---
title: "Abram's rule and additional factors"
subject: "Concrete Technology and Masonry Structure"
chapter: 3
key: "semester_5/concrete_technology_and_masonry_structure:abrams-rule-additional-factors"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/abrams-rule-additional-factors
site: BCT Engineering
---

# Abram's rule and additional factors

Concrete Technology and Masonry Structure · Chapter 3: Mix design of concrete and property of green concrete

- Asked in: 2066 Jestha (old course), 4+6 marks
- Video: [Lec 1 - Factors Affecting Compressive Strength of Concrete | Concrete Technology - Part 1](https://www.youtube.com/watch?v=fH0TdE4BNu4) — DCBA online
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/abrams-rule-additional-factors
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-3

## Question

What is Abram's rule of concrete strength? Describe two additional factors (not included in Abram's rule) which have a significant influence on concrete strength.

## Short answer

Abrams' rule states concrete strength depends only on water-cement ratio; lower w/c gives higher strength. Two additional factors are degree of compaction, where air voids significantly reduce strength (1% voids reduces strength by 5-6%), and age/curing, as strength increases with proper moist curing and suitable temperature, while lack of curing can lower strength by 30-50%.

## Answer

#### Abrams' rule
Abrams (1918) stated that for concrete made with given materials, the **strength depends only on the water-cement ratio**; a lower w/c ratio gives a higher strength:

$$
f_c = \frac{A}{B^{\,w/c}}
$$

($A$ and $B$ are constants for the materials and age; e.g. $A = 96.5$ MPa, $B = 7$). It holds for plastic, fully compacted mixes.

#### Additional factors not covered by the rule
**1. Degree of compaction (air voids)**
Abrams' law assumes complete compaction. Voids left by poor compaction reduce strength severely: 1% of air voids reduces strength by about 5-6%, and 5% voids by up to 30%. The relative strength depends on the density ratio (compacting factor). Stiff mixes with a low w/c need powerful vibration, while a mix with too high a w/c segregates.

**2. Age and curing (temperature and moisture)**
Abrams' law applies at a stated age. Strength increases with age as hydration proceeds (about 60-65% at 7 days, 100% at 28 days, 120-130% at 1 year), but only if the concrete is kept moist (curing) and at a suitable temperature; a lack of curing leaves capillary pores and can reduce strength by 30-50%. A higher temperature gives faster early strength but lower later strength.

**Other factors**: aggregate properties (size, shape, texture, strength, cleanliness), cement type and content, admixtures, aggregate-cement ratio, and the transition zone.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Abrams' rule says strength depends mainly on
   A. aggregate colour
   B. mixer type
   C. water-cement ratio
   D. slump
   Answer: C. For given materials, strength depends on w/c. (Key point: Rule)

2. 1% air voids reduce strength by about
   A. 0.5%
   B. 25%
   C. 50%
   D. 5-6%
   Answer: D. Poor compaction reduces strength; 1% voids cost about 5-6%. (Key point: Compaction)

3. Lack of curing may reduce strength by about
   A. 30-50%
   B. 1%
   C. 5%
   D. not at all
   Answer: A. Without moisture, hydration stops and strength drops by 30-50%. (Key point: Curing effect)

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