C B Shah B.E., M. S.
CEPT University, India
Abstract: Codes of practice do not specifically recommend a limit of maximum temperature for casting& curing concrete.(1) A.C.I. recommended practice for Hot-Weather concreting (605-59) mentions that “Hot weather may adversely affect concrete by causing increased water demand, reduced strength and excessive volume changes which results in cracking. The effect results from high concrete temperatures and rapid evaporation of water from the concrete.” The effects of casting concrete at32.2o C (90oF)on properties in its fresh and hardened states are reported in a paper(2). A study3 was done to find properties of fresh and hardened concrete with uncrushed rounded gravel & sand as aggregates cast and cured at47.8oC(118oF). The findings were compared with those of similar concrete cast and cured at 22.8oC (73oF).Design mixes were developed considering 17.24 MPa (2500psi) and 27.58MPa (4000psi) as base strengths. Properties such as Workability (slump), Density and % Air content of fresh concrete, and Compressive Strength, Secant Modulus E, Dynamic Modulus and Modulus of Rupture of hardened concrete were noted. Relationships between Compressive strength and other properties were discussed.
Research and Development Bulletin RD113T of Portland Cement Association, USA includes a paper(1) by Ronald G Burg on “Influence of casting and curing temperatures on properties of Fresh and Hardened Concrete”. The influences of casting and curing temperatures of 10oC (50oF), 23oC (73oF) and 32oC (90oF )on workability, setting times, unit weight (kg/m3) and % air content in fresh concrete and compressive strength of hardened concrete at ages of 3, 7, 28, and 56 days are presented in the paper. Properties of concrete cast & cured at 10oC and at 32oC were compared with those of concrete cast & cured at 23oC. It is reported in the bulletin that slump & setting time of fresh concrete cast at 10oC were higher than that 23oC and were lower than that of concrete cast at 32oC. Percent compressive strength at 7 days of concrete cast cured at 10oC was lower than that of concrete cast & cured at 23oC but was equal to or more than that of concrete cast & cured at 32oC. Percent compressive strength at 28 days of concrete cast and cured at 32oC was higher than that of concrete cast & cured at 23oC but at later ages of 56 days were lower than that of concrete cast & cured at 23oC.
Another research is reported by Kim el al (4) on “Compressive strength development of concrete with different curing time and temperature.” The curing temperatures were 5oC and 40oC for selected days, while temperature for resting period was kept 20oC. They concluded that concrete subjected to a high temperature at an early age gains higher early age strength but rate of gain of strength decreases at later ages. Concretes subjected to lower temperature during initial days though showed low strength at early age, gained same later age strength.
In Hot Tropical climate like one at 23oN Latitude– Tropic of Cancer- in western parts of India, temperatures above 40oC are common from 10am to 5pmfrom mid-April to June end. On site concrete making & placement in position are usually practiced during this time of day. A study was undertaken at University of Wisconsin, Madison to find “EFFECT OF PLACING AND CURING TEMPERATURE ON PROPERTIES OF CONCRETE”3. This research was done in 1962. Hence all design mixes and testing methodology as well as results correlations are with respect to codes prevalent at that time. Efforts have been made to connect the same with current formulae.
Objective was to find effects of casting and curing concrete at high temperature on its properties in fresh and hardened states. Concrete was cast with natural un-crushed gravel and sand as aggregates.
Concrete mixes 6, 7 were designed (after multiple trials based on ACI 613-54) for compressive strength of 17.24 MPa (2500psi) and 27.58MPa (4000psi) at 28 days. Coarse aggregates quantity was kept constant. Concrete mixes were cast and cured at 22.8oC (73oF) and at118oF (47.8oC).Slump, air content and unit weight of fresh concrete and compressive strength and other properties of hardened concrete were studied.
Presentation of detailed test results and discussion of Compressive Strength is mentioned in this paper. Test results of Static E, Dynamic E, Modulus of Rupture and Modified Cube Strength are also presented in this paper and their relationships with compressive strength are discussed.
Single brand of A.S.T.M. Type I cement was used in the study5. Oxide & Mineral compositions of the cement are shown in table 1 below. Oxide composition was determined by Wisconsin High Commission. Mineral composition was calculated on the basis the oxide composition. Physical properties of cement and aggregates are mentioned in tables 2 and 3 respectively.
Values of specific gravity and % absorption for gravel & sand were taken from previous study3.
Laboratory set up for the study
A room was prepared in subbasement of laboratory to store materials, tools & equipment for casting and curing concrete at 47.8oC (118oF). Temperature in the room was maintained by low pressure steam. The room was unventilated except for opening & closing of a door during work. Water was stored in an aluminum drum covered with a lid which was shut all the time except for taking water for mixing concrete. Water in curing tank was also maintained at 47.8oC (118oF). For casting and curing concrete at 73oF (22.8oC), a space was provided in the laboratory area at first floor. Tables 4 and 5 shows the nomenclature and specifications decided as well as mixture proportion designed respectively for the study.
Casting of concrete
Ingredients were weighed on scale of maximum capacity of 113.4kg (250lb) and least count of 0.057kg (0.125lb) at 22.8oC.Ingredients for casting concrete at 47.8oC were brought to the room in subbasement at least 24 hour before scheduled casting. The concrete was mixed in an electrically driven tilting drum type mixer of 0.0283m3(1cft) capacity rotating at 20 rpm. Ingredients were mixed in dry condition for2 minutes and for 1.5 minutes after adding water. After mixing concrete was discharged in a pan and was turned over 3 times to ensure uniform consistency. Compressive strength and secant modulus were determined by testing cylindrical specimens of 150mm x 300mm were cast. Dynamic modulus of elasticity and modulus of rupture were determined using 75mm x 100mm x 325mm beams. Modified cube strength was determined from 100mm x 100mmsamples recovered from beams tested for modulus of rupture. Following Table 6 and table 7 show the experimental values of properties of concrete in fresh and hardened states respectively. Air contents were measured by Washington Pressure Type Meter. At 47.8oCsmall reduction in slump and an increase in % air content were observed in the concrete.
