Rectangle

The rectangular stress block has zero stress up to a strain of $\varepsilon_c(\text{controlled by}\beta)$ and then a constant stress of $\alpha f_{c d}$.

Code	Version	$a$	$\beta$
ACI318		$\small 1$	$0.85-0.05(f_c-30) / 7 \\ [0.65 : 0.85]$
AS3600	$\small 2001$	$\small 1$	$0.85-0.07\left(f_c-28\right) \\ [0.65: 0.85]$
AS3600	$\small 2009$	$\small 1$	$1.05-0.007 \mathrm{f}_{\mathrm{c}} \\ [0.67: 0.85]$
BS5400		$0.6 / 0.67$	$\small 1$
BS8110		$1$	$0.9$
CSA A23.3		$\small \max \left(0.67,0.85-0.0015 f_c\right)$	$\small \max (0.67,0.97-0.0025\left.f_c\right)$
CSA S6		$\small \max \left(0.67,0.85-0.0015 f_c\right)$	$\small \max (0.67,0.97-0.0025\left.f_c\right)$
EN 1992		if $f_c \leq 50 \mathrm{MPa}$ ; $a = 1$ $\\$ if $f_c>50 \mathrm{MPa}$ ; $a = 1-\frac{(f_c-50)}{200}$	if $f_c \leq 50 \mathrm{MPa}$ ; $\beta = 0.8$ $\\$ if $f_c > 50 \mathrm{MPa}$ ; $\beta = 0.8 - \frac{(f_c - 50)}{400}$
HP CP	$\small > 2004$	$\small 1$	$0.9$
HK CP	$\small 2007 >$	$\small 1$	$0.9 \space \space \space \space \space \space \space \space \space \space f_c\space\leq\space45 \mathrm{MPa} \\0.8 \space \space \space \space \space \space \space \space \space \space f_c\leq70 \mathrm{MPa} \\ 0.72 \space \space \space \space \space \space \space \space f_c \space \leq\space 100 \mathrm{MPa}$
HK SDM		$0.6 / 0.67$	$1$
IRC:112		if $f_c \leq 60 \mathrm{MPa}$ ; $a = 1$ $\\$ if $f_c>60 \mathrm{MPa}$ ; $a = 1- \frac{(f_c-60)}{250}$	if $f_c\leq 60 \mathrm{MPa}$ ; $\beta = 0.8$ $\\$ if $\mathrm{f}_{\mathrm{c}}>60 \mathrm{MPa}$ ; $\beta = 0.8-\frac{(f_c-60)}{500}$
IRS Bridge		$0.6 / 0.67$	$1$
IS 456		$0.8$	$0.84$