CIP 4 - Cracking Concrete Surfaces

1 CIP 4 - Cracking Concrete SurfacesWHAT are Some Forms of Cracks? Concrete , like other construction materials, contractsand expands with changes in moisture and tempera-ture, and deflects depending on load and support condi-tions. Cracks can occur when provisions to accommo-date these movements are not made in design and con-struction. Some forms of common cracks are:Figure A:Plastic shrinkage cracks (CIP 5)Figure B:Cracks due to improper jointing (CIP 6)Figure C: Cracks due to continuous external restraint(Example: Cast-in-place wall restrained alongbottom edge of footing)Figure D: Cracks due to lack of isolation joints (CIP 6)Figure E: D-Cracks from freezing and thawingFigure F:Craze Cracks (See CIP 3)Figure G: Settlement cracksMost random cracks that appear at an early age, al-though unsightly, rarely affect the structural integrity orthe service life of Concrete . Closely spaced patterncracks or D-cracks due to freezing and thawing, thattypically appear at later ages, are an exception and maylead to ultimate majority of Concrete cracks usually occur due toimproper design and construction practices, such as:a.

2 Omission of isolation and contraction joints and im-proper jointing Improper subgrade The use of high slump Concrete or excessive addi-tion of water on the Improper Inadequate or no to Prevent or Minimize Cracking ?WHY Do Concrete Surfaces Crack?All Concrete has a tendency to crack and it is not pos-sible to produce completely crack-free Concrete . How-ever, Cracking can be reduced and controlled if the fol-lowing basic concreting practices are and Formwork. All topsoil and soft spotsshould be removed. The soil beneath the slab shouldbe compacted soil or granular fill, well compactedby rolling, vibrating or tamping. The slab, and there-fore, the subgrade, should be sloped for proper drain-ABCFDGEC racksWallFooting1978, 1989 AND of Cracking in Concrete Structures, ACI 224R,American Concrete Institute, Farmington Hills, for Concrete Floor and Slab Construction, , American Concrete Institute, Farmington Hills, Slab Surface Defects: Causes, Prevention,Repair, IS177, Portland Cement Association, Skokie, Grant T.

3 Halvorson, Troubleshooting Concrete Crack-ing During Construction, Concrete Construction, Octo-ber in Concrete : Causes, Prevention, Repair, A col-lection of articles from Concrete Construction Magazine,June In winter, remove snow and ice prior to placingconcrete and do not place Concrete on a frozensubgrade. Smooth, level subgrades help preventcracking. All formwork must be constructed andbraced so that it can withstand the pressure of theconcrete without movement. Vapor retarders directlyunder a Concrete slab increase bleeding and greatlyincrease the potential for Cracking , especially withhigh-slump Concrete . When a vapor retarder is used,cover it with 3 to 4 inches of a compactible granularfill, such as a crusher-run material to reduce bleed-ing. Immediately prior to Concrete placement, lightlydampen the subgrade, formwork, and the reinforce-ment if severe drying conditions In general, use Concrete with a moder-ate slump (not over 5 inches [125 mm]).

4 Avoidretempering Concrete to increase slump prior toplacement. Higher slump (up to 6 or 7 inches [150to 175 mm]) can be used provided the mixture isdesigned to produce the required strength withoutexcessive bleeding and/or segregation. This is gen-erally accomplished by using water-reducing admix-tures. Specify air-entrained Concrete for outdoorslabs subjected to freezing weather. (See CIP 2) Initial screeding must be promptly fol-lowed by bull floating. DO NOT perform finishingoperations with water present on the surface or be-fore the Concrete has completed bleeding. Do notoverwork or over-finish the surface. For better trac-tion on exterior Surfaces use a broom finish. Whenambient conditions are conducive to a high evapo-ration rate, use means to avoid rapid drying and as-sociated plastic shrinkage Cracking by using windbreaks, fog sprays, and covering the Concrete withwet burlap or polyethylene sheets between Curing is an important step to ensure du-rable crack-resistant Concrete .

5 Start curing as soonas possible. Spray the surface with liquid membranecuring compound or cover it with damp burlap andkeep it moist for at least 3 days. A second applica-tion of curing compound the next day is a good qual-ity assurance Anticipated volumetric changes due to tem-perature and/or moisture should be accommodatedby the construction of contraction joints by sawing,forming or tooling a groove about 1/4 to 1/3 the thick-ness of the slab, with a spacing between 24 to 36times the thickness. Tooled and saw-cut joints shouldbe run at the proper time (CIP 6). A maximum 15feet spacing for contraction joints is often recom-mended. Panels between joints should be square andthe length should not exceed about times thewidth. Isolation joints should be provided wheneverrestriction to freedom of either vertical or horizontalmovement is anticipated such as where floors meetwalls, columns, or footings. These are full-depth jointsand are constructed by inserting a barrier of sometype to prevent bond between the slab and the Over Reinforcement.

6 Providing sufficientconcrete cover (at least 2 inches [50 mm]) to keepsalt and moisture from contacting the steel shouldprevent cracks in reinforced Concrete caused by ex-pansion of rust on reinforcing These Rules to Minimize Cracking1. Design the members to handle all anticipated Provide proper contraction and isolation In slab on grade work, prepare a stable Place and finish according to recommended and established Protect and cure the Concrete properly.