News & Trends Reinforced Concrete ACI updated code ACI318-19 ULS Design

ACI 318-19 Updates for ULS Design of Reinforcement Concrete

Yeong-il SeoApril 4, 2022

ACI 318-19 Updates for ULS Design of Reinforcement Concrete


The new ACI318 Code, ACI318-19 is similar with the previous code, ACI318-14 in the basic approach. but in the current documents, there are a number of significant differences.

Through this brief document, you can discover the differences between the two codes, and understand their factors and design concepts.

In this document, the comparison between ACI 318-19 and ACI318-14 Code has 8 classified sections.

1. New rebar material,

2. Minimum reinforcement provisions

3. New reinforcement strain limit

4. Significant updates to shear provisions

5. Hanger reinforcement provisions

6. New equation for Ie

7. Modification of development length provisions

8. Modification of earthquake-resistance provisions.


Also, the specific standards are based on the ACI318M-19 and ACI318M-14 as SI Units for US Codes.


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√ Table of Contents 

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An Understanding of Structure in 2-Way Slab System

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Structure in 2-Way Slab System


The Building Structural Standard defines a two-way slab system as a concrete slab system in which two rebars are arranged in two directions regardless of the presence or absence of a beam that transmits a load to a column'. Also, in ACI 318, this is expressed as ‘slab systems reinforce flexure in more than one direction, with or without beams between supports’. Therefore, a two-way slab system refers to a slab system in which reinforcing bars are arranged for bending in two or more directions regardless of the presence or absence of beams in columnar rows'. Typical types of two-way slab systems commonly used today include two-way slabs with beams, flat plates, flat slabs, and waffle slabs.


Figure1. Typical Types of 2-Way Slab

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In a certain concrete structure with considerable mass or where a construction progresses rapidly with a number of construction joints, the rate and amount of heat generation due to hydration are important. Non-uniform thermal expansion and contraction due to heat of hydration and cooling of concrete accompanied by changing constraints create undesirable stresses. The stresses may cause detrimental cracking in the concrete, thereby reducing its strength and durability.


Heat of hydration analysis thus becomes important when casting mass concrete structures. It enables us to predict and control temperature and stress distribution within a structure to avoid potential problems. Mass concrete structures requiring heat of hydration analysis depend on their dimensions, shapes, cement types and construction conditions.


In practice, hydration analyses are normally carried out for slabs or mats in excess of 800~1000mm in thickness and walls confined at bottom in excess of about 500mm. Surface cracking may develop initially due to the temperature difference between the surface and center. Through-cracks can also develop as a result of contraction restrained by external boundary conditions in the cooling process of high heat of hydration.


The heat of hydration analysis is largely classified into several sub-analyses. It entails temperature distribution analysis for conduction, convection, heat source, etc.; change in modulus of elasticity due to curing and maturity; and stress analysis for creep and shrinkage. The following outlines the various components affecting the analysis.



1. Heat Transfer Analysis

2. Thermal Stress Analysis

3. Procedure for Heat of Hydration Analysis

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The Objective of This Guide

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What is Post-Tensioning?


Post-tensioning is a technique for reinforcing concrete. Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterward, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete.


Post-tensioning is a form of prestressing. Prestressing simply means that the steel is stressed (pulled or tensioned) before the concrete has to support the service loads. Most precast, prestressed concrete is actually pre-tensioned-the steel is pulled before the concrete is poured. Post-tensioned concrete means that the concrete is poured and then the tension is applied-but it is still stressed before the loads are applied so it is still prestressed.


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