15.5 Construction Materials Testing and Concrete Mix Proportioning

15.5 Construction Materials Testing and Concrete Mix Proportioning

PSAD area 8 of the 2022 table of specifications, "Construction Materials and Testing," has one competency worth 4% (3 items): distinguish the equipment and apparatus used to test construction materials, and proportion concrete mixes. Statistical evaluation of strength tests, with the standard deviation and required average strength $f'_{cr}$, is in Section 3.1. Material properties and handling are in Section 6.1.

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Testing Fresh Concrete

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Testing Hardened Concrete

Example. A $150 \times 300\text{ mm}$ cylinder fails at $P = 250\text{ kN}$ in a splitting test:

$T = \frac{2(250{,}000)}{\pi(300)(150)} = 3.54\text{ MPa}$

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ACI 318 Sampling and Acceptance (NSCP 2015 Chapter 4)

• Strength test: the average strength of at least two $150 \times 300\text{ mm}$ cylinders or three $100 \times 200\text{ mm}$ cylinders, usually at 28 days.
• Sampling frequency: at least once a day for each class of concrete, once for each $110\text{ m}^3$, and once for each $460\text{ m}^2$ of slab or wall surface.
• Acceptance:
  • every average of three consecutive strength tests is at least $f'_c$;
  • no single test falls below $f'_c - 3.5\text{ MPa}$ when $f'_c \le 35\text{ MPa}$, or below $0.90f'_c$ when $f'_c > 35\text{ MPa}$.
• Low-strength investigation: take three cores for each strength test that fell more than 3.5 MPa below $f'_c$. The concrete is considered structurally adequate if:
  • the average of the three cores is at least $0.85f'_c$; and
  • no single core is below $0.75f'_c$.

Example. With $f'_c = 28\text{ MPa}$, cores test at 24.5, 25.0 and 22.0 MPa.

• The average is $23.83 \ge 0.85(28) = 23.8\text{ MPa}$.
• The lowest core is $22.0 \ge 0.75(28) = 21.0\text{ MPa}$.

The concrete is adequate.

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Testing Aggregates

Fineness modulus (FM) is the sum of the cumulative percentages retained on the 150 µm, 300 µm, 600 µm, 1.18 mm, 2.36 mm and 4.75 mm sieves, plus the larger standard sieves when present, divided by 100. ASTM C33 fine aggregate usually has an FM between 2.3 and 3.1. A higher FM means coarser sand.

Example. Cumulative retained percentages are 3 (4.75 mm), 15 (2.36 mm), 35 (1.18 mm), 58 (600 µm), 82 (300 µm) and 96 (150 µm). FM $= 289/100 = 2.89$.

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Testing Reinforcing Steel and Other Materials

• Reinforcing bars are tested in a universal testing machine (UTM) for yield strength, tensile strength and elongation, and are bent around a specified pin to check ductility. Deformed bars sold in the Philippines are produced to the Philippine National Standard PNS 49, and imported bars to ASTM A615 or A706.
• Concrete hollow blocks are tested for compressive strength on the net or gross area as specified.
• Soils for embankments and subgrades are tested by Proctor compaction, field density (sand cone, ASTM D1556) and CBR. These are covered in Section 10.1.

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Mix Proportioning by ACI 211.1 (Absolute Volume)

1. Choose the slump for the type of construction.
2. Choose the nominal maximum aggregate size (NMAS), which must suit the member size and bar spacing.
3. Estimate mixing water and air content from the ACI table for that slump and NMAS.
4. Select the water-cement ratio for the required average strength $f'_{cr}$ (Section 3.1) and for durability.
5. Compute the cement content as water ÷ (w/c).
6. Find the coarse aggregate content: table volume (from NMAS and sand FM) × dry-rodded unit weight.
7. Find the fine aggregate content: 1 m³ minus the absolute volumes of water, cement, coarse aggregate and air.
8. Adjust for aggregate moisture and make trial batches.

Example. The target is $f'_{cr} = 30\text{ MPa}$ in non-air-entrained concrete, with slump 75–100 mm and NMAS 25 mm. The sand has FM = 2.60. The coarse aggregate has a dry-rodded unit weight of $1{,}600\text{ kg/m}^3$. Oven-dry specific gravities are 2.68 (coarse) and 2.64 (fine), and cement is 3.15.

The absolute volumes are:

• water $193/1{,}000 = 0.1930$;
• cement $357.4/3{,}150 = 0.1135$;
• coarse $1{,}104/2{,}680 = 0.4119$;
• air $0.0150$.

Their sum is $0.7334\text{ m}^3$. The fine aggregate is $(1 - 0.7334)(2{,}640) = 704\text{ kg}$.

Moisture adjustment. The sand has 4.0% total moisture with 1.0% absorption, and the gravel has 1.0% with 0.5% absorption.

• Wet sand: $704(1.04) = 732\text{ kg}$.
• Wet gravel: $1{,}104(1.01) = 1{,}115\text{ kg}$.
• Batch water: $193 - 704(0.030) - 1{,}104(0.005) = 166\text{ kg}$.

Philippine volumetric classes. Field estimating in the Philippines commonly uses nominal mixes by volume:

• Class AA, 1 : 1.5 : 3;
• Class A, 1 : 2 : 4;
• Class B, 1 : 2.5 : 5;
• Class C, 1 : 3 : 6.

Cement factors in 40-kg bags per cubic meter come from estimating tables. These nominal mixes are for quantity estimates. Strength-controlled work is proportioned and accepted by tests as described above.