Concrete Terminology and Mechanical Resistance Testing

Concrete Terminology and Definitions

Mixing Water Definition

Mixing Water: Water contained in fresh concrete minus the water absorbed by the aggregate to reach the Saturated Surface Dry (SSD) condition.

Aggregates and Water Absorption

Aggregates (Rocks): Aggregates have pores, and when in contact with water, absorption occurs until they become saturated (often appearing shiny).

Water-Cement Ratio (W/C Ratio)

Water-Cement Ratio (W/C): The ratio between the mass of free water and the mass of cementitious materials (cement and additives).

Kneading (Mixing)

Kneading (Mixing): The process of combining the smaller fractions of concrete materials (the matrix) at one time.

Concrete Mechanical Resistance

Mechanical Resistance

Mechanical Resistance (Strength): The maximum stress that concrete supports, typically expressed in MPa (Megapascals).

Required Average Resistance

Required Average Resistance (f_r): The estimated average result of the mechanical resistance tests.

Average Concrete Resistance

Average Concrete Resistance (f_m): The arithmetic average of the results of the mechanical resistance tests of concrete specimens of the same grade and age.

Confidence Level

Confidence Level: A fraction expressed as a percentage (%) or decimal fraction that indicates the probability that the test results will equal or exceed the specified value (e.g., 95% confidence level implies a 5% failure rate).

Specified Resistance

Specified Resistance (f_c): The required compressive strength (f_r) or flexural strength.

Characteristic Resistance

Characteristic Resistance: A statistically calculated value derived from test results that corresponds to a given confidence level, assuming a normal distribution.

Determining the Water-Cement Ratio

The Water-Cement Ratio (W/C) can be determined based on requirements for strength, durability, or both.

Procedures for Determining Resistance

Resistance determination must be performed using one of the following three procedures:

Procedure 1: Using Previous Test Records

Utilize previous test records to demonstrate the suitability of the proposed concrete batching, provided that the records:

1. Represent the same source materials and conditions.
2. Account for variations in materials and implied dosage adjustments.
3. Include a record of 30 tests covering a period of less than 45 days.

Procedure 2: Testing with Multiple W/C Ratios

Perform concrete tests using three different Water-Cement Ratios (W/C), ensuring the same workability required for the project.

Procedure 3: Determination via Required Average Resistance Table

Determine the Water-Cement Ratio (W/C) based on the required average resistance (f_r) using a standardized table.

Aggregate Testing and Quality Control

Aggregate Sampling and Preparation

Sampling and Preparation: Proper removal and adequate preparation of an aggregate sample are crucial to obtaining representative values during corresponding tests that reflect the original aggregate conditions.

Required Sample Size

Sample Size: A strategic simple or compound sample must be at least twice the size of the sample required for basic laboratory testing. The specific sample size for the work will be detailed in a table.

Sieving and Aggregate Grading

Sieving and Grading: Grading determines the size distribution of an aggregate sample. To determine the grain size, a test sample mass is filtered (sieved), and the mass fractions of the aggregate retained on each sieve are measured.