CE Board Exam Randomizer

⬅ Back to Construction Topics

Aggregates and Batching

Aggregates (fine sand and coarse gravel/crushed stone) make up 60–75% of concrete volume. Their grading is summarized by the fineness modulus (FM), the sum of the cumulative percentages retained on the standard sieves divided by 100. Aggregate is also described by its bulk specific gravity, bulk (dry-rodded) unit weight, and the moisture states: oven-dry, air-dry, saturated-surface-dry (SSD), and wet. Absorption is the water needed to bring oven-dry aggregate to SSD; free (surface) moisture is water beyond SSD that adds to the mixing water.

Batching is the process of proportioning cement, water, aggregates, and admixtures before mixing. It may be done by weight (accurate, preferred) or by volume (using class ratios such as 1:2:4). Surface moisture on aggregates must be deducted from the batch water.

$$\text{FM}=\frac{\sum(\text{cumulative \% retained})}{100}, \qquad \%\,\text{voids}=\left(1-\frac{\gamma_{\text{bulk}}}{G_s\,\gamma_w}\right)\times 100$$
$$\text{Absorption}=\frac{W_{SSD}-W_{OD}}{W_{OD}}\times100, \qquad \text{Free moisture}=\frac{W_{wet}-W_{SSD}}{W_{SSD}}\times100$$

★ Cement Bags for a Class A Mix

A footing requires 4.5 m³ of Class A (1:2:4) concrete, which needs about 9 bags of cement per cubic meter. How many 40-kg bags of cement are required?

$$N=9\ \tfrac{\text{bags}}{\text{m}^3}\times4.5\ \text{m}^3=40.5\ \text{bags}$$

Round up so there is no shortage. Final answer: 41 bags.

★ Fineness Modulus of Sand

A fine-aggregate sample gives the following cumulative percentages retained on the standard sieves (Nos. 4, 8, 16, 30, 50, 100): 2, 12, 30, 58, 78, 92. Determine the fineness modulus.

$$\sum=2+12+30+58+78+92=272$$
$$\text{FM}=\frac{272}{100}=2.72$$

Final answer: FM = 2.72 (typical sand ranges 2.3–3.1).

★★ Percentage of Voids in Coarse Aggregate

A coarse aggregate has a dry-rodded bulk unit weight of 1600 kg/m³ and a bulk specific gravity of 2.65. Determine the percentage of voids.

$$\%\,\text{voids}=\left(1-\frac{1600}{2.65\times1000}\right)\times100$$
$$=\left(1-0.6038\right)\times100=39.6\%$$

Final answer: 39.6% voids.

★★ Aggregate Absorption

An aggregate sample weighs 504 g in the saturated-surface-dry (SSD) condition and 482 g after oven drying. Determine the absorption.

$$\text{Absorption}=\frac{504-482}{482}\times100=\frac{22}{482}\times100$$
$$=4.56\%$$

Final answer: 4.56%. This water is held inside the aggregate and does not count as mixing water.

★★★ Adjusting Mixing Water for Moisture

A concrete mix design calls for 185 kg of water per cubic meter. The sand (620 kg/m³) carries 4% free surface moisture and the gravel (1150 kg/m³) carries 1% free moisture. Determine the water that must be added at the mixer.

Free surface moisture already on the aggregates adds to the mix and must be subtracted from the design water.

$$W_{\text{free}}=0.04(620)+0.01(1150)=24.8+11.5=36.3\ \text{kg}$$
$$W_{\text{add}}=185-36.3=148.7\ \text{kg}$$

Final answer: add 148.7 kg of water per cubic meter.

Scroll to zoom

Exam Generator Problems

Additional board-style practice items for this topic.

Question Bank: w56

MSTE - Construction / Aggregates and Batching / MSTE May 2019

What is the process of proportioning the cement, water, aggregates and additives prior to mixing concrete called?

  1. Batching
  2. Curing
  3. Paving
  4. Mixing
Batching is the process of weighing or measuring out the cement, water, aggregates, and additives according to a mix design before they are mixed. (Mixing blends them; curing maintains moisture for hydration; paving places the concrete.)
$\boxed{\text{Batching}}$

Question Bank: w102

MSTE - Construction / Aggregates and Batching / MSTE November 2019

The most important process to prepare concrete.

  1. batching
  2. admixture
  3. cement-aggregate ratio
  4. water-cement ratio
Batching — the accurate proportioning and measuring of cement, water, aggregates, and admixtures before mixing — is the most important process in preparing concrete.
$\boxed{\text{batching}}$