Effects of Ground Granulated Blast Furnace Slag in Portland Cement ...

6. Guidelines on Use and Summary of Findings
The guidelines and predictive equations outlined below are based on the experiences of this
study. Data from testing with grade 100 GGBFS and four brands of Type I cement with two
types of aggregate was averaged and used to establish the equations. As a result there will be
some variation from these equations particular conditions and materials, but the guidelines
should be a representative guide of expected results.
6.1 Compressive Strength Guidelines
Mantel investigated the correlations between the chemical composition of cements and slags
and their strength performance. After examining five GGBFS samples and eight cements he
found that there was no clear correlation between chemical composition and performance
(Mantel 1994). Mantel recommends that trial mixes be prepared to determine a portland
cement’s adequacy when used with GGBFS. To simplify this process the correlations of
strength with the slag activity tests in section 5.2 were made.
In an attempt to establish general guidelines the results of all compressive tests at each
replacement level were graphed. Figure 17 reveals that compressive strength for GGBFS
concrete tend to be nonlinear as GGBFS level increases reflecting the slower strength
development at early ages. At one year, the strengths of all concretes tend to converge but
GGBFS levels tend to result in slightly lower strengths on average. The following equations
represent the average strength as a function of age for w/cm =0.45:
CS
2
GGBFS
= − 229*( Log(
Age))
+ 1843* Log(
Age)
2205
(Eqn. 3)
0 %
+
2
CS
GGBFS
= − 418*( Log(
Age))
+ 2713* Log(
Age)
1031
(Eqn. 4)
30 %
+
2
CS GGBFS
= − 712*( Log(
Age))
+ 3815* Log(
Age)
22
(Eqn. 5)
50%
−
Where CS = compressive strength in psi and Age = concrete age in days.
The following compressive strength guidelines can be formed from this study:
• Higher 7-day slag activity indices tend to indicate increased 7-day and 28-day
strengths
• Limestone aggregate tended to produce slightly stronger concrete (10% or less).
• GGBFS should not be used at temperatures lower than 40°F as the time to achieve
3000 psi can be unreasonably long for w/cm = 0.45.
• 50% GGBFS concrete has comparable strength to 30% GGBFS concrete after 14
days with w/cm = 0.45.
• While there is delayed early strength development in GGBFS concrete, Grade 100
GGBFS concrete at cement replacement to 50% has comparable (but slightly less)
strength to OPC after 56 days
31