min}}\ge 0.75\cdot f_c^{\prime }$$

Concrete mix design parameters

Yield

Yield is the volume of fresh concrete produced from a batch:

$$\text{Yield}=\frac{\text{Total mass of batched materials}}{\text{Density of freshly mixed concrete}}=\sum \text{(Absolute volumes of concrete ingredients)}$$

Absolute volume

The absolute volume of a concrete ingredient is calculated as:

$$\text{Absolute Volume}=\frac{\text{Mass of loose material}}{\text{Relative density (or specific gravity)}\times \text{Density of water}}$$

Aggregate moisture conditions

Relative densities of aggregates are taken under Saturated Surface-Dry (SSD) conditions.

1. Total moisture (%)

$$\text{Total Moisture (\%)}=100\times \frac{\text{Wet weight}-\text{Oven-dry weight}}{\text{Oven-dry weight}}$$

2. Absorbed moisture (%)

$$\text{Absorbed Moisture (\%)}=100\times \frac{\text{SSD weight}-\text{Oven-dry weight}}{\text{Oven-dry weight}}$$

3. Free Moisture (%)

$$\text{Free Moisture (\%)}=\text{Total Moisture (\%)}-\text{Absorbed Moisture (\%)}$$

Water-cement ratio

The water-cementitious material ratio (w/c) is defined as:

$$w/c=\frac{\text{Mass of water}}{\text{Mass of cementitious materials}}$$

Cementitious materials may include:

• Portland cement
• Blended cement
• Fly ash
• Slag cement
• Silica fume
• Natural pozzolans

Aggregate properties in concrete mix design

SSD (Saturated-surface-dry)

• The condition in which the permeable pores of aggregate particles are filled with water as if submerged, but without any free water on the surface of the particles.

Density (Oven-dry)

• The mass of oven-dry aggregate particles per unit volume, including:
  • Volume of permeable pores
  • Volume of impermeable pores
  • Does not include voids between particles

Density (SSD)

• The mass of saturated-surface-dry aggregate per unit volume, including:
  • Volume of impermeable pores
  • Volume of water-filled permeable pores
  • Does not include voids between particles

Apparent density

• The mass per unit volume of the impermeable portion of the aggregate particles only.

Absorption

• The increase in mass due to water absorbed into pores, not counting surface moisture.
• Expressed as a percentage of dry mass:

$$\text{Absorption (\%)}=\frac{M_{\text{SSD}}-M_{\text{OD}}}{M_{\text{OD}}}\times 100$$

Bulk specific gravity

Oven-dry basis:

• Ratio of oven-dry mass to the mass of water equal to the SSD volume of aggregate:

$$G_{\text{bulk, OD}}=\frac{M_{\text{OD}}}{V_{\text{SSD}}\cdot {\rho }_{\text{water}}}$$

SSD basis:

• Ratio of SSD mass to the mass of water equal to the SSD volume:

$$G_{\text{bulk, SSD}}=\frac{M_{\text{SSD}}}{V_{\text{SSD}}\cdot {\rho }_{\text{water}}}$$

• Typical range for normal-weight aggregates:

$$G_{\text{bulk, SSD}}=2.4\text{ to }2.9$$

Free (surface) moisture

• Moisture content in excess of SSD condition.
• Represents water on the surface of the aggregate particles.

Concrete maturity

Concrete maturity can be estimated using the Time-Temperature Factor method:

$$M=\sum _0^t(T-T_0)\Delta t$$

where

• $M$ = maturity index ($°F$-hours)
• $T$ = average concrete temperature ($°F$) during time interval $\Delta t$
• $T_0$ = datum temperature ($°F$), usually $32°F$ unless otherwise specified
• $t$ = elapsed time ($hr$)
• $\Delta t$ = time intervals ($hr$)

Concrete exposure categories and classes

Freezing and thawing (F)

Class	Condition
F0	Concrete not exposed to freezing-and-thawing cycles
F1	Concrete exposed to freezing-and-thawing cycles with limited exposure to water
F2	Concrete exposed to freezing-and-thawing cycles with frequent exposure to water
F3	Concrete exposed to freezing-and-thawing cycles with frequent exposure to water and exposure to deicing chemicals

Sulfate (S)

Class	Water-soluble sulfate in soil	Dissolved sulfate in water	Condition
S0	${\text{SO}}_4^{2-}<0.10\%$	${\text{SO}}_4^{2-}<150\text{ppm}$	Negligible sulfate exposure
S1	$0.10\le {\text{SO}}_4^{2-}<0.20\%$	$150\le {\text{SO}}_4^{2-}<1500$ or seawater	Mild sulfate exposure
S2	$0.20\le {\text{SO}}_4^{2-}\le 2.00\%$	$1500\le {\text{SO}}_4^{2-}\le 10000$	Moderate sulfate exposure
S3	${\text{SO}}_4^{2-}>2.00\%$	${\text{SO}}_4^{2-}>10000$	Severe sulfate exposure

In contact with water (W)

Class	Condition
W0	Concrete dry in service
W1	Concrete in contact with water where low permeability is not required
W2	Concrete in contact with water where low permeability is required

Corrosion protection of reinforcement(C)

Class	Condition
C0	Concrete dry or protected from moisture
C1	Concrete exposed to moisture, but not to an external source of chlorides
C2	Concrete exposed to moisture and an external source of chlorides (e.g., deicing chemicals, salt, brackish water, seawater, or spray)