Suitable material, such as natural stone, thin brick, ceramic tile, terra cotta, oversized natural or crushed aggregate, may be used as facing materials. Special facing materials shall be properly designed and tested for compatibility with the concrete, precast product, and service conditions.
Once a sample panel has been approved by the architect/engineer, no other source of facing material shall be used for the project unless shown to be equivalent in quality, gradation, and color to the approved sample.
Standard Commentary
4.1 Mix Proportioning C4.1 Mix Proportioning
The properties of concrete mixtures shall be as specified in the project specifications.
Supplementary or replacement cementitious materials other than hydraulic cement may be used in combination with portland or blended cement for economy, reduction of heat of hydration, improved workability, improved strength, and improved durability of the concrete. These materials shall meet the requirements of the following ASTM specifications:
• (ASTM C618) – fly ash, natural pozzolans
• (ASTM C989) – ground granulated blast-furnace slag
• (ASTM C1240) – silica fume
Established concrete mix designs for which strength and performance data exists can be used on the basis of past test results if the concrete is made from the same sources of cement and aggregates.
Much of the skill, knowledge, and technique of producing quality precast concrete elements center around the proper proportioning of the concrete mix.
Before a concrete mix can be properly proportioned, several factors must be known such as the finish, size, and shape of units to be cast. The method of consolidation should be known to help determine the required workability of the mix. The maximum size of the coarse aggregate should be established. The required compressive strength affects the amount of cement to be used as well as the maximum water allowed.
The extent of exposure to severe weather or other harsh environmental conditions will affect the durability requirements of the concrete mix design.
4.1.1 Qualification of New Concrete Mixes C4.1.1 Qualification of New Concrete Mixes Concrete mixes for precast concrete shall be Accepted methods of selecting mix proportioning are
DIVISION 4 CONCRETE
Standard Commentary
Page 4.2 MNL-116 4th Edition
Concrete mixes shall be proportioned and/or evaluated on a per project basis to satisfy project requirements and service conditions such as strength, absorption, volume change, and resistance to freezing and thawing. The mix shall have adequate workability for proper placement and consolidation.
4.1.2 Specified Concrete Strength
Concrete strengths shall be determined on the basis of test specimens at time of stripping, transfer of prestress, or the specified age (typically 28 days). A minimum acceptable strength at time of stripping shall be established by the precast plant engineer or the engineer of record and shall be stated on the drawings. When members are prestressed, the concrete shall have a specified compressive strength, for transfer of the prestressing forces and design.
C4.1.2 Specified Concrete Strength
A minimum design compressive strength for concrete should be determined by the architect/engineer, based on in-service requirements. Consideration for production and erection are the responsibility of the precaster. For structural products, the mix is generally proportioned for strength and durability, with aesthetics as a secondary consideration. Concrete strength is typically specified as compressive strength that is determined at the time of transfer of prestressing forces to the concrete, or as the design strength. A standard age used for determining the official design is 28 days; however, other ages may be specified.
Production requirements for early stripping of units or early prestress transfer and subsequent rapid reuse of forms require high early compressive strengths. The minimum required transportation and erection strength levels are dependent on the shape of the unit, handling, shipping, erection techniques, and the delivery schedule.
These requirements often necessitate such high early strength that the resulting 28-day strengths far exceed the specified design strengths requirement.
Design strengths specified for mixes to be used in prestressed concrete cover a wid e range of strengths, but are generally higher than typical strength requirements for mild reinforced concrete members. In the absence of other specifications, a minimum design strength of 5,000 psi (34.5 MPa) at 28 days is recommended.
4.1.3 Statistical Concrete Strength Considerations
The compressive strength level of the concrete shall be considered satisfactory if the average of each set of any three consecutive strength tests equals or exceeds the specified strength, with no individual test results falling below the specified strength by more than 500 psi (3.5 MPa).
C4.1.3 Statistical Concrete Strength Considerations
The average compressive strength, used as the basis for selecting proportions, should exceed the specified strength by at least:
• 400 psi (2.76 MPa) if the standard deviation is less than 300 psi (2.07 MPa)
• 550 psi (3.79 MPa) if the standard deviation is 300 (2.07 MPa) to 400 psi (2.76 MPa)
• 700 psi (4.82 MPa) if the standard deviation is 400 (2.76 MPa) to 500 psi (3.45 MPa)
• 900 psi (6.20 MPa) if the standard deviation is 500 (3.45 MPa) to 600 psi (4.14 MPa)
These values are based on ACI 318 equations for f′cr
required compressive strength. ACI 318 requires that mixes be proportioned for f′cr as defined by the larger of the following two equations:
f ′cr = f ′c + 1.34s or f ′cr = f ′c + 2.33s – 500 Where
f ′cr = required average compressive strength (psi) f ′c = specified compressive strength (psi) s = standard deviation (psi)
If the standard deviation exceeds 600 psi (4.14 MPa) or if a suitable record of strength test performance is not available, mix proportions should be selected to produce a minimum average strength of:
• 1200 psi (8.27 MPa) greater than the specified strength for required strengths of 5000 psi (34.5 MPa), and below, and
• 1400 psi (9.66 MPa) for required strengths above 5000 psi (34.5 MPa).
DIVISION 4 CONCRETE
Standard Commentary
Page 4.4 MNL-116 4th Edition
4.1.4 Proportioning to Ensure Durability of Concrete
Concrete strength and durability shall be achieved through proper consideration of air entrainment, water content, cementitious material content, and workability. Low water to cementitious material ratio (w/cm) shall be used to provide specified strength, durability, and low absorption. Drying shrinkage characteristics shall be controlled by aggregate size, gradation, mineralogy, aggregate-cement ratio, cement factor, w/cm, additives, and admixtures.
Air entraining admixtures shall conform to the requirements of ASTM C260. Water reducing, retarding, or accelerating admixtures shall conform to the requirements of ASTM C494. High-range water-reducing admixtures (HRWR), also referred to as superplasticizers, shall be in accordance with the requirements of ASTM C494, Type F or G, for normal concrete. For flowing concrete, HRWR shall be in accordance with ASTM C1017, Type 1 or 2. Calcium chloride or admixtures containing chloride ions (Cl¯), other than impurities from admixture ingredients shall not be used in prestressed concrete. This is to ensure against deleterious concentrations of chloride ions that may cause corrosion of the reinforcement.
C4.1.4 Proportioning to Ensure Durability of Concrete
Achieving low absorption rates for the surface of the concrete requires a high-density concrete surface.
Combinations of cement and aggregate shall be selected on the basis of known compatibility determined through performance history or testing.
Incompatible combinations that will result in unacceptable volume changes, cracking, or deterioration such as the use of high-alkali cement with alkali-reactive aggregates, are prohibited.
Not all non-cementitious materials that can be added to concrete will necessarily provide any or all of the benefits noted in 4.1. The effect of adding such materials should be carefully investigated in advance of using the products in concrete mixes for the first time.
Because the aggregate occupies the greatest volume of the mix, it is important to select good quality aggregate.
The use of poor quality aggregates may result in durability concerns for concrete under harsh environmental service conditions.