Discussions of Test results of hardened concrete and their relation with compressive strength
It is probable that test results of hardened concrete would be influenced by different curing and storage conditions and variable moisture contents of the specimens at the time of tests.
Figures 1 and 2 show compressive strength and its internal comparison of all concretes at all ages.
Compressive strength of concrete increased with age all groups. The rate of increase for concrete cast & cured at 47.8oCwas higher in range of about 10-15% up to 7 days than that of concrete cast & cured at 22.8oCfor concretes A, B& C (17.24 MPa). But it was almost same for concretes D and E (27.58 MPa). Eventually 28th and 90th day strength was governed by concrete cast and cured at 22.8oC in all mixes. Addition of water in mix C, though improved workability, gave lower strength on 28th day, but the compressive strengths of similar mixtures (B and C) cast and cured at elevated temperature were observed almost same. Overall observation is that at the ages of 28 and 90 days, % increase of compressive strength of the concrete cast & cured at 47.8oC was less than that of concretes cast at 22.8oC.Results of modified cube strength follow similar trend as of compressive strength up to age of 90 days. Some extra specimens tested at 118 days indicated loss of strength under high curing temperature in dry curing conditions.
Secant Modulus E
Secant E increased with age up to 90 days for placing and curing at 22.8oCfor all groups. Ratios of secant E to ultimate compressive strength (fc’: Compressive strength at 28 days) generally decreased for all groups of concrete. The experimental values of Secant E are close to ACI 318 formula viz. Secant E = W3/2 (33 vfc’) for lower strength concrete. Here W is the unit weight (lbs./cft) of concrete considered in the formula.
Dynamic E increased with age for all groups. Overall, the values of concretes cast and cured at higher temperatures were less than those cast and cured with 22.8oC.
Modulus of rupture (MR)
The values more or less follow the relation of MR = 350 + 0.072*fc’. The values here are almost matching except for the mix C in which extra water was added. With the current formula for modulus of rupture formula, that is 7.5?vfc’, an average value for ?(aggregate factor) is 1.4.The change might be due to gravel as aggregates in concrete. Summarizing all results, the values of modulus of rupture at all ages were about 17-28% of respective compressive strength for mixtures A, B and C, whereas, about 15-18% for D & E. This trend is observed independent to the casting and curing temperatures.
Modified cube strength
Influences of placing and curing temperatures were not well defined. Average values of ratios of modified cube compressive strength and that of fc’ was in the range of 1.45-2.
It is believed that concrete gains strength with age. This may be true for concrete cast and cured at a temperatures not exceeding32.2oC (90oF). Concretes cast & cured at temperatures above 32.2oC may not gain in compressive strength and other properties as may be expected. This study was done under controlled temperature of 47.8oC, wherein on actual site, effect of direct solar radiation will add up to the complications. The disintegration of micro structure due to higher temperature of casting and prolonged curing as well as due to heat of hydration generated within the mass of concrete itself during initial period together may affect the rate of gain of strength of concrete at later stage.
For gravel based concrete, it is recommended to use mixture of higher strength than required at the places where day temperature tends to go beyond 32.2oC, as from experimental results it is revealed that the designed target strength tends to be more than actual strength achieved. With the rapid use of RMC currently, it is recommended that concreting in the area having higher daily temperature should be done during early morning hours or evening hours (after 7 pm) and proper methodology of curing should be adopted immediately after 1 hour of casting.
It is just a recommendation and detailed study should be done incorporating the same. This suggestion has been taken from the study done by Lucy in her dissertation on “Effect of high curing temperatures on the strength, durability and potential of delayed ettringite formation in mass concrete structures”, University of Florida, 20068
1. A.C.I recommended practice for hot weather concreting 605-59.
2. The influence of casting and curing temperature in the properties of fresh and hardened concrete; By Ronald G. Burg; Research and Development Bulletin RD 113 T, Portland cement association, Skokie, Illinois USA1996
3. A Thesis on “EFFECT OF PLACING AND CURING TEMPERATURE ON PROPERTIES OF CONCRETE” by Pankaj A. Desai and Chandrakant B. Shah, for M.S. Degree at University of Wisconsin, Madison, USA in 1962.
4. Compressive strength development of concrete with different curing time and temperature; By J-K Kim, Y-H Moon, S.-H Eo; Cement and Concrete Research, Volume 28, Issue 12, December 1998 – pages 1761-1773
5. A.S.T.M. Part 4, Cement, Concrete Mortars, Road Materials, Water proofing and Soils, 1961.
6. Recommended Practice for Selecting proportions for concrete, ACI 613-54
7. Design and control of concrete mixes, Portland Cement Association.
8. Lucy, Dissertation on “Effect of high curing temperatures on the strength, durability and potential of delayed ettringite formation in mass concrete structures”, University of Florida, 2006.
Author sincerely acknowledge help from his colleagues at Faculty of Technology, CEPT University, for their help in compositing this paper. Author is specifically grateful to Dr. Anal Shah for help in preparing tables and graphs.